Transcriptomic, proteomic and LC-MS analyses reveal anthocyanin biosynthesis during litchi pericarp browning

Anthocyanin is a widely distributed flavonoid pigment that confers plant red color. The biosynthesis of anthocyanin associated with onset of color in developing plants has been well documented. However, whether anthocyanin biosynthesis pathway still functions in fruit at senescent stage remains unclear. In this study we investigated the metabolism of anthocyanin in litchi, a red fruit whose color fades fast after harvest. We showed as the pericarp browning index increased, the total anthocyanin contents decreased in litchi during storage at 20 °C. RNA-seq and proteomics analysis identified 4769 differentially expressed genes (DEG) and 84 differentially expressed proteins (DEP), respectively, in stored fruit compared with that before storage. Among them, 4 DEGs and 1 DEP involved in anthocyanin degradation, and 10 DEGs and 2 DEPs in anthocyanin biosynthesis pathway were upregulated. In particular, chalcone synthase (CHS), the first enzyme involved anthocyanin biosynthesis, and peroxidase (POD), a major enzyme for anthocyanin degradation, showed increased accumulation at both transcript and protein levels. LC-MS analysis showed that the accumulation of five anthocyanins and derivatives were decreased, while one anthocyanin increased in litchi stored for 6 d. These results suggest that anthocyanin degradation is accompanied by biosynthesis during pericarp browning, and that the browning of litchi resulted from more degradation than biosynthesis. The work provides new insight into mechanism underlying litchi browning during storage and possibilities for developing new browning prevention techniques. Additionally, from the evolutional perspective, this finding might imply that plants at senescent stage still try to preserve and disseminate the seeds against stresses.