Breeding for low cadmium accumulation cereals

Breeding for low cadmium accumulation cereals 禾谷类作物低镉积累育种研究进展 first-author corresp ‡Corresponding Author ‡Corresponding Author †E-mail:wufeibo@zju.edu.cn †E-mail:wufeibo@zju.edu.cn wufeibo@zju.edu.cn Department of Agronomy, College of Agriculture and Biotechnology, Zhejiang University, Hangzhou 310058, China Department of Agronomy, College of Agriculture and Biotechnology, Zhejiang University, Hangzhou 310058, China Cadmium (Cd) is an element that is nonessential and extremely toxic to both plants and human beings. Soil contaminated with Cd has adverse impacts on crop yields and threatens human health via the food chain. Cultivation of low-Cd cultivars has been of particular interest and is one of the most cost-effective and promising approaches to minimize human dietary intake of Cd. Low-Cd crop cultivars should meet particular criteria, including acceptable yield and quality, and their edible parts should have Cd concentrations below maximum permissible concentrations for safe consumption, even when grown in Cd-contaminated soil. Several low-Cd cereal cultivars and genotypes have been developed worldwide through cultivar screening and conventional breeding. Molecular markers are powerful in facilitating the selection of low-Cd cereal cultivars. Modern molecular breeding technologies may have great potential in breeding programs for the development of low-Cd cultivars, especially when coupled with conventional breeding. In this review, we provide a synthesis of low-Cd cereal breeding. 镉(Cd)是毒性最强和农田受污染最普遍的重金属之一.土壤镉污染严重影响作物的产量和品质,并可通过食物链富集,从而危害人体健康.选育和种植食用部位镉低积累的作物品种是经济有效利用镉污染土壤,保证农产品安全生产,有效降低镉进入食物链的一条经济、有效的途径.本文主要介绍了近年来国内外在禾谷类作物镉低积累相关数量性状位点(QTL)定位、基因挖掘及其在低镉积累育种中的应用的最新成果;概述了镉低积累作物品种的定义、特征与选育方法,为低镉积累分子育种与生产提供理论指导. Cereals Low Cd accumulation Gene/quantitative trait locus (QTL) mapping Breeding 禾谷类 低镉 数量性状位点(QTL)定位 育种 Introduction 1 Soil cadmium (Cd) contamination poses a serious problem for safe food production and has become a potential agricultural and environmental hazard worldwide. Cd contamination may be caused by industrial emissions or applications of Cd-containing sewage sludge, phosphate fertilizers, or municipal waste (Čásová et al., 2009 2009 ; Roberts, 2014 2014 ; Liu et al., 2015 2015 ; Yousaf et al., 2016 2016 ). Particularly, soils derived from marine shales or contaminated by mine waste are likely to have a high Cd content (Arunakumara et al., 2013 2013 ). Cd has high soil-to-plant mobility and can easily accumulate in plant tissues (Song et al., 2015 2015 ), causing reduced crop yields and threatening human health via the food chain (Clemens et al., 2013 2013 ; Sun et al., 2013 2013 ). In humans, Cd accumulates mainly in the kidneys and has a biological half-life of about 20 years (Clemens et al., 2013 2013 ; Aziz et al., 2015 2015 ). This very slow elimination of accumulated Cd can lead to a variety of serious health issues including anemia, cancer, cardiovascular disease, and renal tubular damage (Satarug et al., 2003 2003 , 2009 2009 ). Extreme cases of chronic Cd toxicity can result in osteomalacia and bone fractures, such as the “Itai-Itai” disease that occurred in Japan during the 1950s and 1960s (Huang et al., 2009 2009 ). As a result of these and other illnesses, Cd contamination in soils has been of increasing concern for decades in many industrialized and developing countries (Arthur et al., 2000 2000 ; Wu et al., 2004 2004 ; Cao et al., 2014a 2014a ). Soil Cd contamination poses considerable risks to human health via the consumption of foods containing high Cd concentrations, especially staple foods such as cereals (Stolt et al., 2003 2003 ). For example, rice has been a major dietary source of Cd for the Japanese population (Tsukahara et al., 2003 2003 ), contributing about 30% of the total dietary Cd intake in Japan during the 1990s (Ikeda et al., 1999 1999 ). Dabeka et al. ( 1987 1987 ) found that the Cd content of cereals was the highest among ten classified food groups in Canada. Grain Cd concentrations as high as 1–2 mg/kg dry weight (DW) have been recorded in rice grown in some areas of central-southern and southwestern China (Du et al., 2013 2013 ; Chen et al., 2018 2018 ). Those levels are much higher than the maximum permissible concentration (MPC) for cereal grains as defined by the Chinese National Standard (NHFPC, 2017 2017 ) and the Codex Alimentarius Commission (CAC, 2019 2019 ). According to national average estimates of exposure, cereals contribute about 32% of the total Cd intake by the Chinese population (WHO, 2011 2011 ). Therefore, it is imperative to reduce Cd accumulation in cereal grains so as to control Cd intake by human beings. The development and planting of low-Cd cultivars is a cost-effective and environmentally friendly approach to minimize soil–plant transfer of Cd and produce safe food for slightly or moderately contaminated soils. For over a decade, breeding programs have been initiated to select for low-Cd crop cultivars (Oliver et al., 1995 1995 ). Many studies have been conducted to screen and develop low-Cd cultivars in a variety of crop species, including rice, barley, maize, and wheat (Oliver et al., 1995 1995 ; Wu and Zhang, 2002 2002 ; Yu et al., 2006 2006 ; Chen et al., 2007a 2007a , 2007b 2007b ; Wu et al., 2007 2007 ; Grant et al., 2008 2008 ; Zeng et al., 2008 2008 ). In this review, we focus on recent advances in selecting low-Cd cereal crops, including: (1) genes related to Cd accumulation; (2) quantitative trait loci (QTLs) associated with Cd accumulation; and (3) breeding low-Cd cultivars. Genes associated with Cd accumulation in cereal crops 1 For developing low-Cd crop cultivars, it is important to identify genes related to Cd accumulation and to understand the mechanisms of Cd uptake and accumulation in plants. In recent years, a number of genes related to Cd transport in cereals have been identified (Table 1 1 ), and progress has been made in understanding the mechanisms of Cd uptake and transport (Chen JG et al., 2019 2019 ). Cd is not essential for plant growth, but its transport in plants is mediated by the transporters for essential elements such as Zn, Mn, Ca, and Fe (Uraguchi and Fujiwara, 2013 2013 ; Clemens and Ma, 2016 2016 ). Some genes related to chelation have also been found to play roles in plant Cd homeostasis. List of reported genes in rice ( Oryza sativa ) related to Cd homeostasis was reviewed by Chen JG et al. ( 2019 2019 ) Gene Gene name Probable function Reference 4 Oryza sativa 1 1   OsNramp1 2 Natural resistance-associated macrophage protein Cd and Fe transporters Takahashi et al., 2011a 2011a , 2011b 2011b   OsNramp5 Cd, Mn, and Fe transporters Ishikawa et al., 2012 2012 ; Sasaki et al., 2012 2012 ; Yang M et al., 2014 2014 ; Tang et al., 2017 2017   OsIRT1 4 Zinc-and iron-regulated transporter 2 Cd and Fe transporters Nakanishi et al., 2006 2006 ; Lee and An, 2009 2009   OsIRT2 Nakanishi et al., 2006 2006   OsZIP1 Cd and Zn transporters Bashir et al., 2012 2012 ; Ramegowda et al., 2013 2013 ; Liu XS et al., 2019 2019   OsZIP7 Cd and Zn accumulation Ricachenevsky et al., 2018 2018 ; Tan et al., 2019 2019   OsHMA2 3 P-type heavy metal ATPase Cd and Zn translocation Satoh-Nagasawa et al., 2012 2012 ; Takahashi et al., 2012 2012 ; Yamaji et al., 2013 2013   OsHMA3 Sequestration of Cd in root Ueno et al., 2010 2010 ; Miyadate et al., 2011 2011 ; Sasaki et al., 2014 2014 ; Yan et al., 2016 2016 ; Shao et al., 2018 2018 ; Lu et al., 2019 2019   OsHMA9 Cd efflux Lee et al., 2007 2007   OsLCD Low cadmium Cd tolerance and accumulation Shimo et al., 2011 2011   OsCd1 Major facilitator superfamily Cd uptake Yan et al., 2019 2019   OsLCT1 Low affinity cation transporter Cd transporter in phloem Uraguchi et al., 2011 2011 , 2014 2014   OsCCX2 Cation/Ca 2+ exchanger 2 Cd tolerance and translocation Hao et al., 2018 2018   OsMTP1 Metal tolerance protein gene Cd translocation Yuan et al., 2012 2012   OsPCS2 Plant chelatase synthase 2 Cd tolerance and accumulation Das et al., 2017 2017   OsPCS1 Plant chelatase synthase 1 Cd tolerance and accumulation Uraguchi et al., 2017 2017   OsMTI-1b Metallothionein-like protein 1b Cd tolerance and accumulation Ansarypour and Shahpiri, 2017 2017   CAL1 Defensin-like protein Cd accumulation in leaf Luo et al., 2018 2018 4 Triticum aestivum   TaHMA2 2 type heavy metal ATPase Cd and Zn transporters Tan et al., 2013 2013   HMA3-B1 Cd accumulation in grains Wiebe et al., 2010 2010 4 Triticum polonicum   TpNRAMP3 2 Natural resistance-associated macrophage protein Cd, Mn, and Co transporters Peng et al., 2018a 2018a   TpNRAMP5 Cd, Mn, and Co transporters Peng et al., 2018b 2018b Zea mays   GRMZM2G175576 Homologous to rice OsHMA2 and OsHMA3 Cd accumulation Zhao XW et al., 2018 2018 4 Hordeum vulgare   HvIRT1 3 Zinc-and iron-regulated transporter Fe, Mn, Zn, and Cd transporters Pedas et al., 2008 2008   HvZIP3 Mn, Zn, and Cd accumulation Sun et al., 2015 2015   HvZIP8 Mn, Zn, and Cd accumulation Sun et al., 2015 2015   HvHMA2 2 P-type heavy metal ATPase Zn, Fe, and Cd transporters Barabasz et al., 2013 2013   HvHMA3 Cd accumulation Wu et al., 2015 2015   HvNramp5 Natural resistance-associated macrophage protein Mn and Cd accumulation in roots and shoots Wu et al., 2016 2016 Transporter-related genes 2 In plants, natural resistance-associated macrophage proteins (NRAMPs) are a family of metal transporters that are integral components of membranes and play important roles in metal uptake, translocation, and intracellular transport (Nevo and Nelson, 2006 2006 ; Sasaki et al., 2012 2012 ). OsNramp5 , a Mn transporter located at the plasma membrane of root cells, was found to be a major transporter of Cd uptake in rice roots exposed to Cd-containing soil medium. Knockout or knockdown of OsNramp5 resulted in more than a 90% reduction in Cd uptake (Sasaki et al., 2012 2012 ; Yang M et al., 2014 2014 ), thereby drastically reducing the Cd content of shoots and grains (Ishimaru et al., 2012 2012 ; Tang et al., 2017 2017 ). Transfer DNA (T-DNA) insertion mutants of OsNramp5 led to large yield reductions of about 11% compared with wild-type plants (Sasaki et al., 2012 2012 ), while OsNramp5 knockout lines developed by ion-beam irradiation (Ishikawa et al., 2012 2012 ) or by the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) system (Tang et al., 2017 2017 ) did not affect grain yield. Orthologues of Nramp5 in other cereal crops have a similar function. TpNramp5 in dwarf Polish wheat functions as a metal transporter for Cd, Mn, and Co. When this gene was expressed in Arabidopsis , it significantly increased Cd, Co, and Mn concentrations in roots, shoots, or the whole plant (Peng et al., 2018b 2018b ). Similarly, knockdown of the orthologue in barley, HvNramp5 , which is preferentially expressed in the outer root cell layers and root tips, resulted in significant reductions of Mn and Cd concentrations in both roots and shoots (Wu et al., 2016 2016 ), but its relative contribution appears to be smaller than that of OsNramp5 in rice. Comparison of the expression levels and transport activities of three Nramp5 orthologues from rice, wheat, and maize, showed that OsNramp5 in rice was expressed at higher levels, resulting in higher Cd transport activity than Nramp5 in wheat or maize (Sui et al., 2018 2018 ). Other NRAMP members in cereal crops also show Cd uptake and transfer activities. The plasma membrane-localized iron transporter, OsNRAMP1, may assist in loading Cd into the xylem. Thus, over-expression of OsNRAMP1 in rice increased Cd accumulation in leaves while reducing Cd levels in roots (Takahashi et al., 2011a 2011a ). TpNramp3 from dwarf Polish wheat has a similar function to TpNramp5 , and is also a transporter for Cd, Mn, and Co. Heterologous expression of TpNramp3 in Arabidopsis resulted in increased Cd, Co, and Mn concentrations in roots, shoots, and whole plants, but did not affect their translocation from roots to shoots (Peng et al., 2018a 2018a ). In addition to the NRAMP family, several members of the zinc/iron-regulated transporter-like proteins (ZIPs) family, the primary zinc and iron transporters in plants, are also involved in Cd uptake. Some Fe transporters have been found to participate in plant Cd uptake. Nakanishi et al. ( 2006 2006 ) found that OsIRT1 and OsIRT2 play important roles in Cd uptake. Over-expression of OsIRT1 resulted in increased Cd accumulation (Lee and An, 2009 2009 ). In Arabidopsis , over-expression of AtIRT1 and AtIRT2 also caused an increase in Cd accumulation (Connolly et al., 2002 2002 ; Vert et al., 2009 2009 ). Similarly, HvIRT1 , a plasma membrane-localized transporter for Fe, Mn, and Zn, also exhibits Cd transport activity when expressed in yeast (Pedas et al., 2008 2008 ). Moreover, OsZIP1 has long been considered a metal uptake transporter for Zn and Cd in rice (Bashir et al., 2012 2012 ; Ramegowda et al., 2013 2013 ). Recent studies showed that over-expression of OsZIP1 reduced concentrations of Zn, Cu, and Cd in rice and improved growth under metal stress (Liu XS et al., 2019 2019 ). The function of OsZIP1 is similar to that of the copper efflux transporter, OsHMA9 , in which it has a Cd efflux function, causing an excretion of Cd from root cells and a reduction of Cd accumulation in rice (Lee et al., 2007 2007 ). OsZIP7 encodes a plasma membrane-localized protein with influx transport activity for both Zn and Cd, and plays an integral role in xylem loading in roots and inter-vascular