Shiyu QIN,Hongen LIU,Zhaojun NIE,Zed RENGEL,Wei GAO,Chang LI and Peng ZHAO,∗College of Resources and Environment,Henan Agricultural University/Key Laboratory of Soil Pollution and Remediation of Henan Province,Zhengzhou 45000(China)

2UWA School of Agriculture and Environment,University of Western Australia,Perth WA 6009(Australia)

ABSTRACTCadmium(Cd)is a toxic heavy metal occurring in the environment naturally and is also generated through various anthropogenic sources and acts as a pollutant.Human health is affected by Cd pollution in farmland soils because food is the main source of Cd intake in the non-smoking population.For crops,Cd toxicity may result from a disturbance in uptake and translocation of mineral nutrients and disturbance in plant metabolism,inhibiting plant growth and development.However,plants have Cd tolerance mechanisms,including restricted Cd uptake,decreased Cd root-to-shoot translocation,enhanced antioxidant enzyme activities,and increased production of phytochelatins.Furthermore,optimal supply of mineral nutrients is one of the strategies to alleviate the damaging effects of Cd on plants and to avoid its entry into the food chain.The emerging molecular knowledge contributes to understanding Cd uptake,translocation,and remobilization in plants.In this review,Cd toxicity and tolerance mechanisms,agricultural practices to minimize Cd accumulation,Cd competition with essential elements(calcium,copper,iron,zinc,and manganese),and genes associated with Cd uptake are discussed in detail,especially regarding how these mineral nutrients and genes play a role in decreasing Cd uptake and accumulation in crop plants.

Key Words: antagonistic interaction,gene regulation,heavy metal,mitigation measures,tolerance,toxicity,uptake and transport

INTRODUCTION

Cadmium(Cd),atomic number 48,is a transition metal along with zinc(Zn)and mercury(Hg).Owing to its unique physical and chemical properties, Cd is widely used in electroplating,timber,and stainless-steel industries,in nickel(Ni)-Cd battery production,and is contained in phosphate fertilizers(Park et al.,2011b;Choppala et al.,2013).The annual Cd production is approximately 30 000 t worldwide(ATSDR,2012).

Cadmium is an extremely poisonous element and has high mobility in the soil-plant system.Therefore,Cd has been studied extensively in agriculture because of its potential toxicity to plants and humans(Liao et al.,2015).Increasing content of Cd is being found in agricultural soils due to the usage of chemical fertilizers,sludge application,sewage irrigation, and atmospheric deposition (Inglezakis et al.,2014;Liang et al.,2017;Dharma-Wardana,2018)(Table I).However, the sources of Cd differ in different places. For example,the potential risk of Cd accumulation in farmland soil from atmospheric deposition and fertilization is relatively low compared to irrigation water,whereas irrigation water and atmospheric deposition were the most important Cd sources in the Yangtze River delta,China(Hou et al.,2014;Jiang et al.,2014).

More recently,Cd contamination in agricultural soils has become one of the most severe and widespread environmental and agricultural problems in Asian countries such as China and India (Fan et al., 2010; Gallego et al., 2012; Mori et al.,2016).Cadmium can be readily taken up by roots and translocated to shoots.Many studies have demonstrated that Cd can enter the food chain easily and become detrimental to human and animal health(Wagner,1993;Satarug et al.,2003).The most serious incident was the outbreak of“Itai-itai disease”in the mid-20th century in Japan,which was caused by the long-term consumption of Cd-contaminated rice(Horiguchi et al.,1994).It was subsequently demonstrated that exposure to Cd appears to be associated with renal health(glomerular damage),bone health(osteomalacia and osteoporosis),and cancers(prostate,kidney,bladder,breast;Järup and Åkesson,2009;Feki-Tounsi and Hamza-Chaffai,2014; Zhang et al., 2014). Therefore, the Cd pollution problem has severe repercussions for food safety,agricultural land quality,and human health(Atafar et al.,2010;Rizwan et al.,2012).

TABLE ICadmium contents in primary pollution sources regarding farmland soilsa)

Different mitigation strategies have been employed in order to reduce Cd toxicity to plants,including the exogenous application of plant growth regulators,inorganic amendments or microbes, selection of low Cd-accumulating cultivars,and seed pretreatment with microwaves and lasers(Rizwan et al.,2016a).In this article,we discuss the key aspects of the current understanding of Cd toxicity and tolerance in plants,focusing on the interaction between Cd and mineral elements.

