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 H₂O₂ 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 H₂O₂ and NO. However, neither serotonin nor N‐acetylserotonin (NAS) was induced by H₂O₂ 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.

中文翻译：

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水稻中镉诱导的褪黑素合成需要光，过氧化氢和一氧化氮：色氨酸脱羧酶和咖啡酸O-甲基转移酶的关键调控作用

在植物中，褪黑素的产生是由寒冷和干旱等刺激引起的，镉（Cd）是水稻中产生褪黑素的最佳诱因。然而，Cd诱导植物褪黑激素合成的机制仍然未知。我们在不同的光照条件下用Cd挑战水稻幼苗，发现连续光照产生最高水平的褪黑激素，而连续黑暗则无法诱导褪黑激素的产生。色氨酸脱羧酶的转录和翻译诱导在镉处理过程中对褪黑激素的光诱导作出了贡献，而光诱导的咖啡酸O的蛋白质水平镉处理可使甲基转移酶（COMT）降低。类似地，镉在体外以剂量依赖的方式抑制了COMT酶的活性。值得注意的是，用H ₂ O ₂生成抑制剂（DPI）或NO清除剂（cPTIO）处理可大大损害Cd诱导的褪黑激素合成。两种抑制剂的组合几乎完全消除了Cd诱导的褪黑激素合成，表明对H ₂ O ₂和NO的绝对需要。然而，H ₂ O ₂既不诱导血清素也不诱导N-乙酰5-羟色胺（NAS）。独自的。相反，NO显着诱导5-羟色胺生成，但不诱导NAS或褪黑激素生成。这表明5-羟色胺没有进入叶绿体，而5-羟色胺N-乙酰基转移酶（SNAT）是组成型表达的。这表明叶绿体SNAT表达可防止暴露于压力下的褪黑激素产生增加，最终导致维持细胞内稳态的褪黑激素水平。