transfer in nodes to preferentially deliver Zn and Cd to developing tissues and grains. Thus, knockout of OsZIP7 resulted in the retention of Zn and Cd in roots and basal nodes (Ricachenevsky et al., 2018 2018 ; Tan et al., 2019 2019 ). Suppression of the zinc transporter genes HvZIP3 and HvZIP8 by RNA interference (RNAi) silencing showed increased Cd accumulation and reduced Zn and Mn concentrations in barley grains (Sun et al., 2015 2015 ). However, considering that the metal concentrations used in some studies were much higher than those found in natural soils or rhizosphere environments, these conclusions should be viewed with caution. For example, the RIT1 gene cloned from pea ( Pisum sativum L.), a homolog of the Arabidopsis IRT1 Fe transporter gene, was previously thought to be associated with Fe uptake, and induced a high rate of Cd 2+ and Zn 2+ influx into the roots of pea seedlings (Cohen et al., 1998 1998 ). However, further research showed that the enhancing effect of RIT1 on Zn and Cd existed only when these elements were present at elevated concentrations, but not at their physiologically relevant soil levels (Cohen et al., 2004 2004 ). P 1B -ATPases, also called heavy metal ATPases (HMAs), play direct roles in plant heavy metal transmembrane transport. OsHMA2 , a transporter for Zn and Cd, is localized in the plasma membrane and mediates Zn/Cd xylem loading and intervascular transport to grains. However, studies showed that both OsHMA2 knockout and overexpression plants had considerably reduced concentrations of Zn and Cd in leaves and grains compared with wild-type rice (Satoh-Nagasawa et al., 2012 2012 ; Takahashi et al., 2012 2012 ; Yamaji et al., 2013 2013 ). HMA2 genes have also been identified in other crops and have a highly conserved function among cereals. For example, barley HvHMA2 has been identified predominantly in the plasma membrane and has a role in pumping Zn and Cd in yeast (Mills et al., 2012 2012 ). Ectopic expression of HvHMA2 in tobacco resulted in modified Zn, Fe, and Cd homeostasis (Barabasz et al., 2013 2013 ). TaHMA2 from bread wheat can transport Zn 2+ and Cd 2+ across membranes, and overexpression of TaHMA2 in Arabidopsis increased Zn/Cd root-to-shoot translocation, and enhanced root length and fresh weight (Tan et al., 2013 2013 ). In contrast, OsHMA3 located in the tonoplast, plays a role in pumping Cd into vacuoles and has a decisive role in reducing the root-to-shoot translocation of Cd (Ueno et al., 2010 2010 ; Miyadate et al., 2011 2011 ). HvHMA3 has been identified as a candidate gene for Cd accumulation in barley shoots and grains (Wu et al., 2015 2015 ). In durum wheat, the HMA3-B1 gene has been found to match perfectly with a major locus Cdu1 , which can explain 80% of the variation in grain Cd accumulation (Knox et al., 2009 2009 ; Zimmerl et al., 2014 2014 ). Sequence analysis of this HMA3 gene in high-and low-Cd durum wheat cultivars showed that a premature stop caused by a 17-bp duplication occurred only in high-Cd cultivars. The close relationship between a severely truncated HMA3 transporter and high Cd phenotype further confirmed that HMA3 was the best candidate gene for the Cdu1 locus (Wiebe, 2012 2012 ; Aprile et al., 2018 2018 ). OsHMA3 has high specificity for Cd rather than broad substrate-specificity. Consequently, loss-of-function or weak alleles of this gene result in increasing Cd root-to-shoot translocation (Ueno et al., 2010 2010 ; Yan et al., 2016 2016 ). In field trials, overexpression of OsHMA3 decreased Cd concentrations in brown rice by more than 90%, but had no significant effect on grain yield or concentrations of other essential micronutrients, including Zn, Fe, Cu, and Mn (Sasaki et al., 2014 2014 ; Lu et al., 2019 2019 ). Given the fact that OsHMA3 and OsHMA2 have different tissue localizations and promoter activities, driving the expression of OsHMA3 by OsHMA2 promoter could specifically reduce Cd accumulation in grains, and mitigate the adverse effect on rice yield induced by OsHMA2 knockout as well (Shao et al., 2018 2018 ). These mutants might be used directly in breeding programs. Recently, nodes in graminaceous plants such as rice and barley were found to play crucial roles in the allocation of multiple mineral nutrients, including essential and nonessential metal elements, by mediating intervascular transfers (Yamaji and Ma, 2014 2014 ). Several node-expressed transporters, mainly in rice, have been found to transport Cd, and among them OsHMA2 is expressed in the phloem of nodes (Satoh-Nagasawa et al., 2012 2012 ; Takahashi et al., 2012 2012 ; Yamaji et al., 2013 2013 ). In addition, the plasma membrane-localized gene, low-affinity cation transporter 1 of rice ( OsLCT1 ), is highly expressed in the uppermost node, and has been shown to mediate the export of phloem Cd and certain other elements into the sieve tube and translocate them from leaf blades and nodes to the grains of rice. Thus, down-regulation of OsLCT1 resulted in an about 50% reduction of Cd in grains compared with that of control plants, whereas the contents of other metals and plant growth were little affected (Uraguchi et al., 2011 2011 , 2014 2014 ). In addition, OsCCX2 , a putative cation/Ca 2+ exchanger (CCX), may function in loading Cd into xylem vessels, thus mediating direct root-derived Cd transport to grains. Knockout of OsCCX2 caused a significant reduction of Cd content in rice grains (Hao et al., 2018 2018 ). Shimo et al. ( 2011 2011 ) found a novel gene, OsLCD (low Cd), which positively regulates Cd tolerance and accumulation in rice. A knockout via a T-DNA insertion reduced Cd accumulation by 43%–55% in grains, but had no effect on Cd accumulation in leaf blades. Yuan et al. ( 2012 2012 ) reported that down-regulation of metal tolerance protein 1 of rice ( OsMTP1 , a bivalent cation transporter of heavy metals including Zn and Cd) markedly reduced heavy metal tolerance and accumulation in various rice tissues. Chelation-related genes 2 In addition to transporter proteins, metal ligands and chelators including metallothioneins (MTs), phytochelatins (PCs), organic acids, and other cysteine (Cys)-rich peptides have a role in metal transport and homeostasis (Ovečka and Takáč, 2014 2014 ). MTs belong to a superfamily of intracellular metal-binding proteins, and bind metals through their Cys residues. The isoform, OsMTI-1b , a rice MT type 1 gene, when expressed in yeast cells, results in increased tolerance to CdCl 2 treatment and increases accumulation of Cd 2+ ions (Ansarypour and Shahpiri, 2017 2017 ). PCs are predominantly associated with detoxification of non-essential toxic metals and metalloids such as Cd and arsenic (As), and thus significantly affect Cd accumulation in cereal plants (Das et al., 2017 2017 ). Uraguchi et al. ( 2017 2017 ) identified two independent OsPCS1 mutant rice lines (a T-DNA and a Tos17 insertion line) and found that they exhibited increased sensitivity to Cd, showing the importance of OsPCS1 -dependent PC synthesis for Cd tolerance in rice. Elemental analyses of rice plants showed decreased Cd accumulation in the grains of both mutants. Recently, a defensin-like protein, CAL1, was found to be involved in Cd accumulation in rice leaves. It promotes Cd secretion into the extracellular space via chelation, then loading into the xylem and long-distance transport, thereby resulting in most Cd being deposited in leaves rather than in grains. This provides a novel and ideal mechanism for developing dual-function rice cultivars that can produce low-Cd grains and be used as Cd hyper-accumulators for remediating contaminated soils (Luo et al., 2018 2018 ). Identification of molecular markers for low Cd accumulation in cereals 1 QTL mapping is acknowledged as an efficient approach for dissecting complicated agronomic traits controlled by multiple genes, and has been successfully applied to detect loci controlling Cd accumulation in some crops. A series of QTLs that control Cd concentrations have been reported in various plants including wheat, maize, barley, and rice (Table 2 2 ). Cd uptake, translocation, and accumulation in grains are complex processes that are controlled by many genes and are affected by environmental factors. Only a small number of genes related to low Cd accumulation have so far been identified, and the underlying knowledge regarding the molecular mechanisms for plant Cd uptake, translocation, and accumulation is still fragmental. Furthermore, the application of these genes to low-Cd plant breeding is still lacking and has little effect on cultivars released by breeders. The challenge now facing breeders is to identify more novel, effective low-Cd genes, and to successfully apply those already identified to produce low-Cd crop cultivars. List of reported genes in rice ( Oryza sativa ) related to Cd homeostasis was reviewed by Chen JG et al. ( 2019 2019 ). PVE: phenotypic variation explained; CSSL: chromosome segment substitution line; RFLP: restriction fragment length polymorphism; SSR: simple sequence repeat; BIL: backcross inbred line; RIL: recombinant inbred line; DH: doubled haploid; CAPS: cleaved amplified polymorphic sequence; SNP: single nucleotide polymorphism; STS: sequence tagged site Crop Mapping population Maker Traits Chromosome QTL PVE (%) Reference Rice 39 CSSLs, Kasalath/Koshihikari 129 RFLPs Cd accumulation in grains 3, 6, 8 Putative QTLs for grain Cd Ishikawa et al., 2005 2005 Rice 98 BILs, Kasalath/Nipponbare RFLP and SSR Cd concentration in leaves and culms 4, 11 qcd4-1 , qcd4-2 , qcd11 Kashiwagi et al., 2009 2009 Rice 127 DHs, JX17/ZYQ8 RFLP and SSR Cd concentration in roots and shoots or their ratio 1, 3, 5, 6, 7, 8, 10 4 QTLs Xue et al., 2009 2009 Rice 184 F 2 , Badari Dhan/Shwe War 141 SSRs Cd concentration in shoots 2, 5, 11 A major QTL 16.1 Ueno et al., 2009b 2009b Rice 177 F 2 , Nipponbare/Anjana Dhan SSR Root-to-shoot Cd translocation 7 A major QTL 85.6 Ueno et al., 2009a 2009a Rice 965 F 2 , Nipponbare/Anjana Dhan SSR Cd concentration in shoots 7 OsHMA3 Ueno et al., 2010 2010 Rice 85 BILs, Sasanishiki/Habataki SSR Cd concentration in shoots and grains 2, 7, 12 qGCd2 , qGCd7 , qSCd12 7.2–35.5 Ishikawa et al., 2010 2010 Rice 144 F 2 , Cho-Ko-Koku 9 Akita 63 SSR Root-to-shoot Cd translocation 7 qCdT7 88.0 Tezuka et al., 2010 2010 Rice F 2 , Cho-Ko-Koku 9 Akita 63 CAPS Root-to-shoot Cd translocation 7 qCdT7 Miyadate et al., 2011 2011 Rice 103 BILs, Koshihikari/Jarjan 169 SSRs Cd concentration in grains 7 qCdp7 31.0–54.0 Abe et al., 2011 2011 Rice 127 DHs, JX17/ZYQ8 RFLP and SSR Cd concentration in grains 3, 4, 6 qCdc3 , qCdc4, qCdc6 10.8–41.7 Zhang et al., 2011 2011 Rice 126 RILs, Fukuhibiki/LAC23 SSR and CAPS Cd concentration in grains 3, 11 gLCdG3 , gLCdG11 8.3–13.9 Sato et al., 2011 2011 Rice 91 RILs, SNU-SG1/Suwon490 124 SSRs Cd concentration in shoots and grains 3, 5, 9, 10, 11 gcc3, srg5, gcc9 , gcc11 , scc10 16.1–24.9 Yan et al., 2013 2013 Rice 46 CSSLs, Koshihikari/LAC23 345 SNPs Cd concentration in shoots 3 glGCd3 Abe et al., 2013 2013 Rice 204 RILs, ZS97B/MY46 Cd concentration in grains 5 qCd5 Huang et al., 2018 2018 Rice DH, YK17/D50 170 SSRs Cd concentration in brown or milled rice 2, 3, 4, 5, 7, 9 32 QTLs Hu et al., 2018 2018 Rice 119 DHs, 3651 BC3F3, Tainan1/Chunjiang06 RFLP Cd concentration in leaves 2 CAL1 13.1 Luo et al., 2018 2018 Rice 115 RILs, Xiang 743/Katy SSR Cd concentration in grains 2, 7 qCd-2 , qCd-7 Liu WQ et al., 2019 2019 Wheat 155 DHs, W9262-260D3/Kofa SSR Cd concentration in grains 5B Cdu1 Knox et al., 2009 2009 Wheat DH, W9262-260D3/Kofa STS Cd concentration in grains 5B Cdu1 and a minor QTL 80.0 Wiebe et al., 2010 2010 Wheat 103 RILs, Ch/Sh Cd concentration in roots 4A, 5D 2 QTLs 10.0–17.5 Ci et al., 2012 2012 QTLs associated with Cd accumulation in rice 2 Ishikawa et al. ( 2005 2005 ) detected three putative QTLs controlling Cd concentrations in brown rice located on chromosomes 3, 6, and 8. Kashiwagi et al. ( 2009 2009 ) found that the interaction of two putative QTLs on chromosome 4 increased Cd concentrations in brown rice. Xue et al. ( 2009 2009 ) identified six QTLs associated with Cd tolerance and three QTLs for root and shoot Cd concentrations in rice seedlings. Ueno et al. ( 2009b 2009b ) identified a major QTL on chromosome 11 controlling the translocation of Cd from roots to shoots in seedlings that explained 16.1% of the detected variation in Cd accumulation. Ueno et al. ( 2009a 2009a ) isolated another novel major QTL with a large effect on Cd distribution in roots and shoots of rice using an F 2 population derived from Anjana Dhan×Nipponbare. This QTL, detected on the short arm of chromosome 7, explained as much as 85.6% of the phenotypic variation. High-resolution mapping and complementation analysis identified OsHMA3 as being the causal gene for this QTL locus (Ueno et al., 2010 2010 ). Ishikawa et al. ( 2010 2010 ) localized a major-effect QTL (named qGCd7 ) to the short arm of chromosome 7, which explained 35.5% of the observed variation, and enhanced grain Cd accumulation in rice grown in a Cd-polluted paddy field. Abe et al. ( 2011 2011 ) detected a major QTL, qCdp7 (accounting for 31%–54% of the phenotypic variance), on chromosome 7, that significantly increased Cd accumulation in both the grain and straw of rice. Zhang et al. ( 2011 2011 ) identified three QTLs for Cd accumulation in brown rice. Sato et al. ( 2011 2011 ) reported two QTLs for increasing Cd accumulation in brown rice: qLCdG11 explained 