The C-Glycosylation of Flavonoids in Cereals

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 UDPglucose-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-Cglucosides 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.

CYP93G2 Is a Flavanone 2-Hydroxylase Required for C-Glycosylflavone Biosynthesis in Rice

“…4C). In fact, the MS/MS spectrum is consistent with those reported for the OsCGT reaction products formed in vitro from 2-hydroxyflavanones (Brazier-Hicks et al, 2009). When the double transformant extracts were acid treated, the C-glucosyl-2-hydroxynarigenin peak was not detected by LC-MS analysis.…”

“…2-Hydroxyflavanones, not flavanones or flavones, were identified to be substrates for C-glucosylation Franz, 1987, 1988). Consistent with this, the rice C-glucosyltransferase OsCGT was recently demonstrated to catalyze the UDP-Glc-dependent C-glucosylation of 2-hydroxyflavanone substrates (Brazier-Hicks et al, 2009). In addition, dehydratase activities that preferentially converted 2-hydroxyflavanone C-glucosides to 6C-glucosylflavones were demonstrated in both rice and wheat extracts (Brazier-Hicks et al, 2009).…”

“…Consistent with this, the rice C-glucosyltransferase OsCGT was recently demonstrated to catalyze the UDP-Glc-dependent C-glucosylation of 2-hydroxyflavanone substrates (Brazier-Hicks et al, 2009). In addition, dehydratase activities that preferentially converted 2-hydroxyflavanone C-glucosides to 6C-glucosylflavones were demonstrated in both rice and wheat extracts (Brazier-Hicks et al, 2009). Apparently, enzyme activities that generate 2-hydroxyflavanones are necessary for channeling flavanones to C-glycosylflavone production in rice.…”

“…In maize, maysin is an O-glycosylated C-glycosylluteolin well known to confer natural resistance in silk tissues toward different lepidopteran larval pests (McMullen et al, 2004). In contrast to the above flavones, tricin (3#,5#-dimethoxylated flavone) is usually detected as O-glycosides in cereals (Cavaliere et al, 2005;Brazier-Hicks et al, 2009). Tricin and its O-glycosides have been shown to function as allelopathic compounds in rice, interfering with weeds and harmful microorganisms in paddy soil (Kong et al, 2004(Kong et al, , 2007.…”

Silencing CHALCONE SYNTHASE in Maize Impedes the Incorporation of Tricin into Lignin and Increases Lignin Content

“…The flavone biosynthesis pathway is shown according to Zhou et al (2008), Corté s-Cruz et al (2003, and Lam et al (2014Lam et al ( , 2015. The enzymes involved in the metabolic route via F2H toward apigenin and luteolin conjugates has been described in rice, wheat, and buckwheat (Fagopyrum esculentum;Brazier-Hicks et al, 2009;Du et al, 2010). The biosynthetic routes via HCT and HQT have been described in switchgrass (Escamilla-Treviñ o et al, 2014) and suggested to be present in maize (Corté s-Cruz et al, 2003).…”