Analysis Of Physiological Function And Gene Expression Of Exogenous Melatonin Enhancing Salt Tolerance Of Rice Seedlings

Salt stress caused by soil salinization on plants is already one of the most important environmental stresses in the world.In recent years,the sea level rise and artificially unreasonable agronomic measures have exacerbated the pace of salinization of cultivated land.As one of the important food crops,rice is relatively sensitive to salinity,and salt stress obviously inhibits its growth,yield and quality.Melatonin(MT)is an emerging plant hormone that can effectively improve the salt tolerance of rice seedlings.In this study,Yandao12 was used as the material.Firstly,the effects of MT spraying on the redox and ion homeostasis of rice seedlings under different salt concentrations were studied.Secondly,the transcriptome sequencing was used to analyze the gene expression of MT regulated rice seedlings under salt stress.Finally,the metabolic response of rice endogenous polyamines(PAS)in improving salt tolerance of rice seedlings under MT spraying was studied.The main results are as follows:1.From the perspective of growth phenotype,dry weight and plant height of rice seedlings,foliar spraying 200 ?M MT effectively reduced the inhibitory effect of salt stress on the growth of rice seedlings.Further analysis showed that MT increased the activities of several antioxidant enzymes,reduced membrane lipid peroxidation,reduced cell damage of plant leaves,and increased the accumulation of N and Si in leaves and roots under salt stress(especially under high salinity).MT also inhibited the absorption of Na+and its transport to the aboveground,and promoted or maintained the absorption of K+and Ca2+and its transport to the aboveground in rice seedlings under salt stress,thereby improving the ion homeostasis of K+/Na+ and Ca2+/Na+ in rice seedlings(especially in leaves).2.The number of differentially expressed genes regulated by MT in rice seedlings leaves and roots under non salt stress was more than that under salt stress.The analysis of GO enrichment showed that the differentially expressed genes regulated by MT in the leaves and roots of rice seedlings under salt stress were mainly related to salt stress response,redox process,antioxidant enzyme activity,H2O2 degradation process and ion transport.At the same time,the analysis of KEGG enrichment showed that MT regulated genes were mainly enriched in the biosynthetic and metabolism of substances,such as phenylpropane synthesis,phenylalanine metabolism,flavonoid biosynthesis,plant hormone signal transduction and so on.In addition,under non salt and salt stress,MT can regulate the differentially expressed genes related to photosynthesis in rice seedlings leaves.MT-regulated transcription factors(TFs)in rice seedlings are mainly WRKY,AP2-EREBP,MYB,and NAC transcription factor families.Under low salt stress,genes encoding these TFs are mainly up-regulated expression patterns,while under high salt stress,genes encoding these TFs are mainly down-regulated expression patterns.3.Under low salt and high salt stress for 7 and 14 days,MT could regulate the gene expression of arginine decarboxylase(ADC),ornithine decarboxylase(ODC)and polyamine oxidase(PAO)in rice seedling leaves and reduce the activities of diamine oxidase(DAO)and polyamine oxidase(PAO),so as to regulate the transformation of polyamine form and content.In the 21-day high-salt treatment of rice leaves,spraying polyamine synthesis inhibitor(D-Arg)reduced the ADC and ODC enzyme activities of the plant leaves,increased its DAO and PAO enzyme activities,reduced the contents of free and bound putrescine(Put),and increased the contents of spermidine(Spd)and spermine(Spm).D-Arg also reduced the content of proline(Pro)in plant leaves and increased the content of Na+.MT can effectively reduce the inhibition of D-Arg on the synthesis of Put by increasing the activities of ADC and ODC in leaves of plants,and promote the transformation of free Put to different forms of PAS,thus increasing the ratio of free and bound(Spd+Spm)/Put in leaves of plants.MT also reduced the oxidative metabolism of PAs in leaves by reducing the activities of DAO and PAO.In addition,even in the presence of D-Arg,MT also increased Pro and K+content,decreased Na+ content(more obvious in leaves).In a word,MT can enhance the salt tolerance of rice seedlings by regulating the distribution and transformation of PAs in different forms and improving the K+/Na+homeostasis of plants.In summary,MT effectively regulated gene expression and physiological and biochemical changes in rice seedlings,significantly improved the oxidative stress and ion toxicity induced by salt stress,thereby promoting its growth and conferring salt tolerance on rice seedlings.