Mechanism Of Exogenous MeJA Regulating Response Of Turnip(Brassica Rapa L.subsp.rapa) Seedlings To NaCl Stress

Turnip(Brassica rapa L.subsp.rapa)is a biennial herb of the genus Brassica in the family Cruciferae.The cultivation of Xinjiang turnip is often subjected to stresses such as salinity and drought,which hinder the growth and development process of the plant.As an exogenous regulatory hormone,jasmonic acid can induce changes in plant resistance-related indicators to resist stress.In this study,we investigated the response mechanism of exogenous MeJA to NaCl stress in turnip seedlings from the aspects of plant physiology,gene expression,metabolite accumulation,and hormone regulation to provide new ideas for the creation of salt-tolerant turnip varieties.The results obtained in this study were as follows:(1)NaCl stress limited the growth of turnip seedlings.The dry and fresh weights of the seedlings were reduced.The MDA content in leaves and roots was increased and cell membranes were damaged.The content of reactive oxygen substances and antioxidant enzyme activities in seedlings increased.NaCl stress reduced the content of chlorophyll and its synthetic precursors in the leaves of turnip seedlings and inhibited photosynthesis in turnip seedlings.(2)Exogenous MeJA inhibited the growth of turnip seedlings and reduced the dry and fresh weight of the plants.The antioxidant enzyme activities in leaf and root tissues had different expression patterns,in which SOD and POD gradually increased with increasing concentration,while CAT and APX activities gradually decreased.Among the reactive oxygen metabolites,H₂O₂ gradually decreased,while O₂·^-content gradually increased.Among the osmoregulatory substances,soluble sugar content increased and soluble protein content decreased.Exogenous spraying MeJA treatment significantly reduced the content of chlorophyll and its precursor substances in the leaves of turnip seedlings and inhibited photosynthesis of seedlings.(3)Exogenous MeJA significantly limited the growth of turnip seedlings under NaCl stress,induced leaf senescence,reduced chlorophyll and its precursor content and inhibited photosynthesis of turnip seedlings.The expression patterns of antioxidant enzymes in leaves and roots varied,with SOD,POD,and APX increasing after N+M treatment,while CAT activity was significantly reduced.Reactive oxygen metabolites(H₂O₂ and O₂·^-)and membrane lipid peroxidation products(MDA)in seedlings were significantly increased by N+M treatment.Taken together,exogenous MeJA induced leaf senescence and inhibited the growth of turnip seedlings under NaCl stress,but increased the activities of some antioxidant enzymes and enhanced the defense mechanism against NaCl stress.(4)Exogenous MeJA treatment inhibited the expression of genes related to chlorophyll synthesis,photosynthesis and carbon metabolism in leaves of turnip seedlings.The expression of genes related to resistance to stress was significantly up-regulated in sulfur metabolism,phytohormone signaling,glutathione metabolism,and phenylpropane biosynthesis.Gene expression in the CTK synthesis pathway was suppressed and CTK-like substances accumulated down-regulatively.Genes expression and hormone contents in the JA and ABA synthesis pathways were significantly up-regulated.NaCl treatment induced significant up-regulation in the expression of most genes and metabolites in the carbon and amino acid metabolism pathways.Most genes in the ABA,JA and Auxin biosynthetic pathways were up-regulated,while CTK synthesis genes were significantly down-regulated.Turnip seedlings were significantly down-regulated in photosynthesis-related genes and significantly up-regulated in amino acid biosynthesis-related genes when co-treated with exogenous MeJA and NaCl stress.From the perspective of metabolite accumulation,organic acids and amino acids were significantly higher in turnip seedlings than in NaCl stress treatment.ABA and JA contents showed up-regulated accumulation.(5)We identified 36 JAZ genes from the turnip genome and classified them into six subfamilies.They were unevenly distributed among 10 chromosomes.Gene structure and conserved motifs of Brr JAZs were similar across the subfamilies,but the differences between the subfamilies were large.Although the proteins varied in length,MW,and Pi,all contained a conserved TIFY or Jas structure.Phylogenetic and collinearity analysis provided some valuable clues to the evolutionary characteristics of Brr JAZ genes.Expression analysis suggested JAZ genes may be involved in the formation of turnip fleshy root,and they also participated in the response to salt and low-temperature stress.MeJA and ABA treatment significantly induced the expression of Brr JAZ genes.Overall,our findings will help understand the biological functions of the Brr JAZ genes in turnip.