Key genes in a "Galloylation-Degalloylation cycle" controlling the synthesis of hydrolyzable tannins in strawberry plants

Abstract

Strawberry fruits, known for their excellent taste and potential health benefits, are particularly valued for their rich content of hydrolyzable tannins (HTs). These compounds play key roles in regulating growth and development. However, the molecular mechanisms underlying HT synthesis in plants remains poorly elucidated. In this study, based on a correlation analysis between the transcriptome and metabolome of HTs, galloyl glucosyltransferase (UGT84A22), serine carboxypeptidase-like acyltransferases (SCPL-ATs), and carboxylesterases (CXEs) were screened. Furthermore, in vitro enzymatic assays confirmed that FaSCPL3-1 acted as a hydrolyzable tannins synthase (HTS), catalyzing the continuous galloylation of glucose to form simple gallotannins (GTs). Additionally, FaCXE1/FaCXE3/FaCXE7 catalyzed the degalloylation of simple GTs and ellagitannins (ETs), and FaUGT84A22 catalyzed the glycosylation of gallic acid (GA) to produce 1-O-β-glucogallin (βG), a galloyl donor. Moreover, in FvSCPL3-1-RNAi transgenic strawberry plants, the contents of simple GT and some ET compounds were reduced, whereas, in FaCXE7 overexpressing strawberry plants, these compounds were increased. These enzymes constituted a biosynthetic pathway of galloyl derivatives, termed the "galloylation-degalloylation cycle"(G-DG cycle). Notably, the overexpression of FaCXE7 in strawberry plants not only promoted HT synthesis but also interfered with plant growth and development by reducing lignin biosynthesis. These findings offer new insights into the mechanisms of HT accumulation in plants, contributing to improving the quality of berry fruits quality and enhancing plant resistance.