Tryptamine 5‐hydroxylase‐deficient Sekiguchi rice induces synthesis of 5‐hydroxytryptophan and N‐acetyltryptamine but decreases melatonin biosynthesis during senescence process of detached leaves

Abstract: Melatonin biosynthesis was examined in Sekiguchi mutant rice lacking functional tryptamine 5‐hydroxylase (T5H) activity, which is the terminal enzyme for serotonin biosynthesis in rice. During senescence process, the leaves of Sekiguchi mutant rice produced more tryptamine and N ‐acetyltryptamine compared with the wild‐type Asahi leaves. Even though T5H activity is absent, Sekiguchi leaves produce low levels of serotonin derived from 5‐hydroxytryptophan, which was found to be synthesized during senescence process. Accordingly, both rice cultivars exhibited similar levels of N ‐acetylserotonin until 6 days of senescence induction; however, only Asahi leaves continued to accumulate N ‐acetylserotonin after 6 days. In contrast, a large amount of N ‐acetyltryptamine was accumulated in Sekiguchi leaves, indicating that tryptamine was efficiently utilized as substrate by the rice arylalkylamine N ‐acetyltransferase enzyme. An increase in N ‐acetyltryptamine in Sekiguchi had an inhibitory effect on synthesis of melatonin because little melatonin was produced in Sekiguchi leaves at 6 days of senescence induction, even in the presence of equivalent levels of N ‐acetylserotonin in both cultivars. The exogenous treatment of 0.1 m m N ‐acetyltryptamine during senescence process completely blocked melatonin synthesis.