Study On The Genotypic Difference Of Cherry Tomato In Response To Cadmium Stress And The Mitigation Effect Of Exogenous Selenium

As one of metal elements with no biological function,cadmium（Cd）pollution not only affects the growth and development of crops,but also threatens human health through the food chain.Therefore,screening and cultivating low-Cd accumulation cultivars and reducing the absorption and transportation of Cd are of great significance to the safe use of contaminated soil and the safe production of crops.Hydroponic experiments were used in this study to screen out cherry cultivars with significant differences in Cd accumulation.We studied the physiological and molecular mechanisms of the differences in Cd tolerance and accumulation of the two cultivars,and investigated the mitigation effect of exogenous selenium（Se）on Cd toxicity in cherry tomato.The main results were as follows:（1）Through hydroponic experiment at the seedling stage,the differences in the shoot growth and Cd content of 30 cherry tomato cultivars under different concentrations of Cd（0,25,50,100μmol/L）were analyzed.The plant height and shoot biomass of all cultivars were decreased with the increase of Cd concentration.The Cd content in the shoots of the 30cherry tomato cultivars differed significantly due to the different genotypes.Cluster analysis of relative plant height,relative shoot dry weight and shoot Cd content of 30 cultivars under different concentrations of Cd stress,we concluded that No.24（Hanluzhe,HLZ）was shoot low-Cd accumulation and Cd-tolerant cultivar;No.26（Lvfeicui,LFC）was shoot high-Cd accumulation and Cd-sensitive cultivar.（2）Comparative transcriptome analysis of leaves and roots of two cherry tomato genotypes with different concentrations of Cd stress（0,10,50μmol/L）for 48 h was carried out.Transcriptome analysis showed that under 0,10,and 50μmol/L Cd stress,the differentially expressed genes（DEGs）in leaves were 1882,3197,and 3544,respectively,with a share of 909 DEGs,and DEGs in roots were 404,942 and 811,with a share of 309DEGs.Therefore,the difference in transcription level between the two cultivars was mainly in leave.GO and KEGG enrichment analysis of DEGs in leaves indicated that these DEGs were mainly in several processes such as plant hormone signal transduction,cell wall biosynthesis and metal transport.In LFC leaves,DEGs had a higher transcription level in auxin signal transduction,while in HLZ leaves,overexpressed DEGs was involved in the process of ethylene signal transduction.Pectin biosynthesis and pectin methyl esterase related genes were overexpressed in LFC leaves,while Cd efflux gene PDR1 on cell wall membrane,Cd transporter gene ABCC3 and CAX3 on vacuolar membrane were up-regulated in HLZ leaves.（3）The effects of different Cd stress（0,10 and 50μmol/L）on growth and Cd content of two genotypes cherry tomato and the role of Se were studied.The results showed that:the higher the Cd concentration,the more severe the growth inhibition of shoot,and LFC was more prominent.The root length and dry weight of HLZ were decreased by 6.74%and 8.00%under high Cd concentration stress,but that in LFC were increased by 30.73%and 10.00%.Cd stress reduced the contents of Ca,Fe,Zn and Cu in plants.The inhibition effect of Cd on Ca uptake in HLZ was significantly lower than that of LFC.Under 50μmol/L Cd stress,Mg content in root and shoots of HLZ was increased by more than 50%,while that of LFC was decreased by 29.83%and 31.78%,respectively.The Cd content in roots,stems and leaves were all increased with the increase of Cd concentration,and more than 50%of Cd in leaves and roots were concentrated in cell walls.Compared with LFC,Cd content in HLZ root organelles and cell walls was lower,and ethanol and water extracted Cd were less,indicating that the Cd in LFC root was more active and easily transport to shoot.Exogenous Se increased the synthesis and accumulation of chlorophyll and carotenoids in cherry tomato leaves under Cd stress,reduced the damage and heat dissipation of Cd to the PSⅡ,and effectively promoted the photosynthetic capacity of leaves.Se application increased the accumulation of Ca,Mg,Fe,Cu and other mineral nutrients in plants under Cd stress,reduced the content of ethanol and water-extracted Cd in roots and leaves,down-regulated the expression of leaf ZIP,NRAMP and YSL2 under high-concentration Cd stress,and up-regulated the expression of CAX3.Therefore,Se not only reduced the total accumulation of Cd in leaves,but also transferred more Cd to inactive organs,reducing the Cd activity and the Cd damage to organelles.（4）The effects of Se on antioxidant system of cherry tomato genotypes under different concentrations of Cd（10 and 50μmol/L）stress were studied using Cd tolerant cultivar HLZ and Cd sensitive cultivar LFC.The results showed that Cd induced aggravated production of reactive oxygen species（ROS）such as H₂O₂ and O₂^-,increased accumulation of malondialdehyde（MDA）,a large amount of electrolyte extravasation,and the content of ROS and membrane peroxidation in LFC were higher than that of HLZ.Cd stress induced a decrease in ascorbic acid（ASA）content,and inhibited superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,ascorbate peroxidase（APX）,monodehydrogenase ascorbate reductase（MDHAR）and glutathione peroxidase（GPX）activities.Under different Cd stress,antioxidant enzyme activity of HLZ was higher than LFC.Cd induced the synthesis of glutathione（GSH）and phytochelatins（PCs）in the leaves.The content of GSH and PCs in HLZ and the synthesis-related enzyme activities were higher than those of LFC.Exogenous Se reduced the content of active oxygen and membrane peroxidation of the two cultivars under Cd stress,increased the content of antioxidants such as ASA and GSH,and the activities of antioxidant enzymes such as SOD,POD,APX and GPX,and promoted ASA-GSH cycle,effectively alleviating the oxidative stress induced by Cd.At the same time,Se promoted the synthesis of GSH and PCs in leaves,improved the chelation of free Cd and its transport to vacuoles,and reduced the activity of Cd and damage to organelles.（5）The effects of Se on fruit yield and quality of two genotypes of cherry tomato under Cd stress（0,10,50μmol/L）were studied.The results showed that Cd content in fruits increased with the increase of Cd concentration,and the content of Cd in HLZ fruits was always lower than that in LFC under different Cd concentrations.High concentration of Cd stress induced smaller size and decreased yield by 25.50%and 35.05%in HLZ and LFC,respectively.Cd induced a decrease in the content of vitamin C,soluble sugar and lycopene in the fruits,and inhibited the accumulation of various mineral elements.Moreover,it had more negative effect on fruit yield and quality of LFC.Application of Se could effectively increase the yield of cherry tomato fruits under Cd stress,increase the content of vitamin C,soluble sugar,and sugar to acid ratio,promote the accumulation of mineral elements,and thereby improve the flavor of the fruit.In addition,Se reduced the accumulation of Cd in the fruit and reduce the safety risk caused by Cd pollution.