Chloroplastic and cytoplasmic overexpression of sheep serotonin N-acetyltransferase in transgenic rice plants is associated with low melatonin production despite high enzyme activity.

Serotonin N-acetyltransferase ( SNAT), the penultimate enzyme in melatonin biosynthesis, catalyzes the conversion of serotonin into N-acetylserotonin. Plant SNAT is localized in chloroplasts. To test SNAT localization effects on melatonin synthesis, we generated transgenic rice plants overexpressing a sheep ( Ovis aries) SNAT ( Oa SNAT) in their chloroplasts and compared melatonin biosynthesis with that of transgenic rice plants overexpressing Oa SNAT in their cytoplasm. To localize the Oa SNAT in chloroplasts, we used a chloroplast targeting sequence ( CTS) from tobacco protoporphyrinogen IX oxidase ( PPO), which expresses in chloroplasts. The purified recombinant CTS: Oa SNAT fusion protein was enzymatically functional and localized in chloroplasts as confirmed by confocal microscopic analysis. The chloroplast-targeted CTS: Oa SNAT lines and cytoplasm-expressed Oa SNAT lines had similarly high SNAT enzyme activities. However, after cadmium and butafenacil treatments, melatonin production in rice leaves was severalfold lower in the CTS: Oa SNAT lines than in the Oa SNAT lines. Notably, enhanced SNAT enzyme activity was not directly proportional to the production of N-acetylserotonin, melatonin, or 2-hydroxymelatonin, suggesting that plant SNAT has a role in the homeostatic regulation of melatonin rather than in accelerating melatonin synthesis. [ABSTRACT FROM AUTHOR]