Physiological And Molecular Mechanisms Of Exogenous Melatonin Inducing Salt Tolerance In Cucumis Sativus L.

Salt stress can inhibit the growth and development of plants,which is one of the main reasons for the decline of crop yield and quality.As an important vegetable crop in China,with the development of modern agriculture,people use a large amount of fertilizer and irrational irrigation,which makes salt stress the main limiting factor for Cucumis sativus cultivation.Many studies on other plants have shown that exogenous or endogenous induction of melatonin can enhance the salt tolerance of plants,but studies on the specific regulatory mechanisms of melatonin to alleviate salt stress have rarely been reported.In this study,using Cucumis sativus seedlings“Jingrun 35”as studied materials to analyzes the physiological and molecular mechanism of exogenous melatonin-induced salt tolerance and study the possible cross-regulation relationship between exogenous melatonin-induced H₂O₂,MAPK cascade pathway and SOS pathway under salt stress in order provides a new theoretical basis for melatonin to alleviate the regulation mechanism of salt stress.The main findings are as follows:1.After pretreated spraying with different concentrations of exogenous melatonin,the degree of wilting and cell relative activity are significantly reduced and100μmol?L^-11 was the most significant in Cucumis sativus seedlings compared with single salt stress.The results show that exogenous melatonin pretreatment can alleviate the damage of cell viability by salt stress,enhance the salt tolerance of Cucumis sativus seedlings,and the mitigation effect of exogenous melatonin was concentration dependent.2.The results of pretreatment with exogenous melatonin then treatment with salt stress compare with salt stress alone,the H₂O₂ content is increased significantly and the superoxide anion content is decreased in Cucumis sativus seedlings.The results show that H₂O₂ may play an important signaling molecule role in the regulation of salt tolerance of Cucumis sativus induced by exogenous melatonin.3.The results of MT+NaCl treatment compare with NaCl stress treatment alone,the electrolyte leakage,malondialdehyde content is decreased significantly and Photosynthetic pigment content,chlorophyll fluorescence parameters,antioxidant enzyme activities and antioxidant enzyme gene expression increased,but the MT+NaCl+DPI,MT+NaCl+DMTU treatment compare with MT+NaCl treatment,the electrolyte leakage,malondialdehyde content is significantly increased and chlorophyll content,chlorophyll fluorescence parameters,and antioxidant activity decreased in Cucumis sativus seedlings.The results show that exogenous melatonin pretreatment can alleviate the damage of Cucumis sativus under salt stress,and the H₂O₂ dependent on NADPH oxidase is involved in the regulation of this mitigation process.4.Compared with NaCl stress treatment,the expression of CsRBOHs genes was significantly induction under MT+NaCl treatment,but compared with MT+NaCl treatment,MT+NaCl+DPI,MT+NaCl+DMTU and MT+NaCl+U0126 treatment the expression of CsRBOHs genes in Cucumis sativus seedlings was inhibited.The results show that under salt stress,exogenous melatonin could induce the expression of NADPH oxidase synthase genes CsRBOHs at the transcriptional level,promote the accumulation of H₂O₂,and the MAPK kinase cascade pathway involved in the regulation of CsRBOHs genes expression by exogenous melatonin.5.Compared with NaCl stress treatment alone,the expression of CsMAPKs genes was significantly up-regulated under MT+NaCl treatment,while compared with MT+NaCl treatment,the expression levels of CsMAPKs genes in the MAPK cascade pathway of Cucumis sativus seedlings treated with MT+NaCl+DPI and MT+NaCl+DMTU treatment were significantly down-regulated.The results show that the MAPK kinase cascade is involved in the regulation of exogenous melatonin induced salt tolerance in Cucumis sativus,and NADPH oxidase dependent H₂O₂ is involved in the regulation of CsMAPKs at the transcriptional level.6.Compared with NaCl treatment alone,the expression of antioxidant enzyme gene and CsSOS1,CsSOS2 and CsSOS3 genes were significantly up-regulated under MT+NaCl treatment,while compared with MT+NaCl treatment,MT+NaCl+DPI,MT+NaCl+DMTU,MT+NaCl+U0126 treatment could inhibit the expression of antioxidant enzyme gene and CsSOS1,CsSOS2 and CsSOS3 genes in Cucumis sativus,and the inhibitory effect of U0126 was more significant.The results show that exogenous melatonin pretreatment can induce the expression of antioxidant enzyme gene and CsSOSs gene under salt stress,and NADPH oxidase dependent H₂O₂ and MAPK cascade pathway are involved in the regulation of antioxidant enzyme genes and CsSOSs genes at the transcriptional level.The above results indicated that exogenous MT pretreatment could induce the expression of NADPH oxidase synthase CsRBOHs genes in Cucumis sativus under salt stress at the transcriptional level,and promote the accumulation of H₂O₂.H₂O₂may further induce expression of CsMAPKs in the MAPK kinase cascade pathway at the transcriptional level.The up-regulated CsMAPKs may promote H₂O₂accumulation by inducing NADPH oxidase synthesis genes CsRBOHs expression,and may be expressed at the transcriptional level by inducing CsSOS1,CsSOS2,CsSOS3 and antioxidant enzymes genes,enhance the salt tolerance of Cucumis sativus.