Transcriptomic and Metabolomic Analyses Reveal That Proanthocyanidin Treatment Delays Pericarp Browning of Litchi Fruit

Proanthocyanidins (PAs) display antioxidant properties and can partially delay pericarp browning in postharvested litchi (<i>Litchi chinensis</i> Sonn.) fruits. In this study, litchi fruit was treated with 3 g/L PA and then stored at 25 °C for 10 days. Differential metabolites and differentially expressed genes were comprehensively analyzed in litchi pericarp across four stages of browning progression. A total of 630 metabolites were identified through untargeted metabolomic analysis, with 237 of them being annotated in the kyoto encyclopedia of genes and genomes (KEGG) database. The abundance of flavonoids (kaempferol), six types of anthocyanins, cyanidins, (-)-epicatechin, and (+)-catechin in the PA biosynthetic pathway was notably reduced in PA-treated pericarp compared to that of the control fruit. Transcriptomic analysis also revealed that five genes in the phenylpropanoid biosynthesis pathways and seven genes of phenoloxidase were significantly downregulated. In addition, correlation analysis revealed significant positive correlations between the browning index and genes like <i>LcFLS</i> (flavonol synthase), <i>LcANS</i> (anthocyanidin synthase), <i>LcUFGT</i> (UDP-glucose:anthocyanidin 3-<i>O</i>-glucosyltransferase), <i>LcPPO</i> (polyphenol oxidase), and <i>LcLAC7</i> (laccase), and a significant negative correlation with PAs and (-)-epicatechin. In conclusion, PA treatment can efficiently delay pericarp browning, mainly by reducing brown substrate accumulation and oxidation, and decreasing PA oxidation and polymerization. This study delves into the regulatory mechanisms through which PA delays litchi pericarp browning.