The Physiological and Molecular Mechanisms of Exogenous Melatonin Promote the Seed Germination of Maize (Zea mays L.) under Salt Stress

Salt stress caused by high concentrations of Na+ and Cl- in soil is one of the most important abiotic stresses in agricultural production, which seriously affects grain yield. The alleviation of salt stress through the application of exogenous substances is important for grain production. Melatonin (MT, N-acetyl-5-methoxytryptamine) is an indole-like small molecule that can effectively alleviate the damage caused by adversity stress on crops. Current studies have mainly focused on the effects of MT on the physiology and biochemistry of crops at the seedling stage, with fewer studies on the gene regulatory mechanisms of crops at the germination stage. The aim of this study was to explain the mechanism of MT-induced salt tolerance at physiological, biochemical, and molecular levels and to provide a theoretical basis for the resolution of MT-mediated regulatory mechanisms of plant adaptation to salt stress. In this study, we investigated the germination, physiology, and transcript levels of maize seeds, analyzed the relevant differentially expressed genes (DEGs), and examined salt tolerance-related pathways. The results showed that MT could increase the seed germination rate by 14.28-19.04%, improve seed antioxidant enzyme activities (average increase of 11.61%), and reduce reactive oxygen species accumulation and membrane oxidative damage. In addition, MT was involved in regulating the changes of endogenous hormones during the germination of maize seeds under salt stress. Transcriptome results showed that MT affected the activity of antioxidant enzymes, response to stress, and seed germination-related genes in maize seeds under salt stress and regulated the expression of genes related to starch and sucrose metabolism and phytohormone signal transduction pathways. Taken together, the results indicate that exogenous MT can affect the expression of stress response-related genes in salt-stressed maize seeds, enhance the antioxidant capacity of the seeds, reduce the damage induced by salt stress, and thus promote the germination of maize seeds under salt stress. The results provide a theoretical basis for the MT-mediated regulatory mechanism of plant adaptation to salt stress and screen potential candidate genes for molecular breeding of salt-tolerant maize.