Mechanisms Of Cadmium Accumulation In Rice And Mitigation Strategies

Cadmium(Cd)is a toxic heavy metal.Contamination of agricultural soil by Cd can affect human health.The phyto-availability of Cd in soil is affected by many factors including soil pH and water content.Thus,it is quite difficult to immobilize Cd in-situ in contaminated soils by applications of chemical compounds.Chemical immobilization or mobilization methods based remediation technologies usually display poor remediation efficiency and limited application range,and the remediation effect may not be long lasting.While physical methods,such as soil removal and replacement,are labor consuming and expensive,and mainly remove contamination to another location.Based on the conception of removal of Cd from contaminated soils,phytoremediation by hyper-accumulators seems to be a promising method in the remediation of Cd polluted soils.The shortcomings of this method include small biomass small,slow growth rate,or insufficiently high metal accumulation ability.Genetic engineering technology may provide a solution to this problem with the recent advance in genome editing.The foundation of this must be laid on well understanding of the mechanisms of Cd uptake and translocation in planta.Therefore,a better understanding Cd uptake and transport in plant is necessary.Among all the Cd exposure souces,entry of Cd into the food-chain poses the largest contribution to the intake of Cd for the general non-smoker population.Reduction of Cd concentration in various foods becomes an urgent task.There is no safety margin in the concentration of Cd in food for human health,we must make efforts to reduce Cd concentration in food as much as we can.Generally,dose of Cd ingestion in food depends on two factors,the average concentration of Cd in food and the amount of food consumption.Wheat,maize and rice are the three most important cereals in the world,providing main sources of carbohydrates.Because of the large consumption of the three cereals,they are the main sources of Cd intake.It is important to decrease Cd concentration in these crops.A better understanding of the molecular mechanisms underpinning Cd accumulation in these three major crops will pave the way to safe production of these crops.Recent studies have identified some key genes in governing Cd concentration in rice,however,little is known about the mechanisms of Cd uptake and translocation in wheat and maize.The present study was carried out a series of physiological and molecular experiments to investigate the Cd uptake ability and the possible mechanisms lie behind.In the present study,we compared Cd uptake abilities among the three major cereal crops in hydroponic cultures under controlled conditions,and explored the possible molecular mechanisms responsible for the differences in Cd uptake.We phenotyped 26 rice cultivars for their Cd accumulation abilities.Using a Chromosome Segment Substitution Lines(CSSLs)population,we identified a QTL in controlling Cd translocation from roots to shoots in rice.We used map-based cloning technology to clone the causal gene and analyze its function.The main results are as follows:1.To get a better understanding of Cd uptake in wheat,maize and rice,we compared Cd uptake abilities in a controlled hydroponic experiments.The results suggest that rice has a stronger Cd uptake ability than wheat and maize.We cloned the Nramp5 genes from rice,wheat and maize.Phylogenetic relationship analysis showed that two genes in wheat,TaNramp5A and TaNramp5D and one gene in maize,ZmNramp5 share close homology with rice OsNramp5.By using metal transport assays in yeast expression,subcellular localization and quantification of gene expression levels,we proposed that Nramp5 functions differently in wheat,maize and rice,which probably explains the difference in Cd uptake among three major cereals.2.Through the combination of two field experiments and a pot experiment,we identified W4 is a stable high Cd accumulation cultivar of rice.By analyzing the Cd accumulation phenotype and marker information of a CSSLs population using WO as the acceptor parent and W4 as the donor parent,we succeeded in identifying a major Quantitative Trait Locus(QTL)controlling Cd translocation from roots to shoots on chromosome 7,which was located between the markers Rc and D9149.By cross two CSSL lines with the background line W0,F2 and F2;3 populations were developed.Using these segregating populations,we cloned the gene controlling Cd translocation from roots to shoots using map-based cloning technology.A new mutation type of OsHMA3 was identified,which has a 42 bp deletion in the coding sequence of OsHMA3 in W4,resulting in 14 amino acid deletion and 1 amino acid substitution.This new type of OsHMA3 lost the function of Cd transport completely.In contrast,OsHMA3 mutation in 80th and 380th aa showed slight transport activities for Cd when expressed in yeast.3.A Single Nucleotide Polymorphisms(SNP)in the last exon of OsNramp5 from WO and W4 led to some functional difference in when expressed in yeast.The two alleles showed no difference in Mn transport,but the W4 allele had a stronger Cd transport activity.In contrast,the rice cultivar WO accumulated a higher Cd concentration in rice roots and showed a higher Cd uptake ability than W4 in a short time uptake kinetic assay.The results suggest that the difference in OsNramp5 does not explain the difference in Cd uptake between WO and W4.By analyzing the Cd accumulation phenotype of the recombination lines between OsNramp5 and OsHMA3 in the F2;3 population,we were able to determine that OsHMA3 was the main causal gene responsible for high Cd accumulation in the rice cultivar W4.In contrast,OsNramp5 may contribute minor effect in phenotype between WO and W4.4.To prevent Cadmiun oading and accumulation in rice grains,genetic engeering measures canbe taken.First,knocking out the major transporter OsNramp5 in rice leads to significant decrease in grain Cadmium content,while,it is likely have no effects when doing the same in wheat and maize,as in the present study we reavealed that Nramp5 play little role in cadmium uptake in wheat and maize.Overexpression OsHMA3 also lead to signigicant cadmium decrease in rice grain,one possible reason for cadmium over-accumulation in wheat grains is the loss-of-function of HMA3 in wheat,thus,trans genetic of dunction HMA3 to wheat may be an effective way to decrease cadmium content in grain.