Melatonin: Biosynthesis, content, and function in horticultural plants and potential application

Derived from tryptophan, melatonin is a nontoxic indolic molecule that is widely identified in species ranging from bacteria to mammals. It plays vital roles in the circadian rhythm, antioxidant activity and immunological enhancement in animals and in photosynthesis, biomass production, osmoregulation during the response to abiotic and biotic stress in plants. Horticultural plants are consideredto be among the most important plants, providing people with fruits, vegetables, beverages, medicinal herbs and ornamentation. Since the presence of melatonin was first observed in horticulture plant Japanese morning glory (Pharbitis nil) in 1993, melatonin levels, synthetic pathways and roles during horticulture plant growth and postharvest storage have attracted increasing attention in recent 20 years. In this review, our aim is to provide an overview of the multiple pathways for the synthesis of melatonin from tryptophan in plants, summarize the levels of melatonin and the influencing factors in horticultural plants, highlight the role of melatonin on the plant growth and development, postharvest quality, and resistance to biotic or abiotic stresses and propose the potential application of melatonin in nutrition and agriculture.