Effects Of Exogenous Melatonin On The Growth Of Soybean Plants Under Salt Stress

Superfluous salt in the soil influences the growth of soybean plants,leading to a decline in yield and quality.The seedling stage is the critical period for the organ morphogenesis of soybean,which determines the tolerance potential of soybean plants to abiotic stress.To investigate the effect of exogenous melatonin on the growth of soybean plants and its mechanism at the seedling stage under saline stress,soybean varieties,Heihe 49（HH49,salt-tolerant variety）and Henong 95（HN95,salt-intolerant variety）,were selected as experimental materials,and pot experiments were conducted to study the effects of foliar spraying of 200μmol·L^-1 melatonin on the morphogenesis,photosynthesis,accumulation and distribution of photosynthetic products,DNA damage repair and cell-cycle arrest of soybean at the seedling stage under 50 mol·L^-1 NaCl stress,so as to analyze the physiological and molecular mechanism of melatonin alleviating the inhibition of saline stress on soybean plant growth.The major results and conclusions are as follows:1.Melatonin treatment showed a significant alleviation effect on the growth inhibition of soybean varieties HH49 and HN95 under NaCl stress.Compared with NaCl treatment,exogenous melatonin significantly（P<0.05）enhanced the plant height,leaf area,total root length,root surface area,root volume,leaf fresh weight,leaf dry weight,root fresh weight and root dry weight of HH49 and HN95 seedlings.Thereinto,the nine indexes of HH49 went up by 11.43%,6.34%,22.53%,36.95%,20%,12.75%,17.65%,10.74%and 29.88%respectively,and the corresponding indexes of HN95 rose by 20.78%、22%、73.52%、115.33%、52.4%、34.83%、46.53%、25.79%and 67.33%respectively.The above results showed that exogenous melatonin had a greater alleviation effect on the growth inhibition of salt-intolerant variety HN95 than that of salt-tolerant variety HH49 under NaCl stress.2.Melatonin treatment significantly relieved membrane lipid peroxidation and osmotic stress in seedling leaves and roots of soybean varieties HH49 and HN95 under NaCl stress.Compared with NaCl treatment,exogenous melatonin remarkably（P<0.05）enhanced the activities of SOD,POD,CAT and APX as well as the contents of soluble protein and soluble sugar in leaves and roots of HH49 and HN95 seedlings,and lowered the contents of H₂O₂,O₂^-and MDA,as well as relative conductivity.3.Melatonin treatment eased the inhibition of NaCl stress on the photosynthetic capacity of seedling leaves of soybean varieties HH49 and HN95.Compared with NaCl treatment,exogenous melatonin significantly（P<0.05）increased photosynthetic pigment content（chlorophyll a,chlorophyll b and carotenoid）,photosynthetic gas exchange parameters（Pn,Gs,Ci and Tr）and chlorophyll fluorescence parameters（Fv/Fm,Fv/Fo,ETR andφPSⅡ）of seedlings leaves of HH49and HN95.4.Melatonin treatment dramatically ameliorated the effects of NaCl stress on carbohydrate metabolism in seedling leaves and roots of soybean varieties HH49 and HN95.Compared with NaCl treatment,exogenous melatonin significantly（P<0.05）increased the contents of soluble sugar,sucrose and starch in leaves and roots of HH49 and HN95 seedlings,significantly（P<0.05）enhanced the activities of sucrose phosphate synthase（SPS）,sucrose synthase（Su Sy）,acid invertase（A-INV）and neutral invertase（N-INV）,significantly（P<0.05）up-regulated expression levels of the related genes（Gm SPS,Gm Su Sy,Gm C-INV andGm A-INV）of sucrose-metabolizing enzymes,significantly（P<0.05）down-regulated expression levels of sucrose transport-related genes（Gm SUC2 andGm SWEET15）,significantly（P<0.05）reduced the activity ofα-amylase in leaves,and down-regulated the expression level of the related gene（Gm AMY3）.The above results indicated that exogenous melatonin promoted carbohydrate accumulation by strengthening sucrose metabolism.5.Melatonin treatment remarkably relieved the DNA damage of the seedling root system of soybean varieties HH49 and HN95 caused by NaCl stress.Compared with NaCl treatment,exogenous melatonin significantly（P<0.05）declined the RAPD polymorphism,the content of 8-OH-dG and the relative density of AP-site of root genomic DNA of HH49 and HN95 under NaCl stress,and enhanced stability of the genome.Moreover,the treatment of melatonin significantly（P<0.05）up-regulated the expression levels of genes involved in DNA damage repair（Gm NBS1,Gm BRCA1,Gm RAD51,Gm MRE11,Gm KU70,Gm KU80,Gm MLH1,Gm MSH2,Gm MSH6,Gm CSA1,Gm RAD4,Gm NTH1,Gm NTH2 andGm NTH3）,8-OH-dG regulation related genes（Gm MMH-2 andGm OGG1）and genes related to cell cycle regulation（Gm RBR1,Gm KRP1,Gm E2Fa,Gm HISTONE H4,Gm ATR,Gm CYCB1;1,andGm CDKA;1）under NaCl treatment,and significantly（P<0.05）down-regulated the expression levels of Gm ATM andGm WEE1 genes,relieving the cell-cycle arrest.6.Melatonin significantly（P<0.05）boosted the yield of soybean varieties HH49 and HN95under NaCl stress.Compared with NaCl treatment,exogenous melatonin enhanced pod number per plant,seeds per plant,100-grain weight and yield per plant of HH49 and HN95 under NaCl stress.Among them,the three indexes of HH49 enhanced by 12.44%、16.85%、6.48%and 17.02%respectively;those of HN95 went up by 17.47%、18.67%、12.88%and 29.84%respectively.According to the above results,the effect of exogenous melatonin on the yield reduction of HN95was greater than that of HH49 under NaCl stress.Taken together,exogenous melatonin can relieve oxidative damage and ameliorate carbohydrate metabolism by advancing the antioxidant defense,photosynthesis and DNA damage repair in leaves and roots of HH49 and HN95 plants under NaCl stress,so as to reduce the inhibition of NaCl stress on the growth and biomass accumulation of HH49 and HN95 seedlings,enhance salt tolerance,and mitigate the decline in production.