Integrative Analysis of the Coloring Mechanism of Red Longan Pericarp through Metabolome and Transcriptome Analyses

The pericarp of longan (<i>Dimocarpus longan</i> Lour.) is rich in secondary metabolites and typically yellow-brown or gray-yellow in appearance. Here, we obtained a specific longan type, called red pericarp (RP) longan, which has a strong red pericarp. To understand the coloring mechanism of RP longan, metabolome and transcriptome data were used to analyze its secondary metabolites and molecular mechanism. From the results of liquid chromatography tandem mass spectrometry, 597 substances were identified in RP longan and 'Shixia' (SX) longan. Among these substances, 33 (mostly including flavonoids) were found in RP longan and 23 (mostly containing phenolic acids) were identified in SX longan. We identified five types of anthocyanins in longan pericarp, including three cyanidin derivatives, one delphinidin derivative, and one pelargonidin derivative. Three cyanidin derivatives, which contained cyanidin 3-<i>O</i>-glucoside, cyanidin 3-<i>O</i>-6″-malonyl-glucoside, and cyanidin <i>O</i>-syringic acid, were the primary components of anthocyanidins, and they only existed in RP longan. Delphinin 3-<i>O</i>-glucoside existed only in SX longan, and pelargonin <i>O</i>-rutinoside existed in RP and SX longan. However, their contents were extremely low. The structural genes <i>F3H</i>, <i>F3'H</i>, <i>UFGT</i>, and <i>GST</i> and the controlling genes containing <i>MYB</i>, <i>bHLH</i>, <i>NAC</i>, and <i>MADS</i> in the biosynthetic pathway of anthocyanin were significantly upregulated in RP longan. In summary, the strong red hue of RP longan is due to the accumulation of cyanidin derivatives in its pericarp, and the genes <i>F3'H</i> and <i>F3'5'H</i> may play an important role in selecting which component of anthocyanins will be synthesized. These results can provide scientific guidance for understanding and developing bioactive compounds from longan fruits.