A laccase gene (LcLac) was involved in polyphenol metabolism and tissue browning of litchi callus

Abstract

Postharvest pericarp browning restricts the shelf life of litchi fruit and was considered as the main bottleneck of litchi industry. Browning is mainly due to the enzymatic catalysis of phenols to form brown substances in the litchi pericarp. (-)-Epicatechin, (-)-gallocatechin, and procyanidin B2 are the main substrates for enzyme catalysis. Previous works have reported that laccase activity was correlated with pericarp browning, and laccase polymerizes substrates in vitro, but the direct evidence was absent. In this research, the sequence of laccase gene LcLac was analyzed, whose expression was correlated with browning index of litchi pericarp at postharvest. Overexpression of LcLac in callus led to higher gene expression and enzymatic activity, and a lower polyphenol content, which contributed to browning of callus. Contrarily, the LcLac gene blocked by CRISPR/Cas9 resulted in an opposite trend of polyphenol contents. LcLac overexpression promoted callus browning, and the gene target block delayed the browning phenomenon. The results clearly indicate that LcLac can be polymerized with polyphenols and is involved in callus browning. LcLac may also participate in polyphenol synthesis and maintain the polyphenol balance in vacuoles.