Exogenous melatonin promotes salt tolerance in smooth bromegrass seedlings: physiological, transcriptomic, and metabolomic evidence

Soil salinization, which severely limits crop yield and quality, has become a global environmental and resource issue. Melatonin plays an important role in plant responses to salt stress. Smooth bromegrass is an important forage with excellent feed value and is widely grown in northern and north-west China for pasture and sand binding. However, the physiological and molecular mechanisms underlying exogenous melatonin regulation of salt stress in smooth bromegrass are not clear. This study compared the phenotype, physiological, transcriptome, and metabolome profiles of two varieties with contrasting salt tolerance attributes under salt and melatonin treatment. After melatonin treatment, the catalase (CAT) and ascorbate peroxidase (APX) activity, proline content, actual photochemical efficiency (Y(II)), relative water content, and fresh weight above ground were significantly higher than under salt treatment, while relative conductivity, H2O2 content, and Na+/K+ ratio were significantly lower than salt treatment. The transcriptome and metabolite profiling analysis of smooth bromegrass seedlings treated without melatonin under salt stress identified the presence of 22522 differentially expressed genes (DEGs) and 862 differentially expressed metabolites (DEMs) in SS, 17809 DEGs and 812 DEMs in ST, while treated with melatonin under salt stress identified the presence of 7033 DEGs and 177 DEMs in SS, 2951 DEGs and 545 DEMs in ST. Furthermore, in response to salt stress, melatonin may be involved in regulating the correlation between DEGs and DEMs in flavonoid biosynthesis, proline biosynthesis, and melatonin biosynthesis. Moreover, melatonin participated in mediating melatonin biosynthesis pathways and affected the expression of ASMT in response to salt stress.