Transcriptome and metabolome profiling revealing anthocyanin and phenolic acid biosynthetic mechanisms in sweet osmanthus pericarp

During the maturation process of sweet osmanthus fruit, its pericarp color gradually changes from green to purple-black. Moreover, the chemical composition changes in the pericarp of sweet osmanthus fruit during its color change; however, the mechanisms by which the major medicinal chemicals and anthocyanins are produced remain unclear. This study adopts a metabolomic approach to analyze the chemical composition of green, light purple, and purple-black pericarps during the sweet osmanthus fruit maturation process. Phenolic acids and lignans were more diverse and higher in the green pericarp than in the purple-black pericarp, especially chlorogenic acid and phillygenin that were upregulated by 1.78-and 4.55-fold, respectively, in the green pencarp, whereas flavonoids and anthocyanins were higher in the purple-black pericarp than in the green pericarp, especially cyanidin-3-O-rutinoside and peonidin-3-O-rutinoside that were upregulated by 6.68- and 16.98-fold, respectively, in the purple-black pericarp. Thus, the purple-black color of the sweet osmanthus pericarp is primarily because of a high content of cyanidin-3-O-rutinoside. Transcriptome sequencing analysis showed that the expression level of genes related to the phenolic acid and lignan synthesis pathways, such as HCT, CCoAOMT, CCR, and F5H, demonstrated significant upregulation in the green pericarp, whereas genes such as CHS, CHI, F3H, DFR, and ANS involved in the flavonoid metabolism pathway were significantly downregulated in the green pericarp. These gene expression differences result in a higher content of phenolic acids and lignans in the green pericarp and a higher content of anthocyanins in the purple-black pericarp. The results of this study provide a theoretical basis for the development and utilization of medicinal components, such as chlomgenic acid, phillygenin, and flavonoids in the pericarp of sweet osmanthus fruit for medicinal, health, and food additive use.