CADMIUM CONCENTRATION AND BIOAVAILABILITY IN SOIL

The soil pH greatly influences the chemistry and mobility of Cd in agricultural soils. Cadmium and its compounds are relatively water-soluble under acidic conditions, but Cd solubility is limited by cadmium carbonate (CdCO3)formation or Cd adsorption onto soil solid phase in neutral to alkaline soils(Santillian-Medrano and Jurinak,1975;Pinto et al.,2004;Kirkham,2006).Cadmium availability to plants is also closely related to other soil factors such as organic matter content,cation exchange capacity,and concentrations of other cations (Benavides et al., 2005). Organic matter converts the soluble/exchangeable Cd into an organicallybound fraction,thus reducing uptake or bioavailability(Liu et al.,2009;Chen et al.,2010).Compost application resulted in approximately 70%lower amount of soluble/exchangeable Cd in Cd-contaminated soil.The cation exchange capacity also affects Cd mobility in soil;an important mechanism in cation sorption is the replacement of structural magnesium(Mg) with Cd at the edges of octahedral sheets, although binding to silanol groups(Si-OH)may also occur(Álvarez-Ayuso and García-Sánchez,2003;Lazarević.,2007;Vega et al.,2009).

In general,Cd concentrations in unpolluted soils are of geogenic origin and are generally below 0.5 mg kg-1dry soil but can reach up to 3.0 mg kg-1or more than 100 mg dm-3in the top 25 cm soil layer owing to various human activities(Tezotto et al.,2012;Szolnoki et al.,2013).In particular,the application of phosphate fertilizers is an important cause of Cd pollution in agricultural soils,with the Cd contents of phosphate fertilizers ranging from less than 3.6 to 527 mg kg-1(Satarug et al., 2003). The Cd2+concentration in the soil solution and diffusion coefficient for Cd2+in aqueous solutions govern the transport of Cd2+to plant roots(Sterckeman et al.,2004).

CADMIUM TOXICITY AND TOLERANCE MECHANISMS IN PLANTS

Fig.1 Cadmium(Cd)toxicity(left side)and plant Cd-tolerance mechanisms(right side).

Cadmium affects plant growth and development due to its toxic effects on different metabolic and physiological processes, but plants have evolved various resistance mechanisms to alleviate Cd toxicity(Fig.1).As reported,Cd is detrimental to plant growth,even at low concentrations(White and Brown, 2010; Dias et al., 2013), causing leaf chlorosis, necrotic lesions, wilting, inhibited root elongation, and reduced biomass (Hussain et al., 2015; Rizwan et al.,2016a,b).Cadmium toxicity impairs gas exchange,decreases photosynthetic pigments, induces water stress,and causes destruction of chloroplast structures(Ci et al.,2009).Moreover,the excess Cd decreases uptake of macroand micronutrients by plants,deteriorating the quality of the edible parts(Rochayati et al.,2011).The formation of reactive oxygen species(ROS),such as superoxide anion(O-2),hydroxyl radical(OH·),and hydrogen peroxide(H2O2),is one of the major Cd toxicity mechanisms in plants(Farooq et al.,2019).As a consequence of oxidative stress,malondialdehyde(MDA)and thiobarbituric acid reactive substances(TBARS)accumulate,and electrolyte leakage occurrs under excess Cd(Younis et al., 2016). Hence, four mechanisms of Cd toxicity in plants have been proposed: i) structural similarity with nutrient cations,such as phosphorus(P)and Zn,resulting in competitive absorption at the root surface;ii)direct interaction of Cd with sulfhydryl(-SH)group,impairing the structure and function of proteins;iii)replacement of the essential cations from specific binding sites;and iv)disturbing the balance between ROS production and scavenging,which consequently damages the macromolecules such as DNA,proteins,lipids,pigments,and other essential cellular molecules (Hossain et al., 2012; DalCorso et al.,2013;Choppala et al.,2014;Singh et al.,2016).

