Suppression of Melatonin 2-Hydroxylase Increases Melatonin Production Leading to the Enhanced Abiotic Stress Tolerance against Cadmium, Senescence, Salt, and Tunicamycin in Rice Plants.

Melatonin 2-hydroxylase (M2H) catalyzes the conversion of melatonin into 2hydroxymelatonin (2OHM), which is present in plants at a higher concentration than melatonin. Although has been cloned, the in vivo function of its product is unknown. Here, we generated stable T homozygous transgenic rice plants in which expression of endogenous was suppressed (RNAi lines). However, we failed to generate overexpression transgenic rice due to failure of somatic embryogenesis. The transcript level showed a diurnal rhythm with a peak at night concomitantly with the peak concentration of 2OHM. RNAi rice showed a reduced mRNA level and 2OHM and melatonin concentrations. The unexpected decrease in the melatonin concentration was caused by redirection of melatonin into cyclic 3hydroxymelatonin via a detour catabolic pathway. Thus, the decrease in the melatonin concentration in RNAi rice led to slowed seedling growth and delayed germination. By contrast, the transient increase in the melatonin concentration was of greater magnitude in the RNAi than the wild-type rice upon cadmium treatment due to possible suppression of melatonin degradation. Due to its higher concentration of melatonin, the RNAi rice displayed tolerance to senescence, salt, and tunicamycin stresses. Therefore, the increase in the melatonin concentration caused by suppression of melatonin degradation or by overexpression of melatonin biosynthetic genes enhances stress tolerance in rice.