Melatonin involves in improving tolerance to abiotic and biotic stresses by regulating various biological processes. However, little is known about the underlying mechanism. Here, we investigated the effects of exogenous melatonin on seed germination in the halophyte Limonium bicolor under salt stress. Specifically, we examined the effect of salt stress on seed germination, melatonin concentration, and changes in the concentrations of nutrients, amylase activity, and hormones in L. bicolor seeds with and without pre-treatment with melatonin. Seed germination was significantly suppressed under a 200 mM NaCl treatment, but pre-treatment with melatonin significantly improved seed germination under salt stress. During seed germination, seeds pre-treated with melatonin contained high levels of melatonin and gibberellic acid (GA), low levels of abscisic acid (ABA), and high levels of amylase and alpha-amylase activity. Melatonin treatment upregulated the expression of key genes involved in GA biosynthesis (GA20ox and GA3ox), downregulated key genes involved in ABA biosynthesis (LbNCED1 and LbNCED3), and upregulated ABA 8'-hydroxylase genes (LbCYP707A1 and LbCYP707A2), which mediate the changes in GA and ABA levels in seeds during germination. A high melatonin concentration in seeds promotes the utilization of nutrients and the synthesis of new proteins to enhance seed germination.
Keywords: Limonium bicolor; germination; hormone; melatonin; salinity.