9.4%–12.9% of the phenotypic variation, while qLCdG3 accounted for 8.3%–13.9%. Yan et al. ( 2013 2013 ) constructed a recombinant inbred (F 7 ) line (RIL) population to identify QTLs controlling Cd accumulation and distribution. Five significant QTLs were detected: scc10 was responsible for Cd accumulation in shoots; gcc3 , gcc9 , and gcc11 were closely associated with Cd accumulation in grains; and sgr5 controlled the translocation of Cd from roots to shoots. Among them, sgr5 had the greatest effect on the distribution of Cd in grains. Using 46 chromosome segment substitution lines (CSSLs), Abe et al. ( 2013 2013 ) detected a major QTL, qlGCd3 , on the long arm of chromosome 3, which was responsible for reduced Cd concentration in rice grains. Using 204 RILs, Hu et al. ( 2018 2018 ) identified 22 QTLs affecting the accumulation of Cd, Cu, Fe, Mn, and Zn in brown rice, among which qCd5 was a novel QTL that regulated low Cd accumulation. Luo et al. ( 2018 2018 ) identified a QTL named CAL1 (Cd accumulation in leaf 1) on chromosome 2 using a doubled haploid (DH) population derived from TN1 and CJ06. Cd levels in grains and leaves of the over-accumulating cultivar (TN1) were 3–4-fold higher than those in the under-accumulating cultivar (CJ06). The authors localized CAL1 and cloned the causal gene using map-based cloning. Liu WQ et al. ( 2019 2019 ) identified two QTLs ( qCd-2 and qCd-7 ) for Cd accumulation in rice grains. McCouch et al. ( 2016 2016 ) established an open-access resource for genome-wide association studies (GWAS) in rice. Using this resource, Zhao JL et al. ( 2018 2018 ) identified 14 QTLs for low Cd accumulation in rice grains evenly distributed in indica and japonica subpopulations consisting of 312 diverse rice accessions. These QTLs co-localized with functional genes, including OsHMA3 , OsNRAMP1 , OsNRAMP5 , and OsLCD . Also, a novel QTL, qCd3-2 , may encode a member of the rice NRAMP gene family ( OsNRAMP2 ). Liu WQ et al. ( 2019 2019 ) used 276 accessions with 416 000 single nucleotide polymorphisms (SNPs) and performed GWAS of grain Cd concentrations in rice grown in heavily multi-contaminated farmland. Seventeen QTLs were found to be responsible for the grain Cd concentration. Yan et al. ( 2019 2019 ) performed GWAS with 127 rice cultivars and identified a QTL on chromosome 3 that explained about 20.7% of the phenotypic variation. Using gene ontology (GO) and yeast spot analyses, they found a member of the major facilitator superfamily, OsCd1 , that takes part in rice Cd uptake and grain Cd accumulation and was the causal gene for QTL3. These results provide a good reference for cloning potentially useful candidate genes and molecular breeding of low-Cd cereal crops. QTLs associated with Cd accumulation in wheat 2 In contrast to multiple-gene control, durum wheat grain Cd accumulation seems to be controlled by a single dominant gene, Cdu1 , which was first found on a linkage group using the random amplified polymorphic DNA marker (OPC20) (Penner et al., 1995 1995 ). Later studies reported that its expression was highly heritable (0.84–0.88) and exhibited no significant effect on agronomic performance or the uptake of other micronutrients (Clarke et al., 1997 1997 , 2002 2002 ). Cdu1 is located on the long arm of chromosome 5B (Knox et al., 2009 2009 ). The same major QTL, which is in the vicinity of Cdu1 , has been repeatedly identified with different mapping populations and markers, explaining most of the Cd variation in durum wheat grain (Wiebe et al., 2010 2010 ; AbuHammad et al., 2016 2016 ; Salsman et al., 2018 2018 ). With regard to the QTLs associated with Cd accumulation in vegetative organs in wheat, Ci et al. ( 2012 2012 ) identified two QTLs for root Cd accumulation at the germination and seedling stages using 103 RILs derived from a cross of Ch×Sh. AbuHammad et al. ( 2016 2016 ) also identified a major QTL contributing a low-Cd uptake allele and explaining 54.3% of phenotypic variation. Oladzad-Abbasabadi et al. ( 2018 2018 ) identified a novel low-Cd uptake locus on chromosome 5BL, and the candidate causal gene was homologous to an aluminum-induced protein-encoded gene rather than to the closely linked Cdu1-B gene. QTLs associated with Cd accumulation in maize and barley 2 In maize, Soric et al. ( 2009 2009 ) detected a QTL associated with Cd uptake on chromosome 2, which could explain 49.8% of the phenotypic variation. Zhao XW et al. ( 2018 2018 ) found 63 loci associated with leaf Cd accumulation through GWAS, and successfully identified a major QTL on chromosome 2 that contained 40 SNPs highly associated with leaf Cd concentration, which could explain over 38% of the phenotypic variation. In barley, Wu et al. ( 2015 2015 ) detected nine QTLs for root Cd accumulation, 21 for shoot Cd accumulation, and 15 for grain Cd accumulation, via GWAS mapping of 100 accessions from the International Barley Core Selected Collection (BCS). Wang et al. ( 2019 2019 ) identified a single QTL, qShCd7H , localized on chromosome 7H, responsible for shoot Cd accumulation, using a DH population derived from a cross between Suyinmai 2 (Cd-sensitive) and Weisuobuzhi (Cd-tolerant). Furthermore, a novel gene, HvPAA1 , related to shoot Cd concentration, was identified from qShCd7H . Functional identification showed that HvPAA1 plays a role in the detoxification of Cd in barley. The results provide a molecular basis for understanding Cd accumulation in barley and will contribute to the development of molecular markers that can be used in breeding for low-Cd-accumulating cultivars. Breeding cereal cultivars with low Cd accumulation in grains 1 Although the use of agronomic practices and chemical regulators can reduce plant Cd uptake, it has been generally accepted that the development of crop cultivars with low Cd accumulation in edible parts is the most efficient approach to deal with medium or slightly Cd polluted farmland. Low-Cd-accumulating cultivars should accumulate Cd in edible parts only to a level below maximum permissible concentrations (MPC) for safe consumption, even when grown in Cd-contaminated soil. The MPCs are set by either the relevant national authorities or regional and international organizations. For example, the Cd standard limit for polished rice set by the Chinese National Standard (NHFPC, 2017 2017 ) is 0.2 mg/kg DW, and the limit set by the Codex Alimentarius Commission (CAC, 2019 2019 ) is 0.4 mg/kg DW. In addition to MPCs, further criteria have also been proposed for the selection of low-Cd cultivars. For instance, the yield and eating quality of selected low-Cd cultivars should not be reduced significantly when grown in Cd-polluted soils. Methodologies for breeding for low-Cd cultivars 2 Breeding for low grain Cd cultivars can be achieved by both conventional and modern breeding approaches. Considerable genetic variation in Cd-accumulating ability has been widely reported in a range of cereal crops (Grant et al., 2008 2008 ; Zhao JL et al., 2018 2018 ; Zhao XW et al., 2018 2018 ). Thus, in conventional breeding, low-Cd cultivars can be selected based on the determination of Cd content, coupled with measurements of morphological and physio-chemical parameters. To handle segregating accessions, such breeding strategies as mass, pure line, and recurrent selection methods can be effectively adopted in developing low-Cd cultivars. In addition, breeders adopt random cross combinations of genotypes to create genetic variation for screening low Cd accumulation genotypes. However, a more efficient strategy is to screen for low-Cd cultivars by measuring the grain Cd content of cultivars that have been or will be in commercial application (Chen et al., 2007a 2007a , 2007b 2007b ; Cao et al., 2014b 2014b ). Cultivars selected as low-Cd types from among existing commercial cultivars with excellent yield and agronomic traits can be directly used in commercial production. Conventional breeding methods have been partially successful in developing low-Cd cultivars. For example, a Cd-pollution-safe spring durum wheat cultivar named “Strongfield” was commercially released in Canada in 2004 (Clarke et al., 2006 2006 ). However, this breeding method typically requires a large amount of effort, and a time-consuming selection process. Recently, molecular breeding approaches have accelerated progress in breeding low-Cd cultivars by exploiting technologies such as marker-assisted selection, allele discovery, allele pyramiding, genome mutation, genome selection, genome-wide association mapping, and gene editing. The adoption of novel technologies, such as genetic modification, marker-assisted selection, and gene editing, has received increasing interest (Randhawa et al., 2013 2013 ; Oladzad-Abbasabadi et al., 2018 2018 ). Furthermore, the successful integration of molecular breeding with conventional breeding methods will accelerate the development of low-Cd cultivars. Genetic differences and screening for low-Cd cereal cultivars—conventional breeding approaches 2 It is well documented that there are significant differences in Cd accumulation among genotypes. Hence, screening crop cultivars for low Cd content is a practical strategy for the selection of low-Cd cultivars and genotypes. This conventional breeding method has found its place in the development of low-Cd cereals. In rice, Arao and Ae ( 2003 2003 ) investigated genotypic variation in grain Cd content, and identified three cultivars with the lowest Cd concentrations. Since then, many researchers have investigated the genotypic and environmental variation in Cd accumulation and distribution in rice and attempted to select low-Cd genotypes (Liu et al., 2005 2005 , 2007 2007 ; He et al., 2006 2006 ; Yu et al., 2006 2006 ; Yan et al., 2010 2010 ; Cao et al., 2014b 2014b ; Pinson et al., 2015 2015 ; Li et al., 2017 2017 ; Zhou et al., 2017 2017 ). In general, durum wheat tends to accumulate more Cd in grains than bread wheat (Stolt et al., 2003 2003 ; Greger and Löfstedt, 2004 2004 ). Wiebe et al. ( 2010 2010 ) even suggested that the risk of high grain Cd threatening human health was limited to durum wheat. Thus, more attention and effort have been paid to breeding low-Cd durum wheat than bread wheat. Several dozen low-Cd durum wheat cultivars have been identified worldwide (Eriksson, 1990 1990 ; Grant et al., 2008 2008 ; Arduini et al., 2014 2014 ; Zimmerl et al., 2014 2014 ; Kubo et al., 2016 2016 ; Perrier et al., 2016 2016 ; Hirzel et al., 2017 2017 , 2018 2018 ; Vergine et al., 2017 2017 ; Oladzad-Abbasabadi et al., 2018 2018 ; Yue et al., 2018 2018 ). Due to the similar chemical properties and electron structures of Zn and Cd (Järup and Åkesson, 2009 2009 ; Nordberg, 2009 2009 ), they often present in soils and move into plants via similar uptake and transport pathways (Waters and Sankaran, 2011 2011 ; Khan et al., 2014 2014 ). Thus, down-regulation of the pathways to reduce Cd accumulation in cereal grains may at the same time inadvertently decrease Zn transport (Palmgren et al., 2008 2008 ; Sebastian and Prasad, 2014 2014 ). Similarly, efforts to remove grain Cd during food processing by leaching and physical separation have reduced the content of some beneficial elements. For example, milling to produce white flour not only reduces Cd concentration by 50.0%, but also leads to reductions of 78.3% in Fe and 69.0% in Zn concentrations (Guttieri et al., 2015a 2015a ). Fortunately, recent research on grain Cd and Zn contents in hard winter wheat germplasm indicated that their accumulation seems to be independently regulated by different regions of the genome, indicating that it may be possible to breed for low-Cd hard winter wheat genotypes without reducing Zn content (Guttieri et al., 2015b 2015b ). In barley, our previous research successfully identified low grain-Cd barley genotypes such as Beitalys and Shang 98-128 (Chang et al., 1982 1982 ; Wu and Zhang, 2002 2002 ; Chen et al., 2007a 2007a , 2007b 2007b ; Wu et al., 2007 2007 ). There is also wide genetic variation in Cd content among maize cultivars. Hinesly et al. ( 1978 1978 ) first reported large genetic variation in leaf and grain Cd concentrations among maize inbred lines, and found that hybrid maize line B73×R805 had the lowest Cd accumulation in grains when grown on sludge-amended soil (Hinesly et al., 1982 1982 ). In a study of 19 inbred maize lines, a large genetic variation observed in shoot Cd concentration was attributed to root-to-shoot translocation rather than to Cd uptake (Florijn and van Beusichem, 1993a 1993a , 1993b 1993b ). From evaluation of Cd concentrations (Zhang et al., 2008 2008 ; Yang YM et al., 2014 2014 ; Fahad et al., 2015 2015 ; Retamal-Salgado et al., 2017 2017 ), Chunyou30 was identified as a low-Cd maize genotype (Zhang et al., 2008 2008 ). In short, conventional breeding methods have had partial success in the development of low-Cd cereals. As they were selected from among commercial cultivars, these low-Cd genotypes can be used either directly or for hybridization to transfer the low Cd accumulation trait (Yu et al., 2006 2006 ). However, with these breeding methods, breeders often prioritize the selection of low-Cd cultivars at the expense of yield and quality. Hence, some selected low-Cd cereal cultivars often exhibit reduced grain yield and lower nutritional quality (Chen et al., 2007a 2007a , 2007b 2007b ). White and Broadley ( 2005 2005 ) and Murgia et al. ( 2013 2013 ) noticed a decrease in essential minerals such as