Action Mechanisms Of Brassinosteroids Ameliorating Zinc Oxide Nanoparticles Stress And Melatonin Involved In Selenium-induced Cadmium Tolerance Stress In Tomato

Due to the rapid development of manufacturing and anthropogenic activities,the contamination of heavy metals in soil is more and more serious,which not only affects the crop production but also affects the safety of food directly.Moreover,with the improvement of industrial technology,more and more novel metal nanoparticules enter into the environment.They have some particular physical and chemical characters in addition to the component of metal,which brings new environment and human risks togather.Plants have evolved complex mechanisms to minimize the damage from heavy metal stress.Plant can alleviate heavy metal stress by regulating hormonal homeostasis and substance metabolism in plant.Therefore,it is scientifically and actually very meaningful in enhancing the tolerance of heavy metal and metal nanoparticle stress,as well as increasing the yield,quality and economic benefits of vegetable production by analyzing the mechanisms of phytohormones and substance metabolism induced heavy metal and metal nanoparticles stress tolerance as well as applying them in crop production.We used tomato(Solarium lycopersicum L.)as plant materials to investigate the effect of brassinosteroids(BR)on alleviating zinc oxide nanoparticles toxicity.Then the effects with different forms of Se on the growth and antioxidant capacity are researched in tomato plants.Finally,the role of melatonin in Se-induced cadmium(Cd)tolerance is analyzed in tomato plants using the technology of virus induced gene silencing(VIGS).The main results are as follows:1.The effects of BR on alleviating ZnO nanoparticles(ZnO NPs)toxicity were invetigated in tomato seedlings.Seedling growth,biomass production and root activity gradually decreased,but Zn accumulation increased with increasing ZnO NPs concentration(10-100 mg/L).The augmentation of BR(5 nM)in media significantly ameliorated 50 mg/L ZnO NPs-induced growth inhibition.Visualization of hydrogen peroxide(H2O2)and quantification of H2O2 and lipid peroxidation in tomato roots confirmed that ZnO NPs induced an oxidative stress.However,BR supplementation substantially reduced ZnO NPs induced oxidative stress.Furthermore,the activities of superoxide dismutase,catalase,ascorbate peroxidase and glutathione reductase were increased by combined treatment of BR and ZnO NPs compared with ZnO NPs only treatment.BR also increased reduced glutathione(GSH)and GSH:GSSG ratio.The changes in relative transcript abundance of corresponding antioxidant genes such as Cu/Zn SOD,CAT1,GSH1 and GR1 were in accordance with the changes in those antioxidants under different treatments.More importantly,combined application of BR and ZnO NPs significantly decreased Zn content in both shoot and root of tomato seedlings as compared with ZnO NPs alone.Taken together,these results adequately indicated that BR could mitigate ZnO NPs toxicity in tomato seedlings.2.The effects of different forms of Se on growth and antioxidant capacity were analyzed in tomato plants.Photosynthetic rate,plant growth and biomass were all increased by treatment with 3 ?M selenocysteine(Se-Cys),Na2SeO3 and Na2SeO4,while no significant effects were observed by treatment with 3?M cysteine(Cys),Na2SO3 and Na2SO4.The absorption of Se in tomato roots was almost same among treatments with different forms of Se,however,compared with Se-Cys and Na2SeO4,the Se content was lower in tomato leaves following treatment with Na2SO3.Different forms of Se increased the total antioxidant capacity by improving the activity of GPX,the content of GSH and the ratio of GSH:GSSG,while greatest effect accounts for Se-Cys.Different forms of Se increased the levels of endogenous melatonin and its precursor chemicals(tryptamine and serotonin)as well as the melatonin biosynthsis genes expression significantly.These results clearly demonstrated that Se enhanced growth and antioxidant capacity in tomato plants.Considering the studied results,the order of efficiency for the forms of Se in achieving positive effects was as follows:Se-Cys>Na2SeO4>Na2SeO3.3.The role of melatonin in Se-induced Cd tolerance was investigated in tomato plamts.Pretreatment with different forms of Se significantly increased plant Cd tolerance,while greatest effect accounts for Se-Cys.Furthermore,Se-Cys and melatonin supplements significantly increased plant Cd tolerance as evidenced by decreased growth inhibition,photoinhibition and electrolyte leakage coupled with the increased level of endogenous melatonin.Se-induced Cd tolerance was compromised in melatonin-deficient plants due to tryptophan decarboxylase(TDC)gene silencing.In control plants,Se treatment increased the levels of glutathione(GSH)and phytochelatins(PCs),as well as the expression of GSH1 and PCS,which were GSH and PC biosynthetic genes respectively,but the effects of Se were compromised in TDC-silenced plants under Cd stress.Our results indicate that melatonin is involved in Se-induced Cd tolerance via the regulation of Cd detoxification.