Melatonin mediates the regulation of ABA metabolism, free-radical scavenging, and stomatal behaviour in two Malus species under drought stress

Melatonin pre-treatment significantly increases the tolerance of both drought-tolerant Malus prunifolia and drought-sensitive M. hupehensis plants. Its beneficial effects include better water conservation in leaves, less electrolyte leakage, steady chlorophyll contents, and greater photosynthetic performance under stress conditions. Melatonin selectively down-regulates MdNCED3, an abscisic acid (ABA) synthesis gene, and up-regulates its catabolic genes, MdCYP707A1 and MdCYP707A2, thereby reducing ABA contents in drought-stressed plants. Melatonin also directly scavenges H2O2 and enhances the activities of antioxidant enzymes to detoxify H2O2 indirectly. These two mechanisms work synergistically to improve the functions of stomata, i.e. causing them to re-open. Plants can effectively regulate their water balance under drought conditions by up-regulating the expression of melatonin synthesis genes MdTDC1, MdAANAT2, MdT5H4, and MdASMT1. Therefore, inducing melatonin production is an important mechanism by which plants can counteract the influence of this abiotic stressor.