Some plant species and genotypes can accumulate relatively large amounts of Cd without any apparent detrimental effects.These plants have developed various Cd tolerance mechanisms,including i)reducing Cd activity in the rhizosphere and decreasing root uptake,ii)decreasing Cd transfer to the above-ground parts,iii)compartmentalization of Cd to exclude it from metabolic pathways, and iv) reducing oxidative stress by maintaining redox homeostasis(Nocito et al.,2011;Choppala et al.,2014;Xu et al.,2017).The cell wall is the first barrier to Cd uptake in plants,and cellular compartmentalization of Cd in the cell wall plays a main role in the Cd adsorption and transport in plant roots.The percentage of Cd accumulation in cell walls was reported to be more than 95%of total Cd content,which indicates that the cell wall may play an important role in preventing Cd uptake and transport(Wang F J et al.,2015;Zhao et al.,2015;Mwamba et al.,2016).

Cadmium transport is also restricted by the production of thiol compounds followed by chelation and the sequestration of Cd chelates in the root vacuoles(Lux et al.,2011;Mendoza-Cózatl et al., 2011). Two main classes of peptides, phytochelatins and metallothioneins, are important Cd chelators(Hall,2002).In plants,the protective role of phytochelatins against Cd toxicity has been indicated in many studies,whereby Cd is bound to the thiol groups of the cysteine residues(Guo et al.,2008;Yadav et al.,2010).In addition,the chelators could facilitate the transport of Cd into the vacuoles(Verbruggen et al.,2009a;Wong and Cobbett,2009).Metallothioneins are helpful in reducing Cd toxicity through metal tolerance or homeostasis and scavenging of ROS in plants (Saraswat and Rai, 2011; Leitenmaier and Küpper,2013).In addition,a high proportion of Cd in the cell wall fraction,precipitation of Cd with phosphate,and binding with pectate or histidyl groups may also contribute to reducing Cd mobility and plant toxicity(Wang P et al.,2015).

Enhanced activities of antioxidant enzymes are a main Cd tolerance mechanism in plants(Nogueirol et al.,2016).The activities of superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), glutathione reductase(GR),and ascorbate peroxidase(APX)are affected by Cd stress in plants.The increase in the activities of these enzymes might help alleviate oxidative stress in Cd toxicity in most plants owing to their function in eliminating ROS,including superoxide radical,hydrogen peroxide,or hydroxyl radical(Benavides et al.,2005;Zeng et al.,2017).The activities of superoxide dismutase and nitric oxide(NO)metabolism are strongly depressed by Cd,and generation of ROS is elevated,because of Cd-induced Ca deficiency(Rodríguez-Serrano et al., 2009). The ascorbate-glutathione and antioxidant enzyme system plays an important role in reducing Cd accumulation under Cd stress(Wu et al.,2017;Qin et al.,2018).Furthermore,overexpression of some genes,such as AtTSB1,OXS3(encoding oxidative stress 3),and ThVHAc1,also improves plant tolerance to Cd stress(Hsiao et al.,2008;Wang C H et al.,2016;Yang G Y et al.,2016).

CADMIUM COMPETITION WITH MINERAL NUTRIENTS FOR UPTAKE AND TRANSPORT

As reported, Cd enters plant cells through transport systems involved in micronutrient uptake(Fig.2).Cadmium addition to growth media affects the calcium(Ca),copper(Cu),iron(Fe),Zn,and/or manganese(Mn)concentrations in plants(López-Millán et al.,2009;Kısa et al.,2016;Chen et al.,2017).In addition,Cd uptake is also affected by the activity of mineral elements, such as Ca, Cu, Fe, Zn, and Mn,in the rhizosphere solution(Han et al.,2006;Lux et al.,2011).

Iron

The most significant visual symptom of Cd toxicity is leaf chlorosis,which is similar to that caused by Fe deficiency,known as“heavy metal induced Fe-deficiency chlorosis”.Because Fe plays a key role in the synthesis of chlorophyll,it is likely that Cd uptake can affect Fe accumulation.Indeed,several studies showed that Fe uptake and translocation from roots to shoot are impaired under Cd stress(Hodoshima et al.,2007;Meda et al.,2007;Gao et al.,2011).Iron supply inhibits Cd uptake in Arabidopsis, whereas Fe deficiency increases Cd accumulation in peanut(Su et al., 2014; He et al.,2017).Furthermore,phosphate or sulfate deprivation decreases Cd uptake by competitively increasing Fe uptake and accumulation in plants, confirming the competitive relationship between Fe and Cd(Astolfiet al.,2012;Yang Y J et al.,2016).On the other hand,Fe deficiency results in increased uptake and accumulation of Cd(Nakanishi et al.,2006;López-Millán et al.,2009),increasing Cd in the cell wall as well as in the organelle and soluble fractions(Su et al.,2014).Differential sensitivity of various transporter genes to Fe deficiency is associated with differential Cd accumulation in low and high Cd-accumulating Solanum species(Xu et al.,2012).

