Exploration of the biosynthesis of galloylated catechins in tea plants

Galloylated flavan-3-ols (galloylated catechins) are not only the functional components that are beneficial to human health but also the characteristic components that endow tea flavor. Tea is an important source for people to ingest galloylated catechins. The galloylated catechin contains the basic structure of flavan-3-ol and a galloyl group. Thus, their biosynthetic pathway involves flavan-3-ol biosynthesis and galloylation and degalloylation reactions. The biosynthetic pathway of flavan-3-ol and its regulation have been studied intensively. Regarding the source of galloyl groups, we propose that there exists a 'galloylation-degalloylation cycle (G-DG)' pathway in tannin-rich plants, which consists of three catalytic steps controlled by gallic acid glucosyltransferase (UGT84A22), serine carboxypeptidyase-like acyltransferase (SCPL-AT), and tannase (TA). It is not only the main terminal pathway for galloylated catechin biosynthesis, but also the main initiation pathway for hydrolyzable tannin biosynthesis. In this paper, we review the research progress on the biosynthesis and regulation of the flavan-3-ol, and G-DG pathways.