Abstract
Melatonin is a ubiquitous regulator in plants and performs a variety of physiological roles, including resistance to abiotic stress, regulation of growth and development, and enhancement of plant immunity. Melatonin exhibits the characteristics of a phytohormone with its pleiotropic effects, biosynthesis, conjugation, catabolism, effective concentration, and the shape and location of its dose-response curves. In addition, CAND2/PMTR1, a phytomelatonin receptor candidate belonging to the G protein-coupled receptors (GPCRs), supports the concept of melatonin as a phytohormone. However, the biochemistry of plant melatonin receptors needs to be further characterized. In particular, some of the experimental findings to date cannot be explained by known GPCR signaling mechanisms, so further studies are needed to explore the possibility of novel signaling mechanisms.