Abstract
Caffeic acid O-methyltransferase (COMT) methylates N-acetylserotonin into melatonin; that is, it has N-acetylserotonin O-methyltransferase (ASMT) activity. The ASMT activity of COMT was first detected in Arabidopsis thaliana COMT (AtCOMT). To confirm the involvement of COMT on melatonin synthesis in other plant species, the ASMT activity of a COMT from rice (Oryza sativa) (OsCOMT) was evaluated. Purified recombinant OsCOMT protein from Escherichia coli was used to validate the high ASMT activity of OsCOMT, similar to that of AtCOMT. The K m and V max values for the ASMT activity of OsCOMT were 243 µM and 2400 pmol min(-1) mg protein(-1), which were similar to those of AtCOMT. Similar to AtCOMT, OsCOMT was localized in the cytoplasm. In vitro ASMT activity was significantly inhibited by either caffeic acid or quercetin in a dose-dependent manner. Analogously, in vivo production of melatonin was significantly inhibited by quercetin in 4-week-old detached rice leaves. Lastly, the transgenic rice plants overexpressing rice COMT showed an increase in melatonin levels whereas transgenic rice plants suppressing the rice COMT had a significant decrease on melatonin levels, suggestive of the direct role of COMT in melatonin biosynthesis in plants.
Highlights
IntroductionMelatonin (N-acetyl-5-methoxytryptamine) has been characterized as an important bioactive molecule that is implicated in animal hormones, and in plant growth and development (Janas and Posmyk, 2013; Arnao and Hernández-Ruiz, 2014; Cipolla-Neto et al, 2014; García et al, 2014; Janjetovic et al, 2014; Luchetti et al, 2014; Hardeland, 2015; Li et al, 2015; Zhang et al, 2015; Zhao et al, 2015)
The key genes include tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), and N-acetylserotonin O-methyltransferase (ASMT)
The conversion of NAS into melatonin is the final step in melatonin biosynthesis and is catalysed by proteins with ASMT enzyme activity
Summary
Melatonin (N-acetyl-5-methoxytryptamine) has been characterized as an important bioactive molecule that is implicated in animal hormones, and in plant growth and development (Janas and Posmyk, 2013; Arnao and Hernández-Ruiz, 2014; Cipolla-Neto et al, 2014; García et al, 2014; Janjetovic et al, 2014; Luchetti et al, 2014; Hardeland, 2015; Li et al, 2015; Zhang et al, 2015; Zhao et al, 2015). The key genes include tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), and N-acetylserotonin O-methyltransferase (ASMT). Among these four genes, SNAT and ASMT play pivotal roles in regulating the relative melatonin levels in plants, because their catalytic activities are very low compared with those of TDC and T5H (Kang et al, 2007; Park et al, 2013a; Park et al, 2013b; Byeon et al, 2014b; Lee et al, 2014). Due to the low ASMT catalytic activity, transgenic rice plants overexpressing three rice ASMT genes showed marginal increases in melatonin levels, even in the exogenous treatment of 1 mM N-acetylserotonin (Park et al, 2013a)
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