Fe, Zn, Mn, and Ca in low-Cd cultivars. Fortunately, there are low-Cd cultivars with normal levels of essential minerals. Luo et al. ( 2018 2018 ) evaluated 212 rice accessions, and identified CJ06 as a low-Cd rice cultivar. Further elemental analysis revealed that CJ06 contains levels of Fe, Mn, Zn, and Cu in grains similar to those in high-Cd-accumulating cultivars (i.e., TN1). However, it may be difficult to breed low-Cd cultivars when undesirable characters are linked to low-Cd-related genes, but problems of this type can now be overcome using novel technologies, including gene editing. Molecular marker-assisted selection for low-Cd cereal cultivars 2 Molecular marker-assisted selection (MAS) should be a practical and acceptable alternative option. A typical example is the markers for a single major locus, Cdu1 , related to Cd accumulation in durum wheat. The markers include a random amplified polymorphic DNA (RAPD) marker (OPC-20), an SNP marker which was able to identify 89% of the low-Cd lines in the Strongfield×Alkabo population, and two user-friendly competitive allele-specific polymerase chain reaction (KASP) markers (Cad-5B and Ex_c1343_2570756) for grain Cd accumulation, with an average prediction accuracy of 84%–88%. These markers have assisted the successful selection of low-Cd durum wheat (Knox et al., 2009 2009 ; Wiebe et al., 2010 2010 ; AbuHammad et al., 2016 2016 ; Salsman et al., 2018 2018 ). Likewise, qShCd7H , a major QTL controlling shoot Cd accumulation in barley, may facilitate the selection of low-Cd barley cultivars in the future (Wang et al., 2019 2019 ). However, an insufficient number of validated markers have been the primary obstacle hindering the application of MAS to the selection of low-Cd progenies of staple food crops. Fortunately, the abundant germplasm resources of the main cereal crops has made GWAS possible, and with advanced linkage mapping technology, QTL identification and verification has become more powerful (Ueda et al., 2015 2015 ; Nicod et al., 2016 2016 ; Zhang et al., 2016 2016 ). In addition, with the rapid development of next-generation sequencing and RNA sequencing (RNA-seq) profiling, the construction of high-density molecular markers and the quantification of expression profiles under Cd exposure have become possible (Wang et al., 2017 2017 ; Aprile et al., 2018 2018 ; Cao et al., 2019 2019 ). Next generation sequencing is expected to enable the identification of further novel QTLs and molecular markers controlling Cd uptake, root-to-shoot translocation, and grain Cd accumulation. Application of gene editing to breeding low-Cd cultivars 2 Progress has been made in the identification and cloning of low-Cd-related genes in the past two decades. For example, AtIRT1 and AtIRT2 (Connolly et al., 2002 2002 ), OsIRT1 and OsIRT2 (Nakanishi et al., 2006 2006 ), and OsNRAMP1 (Curie et al., 2000 2000 ) were identified and found to play roles in Cd uptake. Over-expression of OsNRAMP1 in rice increased Cd accumulation in the leaves, indicating that OsNRAMP1 was an important protein associated with high Cd accumulation in rice (Takahashi et al., 2011a 2011a ). The knockout of OsNRAMP5 or overexpressing OsHMA3 significantly reduced Cd concentration in roots, shoots, and grains when compared with that of their wild-type relatives (Ueno et al., 2010 2010 ; Sasaki et al., 2012 2012 , 2014 2014 ; Yang M et al., 2014 2014 ; Yan et al., 2016 2016 ; Lu et al., 2019 2019 ). The down-regulation of OsLCT1 resulted in an about 50% reduction of Cd in grains compared with that of control plants (Uraguchi et al., 2011 2011 , 2014 2014 ). Manipulating the expression of these transporter genes might be an efficient approach to reduce Cd content in grain crops. However, processes involved in the uptake and transport of Cd are closely associated with those involved in the uptake and transport of essential micronutrients such as Zn, Fe, and Mn (Nakanishi et al., 2006 2006 ; Sasaki et al., 2012 2012 ). Thus, there is still a great challenge, as reducing the Cd concentration in grains may simultaneously interfere with the balance of some essential mineral elements. One typical example is knockout mutations of the major transporter responsible for Mn and Cd uptake in rice—OsNramp5. These resulted in a large reduction of Cd in rice grains compared with that of wild-type plants (Ishikawa et al., 2012 2012 ; Sasaki et al., 2012 2012 ), but simultaneously, a decline in Mn concentration was detected (Ishikawa et al., 2012 2012 ). Because it is an essential micronutrient for plant growth and development, Mn deficiency could inhibit growth and reduce yield. Similarly, the T-DNA insertion lines and RNAi lines of OsNRAMP5 within a japonica cultivar Zhonghua 11, exhibited severe growth inhibition and a significant decrease in grain yield (grain yield of the T-DNA insertion mutant was only 11% of that of wild-type plants) (Sasaki et al., 2012 2012 ). Supplying Mn fertilizer might alleviate Mn deficiency, but this problem may also be solved by inducing different mutation sites or using various genetic backgrounds. A good example is the lcd-kmt mutants of OsNRAMP5 (produced by carbon ion-beam irradiation of a popular Japanese temperate japonica rice cv. Koshihikari), which show few adverse effects on growth with less than 100 mg/kg Mn in their straw (Ishikawa et al., 2012 2012 ). Another example is a Zn and Cd transporter OsHMA2, which plays a role in Zn and Cd loading into the xylem and participates in root-to-shoot translocation of these metals in rice. Overexpression of OsHMA2 driven by a 35S promoter can significantly reduce both Zn and Cd contents in rice grains. However, unexpectedly, when OsHMA2 was expressed under the control of OsSUT1 , Cd concentration in the grains of SUT1-rice was only about half of that in the wild type, while the concentrations of Zn and other elements remained almost the same (Takahashi et al., 2012 2012 ). The OsSUT1 promoter is thought to enhance expression of the Zn transporter, improving its efficiency in accumulating Zn in rice grains (Scofield et al., 2007 2007 ). In addition, OsZIP7 seems to cooperate with OsHMA2 to load Cd and Zn into sieve tubes in the phloem region of nodes, and thus OsSUT1-OsHMA2 is thought to be beneficial for reducing the Cd content of rice grains (Tan et al., 2019 2019 ). However, further research is needed to determine whether OsSUT1-OsZIP7 constructs can reduce Cd concentrations in rice grains. With advances in genomics and gene editing technology, molecular breeding for low Cd accumulation with high yield and quality may become a reality (Shan et al., 2014 2014 ; Li et al., 2018 2018 ). Tang et al. ( 2017 2017 ) and Liu SM et al. ( 2019 2019 ) demonstrated that it is feasible to generate yield-competitive and low-Cd rice lines through CRISPR/Cas9-mediated mutagenesis of OsNramp5 , using the rice cultivars Huazhan and Xidao 1 as the parental wild-type lines. However, genetic modification does not always meet with favorable public perceptions (Chen JG et al., 2019 2019 ; Chen KL et al., 2019 2019 ). Conventional and molecular breeding strategies, used alone and in combination, have been designed to develop low-Cd cereal cultivars to minimize Cd uptake and toxicity. Conventional breeding is still an attractive and potential tool for screening low-Cd crop cultivars. Molecular markers are a powerful technology for the selection of low-Cd cereal cultivars. In addition, modern molecular breeding technologies have great potential in breeding programs for the development of low-Cd cultivars, especially when coupled with conventional breeding. Within the next decade, the emphasis on low-Cd cereal breeding is likely to create more genetic variation for effective selection of low-Cd cultivars and further identification of unknown Cd accumulation-related genes and novel major QTLs controlling grain Cd accumulation. This will lead to a balanced use of phenotypic and genotypic selection and conventional and molecular breeding procedures. Both enhanced evaluation systems and regulations are necessary to determine whether low-Cd cultivars can be introduced into commercial production. References T Abe , F Taguchi-Shiobara , Y Kojima , 等 . Detection of a QTL for accumulating Cd in rice that enables efficient Cd phytoextraction from soil . Breed Sci , 2011 . 61 ( 1 ): 43 - 51 . DOI: 10.1270/jsbbs.61.43 http://doi.org/10.1270/jsbbs.61.43 . Detection of a QTL for accumulating Cd in rice that enables efficient Cd phytoextraction from soil T Abe , Y Nonoue , N Ono , 等 . Detection of QTLs to reduce cadmium content in rice grains using LAC23/Koshihikari chromosome segment substitution lines . Breed Sci , 2013 . 63 ( 3 ): 284 - 291 . 24273423 DOI: 10.1270/jsbbs.63.284 http://doi.org/10.1270/jsbbs.63.284 . Detection of QTLs to reduce cadmium content in rice grains using LAC23/Koshihikari chromosome segment substitution lines WA AbuHammad , S Mamidi , A Kumar , 等 . Identification and validation of a major cadmium accumulation locus and closely associated SNP markers in North Dakota durum wheat cultivars . Mol Breed , 2016 . 36 ( 8 ): 112 DOI: 10.1007/s11032-016-0536-1 http://doi.org/10.1007/s11032-016-0536-1 . Identification and validation of a major cadmium accumulation locus and closely associated SNP markers in North Dakota durum wheat cultivars Z Ansarypour , A Shahpiri . Heterologous expression of a rice metallothionein isoform ( OsMTI-1b ) in Saccharomyces cerevisiae enhances cadmium, hydrogen peroxide and ethanol tolerance . Braz J Microbiol , 2017 . 48 ( 3 ): 537 - 543 . 28223030 DOI: 10.1016/j.bjm.2016.10.024 http://doi.org/10.1016/j.bjm.2016.10.024 . Heterologous expression of a rice metallothionein isoform (OsMTI-1b) in Saccharomyces cerevisiae enhances cadmium, hydrogen peroxide and ethanol tolerance A Aprile , E Sabella , M Vergine , 等 . Activation of a gene network in durum wheat roots exposed to cadmium . BMC Plant Biol, 18:238 , 2018 . DOI: 10.1186/s12870-018-1473-4 http://doi.org/10.1186/s12870-018-1473-4 . Activation of a gene network in durum wheat roots exposed to cadmium T Arao , N Ae . Genotypic variations in cadmium levels of rice grain . Soil Sci Plant Nutr , 2003 . 49 ( 4 ): 473 - 479 . DOI: 10.1080/00380768.2003.10410035 http://doi.org/10.1080/00380768.2003.10410035 . Genotypic variations in cadmium levels of rice grain I Arduini , A Masoni , M Mariotti , 等 . Cadmium uptake and translocation in durum wheat varieties differing in grain-Cd accumulation . Plant Soil Environ , 2014 . 60 ( 1 ): 43 - 49 . . Cadmium uptake and translocation in durum wheat varieties differing in grain-Cd accumulation E Arthur , H Crews , C Morgan . Optimizing plant genetic strategies for minimizing environmental contamination in the food chain . Int J Phytoremediation , 2000 . 2 ( 1 ): 1 - 21 . DOI: 10.1080/15226510008500027 http://doi.org/10.1080/15226510008500027 . Optimizing plant genetic strategies for minimizing environmental contamination in the food chain KKIU Arunakumara , BC Walpola , MH Yoon . Current status of heavy metal contamination in Asia’s rice lands . Rev Environ Sci Bio/Technol , 2013 . 12 ( 4 ): 355 - 377 . DOI: 10.1007/s11157-013-9323-1 http://doi.org/10.1007/s11157-013-9323-1 . Current status of heavy metal contamination in Asia’s rice lands R Aziz , MT Rafiq , TQ Li , 等 . Uptake of cadmium by rice grown on contaminated soils and its bioavailability/toxicity in human cell lines (Caco-2/HL-7702) . J Agric Food Chem , 2015 . 63 ( 13 ): 3599 - 3608 . 25738308 DOI: 10.1021/jf505557g http://doi.org/10.1021/jf505557g . Uptake of cadmium by rice grown on contaminated soils and its bioavailability/toxicity in human cell lines (Caco-2/HL-7702) A Barabasz , A Wilkowska , K Tracz , 等 . Expression of HvHMA2 in tobacco modifies Zn-Fe-Cd homeostasis . J Plant Physiol , 2013 . 170 ( 13 ): 1176 - 1186 . 23664582 DOI: 10.1016/j.jplph.2013.03.018 http://doi.org/10.1016/j.jplph.2013.03.018 . Expression of HvHMA2 in tobacco modifies Zn-Fe-Cd homeostasis K Bashir , Y Ishimaru , NK Nishizawa . Molecular mechanisms of zinc uptake and translocation in rice . Plant Soil , 2012 . 361 ( 1-2 ): 189 - 201 . DOI: 10.1007/s11104-012-1240-5 http://doi.org/10.1007/s11104-012-1240-5 . Molecular mechanisms of zinc uptake and translocation in rice (Codex Alimentarius Commission) CAC . General Standard for Contaminants and Toxins in Food and Feed, Codex Standard 193-1995 . 2019 . General Standard for Contaminants and Toxins in Food and Feed, Codex Standard 193-1995 FB Cao , F Chen , HY Sun , 等 . Genome-wide transcriptome and functional analysis of two contrasting genotypes reveals key genes for cadmium tolerance in barley . BMC Genomics, 15:611 , 2014 . DOI: 10.1186/1471-2164-15-611 http://doi.org/10.1186/1471-2164-15-611 . Genome-wide transcriptome and functional analysis of two contrasting genotypes reveals key genes for cadmium tolerance in barley FB Cao , RF Wang , WD Cheng , 等 . Genotypic and environmental variation in cadmium, chromium, lead and copper in rice and approaches for reducing the accumulation . Sci Total Environ , 2014 . 496 275 - 281 . 