Exogenous strigolactones alleviate the photosynthetic inhibition and oxidative damage of cucumber seedlings under salt stress

Strigolactones (SLs) play crucial roles in plant growth and development and during the response to multiple biotic and abiotic stresses. In this study, we investigated the role of exogenous SLs analog GR24 in amelioration of salt damage and analyzed the interaction between GR24 and other signaling molecules in cucumber seedlings under salt stress by using physiological, biochemical and RNA-seq analysis approaches. Higher chlorophyll contents, stomatal conductance, greater efficiency of photosynthetic were observed in GR24-pretreated plants than in non- GR24-pretreated plants under salt stress. In addition, exogenous GR24 pretreatment can also alleviate salt-induced photodamage, enhance the efficiency of ascorbate-glutathione (AsA-GSH) cycle, and scavenge excessive reactive oxygen species (ROS), thus alleviating oxidative stress under salt stress in cucumber. RNA-seq analysis revealed that there were 1678 differentially expressed genes (DEGs) in D (NaCl)_vs_LD (GR24+NaCl), including 1219 up-regulated genes and 459 down-regulated genes. There were 66 DEGs related to photosynthesis, MAPK cascade pathways, oxidation and antioxidant system screened by GO classification and KEGG enrichment analysis, and the expression levels of these genes were consistent with the changes of physiological parameters. However, the effects of GR24 on the photosynthetic of cucumber seedlings under salt stress, as well as the expression levels of MAPK3/4/6 genes were significantly inhibited by diphenyleneiodonium chloride (DPI, NADPH oxidase inhibitor) or dimethylthiourea (DMTU, H2O2 scavenger). These findings suggested that H2O2 and MAPK cascade pathways might play a positive role in exogenous GR24 alleviating salt stress in cucumber seedlings. In conclusion this study provides insights for further investigating the mechanism of SLs in alleviating adversity stress.