Various chelated Fe forms affect Cd accumulation in plants.For example,accumulation of Cd was higher with Fe(III)citrate than Fe(III)ethylenediaminetetraacetic acid(EDTA)supply(Csog et al.,2011).

Manganese

Manganese is an essential microelement for plant growth,and Mn application could decrease Cd uptake and alleviate Cd toxicity.This antagonism between Mn and Cd was demonstrated in rice (Rahman et al., 2016; Wang et al.,2018).Indeed,Mn is taken up by plants via an active transport mechanism as a divalent cation that can compete with Cd2+because they have some common routes of uptake and transport(Pittman,2005;Sasaki et al.,2012).A Cd-induced water loss,chlorosis,growth inhibition,and nutrient imbalance could be partly recovered by the exogenous application of MnSO4(Peng et al.,2008;Rahman et al.,2016).Increasing the antioxidant enzyme activity in roots together with a high leaf Mn concentration may be responsible for the relative Cd tolerance in Mn-replete conditions(Zornoza et al.,2010).In addition,Mn mitigates Cd toxicity by increasing the Mn/Cd molar ratio, which is associated not only with decreased Cd concentration,but also alleviated lipid peroxidation,decreased water loss and improved photosynthesis(Street et al., 2010; Liu et al., 2013). These effects may be related to the vital role of Mn as part of Mn-SOD and cofactor for Mn-CAT,which participate in plant defense against oxidative stress(Ducic and Polle,2005).

Copper

The positive effects of Cu on reducing Cd toxicity are rarely reported. However, the addition of Cu significantly decreases Cd concentration in shoots and roots of rice(Cui et al.,2008).Also,the addition of Cd to the nutrient solution decreases uptake of Cu,and,conversely,Cd uptake is decreased by Cu application in the solution,in other plant species,such as birch (Betula pendula Roth) (Gussarsson et al., 1995),the Cd/Zn hyperaccumulator Noccaea caerulescens(Küpper and Kochian,2010),and Tulbaghia violacea(Street et al.,2010).Liu et al.(2003)showed that in the Cd-treated rice plants,the Cu concentration decreased in roots,but increased in leaves at ripening, and Cu concentration in both roots and leaves increased at heading.Regression analysis showed significant and positive correlations between Cd and Cu in both roots and leaves(Liu et al.,2003).

Zinc

Zinc and Cd are chemically similar group II-B elements,and Zn has been regarded as an antagonistic element in limiting Cd entry into plants.Uptakes of Zn and Cd show similar tendencies.For instance,some Cd hyperaccumulators also hyperaccumulate Zn, suggesting that Cd and Zn hyperaccumulators rely,at least partly,on common genetic determinants (Verbruggen et al., 2009b; Willems et al.,2010).Similarly,Cd uptake and translocation are enhanced by Zn in the hyperaccumulating ecotype Sedum alfredii(Li et al., 2009). However, many studies have shown that Cd uptake could be suppressed by Zn chelated with amino acids or by Zn-biofortified cultivars(Han et al.,2006;Saifullah et al.,2016;Qaswar et al.,2017).Foliar application of Zn at the booting stage ameliorates the adverse effects of Cd and decreases grain Cd in wheat grown in Cd-contaminated soil (Sarwar et al., 2015). Decreased Cd:Zn ratios could potentially limit Cd enrichment in paddy rice grain during the grain filling stage(de Livera et al.,2011).Soil application of Zn fertilizers could reduce Cd concentration by more than 50%as compared with no Zn application(Fahad et al.,2015).In addition,there was a negative correlation between Zn and Cd concentrations in brown rice(Fahad et al.,2015).Therefore,Zn has been demonstrated as a competing ion in Cd uptake by plants.