25089689 DOI: 10.1016/j.scitotenv.2014.07.064 http://doi.org/10.1016/j.scitotenv.2014.07.064 . Genotypic and environmental variation in cadmium, chromium, lead and copper in rice and approaches for reducing the accumulation ZZ Cao , XY Lin , YJ Yang , 等 . Gene identification and transcriptome analysis of low cadmium accumulation rice mutant ( lcd1 ) in response to cadmium stress using MutMap and RNA-seq . BMC Plant Biol, 19:250 , 2019 . DOI: 10.1186/s12870-019-1867-y http://doi.org/10.1186/s12870-019-1867-y . Gene identification and transcriptome analysis of low cadmium accumulation rice mutant (lcd1) in response to cadmium stress using MutMap and RNA-seq K Čásová , J Černý , J Száková , 等 . Cadmium balance in soils under different fertilization managements including sewage sludge application . Plant Soil Environ , 2009 . 55 ( 8 ): 353 - 361 . . Cadmium balance in soils under different fertilization managements including sewage sludge application AC Chang , AL Page , KW Foster , 等 . A comparison of cadmium and zinc accumulation by four cultivars of barley grown in sludge-amended soils . J Environ Qual , 1982 . 11 ( 2 ): 409 - 412 . DOI: 10.2134/jeq1982.00472425001100030018x http://doi.org/10.2134/jeq1982.00472425001100030018x . A comparison of cadmium and zinc accumulation by four cultivars of barley grown in sludge-amended soils F Chen , FB Wu , J Dong , 等 . Cadmium translocation and accumulation in developing barley grains . Planta , 2007 . 227 ( 1 ): 223 - 232 . 17713783 DOI: 10.1007/s00425-007-0610-3 http://doi.org/10.1007/s00425-007-0610-3 . Cadmium translocation and accumulation in developing barley grains F Chen , J Dong , F Wang , 等 . Identification of barley genotypes with low grain Cd accumulation and its interaction with four microelements . Chemosphere , 2007 . 67 ( 10 ): 2082 - 2088 . 17257649 DOI: 10.1016/j.chemosphere.2006.10.014 http://doi.org/10.1016/j.chemosphere.2006.10.014 . Identification of barley genotypes with low grain Cd accumulation and its interaction with four microelements HP Chen , XP Yang , P Wang , 等 . Dietary cadmium intake from rice and vegetables and potential health risk: a case study in Xiangtan, southern China . Sci Total Environ , 2018 . 639 271 - 277 . 29791880 DOI: 10.1016/j.scitotenv.2018.05.050 http://doi.org/10.1016/j.scitotenv.2018.05.050 . Dietary cadmium intake from rice and vegetables and potential health risk: a case study in Xiangtan, southern China JG Chen , WL Zou , LJ Meng , 等 . Advances in the uptake and transport mechanisms and QTLs mapping of cadmium in rice . Int J Mol Sci , 2019 . 20 ( 14 ): 3417 DOI: 10.3390/ijms20143417 http://doi.org/10.3390/ijms20143417 . Advances in the uptake and transport mechanisms and QTLs mapping of cadmium in rice KL Chen , YP Wang , R Zhang , 等 . CRISPR/Cas genome editing and precision plant breeding in agriculture . Annu Rev Plant Biol , 2019 . 70 667 - 697 . 30835493 DOI: 10.1146/annurev-arplant-050718-100049 http://doi.org/10.1146/annurev-arplant-050718-100049 . CRISPR/Cas genome editing and precision plant breeding in agriculture DW Ci , D Jiang , SS Li , 等 . Identification of quantitative trait loci for cadmium tolerance and accumulation in wheat . Acta Physiol Plant , 2012 . 34 191 - 202 . DOI: 10.1007/s11738-011-0818-5 http://doi.org/10.1007/s11738-011-0818-5 . Identification of quantitative trait loci for cadmium tolerance and accumulation in wheat JM Clarke , D Leisle , GL Kopytko . Inheritance of cadmium concentration in five durum wheat crosses . Crop Sci , 1997 . 37 ( 6 ): 1722 - 1726 . DOI: 10.2135/cropsci1997.0011183X003700060008x http://doi.org/10.2135/cropsci1997.0011183X003700060008x . Inheritance of cadmium concentration in five durum wheat crosses JM Clarke , WA Norvell , FR Clarke , 等 . Concentration of cadmium and other elements in the grain of near-isogenic durum lines . Can J Plant Sci , 2002 . 82 ( 1 ): 27 - 33 . DOI: 10.4141/P01-083 http://doi.org/10.4141/P01-083 . Concentration of cadmium and other elements in the grain of near-isogenic durum lines JM Clarke , TN McCaig , RM DePauw , 等 . Registration of ‘Strongfield’ durum wheat . Crop Sci , 2006 . 46 ( 5 ): 2306 - 2307 . DOI: 10.2135/cropsci2005.12.0454 http://doi.org/10.2135/cropsci2005.12.0454 . Registration of ‘Strongfield’ durum wheat S Clemens , JF Ma . Toxic heavy metal and metalloid accumulation in crop plants and foods . Annu Rev Plant Biol , 2016 . 67 489 - 512 . 27128467 DOI: 10.1146/annurev-arplant-043015-112301 http://doi.org/10.1146/annurev-arplant-043015-112301 . Toxic heavy metal and metalloid accumulation in crop plants and foods S Clemens , MGM Aarts , S Thomine , 等 . Plant science: the key to preventing slow cadmium poisoning . Trends Plant Sci , 2013 . 18 ( 2 ): 92 - 99 . 22981394 DOI: 10.1016/j.tplants.2012.08.003 http://doi.org/10.1016/j.tplants.2012.08.003 . Plant science: the key to preventing slow cadmium poisoning CK Cohen , TC Fox , DF Garvin , 等 . The role of iron-deficiency stress responses in stimulating heavy-metal transport in plants . Plant Physiol , 1998 . 116 ( 3 ): 1063 - 1072 . 9501139 DOI: 10.1104/pp.116.3.1063 http://doi.org/10.1104/pp.116.3.1063 . The role of iron-deficiency stress responses in stimulating heavy-metal transport in plants CK Cohen , DF Garvin , LV Kochian . Kinetic properties of a micronutrient transporter from Pisum sativum indicate a primary function in Fe uptake from the soil . Planta , 2004 . 218 ( 5 ): 784 - 792 . 14648120 DOI: 10.1007/s00425-003-1156-7 http://doi.org/10.1007/s00425-003-1156-7 . Kinetic properties of a micronutrient transporter from Pisum sativum indicate a primary function in Fe uptake from the soil EL Connolly , JP Fett , ML Guerinot . Expression of the IRT1 metal transporter is controlled by metals at the levels of transcript and protein accumulation . Plant Cell , 2002 . 14 ( 6 ): 1347 - 1357 . 12084831 DOI: 10.1105/tpc.001263 http://doi.org/10.1105/tpc.001263 . Expression of the IRT1 metal transporter is controlled by metals at the levels of transcript and protein accumulation C Curie , JM Alonso , M le Jean , 等 . Involvement of NRAMP1 from Arabidopsis thaliana in iron transport . Biochem J , 2000 . 347 749 - 755 . 10769179 DOI: 10.1042/bj3470749 http://doi.org/10.1042/bj3470749 . Involvement of NRAMP1 from Arabidopsis thaliana in iron transport RW Dabeka , AD McKenzie , GMA Lacroix . Dietary intakes of lead, cadmium, arsenic and fluoride by Canadian adults: a 24-hour duplicate diet study . Food Add Contam , 1987 . 4 ( 1 ): 89 - 101 . DOI: 10.1080/02652038709373618 http://doi.org/10.1080/02652038709373618 . Dietary intakes of lead, cadmium, arsenic and fluoride by Canadian adults: a 24-hour duplicate diet study N Das , S Bhattacharya , S Bhattacharyya , 等 . Identification of alternatively spliced transcripts of rice phytochelatin synthase 2 gene OsPCS2 involved in mitigation of cadmium and arsenic stresses . Plant Mol Biol , 2017 . 94 ( 1-2 ): 167 - 183 . 28283922 DOI: 10.1007/s11103-017-0600-1 http://doi.org/10.1007/s11103-017-0600-1 . Identification of alternatively spliced transcripts of rice phytochelatin synthase 2 gene OsPCS2 involved in mitigation of cadmium and arsenic stresses Y Du , XF Hu , XH Wu , 等 . Affects of mining activities on Cd pollution to the paddy soils and rice grain in Hunan province, Central South China . Environ Monit Assess , 2013 . 185 ( 12 ): 9843 - 9856 . 23775460 DOI: 10.1007/s10661-013-3296-y http://doi.org/10.1007/s10661-013-3296-y . Affects of mining activities on Cd pollution to the paddy soils and rice grain in Hunan province, Central South China JE Eriksson . A field study on factors influencing Cd levels in soils and in grain of oats and winter wheat . Water Air Soil Poll , 1990 . 53 ( 1-2 ): 69 - 81 . DOI: 10.1007/BF00154992 http://doi.org/10.1007/BF00154992 . A field study on factors influencing Cd levels in soils and in grain of oats and winter wheat S Fahad , S Hussain , S Saud , 等 . Grain cadmium and zinc concentrations in maize influenced by genotypic variations and zinc fertilization . CLEAN-Soil Air Water , 2015 . 43 ( 10 ): 1433 - 1440 . DOI: 10.1002/clen.201400376 http://doi.org/10.1002/clen.201400376 . Grain cadmium and zinc concentrations in maize influenced by genotypic variations and zinc fertilization PJ Florijn , ML van Beusichem . Uptake and distribution of cadmium in maize inbred lines . Plant Soil , 1993 . 150 ( 1 ): 25 - 32 . DOI: 10.1007/BF00779172 http://doi.org/10.1007/BF00779172 . Uptake and distribution of cadmium in maize inbred lines PJ Florijn , ML van Beusichem . Cadmium distribution in maize inbred lines: effects of pH and level of Cd supply . Plant Soil , 1993 . 153 ( 1 ): 79 - 84 . DOI: 10.1007/BF00010546 http://doi.org/10.1007/BF00010546 . Cadmium distribution in maize inbred lines: effects of pH and level of Cd supply CA Grant , JM Clarke , S Duguid , 等 . Selection and breeding of plant cultivars to minimize cadmium accumulation . Sci Total Environ , 2008 . 390 ( 2-3 ): 301 - 310 . 18036635 DOI: 10.1016/j.scitotenv.2007.10.038 http://doi.org/10.1016/j.scitotenv.2007.10.038 . Selection and breeding of plant cultivars to minimize cadmium accumulation M Greger , M Löfstedt . Comparison of uptake and distribution of cadmium in different cultivars of bread and durum wheat . Crop Sci , 2004 . 44 ( 2 ): 501 - 507 . DOI: 10.2135/cropsci2004.5010 http://doi.org/10.2135/cropsci2004.5010 . Comparison of uptake and distribution of cadmium in different cultivars of bread and durum wheat MJ Guttieri , BW Seabourn , CX Liu , 等 . Distribution of cadmium, iron, and zinc in millstreams of hard winter wheat ( Triticum aestivum L.) . J Agric Food Chem , 2015 . 63 ( 49 ): 10681 - 10688 . 26568286 DOI: 10.1021/acs.jafc.5b04337 http://doi.org/10.1021/acs.jafc.5b04337 . Distribution of cadmium, iron, and zinc in millstreams of hard winter wheat (Triticum aestivum L.) MJ Guttieri , PS Baenziger , K Frels , 等 . Prospects for selecting wheat with increased zinc and decreased cadmium concentration in grain . Crop Sci , 2015 . 55 ( 4 ): 1712 - 1728 . DOI: 10.2135/cropsci2014.08.0559 http://doi.org/10.2135/cropsci2014.08.0559 . Prospects for selecting wheat with increased zinc and decreased cadmium concentration in grain XH Hao , M Zeng , J Wang , 等 . A node-expressed transporter OsCCX2 is involved in grain cadmium accumulation of rice . Front Plant Sci, 9:476 , 2018 . DOI: 10.3389/fpls.2018.00476 http://doi.org/10.3389/fpls.2018.00476 . A node-expressed transporter OsCCX2 is involved in grain cadmium accumulation of rice JY He , C Zhu , YF Ren , 等 . Genotypic variation in grain cadmium concentration of lowland rice . J Plant Nutr Soil Sci , 2006 . 169 ( 5 ): 711 - 716 . DOI: 10.1002/jpln.200525101 http://doi.org/10.1002/jpln.200525101 . Genotypic variation in grain cadmium concentration of lowland rice TD Hinesly , DE Alexander , EL Ziegler , 等 . Zinc and Cd accumulation by corn inbreds grown on sludge amended soil . Agron J , 1978 . 70 ( 3 ): 425 - 428 . DOI: 10.2134/agronj1978.00021962007000030015x http://doi.org/10.2134/agronj1978.00021962007000030015x . Zinc and Cd accumulation by corn inbreds grown on sludge amended soil TD Hinesly , DE Alexander , KE Redborg , 等 . Differential accumulations of cadmium and zinc by corn hybrids grown on soil amended with sewage sludge . Agron J , 1982 . 74 ( 3 ): 469 - 474 . DOI: 10.2134/agronj1982.00021962007400030017x http://doi.org/10.2134/agronj1982.00021962007400030017x . Differential accumulations of cadmium and zinc by corn hybrids grown on soil amended with sewage sludge J Hirzel , J Retamal-Salgado , I Walter , 等 . Cadmium accumulation and distribution in plants of three durum wheat cultivars under different agricultural environments in Chile . J Soil Water Conserv , 2017 . 72 ( 1 ): 77 - 88 . DOI: 10.2489/jswc.72.1.77 http://doi.org/10.2489/jswc.72.1.77 . Cadmium accumulation and distribution in plants of three durum wheat cultivars under different agricultural environments in Chile J Hirzel , J Retamal-Salgado , I Walter , 等 . Effect of soil cadmium concentration on three Chilean durum wheat cultivars in four environments . Arch Agron Soil Sci , 2018 . 64 ( 2 ): 162 - 172 . DOI: 10.1080/03650340.2017.1337892 http://doi.org/10.1080/03650340.2017.1337892 . Effect of soil cadmium concentration on three Chilean durum wheat cultivars in four environments DW Hu , ZH Sheng , QL Li , 等 . Identification of QTLs associated with cadmium concentration in rice grains . J Integr Agric , 2018 . 17 ( 7 ): 1563 - 1573 . DOI: 10.1016/S2095-3119(17)61847-1 http://doi.org/10.1016/S2095-3119(17)61847-1 . Identification of QTLs associated with cadmium concentration in rice grains FL Huang , XJ Wei , JW He , 等 . Mapping of quantitative trait loci associated with concentrations of five trace metal elements in rice ( Oryza sativa ) . Int J Agric Biol , 2018 . 20 ( 3 ): 554 - 560 . . Mapping of quantitative trait loci associated with concentrations of five trace metal elements in rice (Oryza sativa) YZ Huang , Y Hu , YX Liu . Heavy metal accumulation in iron plaque and growth of rice plants upon exposure to single and combined contamination by copper, cadmium and lead . Acta Ecol Sin , 2009 . 29 ( 6 ): 320 - 326 . DOI: 10.1016/j.chnaes.2009.09.011 http://doi.org/10.1016/j.chnaes.2009.09.011 . Heavy metal accumulation in iron plaque and growth of rice plants upon exposure to single and combined contamination by copper, cadmium and lead M Ikeda , ZW Zhang , K Higashikawa , 等 . Background exposure of general women populations in Japan to cadmium in the environment and possible health effects . Toxicol Lett , 1999 . 108 ( 2-3 ): 161 - 166 . 10511257 DOI: 10.1016/S0378-4274(99)00084-3 http://doi.org/10.1016/S0378-4274(99)00084-3 . Background exposure of general women populations in Japan to cadmium in the environment and possible health effects S Ishikawa , N Ae , M Yano . Chromosomal regions with quantitative trait loci controlling cadmium concentration in brown rice ( Oryza sativa ) . New Phytol , 2005 . 