Rice N-acetylserotonin deacetylase regulates melatonin levels in transgenic rice

A reverse melatonin biosynthetic pathway was recently discovered in plants, by which N-acetylserotonin (NAS) is converted into serotonin by N-acetylserotonin deacetylase (ASDAC) rather than into melatonin by N-acetylserotonin O-methyltransferase (ASMT). In this study, we generated transgenic rice plants in which ASDAC was either suppressed or overexpressed to determine whether ASDAC is functionally involved in melatonin biosynthesis. ASDAC-suppressed rice showed increased levels of NAS, 5-methoxytryptamine (5-MT), and melatonin, whereas ASDAC-overexpressed rice exhibited less melatonin synthesis than observed in the wild type. This finding is strong evidence of the role of ASDAC in melatonin biosynthesis in rice. The increased levels of 5-MT, which is produced either by ASDAC from melatonin or by serotonin O-methyltransferase (SOMT) from serotonin in ASDAC-suppressed rice, was ascribed to enhanced SOMT enzyme activity rather than increased transcripts, such as ASMT or caffeic acid O-methyltransferase (COMT) encoding SOMT activity.