Phytomelatonin: an emerging new hormone in plants

Melatonin (N-acetyl-5-methoxytryptamine) synthesized by plants is known as phytomelatonin. It was first detected by several independent research groups in 1995 (Dubbels et al., 1995; Hattori et al., 1995; Kolar et al., 1995; Van Tassel et al., 1995). The biosynthesis of melatonin in animals and plants begins from tryptophan via several similar consecutive enzymatic steps. The first phytomelatonin receptor, PMTR1, was identified in Arabidopsis in 2018 (Wei et al., 2018). Recently, several independent research groups have discovered that PMTR1 and its homologous proteins are required for perceiving phytomelatonin signaling in stomatal closure, seed germination and seedling growth, flowering, leaf senescence, and in responding to various biotic and abiotic stresses in Arabidopsis, tobacco, alfalfa, maize, and cassava (Bai et al., 2022; Chen et al., 2022). Phytomelatonin is therefore comparable to the more well-known plant hormones and regulates nearly all aspects of plant life history (Box 1).