| China is a country with a large population and little arable land per capita.Thus,ensuring food production and security are related to national development and people’s livelihood.Soil salinization is gradually becoming the main abiotic stress affecting agriculture production in China.It’s of great significance for promoting the exploitation and utilization of saline-alkali land,increasing the utilization area of cultivated land,and ensuring food security to analyze the salt tolerance mechanism of plants,explore key salt tolerance genes,cultivate new varieties of salt resistant crops by molecular breeding methods,and find simple and efficient methods of improving the salt tolerance capacity of plants.As the basic component of protein,amino acids can not only provide energy for plants life activity,but also serve as signal molecules to regulate plants growth and development and respond to a variety of biotic stresses,such as bacteria,fungi,and viruses and so on.However,the important roles and molecular mechanisms of Met(Methionine)in plants abiotic stress response have been rarely studied.In this study,mainly using the model plant Arabidopsis thaliana as the material,it was found that Met could improve the salt tolerance of plants,and the important roles of Met in regulating plants salt stress response and its molecular regulatory mechanism were systematically studied.The main research results are as follows:1.Salt stress induces an increase in Met synthesis of plants.Both HMTs(HOMOCYSTEINE-S-METHYLTRANSFERASES)and MSs(METHIONINE SYNTHASES),the key genes of Met synthesis pathway,were induced by salt stress,leading to an increase in Met levels of plants under salt stress.Both ABA and H2O2 induced the transcriptional expression of HMTs and MSs,and the ability of salt stressinduced upregulation of HMTs and MSs was obviously inhibited in the ABA signal mutant abi5-7 and the ROS(REACTIVE OXYGEN SPECIES)synthesis reduction mutant rbohd/f,implying that salt stress regulates Met synthesis through ABA and ROS pathways.2.Met improves plants salt resistance.According to the transcriptome of salt treatment,we found that Met synthesis pathway might be induced by salt stress,suggesting that Met may participate in the response of plants to salt stress.Phenotypic analysis of salt stress by exogenous application of Met revealed that Met could improve the seed germination rate and cotyledon green ratio of Arabidopsis under salt stress.Analysis of salt stress phenotypes using Arabidopsis Met synthesis mutants hmtl-3 and ms1-3 revealed that both hmt1-3 and ms1-3 mutants exhibited significant salt stress hypersensitivity phenotypes,indicating that Met can improve salt tolerance in plants.3.Met activates ABA signaling.The transcriptome analysis results of Met treatment and ABA treatment showed that there was a significant overlap between Metregulated and ABA-regulated genes,among which 451 genes were up-regulated,accounting for approximately 26%of the total Met-activated genes,and 3 15 genes were down-regulated,accounting for approximately 22.5%of the total Met inhibitory genes.Additionally,Met activated the expression of genes related to ABA synthesis,transportation,and signal transduction,thereby increasing endogenous ABA content.This result indicated that Met may enhance the salt resistance of plants by promoting ABA synthesis and signaling,thereby activating the expression of salt stress response genes.4.Met inhibits primary root elongation of plants.In order to improve survival probability under salt stress,plants usually adopt a balance strategy between regulating growth and salt resistance.On the one hand,the salt stress signaling pathway is activated immediately to improve plants salt tolerance.On the other hand,plants reduce growth and salt absorption by inhibiting root growth to allocate more energy for improving plants salt tolerance.This study found that salt stress could increase the level of Met in plants and Met inhibited the growth of plants primary root in a concentrationdependent manner,while its metabolite S-adenosylmethionine(SAM)had no similar effect.Further analysis revealed that although Met was considered as a precursor of ethylene,its inhibition of primary root elongation was not achieved through ethylene signaling,but through inhibition of auxin and cytokinin signaling pathways.Met treatment inhibited GUS signal of DR5:GUS plants and GFP signal of TCSn:GFP plants,which indicated that Met inhibits auxin signal and cytokinin signal.Mutants with reduced auxin synthesis,transport mutants,and signal transduction mutants could aggravate the inhibition of Met on primary root growth,while the mutant with increased auxin synthesis and exogenous auxin treatment could alleviate the inhibition of Met on primary root growth,indicating that Met inhibited primary root elongation by inhibiting the auxin signaling pathway.Cytokinin synthesis mutants and signal transduction mutants could also aggravate the inhibition of Met on primary root growth,while ahp6(cytokinin signal suppressor)mutants,B-type cytokinin response factor overexpression transgenic plants,and exogenous cytokinin treatment could alleviate the inhibition of Met on primary root growth,implying that Met also inhibited primary root growth by inhibiting cytokinin signaling pathways.Cell cycle regulation also mediates the process of Met inhibiting primary root growth.Met treatment weakened the GUS signal at the root tip of pCYCB1;1:GUS plants,and transcriptome analysis of Met treated Arabidopsis showed that nearly 90%of the expression of cell cycle related genes was significantly inhibited.The e2fa and e2fb mutants were hypersensitive to Met,while 35S:E2Fa/35S:Dpa alleviated the inhibition of Met on the primary root.In addition,Met inhibited the expression of homeostasis maintenance factors such as WOX5,SCR,and SHR in root tip stem cells,suggesting that Met may inhibit primary root elongation by affecting the activity of root tip meristem tissue.To sum up,this study found that salt stress could induce the expression of Met synthesis genes and promote Met synthesis,which contributed to improving salt resistance of plants.On the one hand,Met may act as an important signaling molecule that activates salt stress responses through ABA signaling pathway,enhancing plants salt tolerance by regulating the expression of salt stress response genes.On the other hand,plants respond to increased Met accumulation under salt stress and inhibits root growth in a concentration-dependent manner,adjusting the balance between growth and salt resistance,thereby allocating more energy to plants salt resistance responses. |
