Elucidation and reconstitution of hydrolyzable tannin biosynthesis.

Hydrolyzable tannins (HTs) are a class of polyphenols produced mostly in core eudicot plants. They accumulate in various plant tissues and are considered to function as defense compounds that protect against herbivory, infections, and toxic metals (specifically aluminum ions). Moreover, HTs have industrial and pharmaceutical uses that benefit humans. Elucidating and reconstituting the biosynthesis of HTs is necessary for genetically engineering in planta functions and for efficiently producing HTs for human use. The biosynthesis of HTs is initiated by the formation of gallic acid from the shikimate pathway intermediate 3-dehydroshikimic acid, which is catalyzed by bifunctional dehydroquinate dehydratase/shikimate dehydrogenases (DQD/SDHs). In the second step, UDP glycosyltransferases (UGTs) esterify gallic acid with glucose to form β-glucogallin (1-O-galloyl-β-D-glucose). β-glucogallin is then converted to 1,2,3,4,6-penta-O-galloyl-β-D-glucose through a series of galloylation steps that are catalyzed by galloyltransferases, using β-glucogallin as a galloyl donor. Laccases subsequently catalyze the oxidative coupling between adjacent galloyl groups to form hexahydroxydiphenoyl (HHDP) groups, which are characteristic components of ellagitannins. Furthermore, monomeric ellagitannins can undergo oligomerization via intermolecular oxidative coupling, which is also catalyzed by laccases. To reconstitute the HT biosynthetic pathway in HT-non-accumulating plants, DQD/SDHs and UGTs from Eucalyptus camaldulensis were heterologously co-expressed in Nicotiana benthamiana leaves, which resulted in the production of gallic acid and β-glucogallin. In future studies, this transgenic system will be used to identify genes encoding galloyltransferases and laccases to further elucidate and reconstitute the HT biosynthetic pathway.