**1. Introduction**

Melatonin, *N*-acetyl-5-methoxytryptamine, is an indole-like tryptamine. In 1958, Lerner et al. [1] extracted melatonin from the pineal gland of cattle and found that it was a kind hormone-like substance, which was widely involved in the growth and development regulation, as well as signal transduction, in animals. Before the 1990s, melatonin was recognized as an animal hormone. However, Balzer and Hardeland [2] successfully isolated and identified melatonin from single-celled algae, indicating the existence of natural melatonin in the plant kingdom. Since then, melatonin has become a research hotspot and received extensive attention from researchers in the field of plant and agricultural science. With the rapid development of material separation and identification technology, plant physiology and cell biology, molecular biology, sequencing technology and other research methods, scientists have conducted more comprehensive and in-depth studies on the synthesis pathway, concentration, distribution and biological function of melatonin in plants. Moreover, melatonin has a variety of regulatory effects in plants and has many benefits for human health. Therefore, the research on melatonin in horticultural crops is fast increasing, which is no less than that in model plants of *Arabidopsis thaliana*. Therefore, this review comprehensively and systematically introduced the research progress on melatonin and its function in horticultural

crops, and looked forward to the future research to provide some theoretical basis for the application of melatonin in horticultural crop production.
