Selenium Regulated Plant Growth And Cadmium Response In Root Of Rape Under Cadmium Stress

Cadmium(Cd)is a highly toxic metal element.The accumulation of Cd in soil increases Cd uptake by plants.Cd accumulated in the edible parts of plants will transfer to the food chain,thus threatening human health.Selenium(Se)is an essential micronutrient for animals and humans.The proper level of Se can improve the yield and quality of crops,alleviate the oxidative damage induced by heavy metals and reduce the accumulation of heavy metals in plants.Root is the primary plant organ that directly contacts with Cd pollutants in soil.The accumulation and translocation of Cd in plants are highly related to root uptake.Since Se plays a protective role in plants by decreasing Cd accumulation,we speculated that Se might regulate the growth of root under Cd stress.This study is intended to investigate the effect of Se on the growth of rape under Cd stress.We focused on the root growth and the uptake and distribution of Cd in root,aiming to investigate the mechanism of the Se-Cd interaction on the uptake and distribution of Cd in rape roots.The main results are as follows:(1)Exogenous Se improved the growth of rape under Cd stress.Compared with the treatments without Cd stress,the regulation effect of Se on plant biomass was more obvious at 5 ?mol/L Cd level than that of 1 ?mol/L Cd level.The results showed that Se was benefical to enhance the net photosynthetic rate,stomatal conductance and transpiration rate,and increase the contents of chlorophyll a,chlorophyll b and carotenoid,which might contribute to maintaining the stability of photosynthetic characteristics.Under 1 ?mol/L Cd level,exogenous Se showed no significant effects on the Cd concentrations in shoots and roots.Under 5 ?mol/L Cd level,with the treatment of 0.1 and 1 ?mol/L Se,the shoot Cd concentrations decreased by 4.02% and 21.06%,and the root Cd concentrations significantly reduced by 20.18% and 24.77% for rape cultivar L338,respectively;the shoot Cd concentrations decreased by 4.28% and 12.41%,and the root Cd concentrations significantly reduced by 4.79% and 27.28% for rape cultivar L351,respectively.The enhancement of photosynthesis and the reduction of Cd concentration are conducive to maintaining the relative stability of plant growth.(2)Se is beneficial to maintain the growth and morphology of rape roots under Cd stress.Cd caused irreversible damage to the root morphology of rapes.Under 5 ?mol/L Cd stress,Se treatment slightly increased the surface area,mean diameter and volume of rape roots,while decreased the numbers of root tips.Judging from the color depth of histochemical staining in root tips,1 ?mol/L Se treatment alleviated the plasma membrane damage and superoxide ion accumulation induced by Cd(1 and 5 ?mol/L)stress.These results suggest that the effect of Se on the growth and morphology of rape roots may be involved in the regulation effect of Se on the uptake and distribution of Cd in roots.(3)Under 5 ?mol/L Cd stress,Se changed the chemical forms and subcellular distribution of Cd in rape roots and increased the content of polysaccharides in root cell wall,which was facilitated to the retention of Cd in roots.Under 5 ?mol/L Cd level,with the treatment of 0.1 and 1 ?mol/L Se,the Cd concentrations in soluble fractions decreased by 1.00% and 6.94%,and the Cd concentrations in cell wall increased by 29.01% and 10.29% for rape cultivar L338,respectively;the Cd concentrations in soluble fractions decreased by 11.09% and 18.39%,and the Cd concentrations in cell wall increased by 0.40% and 15.86% for rape cultivar L351,respectively.Exogenous Se affected the Cd concentrations of different chemical forms in rape roots.In addition,exogenous Se increased the content of pectin and hemicellulose 2 in root cell wall,which was consistent with the increase of Cd concentration in root cell wall.Based on the above results,we speculate that the effects of Se on the Cd distribution at the subcellular level and the change of polysaccharide composition in cell wall are related to the increased retention of Cd in rape roots.(4)Se regulated the expression of genes related to Cd uptake and transport in rape roots.At the levels of 1 and 5 ?mol/L Cd,exogenous Se treatment significantly reduced the expression of Cd uptake genes BnIRT1;7 and BnIRT1;8 and the Cd transport gene BnHMA4;3 in rape roots.At 5 ?mol/L Cd level,exogenous Se treatment significantly reduced the expression of Cd transport gene BnHMA2;1 in rape roots.The results suggest that Se may be involved in the regulation of gene expression relating to Cd uptake and transport in rape roots.(5)Se application in soil reduced the accumulation of Cd in seeds.In Cd-contaminated soils,the effect of Se application on the dry matter accumulation of two different cultivars of rapes was not significant.The change of Cd concentrations in roots,stems and pods was different due to the different levels of Se and Cd and the different plant parts.However,the dose level of Se application in the experiment showed the potential to reduce the Cd accumulation in seeds.At 0.5 mg/kg soil Cd pollution level,with the application of 1 and 5 mg/kg Se in soil,the seed Cd concentrations decreased by 42.11% and 68.42% in rape cultivar L338,and reduced by 20% and 20% in rape cultivar L351,respectively.At 5 mg/kg soil Cd pollution level,the application of Se(0.1,1 and 5 mg/kg)in soil obviously reduced the Cd concentrations in seeds of the two rapes,ranging from 11.94% to 26.87%.The results might provide basis for exploring effective measures to reduce Cd accumulation in rapes grown in Cd pollution areas.