CnHY5 Represses CnSNAT1 Transcription to Regulate Melatonin Biosynthesis in Coconut (Cocos nucifera L.)

Melatonin is a vital bioactive molecule with antioxidant and anti-aging effects, contributing to human health. Coconut (Cocos nucifera L.) is a potential dietary source of melatonin, while the regulatory mechanism of melatonin biosynthesis remains poorly understood. The melatonin variation in coconut flesh during fruit development was identified, i.e., the melatonin content significantly increased in coconut flesh from the commercial maturity stage to the physiological maturity stage, and a similar increasing trend was observed in the expression of CnSNAT1, encoding a melatonin biosynthetic enzyme. Evidence showed that CnHY5 was an upstream regulator of CnSNAT1. CnHY5 is localized in the nucleus and directly binds to the G-box motif of CnSNAT1 promoter. This binding repressed CnSNAT1 transcription and reduced melatonin production. The inhibitory effects of CnHY5 on CnSNAT1 expression and melatonin accumulation were supported in transgenic Arabidopsis plants and apple calli overexpressing CnHY5. Collectively, we identified a CnHY5-CnSNAT1 transcriptional regulatory pathway involved in melatonin biosynthesis during coconut fruit maturation. Our results also showed that this regulatory pathway was probably conserved across the species in plants. These findings provide new insights into the transcriptional regulation of melatonin biosynthesis in tropical crops and offer a potential molecular basis for the development of melatonin-enriched coconut cultivars.