Functional Characterization of the Ciliate Serotonin -Acetyltransferase, a Pivotal Enzyme in Melatonin Biosynthesis and Its Overexpression Leads to Peroxidizing Herbicide Tolerance in Rice

Serotonin -acetyltransferase (SNAT) is a pivotal enzyme for melatonin biosynthesis in all living organisms. It catalyzes the conversion of serotonin to -acetylserotonin (NAS) or 5-methoxytrypytamine (5-MT) to melatonin. In contrast to animal- and plant-specific genes, a novel clade of archaeal genes has recently been reported. In this study, we identified homologues of archaeal genes in ciliates and dinoflagellates, but no animal- or plant-specific homologues. Archaeal homologue from the ciliate was annotated as a putative -acetyltransferase. To determine whether the putative () exhibits SNAT enzyme activity, we chemically synthesized and expressed the full-length coding sequence (CDS) in , from which the recombinant SlSNAT protein was purified by Ni affinity column chromatography. The recombinant SlSNAT exhibited SNAT enzyme activity toward serotonin ( = 776 µM) and 5-MT ( = 246 µM) as substrates. Furthermore, -overexpressing (SlSNAT-OE) transgenic rice plants showed higher levels of melatonin synthesis than wild-type controls. The SlSNAT-OE rice plants exhibited delayed leaf senescence and tolerance against treatment with the reactive oxygen species (ROS)-inducing herbicide butafenacil by decreasing hydrogen peroxide (HO) and malondialdehyde (MDA) levels, suggesting that melatonin alleviates ROS production in vivo.