Effects Of Exogenous Strigolactones On Salt Stress Tolerance Of Cucumber

Environmental stress can inhibit seed germination,delay plant growth,accelerate senescence,and cause plant death.Salt stress is an environmental stress factor that severely restricts agricultural production.Plants respond to salt stress through a variety of biochemical and molecular mechanisms.These mechanisms include regulating ion absorption and accumulation,alleviating membrane lipid damage,maintaining osmotic balance,enhancing photosynthesis and oxidase（antioxidant enzyme and non-antioxidant enzyme）activity,changing the expression of salt tolerance genes,and strengthening plant salt stress tolerance.In addition,the level of plant hormones（auxin,ethylene,jasmonic acid,gibberellic acid,abscisic acid,etc.）or other signaling molecules（hydrogen peroxide and calcium ions）sensing and transduction can also adjust plant growth strategies to adapt to salt stress.Cucumber is an important vegetable crop,and the yield of cucumber grown in saline-alkali land is obviously limited.Therefore,the study of salt tolerance mechanism has theoretical guiding significance for increasing the yield of cucumber seedlings under salt stress.Strigolactones/SLs are a class of carotenoid terpenoid lactones isolated from plant root exudates,which can participate in the growth and development of plants and the defense response to abiotic stress.However,the current research on the molecular mechanism of SLs-mediated plant response to salt stress is still not systematic,and there are relatively few studies on the signal pathways and signal molecules involved in the process of SLs in regulating the response of plants to salt stress.In this study,the cucumber variety"Jingrun 35"was used as the experimental material to spray GR24（SLs analogue）on cucumber seedling leaves under salt stress to study the effects of exogenous GR24 on cucumber seedling growth phenotype,osmotic adjustment,membrane lipid damage,and ionization.The influence of content,photosynthesis,stomata opening,oxidative defense ability and stress resistance gene expression level,and ranscriptome（RNA-Seq）was used to analyze the effects of exogenous GR24 on the salt tolerance genes and metabolic pathways of cucumber seedlings.In addition,the addition of NADP oxidase inhibitor DPI,H₂O₂scavenger DMTU,Ca²⁺chelator EGTA,and Ca²⁺channel inhibitor La Cl₃was also used to explore whether the signal molecules H₂O₂and Ca²⁺are involved in regulating the salt tolerance of cucumber seedlings.Analyzing the mechanism of SLs inducing salt tolerance of cucumber seedlings from the physiological,biochemical and genetic levels provides new insights for the resistance mechanisms of SLs to alleviate salt stress.The main results are as follows:The main results were as follows:（1）compared with salt stress alone,the growth of cucumber seedlings pretreated with exogenous GR24 was significantly inhibited,the contents of MDA,ASA and GSH increased significantly,while the GSSG and the ratio of GSH/GSSG decreased significantly,indicating that exogenous GR24 could alleviate salt stress,reduce membrane damage and improve salt tolerance by affecting ASA-GSH cycle.In addition,under salt stress,the stomatal opening of cucumber seedlings decreased,and the total chlorophyll content,chlorophyll a,chlorophyll b,carotenoid content,RWC,Tr,Gs,Pn,Ci,Pn,Fv’/Fm’,ETR,Y（II）,q P,q L and Fv/Fm decreased,while NPQ,Y（NPQ）and qN increased.GR24 pretreatment alleviated the decrease of photosynthetic rate and the increase of stomatal opening caused by salt stress.Chlorophyll content,chlorophyll a,chlorophyll b,carotenoid content and RWC,Tr,Gs,Pn,Ci,Pn,Fv’/Fm’,ETR,Y（II）,q P,q L and Fv/Fm increased significantly,NPQ,Y（NPQ）and qN increased significantly,while DPI and DMTU inhibited the mitigation effect of GR24 on photosynthesis of cucumber seedlings under salt stress.The results showed that exogenous GR24 increased osmotic regulation,gas exchange parameters and light and light parameters of cucumber seedlings under salt stress,and enhanced photosynthesis of cucumber seedlings,and H₂O₂was involved in this process.（2）Salt stress caused membrane damage,ion imbalance,osmotic stress and oxidative stress,while exogenous GR24 pretreatment alleviated membrane damage,maintained ion homeostasis,enhanced osmotic regulation and antioxidant enzyme activities,and improved ROS scavenging ability of cucumber seedlings under salt stress.q RT-PCR analysis showed that antioxidant enzyme genes（CsCAT,Cs SOD,Cs POD and Cs APX）,NADPH gene（Cs RBOHA-F）,MAPK gene（Cs MAPK3,Cs MAPK4 and Cs MAPK6）,SOS gene（Cs SOS1,Cs SOS2 and Cs SOS3）and CDPK related genes（CsCDPK1-6,CsCDPK10-14 and CsCDPK16-19）played an important role in response to salt stress.Exogenous GR24 induced significant changes in the expression level of these genes.In addition,the application of DPI,DMTU,EGTA and La Cl₃showed that Ca²⁺and H₂O₂mediated salt stress signal transduction in cucumber seedlings.（3）RNA-Seq sequencing analysis showed that there were 10726,7072 and1678 differentially expressed genes（DEGs）in CK（control）＿vs＿D（Na Cl）,CK＿vs＿LD（GR24+Na Cl）and D＿vs＿LD,respectively.GO functional annotation shows that DEGs is mainly enriched in metabolic process,cellular process,biological regulation,stimulation response,signal transduction,membrane and antioxidation and so on.KEGG enrichment analysis showed that DEGs was mainly enriched in starch,sucrose metabolism,carbon metabolism,photosynthesis,carbon fixation in photosynthetic organisms,plant hormone signal transduction and so on.DEGs in D＿vs＿LD mainly includes proteins and metabolites related to photosynthesis,peroxidation,glutathione metabolism,Ca²⁺signal transduction and MAPK cascade signal transduction,as well as related proteins and transcription factors that enhance salt tolerance by stimulating other plant hormones（IAA,gibberellin,BRs,ABA,etc.）signal transduction.In addition,some DEGs are also involved in the metabolic process of salt stress,including glycolysis,carbohydrate metabolism,TCA cycle and secondary metabolic pathways.