Flavanone Glycoside Biosynthesis in Citrus

Abstract

Previous indirect evidence suggested that the biosynthesis of flavonoids in Citrus may not proceed via the usual chalcone synthase reaction and that glycosylation occurs during chalcone formation and not afterward, as has been reported in other species. We detected chalcone-synthase and UDP-glucose:flavanone-7-O-glucosyl-transferase activities in cell-free extracts of Citrus. The glucosylated flavanone was further rhamnosylated when exogenous UDP-glucose and NADPH were added to the extract. Chalcone-synthase activity was detected in cellfree extracts derived from young leaves and fruits. Young fruits (2 millimeter diameter) had the highest chalcone synthase activity. UDP-glucose:flavanone-7-0-glucosyl-transferase activity was measured in cell-free extracts derived from young leaves and fruits of Citrus mitis and Citrus maxima. The highest UDP-glucose:flavanone-7-O-glucosyl-transferase activity was found in young C. maxima leaves. These data indicate that Citrus contains a flavonoid pathway similar to that studied in other species.Several Citrus spp. (Rutaceae) contain bitter tasting flavanone-disaccharides (18). They are used as food additives for their flavor or can be converted by hydrogenation into the extremely sweet dihydrochalcone derivatives (18). Little information is available on the biosynthetic pathways to flavanone-glycosides in Citrus (16). Chalcone-synthase is a key enzyme in the flavonoid pathway in several plants (13), but has not previously been reported in Citrus tissues. Chalconesynthase catalyzes the condensation of p-coumaryl-CoA with three molecules of malonyl-CoA yielding naringenin-chalcone (20). The substrate specificity of a 'naringin-chalconecyclase' activity, from immature grapefruit fruit (25), suggested that the flavonoid pathways in Citrus, might be different from the more studied flavonoid pathways of other plants (13,14) (Fig. 1). They interpreted their results (25) as suggesting that glucosylation and rhamnosylation occurred during chalcone formation in Citrus, as the aglycones were not ' Supported by a Minerva foundation travel grant. Present address:

Citrus fruit flavor and aroma biosynthesis: isolation, functional characterization, and developmental regulation of Cstps1, a key gene in the production of the sesquiterpene aroma compound valencene

“…Thus, the data in the literature are inconclusive as to the evolutionary source of valencene production, but suggest that putative enzymes converting valencene to nootkatone were inherited from pummelo. Similarly, the bitter¯avanone neohesperidoside¯avor compounds and their corresponding biosynthetic enzymes are distinctly associated with species/varieties of pummelo origin (Bar-Peled et al, 1991;Lewinsohn et al, 1989;Ortuno et al, 1995).…”

Cloning and Characterization of Vitis viniferaUDP-Glucose:Flavonoid 3-O-Glucosyltransferase, a Homologue of the Enzyme Encoded by the Maize Bronze-1Locus That May Primarily Serve to Glucosylate Anthocyanidins in Vivo

“…The most widely studied groups of plant glycosyltransferases are those associated with the biosynthesis of flavonoid glucosides, including flavonol glucosides, flavanone glucosides, and anthocyanins (3,4,9,10). Earliest reports included the detection of an anthocyanidin and flavonol glucosylating activity in endosperm extracts of maize (Zea mays), (11)(12)(13).…”

Juvenile-Specific Localization and Accumulation of a Rhamnosyltransferase and Its Bitter Flavonoid in Foliage, Flowers, and Young Citrus Fruits

“…Indeed, there is high chalcone synthase activity in young Citrus leaves (Lewinsohn et al, 1989b). Furthermore, there was a higher specific activity of 7-O-glucosyltransferase in young leaves of calamondin and grapefruit than in slightly more expanded leaves (Lewinsohn et al, 1989b;McIntosh et al, 1990).…”

“…Hesperetin is then glucosylated to hesperitin-7-O-glucoside (Lewinsohn et al, 1989b;McIntosh et al, 1990) and is finally rhamnosylated to neohesperidin by the same rhamnosyltransferase, producing naringin (Lewinsohn et al, 1989b;Bar-Peled et al, 1991). The accumulation and distribution of the major flavanoneglycoside products have been investigated during different stages of growth of grapefruit and sour orange without parallel studies of the enzymes involved.…”