Molybdenum(Mo)

Molybdenum is an essential element required in very small amounts(Kaiser et al.,2005).The Mo content in Tulbaghia violacea is not significantly affected by the presence of Cd(Street et al.,2010).However,the correlation between Mo and Cd within a tissue appears to be highly variable among species,accessions,tissues,and environments.For example,the positive correlations between Cd and Mo were found in roots and leaves,including in various accessions of

Mimulus guttatus and Arabidopsis thaliana(Baxter et al.,2012; Lowry et al., 2012). However, under Fe deficiency and in the Fe-deficient mutant frd3,uptakes of Cd and Mo are negatively correlated,potentially because of the negative correlation between the activity of Mo-transporter MOT1 and Fe-transporter IRT1(Baxter,2009;Lowry et al.,2012).

The Mo-dependent enzymes are involved in carbon(C),nitrogen(N),and sulfur(S)metabolisms,including nitrate assimilation, purine synthesis, phytohormone production,and sulfite detoxification (Tejada-Jiménez et al., 2013).Indeed,N and S could influence Cd uptake and accumulation(Han et al.,2018;Yu et al.,2019).Therefore,Mo can affect Cd absorption indirectly by influencing the N and S metabolic pathways.A current poor understanding of the relationship between Mo and Cd makes further studies highly desirable.

Other elements

Some other elements,such as the essential element Ca and the beneficial elements silicon(Si)and selenium(Se),are also connected with Cd uptake by plants.For example,Cd2+could enter the faba bean guard cells through the Ca2+channels to affect plant water loss,gas exchange,and stomatal behavior (Perfus-Barbeoch et al., 2002). He et al. (2017)reported that Cd uptake was inhibited along with Ca by a Cachannel blocker and an ATP-ase inhibitor but was promoted by the Ca or Fe deficiency treatment, suggesting that Cd likely enters amaranth roots via the Ca uptake pathway.

Silicon decreases Cd accumulation in plants,especially in shoots,because Si addition enhances phytochelatin(PC)content, upregulates PCS1 (encoding PC) and IRT1 and NRAMP1(encoding Fe and metal transporters)(Greger et al.,2016).Ma et al.(2011)reported that a hemicellulosebound form of Si with net negative charges was responsible for inhibition of Cd uptake in rice cells by Cd complexation with Si-hemicellulose matrix and subsequent co-deposition.

Selenium is an essential element for humans and animals,but not for plants.Its application leads to significantly increased accumulation in rice grain,and markedly decreased Cd concentration in rice tissues(Hu et al.,2014).The mechanism of Se minimizing Cd accumulation in rice could be because of decreased metal bioavailability in soil or increased activities of SOD,POD,and H+-and Ca2+-ATPase in rice(Lin et al.,2012;Hu et al.,2014).

AGRICULTURAL PRACTICES TO MINIMIZE CADMIUM ACCUMULATION IN PLANTS

There are different agricultural practices to reduce Cd accumulation in plants,such as utilizing growing seasons and geographic patterns (Kubo et al., 2008), co-planting phytoextractor plants and food plants(Tang et al.,2012),lime and peat application(Chen Y H et al.,2016;Miura et al.,2016),exogenous application of microbes,fungi or amino acids(Shahabivand et al.,2012;Muehe et al.,2015;Zouari et al.,2016),and grafting(Alves et al.,2017).The application of inorganic fertilizers can reduce the absorption of Cd by crops.For example,Cd concentration decreases in plants growing in Cd-contaminated soil after the application of various minerals,S,and slow-release N fertilizers(Mostofa et al., 2015; Rizwan et al., 2016a; Zhang et al., 2016).The application of Zn fertilizer significantly decreases Cd uptake and translocation in both dicotyledonous and monocotyledonous plants (Jiao et al., 2004). Foliar-applied Zn decreases Cd concentration in roots, straw, and grain of wheat,especially when applied at the booting stage and grain filling(Tavarez et al.,2005;Saifullah et al.,2016;Forster et al.,2018).Selenium fertilization(either to soil or foliage)markedly decreases Cd uptake and accumulation in plant tissues (Hu et al., 2014; Xiong et al., 2015). In addition,foliar spraying with Si and Se is effective in reducing Cd accumulation in rice cultivars(Gao et al.,2018).Fertilization not only provides plant nutrients,but also changes the speciation and thus bioavailability of heavy metals(Tu et al.,2000;Ning et al.,2017).

