Zinc Treatment of Tea Plants Improves the Synthesis of Trihydroxylated Catechins via Regulation of the Zinc-Sensitive Protein CsHIPP3

The tea plant (Camellia sinensis [L.] O. Kussntze) is a global economic crop. Zinc treatment of tea plants can enhance catechin biosynthesis. However, the underlying molecular mechanism behind catechin formation through zinc regulation remains unclear. This study identified a zinc-responsive protein, C. sinensis heavy metal-associated isoprenylated plant protein 3 (CsHIPP3), from zinc-treated tea seedlings. CsHIPP3 expression was positively correlated with trihydroxylated catechin (TRIC) content. CsF3′5′H1 is a crucial regulator of the TRIC synthesis pathway. The interaction between CsHIPP3 and CsF3′5′H1 was assessed using bimolecular fluorescence complementation, firefly luciferase complementation imaging, and pulldown experiments. CsHIPP3 knockdown using virus-induced gene silencing technology decreased the content of each component of TRICs. Compared with the control, the relative catechin content was reduced by 40.12–55.39%. Co-overexpression of CsHIPP3 and CsF3′5′H1 significantly elevated the TRIC content in tea leaves and calli. Moreover, the TRIC content in transient co-overexpression leaves was 1.44-fold higher than that of the control group, and tea callus was 50.83% higher in transient co-overexpression than in the wild type. Thus, zinc-regulated TRIC synthesis in a zinc-rich environment was mediated by binding CsHIPP3 with CsF3′5′H1 to promote TRIC synthesis and accumulation.