168 ( 2 ): 345 - 350 . 16219074 DOI: 10.1111/j.1469-8137.2005.01516.x http://doi.org/10.1111/j.1469-8137.2005.01516.x . Chromosomal regions with quantitative trait loci controlling cadmium concentration in brown rice (Oryza sativa) S Ishikawa , T Abe , M Kuramata , 等 . A major quantitative trait locus for increasing cadmium-specific concentration in rice grain is located on the short arm of chromosome 7 . J Exp Bot , 2010 . 61 ( 3 ): 923 - 934 . 20022924 DOI: 10.1093/jxb/erp360 http://doi.org/10.1093/jxb/erp360 . A major quantitative trait locus for increasing cadmium-specific concentration in rice grain is located on the short arm of chromosome 7 S Ishikawa , Y Ishimaru , M Igura , 等 . Ion-beam irradiation, gene identification, and marker-assisted breeding in the development of low-cadmium rice . Proc Natl Acad Sci USA , 2012 . 109 ( 47 ): 19166 - 19171 . DOI: 10.1073/pnas.1211132109 http://doi.org/10.1073/pnas.1211132109 . Ion-beam irradiation, gene identification, and marker-assisted breeding in the development of low-cadmium rice Y Ishimaru , R Takahashi , K Bashir , 等 . Characterizing the role of rice NRAMP5 in manganese, iron and cadmium transport . Sci Rep, 2:286 , 2012 . DOI: 10.1038/srep00286 http://doi.org/10.1038/srep00286 . Characterizing the role of rice NRAMP5 in manganese, iron and cadmium transport L Järup , A Åkesson . Current status of cadmium as an environmental health problem . Toxicol Appl Pharmacol , 2009 . 238 ( 3 ): 201 - 208 . 19409405 DOI: 10.1016/j.taap.2009.04.020 http://doi.org/10.1016/j.taap.2009.04.020 . Current status of cadmium as an environmental health problem T Kashiwagi , K Shindoh , N Hirotsu , 等 . Evidence for separate translocation pathways in determining cadmium accumulation in grain and aerial plant parts in rice . BMC Plant Biol, 9:8 , 2009 . DOI: 10.1186/1471-2229-9-8 http://doi.org/10.1186/1471-2229-9-8 . Evidence for separate translocation pathways in determining cadmium accumulation in grain and aerial plant parts in rice MA Khan , N Castro-Guerrero , DG Mendoza-Cozatl . Moving toward a precise nutrition: preferential loading of seeds with essential nutrients over non-essential toxic elements . Front Plant Sci, 5:51 , 2014 . DOI: 10.3389/fpls.2014.00051 http://doi.org/10.3389/fpls.2014.00051 . Moving toward a precise nutrition: preferential loading of seeds with essential nutrients over non-essential toxic elements RE Knox , CJ Pozniak , FR Clarke , 等 . Chromosomal location of the cadmium uptake gene ( Cdu1 ) in durum wheat . Genome , 2009 . 52 ( 9 ): 741 - 747 . 19935921 DOI: 10.1139/G09-042 http://doi.org/10.1139/G09-042 . Chromosomal location of the cadmium uptake gene (Cdu1) in durum wheat K Kubo , H Kobayashi , M Fujita , 等 . Varietal differences in the absorption and partitioning of cadmium in common wheat ( Triticum aestivum L.) . Environ Exp Bot , 2016 . 124 79 - 88 . DOI: 10.1016/j.envexpbot.2015.12.007 http://doi.org/10.1016/j.envexpbot.2015.12.007 . Varietal differences in the absorption and partitioning of cadmium in common wheat (Triticum aestivum L.) S Lee , G An . Over-expression of OsIRT1 leads to increased iron and zinc accumulations in rice . Plant Cell Environ , 2009 . 32 ( 4 ): 408 - 416 . 19183299 DOI: 10.1111/j.1365-3040.2009.01935.x http://doi.org/10.1111/j.1365-3040.2009.01935.x . Over-expression of OsIRT1 leads to increased iron and zinc accumulations in rice S Lee , YY Kim , Y Lee , 等 . Rice P 1B -type heavy-metal ATPase, OsHMA9, is a metal efflux protein . Plant Physiol , 2007 . 145 ( 3 ): 831 - 842 . 17827266 DOI: 10.1104/pp.107.102236 http://doi.org/10.1104/pp.107.102236 . Rice P1B-type heavy-metal ATPase, OsHMA9, is a metal efflux protein K Li , HY Yu , TX Li , 等 . Cadmium accumulation characteristics of low-cadmium rice ( Oryza sativa L.) line and F 1 hybrids grown in cadmium-contaminated soils . Environ Sci Pollut Res , 2017 . 24 ( 21 ): 17566 - 17576 . DOI: 10.1007/s11356-017-9350-5 http://doi.org/10.1007/s11356-017-9350-5 . Cadmium accumulation characteristics of low-cadmium rice (Oryza sativa L.) line and F1 hybrids grown in cadmium-contaminated soils S Li , SM Liu , HW Fu , 等 . High-resolution melting-based TILLING of γ ray-induced mutations in rice . J Zhejiang Univ-Sci B (Biomed & Biotechnol) , 2018 . 19 ( 8 ): 620 - 629 . DOI: 10.1631/jzus.B1700414 http://doi.org/10.1631/jzus.B1700414 . High-resolution melting-based TILLING of γ ray-induced mutations in rice JG Liu , QS Zhu , ZJ Zhang , 等 . Variations in cadmium accumulation among rice cultivars and types and the selection of cultivars for reducing cadmium in the diet . J Sci Food Agric , 2005 . 85 ( 1 ): 147 - 153 . DOI: 10.1002/jsfa.1973 http://doi.org/10.1002/jsfa.1973 . Variations in cadmium accumulation among rice cultivars and types and the selection of cultivars for reducing cadmium in the diet JG Liu , M Qian , GL Cai , 等 . Uptake and translocation of Cd in different rice cultivars and the relation with Cd accumulation in rice grain . J Hazard Mater , 2007 . 143 ( 1-2 ): 443 - 447 . 17079078 DOI: 10.1016/j.jhazmat.2006.09.057 http://doi.org/10.1016/j.jhazmat.2006.09.057 . Uptake and translocation of Cd in different rice cultivars and the relation with Cd accumulation in rice grain SM Liu , J Jiang , Y Liu , 等 . Characterization and evaluation of OsLCT1 and OsNramp5 mutants generated through CRISPR/Cas9-mediated mutagenesis for breeding low Cd rice . Rice Sci , 2019 . 26 ( 2 ): 88 - 97 . DOI: 10.1016/j.rsci.2019.01.002 http://doi.org/10.1016/j.rsci.2019.01.002 . Characterization and evaluation of OsLCT1 and OsNramp5 mutants generated through CRISPR/Cas9-mediated mutagenesis for breeding low Cd rice WQ Liu , XW Pan , YC Li , 等 . Identification of QTLs and validation of qCd-2 associated with grain cadmium concentrations in rice . Rice Sci , 2019 . 26 ( 1 ): 42 - 49 . DOI: 10.1016/j.rsci.2018.12.003 http://doi.org/10.1016/j.rsci.2018.12.003 . Identification of QTLs and validation of qCd-2 associated with grain cadmium concentrations in rice WX Liu , SH Shang , F Xue , 等 . Modulation of exogenous selenium in cadmium-induced changes in antioxidative metabolism, cadmium uptake, and photosynthetic performance in the 2 tobacco genotypes differing in cadmium tolerance . Environ Toxicol Chem, 2015 , 2015 . 34 ( 1 ): 92 - 99 . DOI: 10.1002/etc.2760 http://doi.org/10.1002/etc.2760 . Modulation of exogenous selenium in cadmium-induced changes in antioxidative metabolism, cadmium uptake, and photosynthetic performance in the 2 tobacco genotypes differing in cadmium tolerance XS Liu , SJ Feng , BQ Zhang , 等 . OsZIP1 functions as a metal efflux transporter limiting excess zinc, copper and cadmium accumulation in rice . BMC Plant Biol, 19:283 , 2019 . DOI: 10.1186/s12870-019-1899-3 http://doi.org/10.1186/s12870-019-1899-3 . OsZIP1 functions as a metal efflux transporter limiting excess zinc, copper and cadmium accumulation in rice CN Lu , LX Zhang , Z Tang , 等 . Producing cadmium-free Indica rice by overexpressing OsHMA3 . Environ Int , 2019 . 126 619 - 626 . 30856449 DOI: 10.1016/j.envint.2019.03.004 http://doi.org/10.1016/j.envint.2019.03.004 . Producing cadmium-free Indica rice by overexpressing OsHMA3 JS Luo , J Huang , DL Zeng , 等 . A defensin-like protein drives cadmium efflux and allocation in rice . Nat Commun, 9:645 , 2018 . DOI: 10.1038/s41467-018-03088-0 http://doi.org/10.1038/s41467-018-03088-0 . A defensin-like protein drives cadmium efflux and allocation in rice SR McCouch , MH Wright , CW Tung , 等 . Open access resources for genome-wide association mapping in rice . Nat Commun, 7:10532 , 2016 . DOI: 10.1038/ncomms10532 http://doi.org/10.1038/ncomms10532 . Open access resources for genome-wide association mapping in rice RF Mills , KA Peaston , J Runions , 等 . HvHMA2, a P 1B -ATPase from barley, is highly conserved among cereals and functions in Zn and Cd transport . PLoS ONE, 7:e42640 , 2012 . DOI: 10.1371/journal.pone.0042640 http://doi.org/10.1371/journal.pone.0042640 . HvHMA2, a P1B-ATPase from barley, is highly conserved among cereals and functions in Zn and Cd transport H Miyadate , S Adachi , A Hiraizumi , 等 . OsHMA3, a P 1B -type of ATPase affects root-to-shoot cadmium translocation in rice by mediating efflux into vacuoles . New Phytol , 2011 . 189 ( 1 ): 190 - 199 . 20840506 DOI: 10.1111/j.1469-8137.2010.03459.x http://doi.org/10.1111/j.1469-8137.2010.03459.x . OsHMA3, a P1B-type of ATPase affects root-to-shoot cadmium translocation in rice by mediating efflux into vacuoles I Murgia , L de Gara , MA Grusak . Biofortification: how can we exploit plant science and biotechnology to reduce micronutrient deficiencies? . Front Plant Sci, 4:429 , 2013 . DOI: 10.3389/fpls.2013.00429 http://doi.org/10.3389/fpls.2013.00429 . Biofortification: how can we exploit plant science and biotechnology to reduce micronutrient deficiencies? H Nakanishi , I Ogawa , Y Ishimaru , 等 . Iron deficiency enhances cadmium uptake and translocation mediated by the Fe 2+ transporters OsIRT1 and OsIRT2 in rice . Soil Sci Plant Nutr , 2006 . 52 ( 4 ): 464 - 469 . DOI: 10.1111/j.1747-0765.2006.00055.x http://doi.org/10.1111/j.1747-0765.2006.00055.x . Iron deficiency enhances cadmium uptake and translocation mediated by the Fe2+ transporters OsIRT1 and OsIRT2 in rice Y Nevo , N Nelson . The NRAMP family of metal-ion transporters . Biochim Biophys Acta , 2006 . 1763 ( 7 ): 609 - 620 . 16908340 DOI: 10.1016/j.bbamcr.2006.05.007 http://doi.org/10.1016/j.bbamcr.2006.05.007 . The NRAMP family of metal-ion transporters (National Health and Family Planning Commission) NHFPC . National Safety Standard for Contaminants in Foods, GB2762-2017 . 2017 . National Standards of People’s Republic of China . (in Chinese) . National Safety Standard for Contaminants in Foods, GB2762-2017 J Nicod , RW Davies , N Cai , 等 . Genome-wide association of multiple complex traits in outbred mice by ultra-low-coverage sequencing . Nat Genet , 2016 . 48 ( 8 ): 912 - 918 . 27376238 DOI: 10.1038/ng.3595 http://doi.org/10.1038/ng.3595 . Genome-wide association of multiple complex traits in outbred mice by ultra-low-coverage sequencing GF Nordberg . Historical perspectives on cadmium toxicology . Toxicol Appl Pharmacol , 2009 . 238 ( 3 ): 192 - 200 . 19341754 DOI: 10.1016/j.taap.2009.03.015 http://doi.org/10.1016/j.taap.2009.03.015 . Historical perspectives on cadmium toxicology A Oladzad-Abbasabadi , A Kumar , S Pirseyedi , 等 . Identification and validation of a new source of low grain cadmium accumulation in durum wheat . G3 , 2018 . 8 ( 3 ): 923 - 932 . 29352079 DOI: 10.1534/g3.117.300370 http://doi.org/10.1534/g3.117.300370 . Identification and validation of a new source of low grain cadmium accumulation in durum wheat DP Oliver , JW Gartrell , KG Tiller , 等 . Differential responses of Australian wheat cultivars to cadmium concentration in wheat grain . Aust J Agric Res , 1995 . 46 ( 5 ): 873 - 886 . DOI: 10.1071/AR9950873 http://doi.org/10.1071/AR9950873 . Differential responses of Australian wheat cultivars to cadmium concentration in wheat grain M Ovečka , T Takáč . Managing heavy metal toxicity stress in plants: biological and biotechnological tools . Biotechnol Adv , 2014 . 32 ( 1 ): 73 - 86 . 24333465 DOI: 10.1016/j.biotechadv.2013.11.011 http://doi.org/10.1016/j.biotechadv.2013.11.011 . Managing heavy metal toxicity stress in plants: biological and biotechnological tools MG Palmgren , S Clemens , LE Williams , 等 . Zinc biofortification of cereals: problems and solutions . Trends Plant Sci , 2008 . 13 ( 9 ): 464 - 473 . 18701340 DOI: 10.1016/j.tplants.2008.06.005 http://doi.org/10.1016/j.tplants.2008.06.005 . Zinc biofortification of cereals: problems and solutions P Pedas , CK Ytting , AT Fuglsang , 等 . Manganese efficiency in barley: identification and characterization of the metal ion transporter HvIRT1 . Plant Physiol , 2008 . 148 ( 1 ): 455 - 466 . 18614714 DOI: 10.1104/pp.108.118851 http://doi.org/10.1104/pp.108.118851 . Manganese efficiency in barley: identification and characterization of the metal ion transporter HvIRT1 F Peng , C Wang , YR Cheng , 等 . Cloning and characterization of TpNRAMP3 , a metal transporter from Polish wheat ( Triticum polonicum L.) . Front Plant Sci, 9:1354 , 2018 . DOI: 10.3389/fpls.2018.01354 http://doi.org/10.3389/fpls.2018.01354 . Cloning and characterization of TpNRAMP3, a metal transporter from Polish wheat (Triticum polonicum L.) F Peng , C Wang , JS Zhu , 等 . Expression of TpNRAMP5, a metal transporter from polish wheat ( Triticum polonicum L.), enhances the accumulation of Cd, Co and Mn in transgenic Arabidopsis plants . Planta , 2018 . 247 ( 6 ): 1395 - 1406 . 29523961 DOI: 10.1007/s00425-018-2872-3 http://doi.org/10.1007/s00425-018-2872-3 . Expression of TpNRAMP5, a metal transporter from polish wheat (Triticum polonicum L.), enhances the accumulation of Cd, Co and Mn in transgenic Arabidopsis plants GA Penner , LJ Bezte , D Leisle , 等 . Identification of RAPD markers linked to a gene governing cadmium uptake in durum wheat . Genome , 1995 . 38 ( 3 ): 543 - 547 . 18470188 DOI: 10.1139/g95-070 http://doi.org/10.1139/g95-070 . Identification of RAPD markers linked to a gene governing cadmium uptake in durum wheat F Perrier , B Yan , F Candaudap , 等 . Variability in grain cadmium concentration among durum wheat cultivars: impact of aboveground biomass partitioning . Plant Soil , 2016 . 