Organic manure(OM)amendments improve soil quality and reduce Cd availability in soil solution and could also effectively decrease Cd accumulation in brown rice grown in Cd-contaminated paddy fields(Xiao et al.,2017;Khan M A et al.,2017).However,some residues from the biogas production based on human urine,farm manure,and sewage sludge have a potential risk of increasing soil Cd mobilization,which strongly affects plant Cd concentration(Dahlin et al.,2016;Yousaf et al.,2016).Compost and biogas slurry applications could also trap Cd in the soil by stabilization or extraction,and thus could decrease Cd uptake by wheat and maize(Ahmad et al.,2015).In addition,organic fertilizer(cow manure)applied alone or in combination with inorganic fertilizers(urea,triple superphosphate,and potassium chloride)reduces Cd uptake by plants(Liu et al.,2017).

Biochar is a carbon-rich charcoal produced by the slow pyrolysis of biomass at low to medium temperatures(450-650◦C)under oxygen-limited conditions;it is a good soil amendment for Cd-contaminated soils(Roberts et al.,2015;Ali et al.,2017).Biochar decreases the bioactivity and availability of many heavy metals, including Cd (Park et al.,2011a).Biochar application decreases Cd uptake in many plant species,such as pakchoi(Yasmin et al.,2017),wheat(Yousaf et al., 2016; Abbas et al., 2017), and rice (Bian et al., 2016). This effect could be related to the biochar porous structure and various functional groups (C-H and O-H),thereby increasing the adsorption of metals(Huang et al., 2011). In addition, a biochar-mediated decrease in Cd concentration in plants might be due to biochar being mostly alkaline and having high porosity and cation exchange capacity(CEC)(Chen D et al.,2016;Olmo et al.,2016;Khan K Y et al.,2017).Biochar might decrease the exchangeable fraction of Cd in soil by adsorption and complexation reactions,thus reducing their bioaccessibility and plant uptake.

Selection and breeding of low Cd-accumulating cultivars could effectively reduce Cd accumulation in plants(Wang J B et al.,2015;Wang A Y et al.,2016;Dai and Yang,2017).Yu et al.(2006)reported that grain Cd concentrations in 43 paddy rice cultivars were highly correlated between high and low levels of Cd exposure experiments,suggesting Cd accumulation in rice grain is genotype-dependent,and the selection of pollution-safe cultivars is possible.

Overall, low Cd accumulation in plants could be associated with several physiological processes,such as low Cd uptake by roots,high production of Cd-chelates in the cytoplasm of root cells and the sequestration of Cd-chelates in the vacuole,and low transport from roots to shoot(Lux et al.,2011).

In conclusion, it is likely that a combination of various agricultural strategies is needed to minimize the Cd concentration in plants based on local conditions.

GENES ASSOCIATED WITH CADMIUM UPTAKE

Up to now,few genes coding for Cd-specific transporters have been reported (Table II). Cadmium is taken up by roots via cation transport systems normally involved in uptake of essential nutrients,such as members of the zincregulated/iron-regulated transporter protein (ZIP), heavy metal transporting ATPases(HMA),and natural resistanceassociated macrophage protein(NRAMP)families(Williams and Mills, 2005; Küpper and Kochian, 2010; Hassan and Aarts,2011).

Cadmium absorption from the soil is thought to occur mainly via the ZIP family transporters.Members ZIP8 and AtIRT1 have been suggested to be responsible for Cd uptake by roots(Clemens et al.,2013;Ogo et al.,2014).In rice,Fe transporters such as OsIRT1 and OsIRT2 are also involved in Cd uptake and transport (Nakanishi et al., 2006). Zinc transporters such as OsZIP1, ZNT1, ZNT5, and MTP1(metal-transport protein) could also transport Cd and be part of plant acclimation to Cd toxicity(Pence et al.,2000;Küpper and Kochian,2010;Clemens et al.,2013).

The P1B-type ATPases,also known as the heavy metal transporting ATPases (HMAs), play an important role in transporting Cd against the electrochemical gradient using the energy provided by ATP hydrolysis.The HMAs family contains eight members,named AtHMA1 to AtHMA8,and is clustered into two classes:members transporting monovalent (Cu/Ag group) and divalent cations (Zn/Co/Cd/Pb group). Three of these transporters, AtHMA2, AtHMA3,and AtHMA4, are closely related to each other and their sequences are most similar to the divalent heavy metal cation transporters of prokaryotes(Hussain et al.,2004;Verret et al.,2004;Morel et al.,2009).The AtHMA4 plays a key role in Cd translocation in non-hyperaccumulating dicots,such as Arabidopsis thaliana,and AtHMA3 is a vacuolar transporter(Morel et al.,2009;Wong and Cobbett,2009).The homologous genes of these transporters play an important role in translocation of Cd from the symplasm into the xylem in rice or wheat,such as OsHMA2,OsHMA3,and TaHMA2(Ueno et al.,2010;Miyadate et al.,2011;Tan et al.,2013).

