Compared with other five traditional hormones, the identification of melatonin（MT）was done in only two decades. MT is an emerging plant hormone, which is a pleiotropic signaling molecule with multiple physiological functions in organisms. In plants, MT is called phytomelatonin, which is not only involved in the regulation of seed germination, root system architecture, stomatal movement, biological rhythm, flowering, and senescence, but also in the response of plants to biotic and abiotic stresses by activating the activities of antioxidant system, scavenging ROS, followed by reducing osmotic stress, oxidative stress, protein denaturation, and cell damage caused by biotic and abiotic stress, thus plants respond to biotic and abiotic stresses. Based on the latest research progress of MT metabolism in plant and its response to abiotic stress, the anabolism and catabolism of MT in plants are summarized. The plant resistance to high temperature, low temperature, drought, salt, heavy metals, ultraviolet radiation and waterlogging is improved ultimately under stress by MT directly removing ROS and/or triggering the signal transduction pathway, up-regulating resistance related gene expression, and then activating the vitality of osmoregulation system and antioxidant system, promoting the synthesis of stress protein and secondary metabolic substances, stable photosynthesis and carbon metabolism, and thus reducing the accumulation of ROS and cell oxidative damage. This paper lays the theoretical foundation for understanding the metabolism, physiological function, and cellular signal transduction of MT in plants, and point out the future research directions.