404 ( 1-2 ): 307 - 320 . DOI: 10.1007/s11104-016-2847-8 http://doi.org/10.1007/s11104-016-2847-8 . Variability in grain cadmium concentration among durum wheat cultivars: impact of aboveground biomass partitioning SRM Pinson , L Tarpley , WG Yan , 等 . Worldwide genetic diversity for mineral element concentrations in rice grain . Crop Sci , 2015 . 55 ( 1 ): 294 - 311 . DOI: 10.2135/cropsci2013.10.0656 http://doi.org/10.2135/cropsci2013.10.0656 . Worldwide genetic diversity for mineral element concentrations in rice grain Y Ramegowda , R Venkategowda , P Jagadish , 等 . Expression of a rice Zn transporter, OsZIP1 , increases Zn concentration in tobacco and finger millet transgenic plants . Plant Biotechnol Rep , 2013 . 7 ( 3 ): 309 - 319 . DOI: 10.1007/s11816-012-0264-x http://doi.org/10.1007/s11816-012-0264-x . Expression of a rice Zn transporter, OsZIP1, increases Zn concentration in tobacco and finger millet transgenic plants HS Randhawa , M Asif , C Pozniak , 等 . Application of molecular markers to wheat breeding in Canada . Plant Breed , 2013 . 132 ( 5 ): 458 - 471 . DOI: 10.1111/pbr.12057 http://doi.org/10.1111/pbr.12057 . Application of molecular markers to wheat breeding in Canada J Retamal-Salgado , J Hirzel , I Walter , 等 . Bioabsorption and bioaccumulation of cadmium in the straw and grain of maize ( Zea mays L.) in growing soils contaminated with cadmium in different environment . Int J Environ Res Public Health , 2017 . 14 ( 11 ): 1399 DOI: 10.3390/ijerph14111399 http://doi.org/10.3390/ijerph14111399 . Bioabsorption and bioaccumulation of cadmium in the straw and grain of maize (Zea mays L.) in growing soils contaminated with cadmium in different environment FK Ricachenevsky , T Punshon , S Lee , 等 . Elemental profiling of rice FOX lines leads to characterization of a new Zn plasma membrane transporter, OsZIP7 . Front Plant Sci, 9:865 , 2018 . DOI: 10.3389/fpls.2018.00865 http://doi.org/10.3389/fpls.2018.00865 . Elemental profiling of rice FOX lines leads to characterization of a new Zn plasma membrane transporter, OsZIP7 TL Roberts . Cadmium and phosphorous fertilizers: the issues and the science . Procedia Eng , 2014 . 83 52 - 59 . DOI: 10.1016/j.proeng.2014.09.012 http://doi.org/10.1016/j.proeng.2014.09.012 . Cadmium and phosphorous fertilizers: the issues and the science E Salsman , A Kumar , W AbuHammad , 等 . Development and validation of molecular markers for grain cadmium in durum wheat . Mol Breed , 2018 . 38 ( 3 ): 28 DOI: 10.1007/s11032-018-0788-z http://doi.org/10.1007/s11032-018-0788-z . Development and validation of molecular markers for grain cadmium in durum wheat A Sasaki , N Yamaji , K Yokosho , 等 . Nramp5 is a major transporter responsible for manganese and cadmium uptake in rice . Plant Cell , 2012 . 24 ( 5 ): 2155 - 2167 . 22589467 DOI: 10.1105/tpc.112.096925 http://doi.org/10.1105/tpc.112.096925 . Nramp5 is a major transporter responsible for manganese and cadmium uptake in rice A Sasaki , N Yamaji , JF Ma . Overexpression of OsHMA3 enhances Cd tolerance and expression of Zn transporter genes in rice . J Exp Bot , 2014 . 65 ( 20 ): 6013 - 6021 . 25151617 DOI: 10.1093/jxb/eru340 http://doi.org/10.1093/jxb/eru340 . Overexpression of OsHMA3 enhances Cd tolerance and expression of Zn transporter genes in rice S Satarug , JR Baker , S Urbenjapol , 等 . A global perspective on cadmium pollution and toxicity in non-occupationally exposed population . Toxicol Lett , 2003 . 137 ( 1-2 ): 65 - 83 . 12505433 DOI: 10.1016/S0378-4274(02)00381-8 http://doi.org/10.1016/S0378-4274(02)00381-8 . A global perspective on cadmium pollution and toxicity in non-occupationally exposed population S Satarug , SH Garrett , MA Sens , 等 . Cadmium, environmental exposure, and health outcomes . Environ Health Persp , 2009 . 118 ( 2 ): 182 - 190 . DOI: 10.1289/ehp.0901234 http://doi.org/10.1289/ehp.0901234 . Cadmium, environmental exposure, and health outcomes H Sato , S Shirasawa , H Maeda , 等 . Analysis of QTL for lowering cadmium concentration in rice grains from ‘LAC23’ . Breed Sci , 2011 . 61 ( 2 ): 196 - 200 . DOI: 10.1270/jsbbs.61.196 http://doi.org/10.1270/jsbbs.61.196 . Analysis of QTL for lowering cadmium concentration in rice grains from ‘LAC23’ N Satoh-Nagasawa , M Mori , N Nakazawa , 等 . Mutations in rice ( Oryza sativa ) heavy metal ATPase 2 ( OsHMA2 ) restrict the translocation of zinc and cadmium . Plant Cell Physiol , 2012 . 53 ( 1 ): 213 - 224 . 22123790 DOI: 10.1093/pcp/pcr166 http://doi.org/10.1093/pcp/pcr166 . Mutations in rice (Oryza sativa) heavy metal ATPase 2 (OsHMA2) restrict the translocation of zinc and cadmium GN Scofield , T Hirose , N Aoki , 等 . Involvement of the sucrose transporter, OsSUT1, in the long-distance pathway for assimilate transport in rice . J Exp Bot , 2007 . 58 ( 12 ): 3155 - 3169 . 17728297 DOI: 10.1093/jxb/erm153 http://doi.org/10.1093/jxb/erm153 . Involvement of the sucrose transporter, OsSUT1, in the long-distance pathway for assimilate transport in rice A Sebastian , MNV Prasad . Cadmium minimization in rice. A review . Agron Sustain Dev , 2014 . 34 155 - 173 . DOI: 10.1007/s13593-013-0152-y http://doi.org/10.1007/s13593-013-0152-y . Cadmium minimization in rice. A review QW Shan , YP Wang , J Li , 等 . Genome editing in rice and wheat using the CRISPR/Cas system . Nat Protoc , 2014 . 9 ( 10 ): 2395 - 2410 . 25232936 DOI: 10.1038/nprot.2014.157 http://doi.org/10.1038/nprot.2014.157 . Genome editing in rice and wheat using the CRISPR/Cas system JF Shao , JX Xia , N Yamaji , 等 . Effective reduction of cadmium accumulation in rice grain by expressing OsHMA3 under the control of the OsHMA2 promoter . J Exp Bot , 2018 . 69 ( 10 ): 2743 - 2752 . 29562302 DOI: 10.1093/jxb/ery107 http://doi.org/10.1093/jxb/ery107 . Effective reduction of cadmium accumulation in rice grain by expressing OsHMA3 under the control of the OsHMA2 promoter H Shimo , Y Ishimaru , G An , 等 . Low cadmium ( LCD ), a novel gene related to cadmium tolerance and accumulation in rice . J Exp Bot , 2011 . 62 ( 15 ): 5727 - 5734 . 21908474 DOI: 10.1093/jxb/err300 http://doi.org/10.1093/jxb/err300 . Low cadmium (LCD), a novel gene related to cadmium tolerance and accumulation in rice WE Song , SB Chen , JF Liu , 等 . Variation of Cd concentration in various rice cultivars and derivation of cadmium toxicity thresholds for paddy soil by species-sensitivity distribution . J Integr Agric , 2015 . 14 ( 9 ): 1845 - 1854 . DOI: 10.1016/S2095-3119(14)60926-6 http://doi.org/10.1016/S2095-3119(14)60926-6 . Variation of Cd concentration in various rice cultivars and derivation of cadmium toxicity thresholds for paddy soil by species-sensitivity distribution R Soric , Z Loncaric , V Kovacevic , 等 . A major gene for leaf cadmium accumulation in maize ( Zea mays L . 2009 . ). The Proceedings of International Plant Nutrition Colloquium XVI, UC Davis: Department of Plant Sciences . . A major gene for leaf cadmium accumulation in maize (Zea mays L JP Stolt , FEC Sneller , T Bryngelsson , 等 . Phytochelatin and cadmium accumulation in wheat . Environ Exp Bot , 2003 . 49 ( 1 ): 21 - 28 . DOI: 10.1016/S0098-8472(02)00045-X http://doi.org/10.1016/S0098-8472(02)00045-X . Phytochelatin and cadmium accumulation in wheat FQ Sui , JD Chang , Z Tang , 等 . Nramp5 expression and functionality likely explain higher cadmium uptake in rice than in wheat and maize . Plant Soil , 2018 . 433 ( 1-2 ): 377 - 389 . DOI: 10.1007/s11104-018-3849-5 http://doi.org/10.1007/s11104-018-3849-5 . Nramp5 expression and functionality likely explain higher cadmium uptake in rice than in wheat and maize HY Sun , FB Cao , NB Wang , 等 . Differences in grain ultrastructure, phytochemical and proteomic profiles between the two contrasting grain Cd-accumulation barley genotypes . PLoS ONE, 2013 , 2013 . 8 ( 11 ): e79158 DOI: 10.1371/journal.pone.0079158 http://doi.org/10.1371/journal.pone.0079158 . Differences in grain ultrastructure, phytochemical and proteomic profiles between the two contrasting grain Cd-accumulation barley genotypes HY Sun , ZH Chen , F Chen , 等 . DNA microarray revealed and RNAi plants confirmed key genes conferring low Cd accumulation in barley grains . BMC Plant Biol, 15:259 , 2015 . DOI: 10.1186/s12870-015-0648-5 http://doi.org/10.1186/s12870-015-0648-5 . DNA microarray revealed and RNAi plants confirmed key genes conferring low Cd accumulation in barley grains R Takahashi , Y Ishimaru , T Senoura , 等 . The OsNRAMP1 iron transporter is involved in Cd accumulation in rice . J Exp Bot , 2011 . 62 ( 14 ): 4843 - 4850 . 21697258 DOI: 10.1093/jxb/err136 http://doi.org/10.1093/jxb/err136 . The OsNRAMP1 iron transporter is involved in Cd accumulation in rice R Takahashi , Y Ishimaru , H Nakanishi , 等 . Role of the iron transporter OsNRAMP1 in cadmium uptake and accumulation in rice . Plant Signal Behav , 2011 . 6 ( 11 ): 1813 - 1816 . 22067109 DOI: 10.4161/psb.6.11.17587 http://doi.org/10.4161/psb.6.11.17587 . Role of the iron transporter OsNRAMP1 in cadmium uptake and accumulation in rice R Takahashi , Y Ishimaru , H Shimo , 等 . The OsHMA2 transporter is involved in root-to-shoot translocation of Zn and Cd in rice . Plant Cell Environ , 2012 . 35 ( 11 ): 1948 - 1957 . 22548273 DOI: 10.1111/j.1365-3040.2012.02527.x http://doi.org/10.1111/j.1365-3040.2012.02527.x . The OsHMA2 transporter is involved in root-to-shoot translocation of Zn and Cd in rice JJ Tan , JW Wang , TY Chai , 等 . Functional analyses of TaHMA2, a P 1B -type ATPase in wheat . Plant Biotechnol J , 2013 . 11 ( 4 ): 420 - 431 . 23294838 DOI: 10.1111/pbi.12027 http://doi.org/10.1111/pbi.12027 . Functional analyses of TaHMA2, a P1B-type ATPase in wheat LT Tan , YX Zhu , T Fan , 等 . OsZIP7 functions in xylem loading in roots and inter-vascular transfer in nodes to deliver Zn/Cd to grain in rice . Biochem Biophys Res Commun , 2019 . 512 ( 1 ): 112 - 118 . 30871778 DOI: 10.1016/j.bbrc.2019.03.024 http://doi.org/10.1016/j.bbrc.2019.03.024 . OsZIP7 functions in xylem loading in roots and inter-vascular transfer in nodes to deliver Zn/Cd to grain in rice L Tang , BG Mao , YK Li , 等 . Knockout of OsNramp5 using the CRISPR/Cas9 system produces low Cd-accumulating indica rice without compromising yield . Sci Rep, 7:14438 , 2017 . DOI: 10.1038/s41598-017-14832-9 http://doi.org/10.1038/s41598-017-14832-9 . Knockout of OsNramp5 using the CRISPR/Cas9 system produces low Cd-accumulating indica rice without compromising yield K Tezuka , H Miyadate , K Katou , 等 . A single recessive gene controls cadmium translocation in the cadmium hyperaccumulating rice cultivar Cho-Ko-Koku . Theor Appl Genet , 2010 . 120 ( 6 ): 1175 - 1182 . 20039013 DOI: 10.1007/s00122-009-1244-6 http://doi.org/10.1007/s00122-009-1244-6 . A single recessive gene controls cadmium translocation in the cadmium hyperaccumulating rice cultivar Cho-Ko-Koku T Tsukahara , T Ezaki , J Moriguchi , 等 . Rice as the most influential source of cadmium intake among general Japanese population . Sci Total Environ , 2003 . 305 ( 1-3 ): 41 - 51 . 12670756 DOI: 10.1016/S0048-9697(02)00475-8 http://doi.org/10.1016/S0048-9697(02)00475-8 . Rice as the most influential source of cadmium intake among general Japanese population Y Ueda , F Frimpong , YT Qi , 等 . Genetic dissection of ozone tolerance in rice ( Oryza sativa L.) by a genome-wide association study . J Exp Bot , 2015 . 66 ( 1 ): 293 - 306 . 25371505 DOI: 10.1093/jxb/eru419 http://doi.org/10.1093/jxb/eru419 . Genetic dissection of ozone tolerance in rice (Oryza sativa L.) by a genome-wide association study D Ueno , E Koyama , I Kono , 等 . Identification of a novel major quantitative trait locus controlling distribution of Cd between roots and shoots in rice . Plant Cell Physiol , 2009 . 50 ( 12 ): 2223 - 2233 . 19884249 DOI: 10.1093/pcp/pcp160 http://doi.org/10.1093/pcp/pcp160 . Identification of a novel major quantitative trait locus controlling distribution of Cd between roots and shoots in rice D Ueno , I Kono , K Yokosho , 等 . A major quantitative trait locus controlling cadmium translocation in rice ( Oryza sativa ) . New Phytol , 2009 . 182 ( 3 ): 644 - 653 . 19309445 DOI: 10.1111/j.1469-8137.2009.02784.x http://doi.org/10.1111/j.1469-8137.2009.02784.x . A major quantitative trait locus controlling cadmium translocation in rice (Oryza sativa) D Ueno , N Yamaji , I Kono , 等 . Gene limiting cadmium accumulation in rice . Proc Natl Acad Sci USA , 2010 . 107 ( 38 ): 16500 - 16505 . DOI: 10.1073/pnas.1005396107 http://doi.org/10.1073/pnas.1005396107 . Gene limiting cadmium accumulation in rice S Uraguchi , T Fujiwara . Rice breaks ground for cadmium-free cereals . Curr Opin Plant Biol , 2013 . 16 ( 3 ): 328 - 334 . 23587938 DOI: 10.1016/j.pbi.2013.03.012 http://doi.org/10.1016/j.pbi.2013.03.012 . Rice breaks ground for cadmium-free cereals S Uraguchi , T Kamiya , T Sakamoto , 等 . Low-affinity cation transporter ( OsLCT1 ) regulates cadmium transport into rice grains . Proc Natl Acad Sci USA , 2011 . 108 ( 52 ): 20959 - 20964 . DOI: 10.1073/pnas.1116531109 http://doi.org/10.1073/pnas.1116531109 . Low-affinity cation transporter (OsLCT1) regulates cadmium transport into rice grains S Uraguchi , T Kamiya , S Clemens , 等 . Characterization of OsLCT1, a cadmium transporter from indica rice ( Oryza sativa ) . Physiol Plant , 2014 . 151 ( 3 ): 339 - 347 . 24627964 DOI: 10.1111/ppl.12189 http://doi.org/10.1111/ppl.12189 . Characterization of OsLCT1, a cadmium transporter from indica rice (Oryza sativa) S Uraguchi , N Tanaka , C Hofmann , 等 . Phytochelatin synthase has contrasting effects on cadmium and arsenic accumulation in rice grains . Plant Cell Physiol , 2017 . 58 ( 10 ): 1730 - 1742 . 