Cadmium-induced melatonin synthesis in rice requires light, hydrogen peroxide, and nitric oxide: Key regulatory roles for tryptophan decarboxylase and caffeic acid O-methyltransferase

In plants, melatonin production is induced by stimuli such as cold and drought, and cadmium (Cd) is the best elicitor of melatonin production in rice. However, the mechanism by which Cd induces melatonin synthesis in plants remains unknown. We challenged rice seedlings with Cd under different light conditions and found that continuous light produced the highest levels of melatonin, while continuous dark failed to induce melatonin production. Transcriptional and translational induction of tryptophan decarboxylase contributed to the light induction of melatonin during Cd treatment, whereas the protein level of light-induced caffeic acid O-methyltransferase (COMT) was decreased by Cd treatment. In analogy, COMT enzyme activity was inhibited in vitro by Cd in a dose-dependent manner. Notably, the Cd-induced melatonin synthesis was significantly impaired by treatment with either an H₂ O₂ production inhibitor (DPI) or an NO scavenger (cPTIO). The combination of both inhibitors almost completely abolished Cd-induced melatonin synthesis, suggesting an absolute requirement for H₂ O₂ and NO. However, neither serotonin nor N-acetylserotonin (NAS) was induced by H₂ O₂ alone. In contrast, NO significantly induced serotonin production but not NAS or melatonin production. This indicated that serotonin did not enter chloroplasts, where serotonin N-acetyltransferase (SNAT) is constitutively expressed. This suggests that chloroplastidic SNAT expression prevents increased melatonin production after exposure to stress, ultimately leading to the maintenance of a steady-state melatonin level inside cells.