Metabolomic and transcriptomic analysis reveal high solar irradiance inhibited the melanin formation in persimmon fruit peel

During fruit development of Diospyros kaki cv. 'Heishi', the peel color changed to orange-red when undergone high solar irradiance, which was different from turning black in normal condition. Limited information on this phenomenon of persimmon fruit has been reported. In the present study, we explored 'Heishi' persimmon fruit under two conditions of extreme solar irradiance during development. Their peel characteristics were evaluated, including organelle ultra-structure, polyphenol oxidase (PPO) activity, transcriptome profile, and metabolomics. The results showed that high solar irradiance severely degraded or ruptured chloroplast, and most of the analyzed metabolites were higher accumulated in high solar irradiance stressed peel than in the normal tissues. Significantly accumulated phenolics and carotenoids may contribute to photoprotection by scavenging high solar irradiance stress triggered ROS products. RNA-Seq analysis showed that many differentially expressed genes (DEGs) related to the accumulation of the analyzed metabolites were up-regulated under stress. The stress also significantly inhibited PPO activity at stages of melanin formation. The reduced melanin and significantly accumulated carotenoids (especially lycopene and beta-carotene) finally resulted in orange-red fruit peel.