29016913 DOI: 10.1093/pcp/pcx114 http://doi.org/10.1093/pcp/pcx114 . Phytochelatin synthase has contrasting effects on cadmium and arsenic accumulation in rice grains M Vergine , A Aprile , E Sabella , 等 . Cadmium concentration in grains of durum wheat ( Triticum turgidum L. subsp . durum) J Agric Food Chem , 2017 . 65 ( 30 ): 6240 - 6246 . DOI: 10.1021/acs.jafc.7b01946 http://doi.org/10.1021/acs.jafc.7b01946 . Cadmium concentration in grains of durum wheat (Triticum turgidum L. subsp G Vert , M Barberon , E Zelazny , 等 . Arabidopsis IRT2 cooperates with the high-affinity iron uptake system to maintain iron homeostasis in root epidermal cells . Planta , 2009 . 229 ( 6 ): 1171 - 1179 . 19252923 DOI: 10.1007/s00425-009-0904-8 http://doi.org/10.1007/s00425-009-0904-8 . Arabidopsis IRT2 cooperates with the high-affinity iron uptake system to maintain iron homeostasis in root epidermal cells XK Wang , X Gong , FB Cao , 等 . HvPAA1 encodes a P-Type ATPase, a novel gene for cadmium accumulation and tolerance in barley ( Hordeum vulgare L.) . Int J Mol Sci , 2019 . 20 ( 7 ): 1732 DOI: 10.3390/ijms20071732 http://doi.org/10.3390/ijms20071732 . HvPAA1 encodes a P-Type ATPase, a novel gene for cadmium accumulation and tolerance in barley (Hordeum vulgare L.) Y Wang , XL Wang , C Wang , 等 . Transcriptomic profiles reveal the interactions of Cd/Zn in dwarf Polish wheat ( Triticum polonicum L.) roots . Front Physiol, 8:168 , 2017 . DOI: 10.3389/fphys.2017.00168 http://doi.org/10.3389/fphys.2017.00168 . Transcriptomic profiles reveal the interactions of Cd/Zn in dwarf Polish wheat (Triticum polonicum L.) roots BM Waters , RP Sankaran . Moving micronutrients from the soil to the seeds: genes and physiological processes from a biofortification perspective . Plant Sci , 2011 . 180 ( 4 ): 562 - 574 . 21421405 DOI: 10.1016/j.plantsci.2010.12.003 http://doi.org/10.1016/j.plantsci.2010.12.003 . Moving micronutrients from the soil to the seeds: genes and physiological processes from a biofortification perspective PJ White , MR Broadley . Biofortifying crops with essential mineral elements . Trends Plant Sci , 2005 . 10 ( 12 ): 586 - 593 . 16271501 DOI: 10.1016/j.tplants.2005.10.001 http://doi.org/10.1016/j.tplants.2005.10.001 . Biofortifying crops with essential mineral elements (World Health Organization) WHO . WHO Food Additives Series 64: Safety Evaluation of Certain Food Additives and Contaminants . 2011 . Seventy-third Meeting of the Joint FAO/WHO Expert Committee on Food Additives (JECFA), Geneva . . WHO Food Additives Series 64: Safety Evaluation of Certain Food Additives and Contaminants K Wiebe . Molecular characterization of Cdu-b1 , a major locus controlling cadmium accumulation in durum wheat ( Triticum turgidum L. var durum ) grain. MS T , 2012 . University of Saskatchewan, Saskatoon, Canada , . Molecular characterization of Cdu-b1, a major locus controlling cadmium accumulation in durum wheat (Triticum turgidum L. var durum) grain. MS T K Wiebe , NS Harris , JD Faris , 等 . Targeted mapping of Cdu1 , a major locus regulating grain cadmium concentration in durum wheat ( Triticum turgidum L. var durum ) . Theor Appl Genet , 2010 . 121 ( 6 ): 1047 - 1058 . 20559817 DOI: 10.1007/s00122-010-1370-1 http://doi.org/10.1007/s00122-010-1370-1 . Targeted mapping of Cdu1, a major locus regulating grain cadmium concentration in durum wheat (Triticum turgidum L. var durum) DZ Wu , K Sato , JF Ma . Genome-wide association mapping of cadmium accumulation in different organs of barley . New Phytol , 2015 . 208 ( 3 ): 817 - 829 . 26061418 DOI: 10.1111/nph.13512 http://doi.org/10.1111/nph.13512 . Genome-wide association mapping of cadmium accumulation in different organs of barley DZ Wu , N Yamaji , M Yamane , 等 . The HvNramp5 transporter mediates uptake of cadmium and manganese, but not iron . Plant Physiol , 2016 . 172 ( 3 ): 1899 - 1910 . 27621428 DOI: 10.1104/pp.16.01189 http://doi.org/10.1104/pp.16.01189 . The HvNramp5 transporter mediates uptake of cadmium and manganese, but not iron FB Wu , G Zhang . Genotypic differences in effect of Cd on growth and mineral concentrations in barley seedlings . Bull Environ Contam Toxicol , 2002 . 69 ( 2 ): 219 - 227 . 12107698 DOI: 10.1007/s00128-002-0050-5 http://doi.org/10.1007/s00128-002-0050-5 . Genotypic differences in effect of Cd on growth and mineral concentrations in barley seedlings FB Wu , HX Wu , GP Zhang , 等 . Differences in growth and yield in response to cadmium toxicity in cotton genotypes . J Plant Nutr Soil Sci , 2004 . 167 ( 1 ): 85 - 90 . DOI: 10.1002/jpln.200320320 http://doi.org/10.1002/jpln.200320320 . Differences in growth and yield in response to cadmium toxicity in cotton genotypes FB Wu , GP Zhang , P Dominy , 等 . Differences in yield components and kernel Cd accumulation in response to Cd toxicity in four barley genotypes . Chemosphere , 2007 . 70 ( 1 ): 83 - 92 . 17675207 DOI: 10.1016/j.chemosphere.2007.06.051 http://doi.org/10.1016/j.chemosphere.2007.06.051 . Differences in yield components and kernel Cd accumulation in response to Cd toxicity in four barley genotypes DW Xue , MC Chen , GP Zhang . Mapping of QTLs associated with cadmium tolerance and accumulation during seedling stage in rice ( Oryza sativa L.) . Euphytica , 2009 . 165 ( 3 ): 587 DOI: 10.1007/s10681-008-9785-3 http://doi.org/10.1007/s10681-008-9785-3 . Mapping of QTLs associated with cadmium tolerance and accumulation during seedling stage in rice (Oryza sativa L.) N Yamaji , JF Ma . The node, a hub for mineral nutrient distribution in graminaceous plants . Trends Plant Sci , 2014 . 19 ( 9 ): 556 - 563 . 24953837 DOI: 10.1016/j.tplants.2014.05.007 http://doi.org/10.1016/j.tplants.2014.05.007 . The node, a hub for mineral nutrient distribution in graminaceous plants N Yamaji , JX Xia , N Mitani-Ueno , 等 . Preferential delivery of zinc to developing tissues in rice is mediated by P-type heavy metal ATPase OsHMA2 . Plant Physiol , 2013 . 162 ( 2 ): 927 - 939 . 23575418 DOI: 10.1104/pp.113.216564 http://doi.org/10.1104/pp.113.216564 . Preferential delivery of zinc to developing tissues in rice is mediated by P-type heavy metal ATPase OsHMA2 HL Yan , WX Xu , JY Xie , 等 . Variation of a major facilitator superfamily gene contributes to differential cadmium accumulation between rice subspecies . Nat Commun , 2019 . 10 ( 1 ): 2562 31189898 DOI: 10.1038/s41467-019-10544-y http://doi.org/10.1038/s41467-019-10544-y . Variation of a major facilitator superfamily gene contributes to differential cadmium accumulation between rice subspecies JL Yan , PT Wang , P Wang , 等 . A loss-of-function allele of OsHMA3 associated with high cadmium accumulation in shoots and grain of Japonica rice cultivars . Plant Cell Environ , 2016 . 39 ( 9 ): 1941 - 1954 . 27038090 DOI: 10.1111/pce.12747 http://doi.org/10.1111/pce.12747 . A loss-of-function allele of OsHMA3 associated with high cadmium accumulation in shoots and grain of Japonica rice cultivars YF Yan , DH Choi , DS Kim , 等 . Genotypic variation of cadmium accumulation and distribution in rice . J Crop Sci Biotechnol , 2010 . 13 ( 2 ): 69 - 73 . DOI: 10.1007/s12892-010-0036-5 http://doi.org/10.1007/s12892-010-0036-5 . Genotypic variation of cadmium accumulation and distribution in rice YF Yan , P Lestari , KJ Lee , 等 . Identification of quantitative trait loci for cadmium accumulation and distribution in rice ( Oryza sativa ) . Genome , 2013 . 56 ( 4 ): 227 - 232 . 23706075 DOI: 10.1139/gen-2012-0106 http://doi.org/10.1139/gen-2012-0106 . Identification of quantitative trait loci for cadmium accumulation and distribution in rice (Oryza sativa) M Yang , YY Zhang , LJ Zhang , 等 . OsNRAMP5 contributes to manganese translocation and distribution in rice shoots . J Exp Bot , 2014 . 65 ( 17 ): 4849 - 4861 . 24963001 DOI: 10.1093/jxb/eru259 http://doi.org/10.1093/jxb/eru259 . OsNRAMP5 contributes to manganese translocation and distribution in rice shoots YM Yang , ZR Nan , ZJ Zhao . Bioaccumulation and translocation of cadmium in wheat ( Triticum aestivum L.) and maize ( Zea mays L.) from the polluted oasis soil of Northwestern China . Chem Spec Bioavailab , 2014 . 26 ( 1 ): 43 - 51 . DOI: 10.3184/095422914X13888342841789 http://doi.org/10.3184/095422914X13888342841789 . Bioaccumulation and translocation of cadmium in wheat (Triticum aestivum L.) and maize (Zea mays L.) from the polluted oasis soil of Northwestern China B Yousaf , GJ Liu , RW Wang , 等 . Bioavailability evaluation, uptake of heavy metals and potential health risks via dietary exposure in urban-industrial areas . Environ Sci Pollut Res , 2016 . 23 ( 22 ): 22443 - 22453 . DOI: 10.1007/s11356-016-7449-8 http://doi.org/10.1007/s11356-016-7449-8 . Bioavailability evaluation, uptake of heavy metals and potential health risks via dietary exposure in urban-industrial areas H Yu , JL Wang , W Fang , 等 . Cadmium accumulation in different rice cultivars and screening for pollution-safe cultivars of rice . Sci Total Environ , 2006 . 370 ( 2-3 ): 302 - 309 . 16870236 DOI: 10.1016/j.scitotenv.2006.06.013 http://doi.org/10.1016/j.scitotenv.2006.06.013 . Cadmium accumulation in different rice cultivars and screening for pollution-safe cultivars of rice LY Yuan , SG Yang , BX Liu , 等 . Molecular characterization of a rice metal tolerance protein , OsMTP1 . Plant Cell Rep , 2012 . 31 ( 1 ): 67 - 79 . 21892614 DOI: 10.1007/s00299-011-1140-9 http://doi.org/10.1007/s00299-011-1140-9 . Molecular characterization of a rice metal tolerance protein, OsMTP1 JY Yue , XJ Wei , HZ Wang . Cadmium tolerant and sensitive wheat lines: their differences in pollutant accumulation, cell damage, and autophagy . Biol Plant , 2018 . 62 ( 2 ): 379 - 387 . DOI: 10.1007/s10535-018-0785-4 http://doi.org/10.1007/s10535-018-0785-4 . Cadmium tolerant and sensitive wheat lines: their differences in pollutant accumulation, cell damage, and autophagy FR Zeng , Y Mao , WD Cheng , 等 . Genotypic and environmental variation in chromium, cadmium and lead concentrations in rice . Environ Pollut , 2008 . 153 ( 2 ): 309 - 314 . 17905495 DOI: 10.1016/j.envpol.2007.08.022 http://doi.org/10.1016/j.envpol.2007.08.022 . Genotypic and environmental variation in chromium, cadmium and lead concentrations in rice L Zhang , L Zhang , FB Song . Cadmium uptake and distribution by different maize genotypes in maturing stage . Commun Soil Sci Plant Anal , 2008 . 39 ( 9-10 ): 1517 - 1531 . DOI: 10.1080/00103620802006651 http://doi.org/10.1080/00103620802006651 . Cadmium uptake and distribution by different maize genotypes in maturing stage XH Zhang , ML Warburton , T Setter , 等 . Genome-wide association studies of drought-related metabolic changes in maize using an enlarged SNP panel . Theor Appl Genet , 2016 . 129 ( 8 ): 1449 - 1463 . 27121008 DOI: 10.1007/s00122-016-2716-0 http://doi.org/10.1007/s00122-016-2716-0 . Genome-wide association studies of drought-related metabolic changes in maize using an enlarged SNP panel XQ Zhang , GP Zhang , LB Guo , 等 . Identification of quantitative trait loci for Cd and Zn concentrations of brown rice grown in Cd-polluted soils . Euphytica , 2011 . 180 ( 2 ): 173 - 179 . DOI: 10.1007/s10681-011-0346-9 http://doi.org/10.1007/s10681-011-0346-9 . Identification of quantitative trait loci for Cd and Zn concentrations of brown rice grown in Cd-polluted soils JL Zhao , W Yang , SH Zhang , 等 . Genome-wide association study and candidate gene analysis of rice cadmium accumulation in grain in a diverse rice collection . Rice , 2018 . 11 ( 1 ): 61 30465288 DOI: 10.1186/s12284-018-0254-x http://doi.org/10.1186/s12284-018-0254-x . Genome-wide association study and candidate gene analysis of rice cadmium accumulation in grain in a diverse rice collection XW Zhao , LX Luo , YH Cao , 等 . Genome-wide association analysis and QTL mapping reveal the genetic control of cadmium accumulation in maize leaf . BMC Genomics, 19:91 , 2018 . DOI: 10.1186/s12864-017-4395-x http://doi.org/10.1186/s12864-017-4395-x . Genome-wide association analysis and QTL mapping reveal the genetic control of cadmium accumulation in maize leaf Q Zhou , GS Shao , YX Zhang , 等 . The difference of cadmium accumulation between the indica and japonica subspecies and the mechanism of it. Plant Growth Regul , 81(3):523-532 . https://doiorg/101007/s10725-016-0229-0 , 2017 . The difference of cadmium accumulation between the indica and japonica subspecies and the mechanism of it. Plant Growth Regul, 81(3):523-532 S Zimmerl , J Lafferty , H Buerstmayr . Assessing diversity in Triticum durum cultivars and breeding lines for high versus low cadmium content in seeds using the CAPS marker usw47 . Plant Breed , 2014 . 133 ( 6 ): 712 - 717 . DOI: 10.1111/pbr.12218 http://doi.org/10.1111/pbr.12218 . Assessing diversity in Triticum durum cultivars and breeding lines for high versus low cadmium content in seeds using the CAPS marker usw47 The authors would like to thank Prof. Guo-ping ZHANG (Zhejiang University, Hangzhou, China) for instructing the writing of this manuscript. Compliance with ethics guidelines Qin CHEN and Fei-bo WU declare that they have no conflict of interest. This article does not contain any studies with human or animal subjects performed by either of the authors. 10.1631/jzus.B1900576 Project supported by the Key Research Foundation of Science and Technology Department of Zhejiang Province of China (No. 2016C02050-9-7) 2019-09-30 2020-01-15 2022-01-05 Copyright © Zhejiang University and Springer-Verlag GmbH Germany, part of Springer Nature 2020 2020 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 06 21