Exogenous melatonin priming promotes seed germination by enhancing reserve mobilization and hormone metabolism of late sown wheat

Low temperature (LT) during germination of late sown wheat can severely inhibit germination and seedling establishment, and it's important to explore approaches to improve LT tolerance in wheat germination. Yangmai16 (LT tolerant) and Xumai33 (LT sensitive) wheat cultivars were primed with 100 mu mol/L melatonin (MT) to investigate the effects of exogenous MT on hormone and substance metabolism during seed germination under LT stress. LT delayed the germination time of both cultivars and reduced the vigor index (VI), but these changes were more pronounced in Xumai33 than in Yangmai16. MT improved VI of seeds of different cultivars, reduced germination time, and promoted radicle and coleoptile growth under the LT treatment. MT improved the amylase activity, soluble sugar, and free amino acid contents and up-regulated the relative expression of GA synthesis genes (TaGA20ox1 and TaGA3ox2) and ABA catabolic genes (TaCYP707A1 and TaCYP707A2) while down-regulated the relative expression of GA catabolic genes (TaGA2ox6 and TaGA2ox9) and ABA biosynthesis genes (TaNCED and TaABI3) leading to an increase in GA content and a decrease in ABA content to improve GA/ABA ratio under LT treatment. Moreover, Yangmai16 had stronger material mobilization and osmoregulation abilities than Xumai33 under LT stress. These results proved that MT priming could be a potential method to improve GA/ABA ratio, starch metabolism, and osmoregulation to enhance reserve mobilization and hormone metabolism to promote seed germination and seedling growth in wheat.