Cyclic 3-hydroxymelatonin exhibits diurnal rhythm and cyclic 3-hydroxymelatonin overproduction increases secondary tillers in rice by upregulating MOC1 expression

Cyclic 3-hydroxymelatonin (c3OHM) is a major metabolite of melatonin in plants produced by the enzymatic action of melatonin 3-hydroxylase (M3H). However, the function of c3OHM in plants is unclear. Here, we report that M3H mRNA and c3OHM levels display diurnal rhythms with peaks at night, but not in a circadian manner. This diurnal rhythmicity occurred predominantly in the late vegetative growth stage (8 weeks after germination), but was absent in the early vegetative growth stage. Transgenic rice plants overexpressing or underexpressing M3H were generated to investigate the physiological roles of diurnal production of c3OHM. The M3H-overexpression (OE) line exhibited higher M3H activity and c3OHM production than the wild-type, and vice versa for the M3H‑underexpression rice (RNAi). The seedling growth phenotype of the OE and RNAi lines was comparable to that of the wild-type but exhibited pleiotropic phenotypic defects at the reproductive stage, such as decreased height, biomass, grain yield, and fertility. Of note, the OE rice showed significantly increased numbers of secondary tillers and panicles. The increase in tiller number of the OE line was linked to increased expression of tiller-related genes, such as MOC1 and TB1, suggesting that the diurnal rhythm of c3OHM production is associated with the tiller number, a pivotal agronomic trait governing grain yield in rice.