Melatonin is Involved in Skotomorphogenesis by Regulating Brassinosteroid Biosynthesis in Rice Plants

Serotonin N-acetyltransferase (SNAT) is the penultimate enzyme in melatonin biosynthesis catalyzing the conversion of serotonin into N-acetylserotonin. In plants, SNAT is encoded by 2 isogenes of which SNAT1 is constitutively expressed and its overexpression confers increased yield in rice. However, the role of SNAT2 remains to be clarified. In contrast to SNAT1, the diurnal rhythm of SNAT2 mRNA expression peaks at night. In this study, transgenic rice plants in which SNAT2 expression were suppressed by RNAi technology showed a decrease in melatonin and a dwarf phenotype with erect leaves, reminiscent of brassinosteroids (BR)-deficient mutants. Of note, the dwarf phenotype was dependent on the presence of dark, suggesting that melatonin is involved in dark growth (skotomorphogenesis). In support of this suggestion, SNAT2 RNAi lines exhibited photomorphogenic phenotypes such as inhibition of internodes and increased expression of light-inducible CAB genes in the dark. The causative gene for the melatonin-mediated BR biosynthetic gene was DWARF4, a rate-limiting BR biosynthetic gene. Exogenous melatonin treatment induced several BR biosynthetic genes, including DWARF4, D11, and RAVL1. As expected from the erect leaves, the SNAT2 RNAi lines produced less BR than the wild type. Our results show for the first time that melatonin is a positive regulator of dark growth or shade outgrowth by regulating BR biosynthesis in plants.