Abstract
Ellagic acid/ellagitannins are plant polyphenolic antioxidants that are synthesized from gallic acid and have been associated with a reduced risk of cancer and cardiovascular diseases. Here, we report the identification and characterization of five glycosyltransferases (GTs) from two genera of the Rosaceae family (Fragaria and Rubus; F. × ananassa FaGT2*, FaGT2, FaGT5, F. vesca FvGT2, and R. idaeus RiGT2) that catalyze the formation of 1-O-galloyl-β-D-glucopyranose (β-glucogallin) the precursor of ellagitannin biosynthesis. The enzymes showed substrate promiscuity as they formed glucose esters of a variety of (hydroxyl)benzoic and (hydroxyl)cinnamic acids. Determination of kinetic values and site-directed mutagenesis revealed amino acids that affected substrate preference and catalytic activity. Green immature strawberry fruits were identified as the main source of gallic acid, β-glucogallin, and ellagic acid in accordance with the highest GT2 gene expression levels. Injection of isotopically labeled gallic acid into green fruits of stable transgenic antisense FaGT2 strawberry plants clearly confirmed the in planta function. Our results indicate that GT2 enzymes might contribute to the production of ellagic acid/ellagitannins in strawberry and raspberry, and are useful to develop strawberry fruit with additional health benefits and for the biotechnological production of bioactive polyphenols.
Highlights
Plant phenols are mostly products of the phenylpropanoid and shikimate pathway, and comprise a large variety of compounds (Quideau et al, 2011)
Our results indicate that GT2 enzymes might contribute to the production of ellagic acid/ellagitannins in strawberry and raspberry, and are useful to develop strawberry fruit with additional health benefits and for the biotechnological production of bioactive polyphenols
Sequence alignment and a phylogenetic comparison of three UDP-glucose:gallic acid GTs from Vitis vinifera (VvgGT1–VvgGT3; Khater et al, 2012) with putative GTs encoded by the F. vesca genome (Shulaev et al, 2011) yielded two sequences whose closest homologs in F.×ananassa have been named FaGT2 and FaGT5 (Supplementary Fig. S2)
Summary
Plant phenols are mostly products of the phenylpropanoid and shikimate pathway, and comprise a large variety of compounds (Quideau et al, 2011). Among the bioactive polyphenols are tannins, which can be divided into condensed tannins (proanthocyanidins) and hydrolyzable tannins (ellagitannins and gallotannins). The latter are heterogeneous polymers formed from phenolic acids, especially gallic acid, esterified with simple sugar molecules (Niemetz and Gross, 2005).
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