The Role Of Hydrogen Peroxide In Spermidine-Induced Autophagy To Enhance Salt Tolerance In Cucumber Seedlings

With the development of China’s protected cultivation industry,unreasonable cultivation management has led to the problem of soil secondary salinization.The cucumber cultivation area in protected horticulture is large and the planting range is wide,the cucumber root system is weak,and it is sensitive to salt damage in the rhizosphere soil.Secondary salinized soil significantly inhibits the growth of cucumber,which seriously affects the yield and quality of cucumber.Therefore,analyzing the regulation mechanism of salt tolerance in cucumber is of great practical significance to alleviate or solve the stress hazards of cucumber and other crops,improve yield and quality,and ensure a balanced supply of vegetables in China.Spermidine(Spd),as a new plant growth regulator,can effectively alleviate the damage caused by salt stress on plants.Autophagy is an evolutionarily conserved cell degradation process that can engulf harmful components within cells and transport them to the vacuole for degradation and reuse,which plays a vital role in plant development and stress response.In recent years,Spd regulation of autophagy in animals has become a research hotspot,but research in plants,especially horticultural plants,is still scarce.In this paper,cucumber ’Jinchun No.4’ was used as a material,and hydroponic methods were used to study the effects of Spd on autophagy and its relationship with H2O2 under 75 mM NaCl stress through physiological and biochemical and molecular biological methods such as plant chlorophyll fluorescence,transmission electron microscopy(TEM),and confocal microscopy.The findings are as follows:1.Spd relieves salt stress in cucumber seedlings.Salt stress significantly inhibited cucumber growth,and photosynthetic rate and chlorophyll fluorescence were significantly reduced compared to the control.Relative electrical conductivity(EL),Na+ content,and Na+/K+of the leaves were significantly increased.Foliar application of Spd significantly promoted cucumber growth and inhibited the increase of EL,Na+ content,Na+/K+ induced by salt stress.These results indicate that exogenous Spd can improve the salt tolerance of cucumber.2.Spd-induced autophagy alleviates salt stress in cucumber seedlings.There were no significant differences in Spd-treated cucumber autophagy-related genes and proteins,insoluble proteins and their ubiquitination levels under control.The contents of autophagosomes,expression of autophagy-related genes of cucumber treated with Spd under salt stress significantly increased,which suppressed the increase of insoluble protein and ubiquitination caused by salt stress.These results indicate that Spd can induce autophagy to alleviate the damage of cucumber under salt stress.3.Spd induces H2O2 as a signal to activate autophagy to alleviate salt stress in cucumber seedlings.Spd treatment up-regulated the expression of Respiratory burst oxidase homologl(RBOH)in the leaves and roots of cucumber,increased H2O2 content,and autophagosome content,autophagy-related genes and their proteins significantly increased.However,after pretreatment with dimethylthiourea(DMTU,H2O2 scavenger)or diphenyliodine(DPI,an inhibitor of NADPH oxidase),the content of autophagosomes,autophagy-related genes and their protein phases significantly decreased,and dry matter accumulation,photosynthetic rate significantly decreased,and electrical conductivity significantly increased.These results indicate that Spd induces autophagy with H2O2 as a signal to alleviate salt stress in cucumber.