The Response And Control Mechanism Of Cadmium Stress During Seedlings Stage Of Winter Wheat

Cadmium（Cd）is one of the most toxic heavy metals,which can be absorbed and accumulated by plants and bio-amplified by the food chain,bringing potential hazards to human and other animal health.According to the"National Soil Pollution Status Survey Bulletin"in 2014,the over-standard rate of Cd in China is as high as 7.0%,ranking first in heavy metal pollution.At present,the treatment of soil Cd pollution is still one of the world’s problems.As the main food crop in China,it is of great significance to ensure the quality and safety of wheat.A large number of related researches have been carried out by experts at home and abroad,but the field trials or barrel experiment studies are susceptible to external factors,which can not truly reflect the response of plants to Cd and the reasons for the differences among different varieties.Therefore,winter wheat cultivars with high-and low-Cd accumulation in grain（XM and YM）were selected as test crops in this study.The response of XM and YM seedlings to different Cd concentrations and the mechanism of Cd inhibition and control were studied in a single hydroponic environment to provide theoretical basis for safe production of wheat and safe utilization of low Cd contaminated soil in the future.The main results are as follows:（1）The results of adsorption on root surface,transport and subcellular distribution showed that the total amount of Cd adsorbed on the surface of XM root was greater than YM under the treatments of 5and 50μmol.L^-1Cd,but the opposite was observed under 100μmol.L^-1Cd treatment.The Cd adsorbed on the surface of XM and YM roots was mainly exchanged and complexed.The transfer coefficients of YM were higher than XM at different Cd concentrations,and the differences were significant at 5 and50μmol.L^-1（P<0.01）.The ratio of Cd in root cell wall（FI）and cytosol component（FIII）and cytosol component（FIII）of shoots were higher than XM at different Cd concentrations.Compared with XM,YM had better tolerance to Cd stress than XM.（2）The ultrastructure of root tip cells and expression of antioxidant enzymes-related genes showed that the subcellular structure of root tip cells was damaged more seriously with the increase of Cd concentration,and at 100μmol.L^-1 Cd level,the root tip cells were vacuolated.Under different Cd treatments,the relative expression of POD,CAT and APX genes in root and shoot tissues of YM were significantly higher than that of XM（P<0.05）,while the relative expression of SOD genes in root and shoot tissues of YM was lower than that of XM.Compared with XM,YM had stronger antioxidant capacity.（3）Sequencing analysis of the transcriptome showed that 1427 DEGs were obtained after Cd stress treatment,of which 429 genes were up-regulated DEGs and 998 genes were down-regulated DEGs.Through GO annotation and KEGG pathway analysis,DEGs were significantly enriched in glutathione metabolism,sulfur metabolism,ABC transporter,nitrogen metabolism and other pathways,these DEGs and significantly enriched metabolic pathways are related to the absorption,translocation and detoxification of Cd in wheat plants.（4）The study on the inhibition of Cd absorption and accumulation by different N and S elements showed that NO₃^-could reduce the Cd content in the shoots and roots of XM and YM,but when the N concentration reached 2NO₃^-,it will increase of Cd content in the above-ground tissues and roots of XM and YM.Compared with SO₄^2-,S can reduce the content of Cd in plants.Under different S concentrations,the Cd content in XM and YM roots was the lowest under S+Cd treatment,while that in the shoots was lowest under 0+Cd treatment.The interaction of different N forms and different S forms showed that the content of Cd in shoots of XM was the lowest under the interaction of NO₃^-+SO₄^2-,and YM was the lowest under the interaction of NH₄⁺+S.The interaction between NO₃^-and S or SO₄^2-has a variety of effects on the control effect of Cd.（5）The inhibition study of Cd absorption and accumulation by PEG6000 treatment showed that the content and accumulation of Cd in roots,shoots and whole plants of XM and YM were inversely proportional to the concentration of PEG6000.Using PEG6000 could effectively reduce the content of Cd in roots and shoots and the accumulation of Cd in plants.There was no significant difference in Cd content between XM and YM under the same treatment（P>0.05）.In summary,the retention and fixation of Cd by cell wall（FI）and cytosol fraction（FIII）of roots is the key to reduce Cd toxicity and improve Cd resistance of plants,while the binding and compartmentalization of cytosol fraction in shoots to Cd may be a key factor for the low accumulation of Cd in the grain.Compared with XM,YM has higher tolerance to Cd and its antioxidant capacity is also higher than XM.Using N element,S element and their interaction,and PEG6000 can achieve the control of Cd absorption and accumulation,and reduce the Cd content in plants.