Functional studies of the natural resistance-associated macrophage proteins(Nramps)showed they could transport metal ions,such as Mn2+,Zn2+,Cu2+,Fe2+,Cd2+,Ni2+,Co2+, and Al3+(Colangelo and Guerinot, 2006; Xia et al., 2010). Proteins Nramp2, Nramp3, and Nramp4 are involved in Mn and Fe uptake/homeostasis in Arabidopsis,whereas Nramp1, Nramp5, and Nramp 6 are the major transporters responsible for Cd uptake by rice(Thomine et al.,2000;Cailliatte et al.,2010;Lanquar et al.,2010;Sasaki et al.,2012).Protein Nramp1 was suggested to be involved in cellular Cd uptake and Cd transport within the plant(Takahashi et al.,2011).The AtNramp 6 is an intracellular metal transporter and affects distribution/availability of Cd within the cell(Cailliatte et al.,2009).In rice,OsNramp5 was identified as the major Cd uptake transporter(Sasaki et al.,2012).

TABLE IISummary of transporters related to Cd uptake and transport

Cadmium also might enter root cells as Cd-chelates through yellow stripe 1-like(YSL)proteins that belong to the oligopeptide transporters(OPT)superfamily of oligopeptide transporters(Curie et al.,2009;Wong and Cobbett,2009).Over-expression of cation exchangers AtCAX2 or AtCAX4 in tobacco enhances Cd transport into root tonoplast vesicles and increases Cd accumulation in roots,suggesting that Cd can be translocated across the tonoplast by AtCAX2 and AtCAX4(Korenkov et al.,2007).

Protein OsLCD is localized in the cytoplasm and nucleus and is expressed mainly in the vascular tissues in the roots and phloem companion cells in the leaves of rice.Decreased Cd concentration in grains in the lcd mutant indicates OsLCD may be involved in Cd transport within rice(Shimo et al.,2011).

In addition,Cd might be transported into grains via lowaffinity cation channels, such as TaLCT1 and OsLCT1,localized in the plasma membrane in wheat and rice,respectively (Schachtman et al., 1997; Uraguchi et al.,2011). Furthermore, Cd might enter root cells via cation channels, such as depolarization-activated Ca channels(DACC),hyperpolarization-activated Ca channels(HACC),and voltage-insensitive cation channels(VICC),all of which are relatively non-selective among cations, or through orthologues of the wheat TaLCT1 transporter (White and Broadley,2003;Plaza et al.,2007;DalCorso et al.,2008;Pedas et al.,2008;Verbruggen et al.,2009b).

CONCLUSIONS AND FUTURE PROSPECTS

The harmful effects of Cd on plant growth and development are well established.The tolerance mechanisms are based on: i)decreasing Cd uptake and translocation from roots to shoot,ii)compartmentation of Cd in the cell walls and sequestration of Cd chelates in the root-cell vacuoles,iii)enhancing concentrations of antioxidants and activity of antioxidative enzymes,and iv)up-regulating phytochelatin synthesis and altering expression of heavy metal transporter genes.There is an antagonistic interaction between Cd and metallic nutrients(such as Fe,Mn,Cu,and Zn)in uptake,and there is an indirect impedance effect of beneficial elements(Se and Si) in Cd uptake by plants. These effects are dependent on element concentration, root environment, and genotype.It has been demonstrated that mineral nutrients play an important role in minimizing Cd uptake and transport in plants.However,the genes related to Cd uptake or transport will play a decisive role in decreasing Cd accumulation and enhancing resistance in plants.Further studies are needed to elucidate the molecular nature of the processes contributing to Cd toxicity and tolerance in plants,with a particular emphasis on decreasing Cd accumulation in edible parts.

ACKNOWLEDGEMENT

This work was supported by the Science and Technology Innovation Fund of Henan Agricultural University, China(No.KJCX2019A19),the National Key R&D Program of China (No. 2017YFD0201700), and the Doctoral Scientific Research Foundation of Henan Agricultural University,China(No.30500592).