Relationship between melatonin and abscisic acid in response to salt stress of tomato

•Although MT, ABA, and MT + ABA treatments effectively enhanced salt stress tolerance, the mechanisms were different.•MT + ABA treatments increased salt tolerance more than either ABA or MT treatments alone.•MT + ABA treatments mainly regulated hormone metabolism and the expression of salt-regulated genes to promote the growth of tomato seedlings under salt stress. Melatonin (MT) and abscisic acid (ABA) play an essential role in improving salt stress tolerance and are closely related in plants. However, few studies have investigated the effects of MT + ABA treatments on salt resistance in tomato seedlings. The purpose of this study was to compare the similarities and differences in the physiological mechanisms of alleviating tomato salt stress by exogenous MT or ABA treatments alone. The other objective is to explore whether they interact with each other when fulfilling their biological functions. In the study, tomato seedlings were treated with 200 mM NaCl after application of MT, ABA, and MT + ABA. Here, the effects of hormones on antioxidants, osmotic regulation, photosynthetic performance, and the expression of redox and metabolism-related genes in tomato were identified. The results showed that MT, ABA, and MT + ABA treatments effectively enhanced antioxidant capacity to reduce reactive oxygen species (ROS) and malondialdehyde (MDA) levels in tomato seedlings. The contents of proline, endogenous MT, and photosynthetic pigment were found to be increased levels, and the expression of redox genes, salt tolerance-related genes, and MT biosynthesis genes were significantly up-regulated. ABA catabolism was increased and anabolism was down-regulated, resulting in a reduction of endogenous ABA content. However, there were different mechanisms for alleviating salt stress in tomato seedlings treated with MT, ABA and MT + ABA. MT treatments enhanced the ascorbic acid- glutathione (AsA-GSH) cycle and appropriately improved stomatal conductance under salt stress. ABA treatments increased proline content and decreased stomatal conductance. MT + ABA treatments mainly regulated hormone metabolism and improved the expression of salt-regulated genes. In summary, MT, ABA, and MT + ABA had different key pathways in response to salt stress. Notably, the effects of MT + ABA and MT treatments were similar. However, MT + ABA treatments were more beneficial to promoting the growth of tomato seedlings under salt stress. [Display omitted]