LAUSR

Phytomelatonin is a new plant hormone, and its primary functions in plant growth and development remain relatively poorly appraised. Phytomelatonin is a master regulator of the reactive oxygen species (ROS) signaling and acts as a darkness signal in circadian stomatal closure. Plants exhibit at least three interrelated patterns of interactions between phytomelatonin and ROS production. Exogenous melatonin could induce flavonoid biosynthesis, which might be required for maintenance of antioxidant capacity under stress, after harvest and in leaf senescence conditions. However, several genetic studies provided direct evidence that phytomelatonin plays a negative role in the biosynthesis of flavonoids under normal growth conditions. Phytomelatonin delays flowering time in both dicot and monocot plants, probably via its receptor PMTR1 and interactions with the gibberellin (GA), strigolactone (SL) and ROS signaling pathways. Furthermore, phytomelatonin signaling also functions in hypocotyl and shoot growth in skotomorphogenesis and UV-B exposure; the G protein α-subunit (arabidopsis GPA1 and rice RGA1) and Constitutive Photomorphogenic1 (COP1) are important signal components during this process. Taken together, phytomelatonin acts as a darkness signal with important regulatory roles in circadian stomatal closure, flavonoid biosynthesis, flowering, and hypocotyl and shoot growth.