Light-regulated melatonin biosynthesis in rice during the senescence process in detached leaves

The effect of light on melatonin biosynthesis was examined in detached rice (Oryza sativa cv. Asahi) leaves during the senescence process. The detached leaves were exposed to senescence treatment either in constant darkness or in constant light, and subjected to HPLC analysis for melatonin and its precursors. Higher melatonin levels were detected in rice leaves under constant light while very low levels were observed in constant darkness. Levels of the melatonin intermediates, tryptamine, serotonin, and N-acetylserotonin significantly decreased in the dark compared to those in the light. Furthermore, relative mRNA levels of melatonin biosynthetic genes and their corresponding proteins decreased accordingly in constant darkness. The most striking difference between constant light and dark was observed in levels of the protein tryptamine 5-hydroxylase. These results suggest that melatonin biosynthesis during senescence is dependent on light signals in rice leaves, contrary to the response found in animals.