CsSNAT positively regulates salt tolerance and growth of cucumber by promoting melatonin biosynthesis

Abstract Salt stress is an important limiting factor in agricultural production. Previous studies have shown that exogenous melatonin (N-acetyl-5-methoxytryptamine) promotes plant defense against various abiotic stresses in a concentration dependent manner. It is an efficient and convenient way to improve the resistance of plants by modulating the endogenous melatonin at a moderately high level by utilizing the plant's own regulatory mechanism. Serotonin N-acetyltransferase (SNAT) is a key enzyme in the melatonin synthesis pathway. In this study, CsSNAT overexpression lines and CsSNAT RNA interference (RNAi) lines were generated. The content of endogenous melatonin increased 2-3-fold in overexpression lines (OE-15 and OE-40) and decreased 0.5-0.6-fold in RNAi lines (RNAi-5 and RNAi-7). After three days of treatment with 100 mM NaCl, the overexpression lines exhibited stronger salt tolerance compared to wild type and RNAi lines. The malondialdehyde (MDA) content, relative electrical conductivity (REC), proline content, maximum photochemical efficiency of photosystem II (Fv/Fm), reactive oxygen species (ROS) accumulation, and the activities of antioxidant enzymes showed more significant salt stress injury in the RNAi lines. Under salt stress, there was significantly higher expression of stress resistance-related genes such as CsSOD, CsCAT, CsP5CS1, CsP5CS2, CsPIP, and CsHSP90 in the overexpression lines compared to the wild type and the RNAi lines. Further analysis revealed that overexpression of the CsSNAT gene in cucumber promoted seed germination. The growth of cucumber seedlings of the RNAi lines was severely inhibited. Taken together, our results suggest that CsSNAT positively regulates salt tolerance and growth of cucumber seedlings by promoting melatonin synthesis.