| Cadmium(Cd)is noted as the seventh eco-toxic heavy metal that negatively affects arable lands,plants,and water bodies owing to its high solubility in water and readily uptake in plant tissues.In China,about 13330 ha of farmland has been heavily contaminated with Cd,thereby making the lands unfit for agricultural purposes.One possible strategy to overcome this constraint is the breeding of tolerant cultivars and these cultivars can subsequently be used for phytoremediation of Cd-contaminated soils.Castor(Ricinus communis L.)is a non-edible oilseed plant that can tolerate and grow on Cd-contaminated soils owing to its strong antioxidant and root system,high biomass as well as accumulation capacity.Several studies regarding the physiological mechanisms for Cd tolerance have been conducted in castor.However,studies underlying the genetic and biochemical mechanism of Cd tolerance in castor have not been fully elucidated.Thus,in this study,genome-wide association study on Cd tolerance was conducted with a natural population composed of 175 castor accessions genotyped by 143 polymorphic SSR markers,using the relative Cd tolerance index of seven(7)traits as an indicator,to identify Cd tolerance genetic loci,explore beneficial alleles and potential candidate genes.Furtherly,the differential expression of six(6)candidate genes and superoxide dismutase(SOD),catalase(CAT),glutathione(GSH),and malondialdehyde(MDA)between the extremely cadmium tolerant and sensitive accessions were analyzed to unravel the biochemical and genetic mechanism of castor Cd tolerance.The results are as follows:1.The relative Cd tolerance index of seven(7)traits significantly varied among 175 accessions,with a range of 0.08 ～ 0.99.Among the traits,the root dry weight and shoot length were highly sensitive and tolerant to Cd stress,respectively.2.A total of 736 alleles were amplified from 175 accessions by 143 SSR primers,with an average value of 5.10.The average gene diversity and polymorphism information content were0.67 and 0.62,respectively.Population structure analysis divided the 175 accessions into 3subgroups and this was consistent with the phylogenetic analysis.3.27 significant(-log10 P >1.5)loci were detected to be associated with all the seven(7)Cd tolerance trait by association mapping and the phenotypic variance explained by these loci varied from 5.63% ～ 20.47%,with an average value of 12.31%.10 loci were associated with dry shoot weight and one(1)with root length.Two(2)loci(RCM223 and RCM644)were both associated with root and shoot fresh weight,suggesting a pleiotropic effect.4.Mining of beneficial alleles revealed 29 and 181 accessions that contributed prominently to Cd-tolerance and Cd-sensitivity,respectively and this may be useful in breeding Cd-tolerant cultivars in castor.5.Eight(8)novel putative candidate genes involved in stress responses and developmental processes,associated with six of the traits,were annotated by analyzing the scaffold regions of the significant loci with conserved amino sequences of the castor genome.q RT-PCR analyses revealed that all the selected six(6)candidate genes were differentially expressed and accessionspecific.The gene PHT2;1 was highly expressed in the tolerant accession signifying its potential role in enhancing castor tolerance to Cd stress.6.There were significant variations between the extremely Cd-tolerant accession 16-024 and extremely Cd-sensitive accession S2-4 and the expression of SOD,CAT,GSH,and MDA were accession-specific,suggesting the antioxidant capacity of Cd-tolerant accession.In summary,it can be concluded that Cd toxicity induces oxidative stress in castor,however strong antioxidant systems are responsible to enhance Cd tolerance in this plant.Also,Cd tolerance of castor varies among different accessions and traits or organs.SSR markers are desirable for genetic diversity and GWAS mapping.To our knowledge,this is the first study of GWAS mapping and identification of novel genes in castor under Cd stress.These results might facilitate the study of the molecular and genetic basis of Cd tolerance and the cloning of candidate Cd tolerance genes and cultivars,which could be useful for the remediation of Cdcontaminated environments. |
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