Abstract
Flavonoids normally accumulate in plants as O-glycosylated derivatives, but several species, including major cereal crops, predominantly synthesize flavone-C-glycosides, which are stable to hydrolysis and are biologically active both in planta and as dietary components. An enzyme (OsCGT) catalyzing the UDP-glucose-dependent C-glucosylation of 2-hydroxyflavanone precursors of flavonoids has been identified and cloned from rice (Oryza sativa ssp. indica), with a similar protein characterized in wheat (Triticum aestivum L.). OsCGT is a 49-kDa family 1 glycosyltransferase related to known O-glucosyltransferases. The recombinant enzyme C-glucosylated 2-hydroxyflavanones but had negligible O-glucosyltransferase activity with flavonoid acceptors. Enzyme chemistry studies suggested that OsCGT preferentially C-glucosylated the dibenzoylmethane tautomers formed in equilibrium with 2-hydroxyflavanones. The resulting 2-hydroxyflavanone-C-glucosides were unstable and spontaneously dehydrated in vitro to yield a mixture of 6C- and 8C-glucosyl derivatives of the respective flavones. In contrast, in planta, only the respective 6C-glucosides accumulated. Consistent with this selectivity in glycosylation product, a dehydratase activity that preferentially converted 2-hydroxyflavanone-C-glucosides to the corresponding flavone-6C-glucosides was identified in both rice and wheat. Our results demonstrate that cereal crops synthesize C-glucosylated flavones through the concerted action of a CGT and dehydratase acting on activated 2-hydroxyflavanones, as an alternative means of generating flavonoid metabolites.
Highlights
Ical activity, the C-glycosyltransferases (CGTs)2 responsible for forming these glycosidic bonds have attracted relatively little attention
The rice enzyme OsCGT catalyzes the formation of the precursors of flavone-C-glucosides, with a protein with similar activities and physical characteristics identified in wheat
None of the group E UGTs are known to C-glucosylate acceptors, with UGT72B1 showing no activity toward the 2-hydroxyflavanone substrates in our hands
Summary
Ical activity, the C-glycosyltransferases (CGTs)2 responsible for forming these glycosidic bonds have attracted relatively little attention. Total glucosyltransferase (OGT plus CGT) activities were determined in wheat and rice using UDP-[14C]glucose as the donor and the flavones chrysin, apigenin, and luteolin as acceptors (Fig. 1).
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