Study Unveils the Complete Biosynthetic Loop Regulating Tannin Production in Strawberries

Hydrolyzable tannins (HTs), including gallotannins and ellagitannins, are essential polyphenolic compounds that influence fruit taste, antioxidant capacity, and stress tolerance. Although condensed tannin biosynthesis has been well studied, the enzymatic processes governing HT formation remain unclear. Previous research suggested that plant esterases and acyltransferases may participate in galloylation and degalloylation, but the genes involved were unidentified. Understanding these pathways is vital, as HT metabolism is closely linked to carbohydrate distribution and secondary metabolism. Due to these challenges, a comprehensive study on the genes and mechanisms controlling tannin biosynthesis in strawberries was urgently needed.

A research team from Anhui Agricultural University published a paper (DOI: 10.1093/hr/uhae350) in Horticulture Research on December 16, 2024. The study unveiled the complete biosynthetic loop regulating tannin production in strawberries, integrating transcriptomic, metabolomic, and enzymatic analyses. By identifying critical genes in the G-DG cycle, the researchers demonstrated how metabolic recycling of galloyl groups drives tannin accumulation and affects neighboring pathways, offering new strategies for improving fruit quality and plant resilience.

Using a combined transcriptome–metabolome approach, the researchers correlated tannin accumulation with the expression of UGT84A22, SCPL-AT, and CXE gene families. They found that FaUGT84A22-1 catalyzes the glycosylation of gallic acid to produce 1-O-β-glucogallin (βG), the galloyl donor; FaSCPL3-1 continuously attaches galloyl groups to glucose, forming gallotannins such as pentagalloylglucose (PGG); and FaCXE1/FaCXE3/FaCXE7 hydrolyze PGG and ellagitannins to regenerate gallic acid, completing the G-DG cycle.

Transgenic validation provided striking evidence: RNA interference of FvSCPL3-1 reduced tannin levels, while overexpression of FaCXE7 increased them but caused soft stems and delayed development due to reduced lignin biosynthesis. Transcriptomic data revealed that excessive cycling diverts carbohydrate flux away from lignin and flavonoid pathways, reshaping plant growth. Together, these discoveries demonstrate how the G-DG cycle integrates biosynthetic and catabolic reactions to fine-tune tannin homeostasis in strawberries, highlighting the metabolic balance between flavor compounds and structural integrity.

Our findings redefine how plants manage polyphenol metabolism. The galloylation–degalloylation cycle is not merely a linear biosynthetic process but a self-sustaining loop that controls both the production and recycling of tannins. This insight bridges a long-standing gap in plant secondary metabolism research and provides a genetic roadmap for improving fruit quality traits such as flavor and antioxidant potential. It also reveals how metabolic competition shapes growth–defense trade-offs in strawberries."

Prof. Liping Gao, corresponding author of the study

This discovery lays the groundwork for metabolic engineering of fruit crops with enhanced flavor, nutritional value, and stress resistance. By fine-tuning the activity of genes within the G-DG cycle, breeders can potentially increase beneficial tannin content while minimizing negative effects on lignin or growth. The identified enzymes-FaUGT84A22, FaSCPL3-1, and FaCXE7-serve as genetic targets for optimizing strawberry varieties and other tannin-rich plants such as tea or pomegranate. Moreover, understanding this metabolic balance offers a valuable framework for designing precision breeding strategies that align flavor improvement with plant resilience under environmental stress.