Abstract
Fruits of sweet orange (Citrus sinensis), a popular commercial Citrus species, contain high concentrations of flavonoids beneficial to human health. These fruits predominantly accumulate O-glycosylated flavonoids, in which the disaccharides [neohesperidose (rhamnosyl-α-1,2-glucose) or rutinose (rhamnosyl-α-1,6-glucose)] are linked to the flavonoid aglycones through the 3- or 7-hydroxyl sites. The biotransformation of the flavonoid aglycones into O-rutinosides or O-neohesperidosides in the Citrus plants usually consists of two glycosylation reactions involving a series of uridine diphosphate-sugar dependent glycosyltransferases (UGTs). Although several genes encoding flavonoid UGTs have been functionally characterized in the Citrus plants, full elucidation of the flavonoid glycosylation process remains elusive. Based on the available genomic and transcriptome data, we isolated a UGT with a high expression level in the sweet orange fruits that possibly encodes a flavonoid glucosyltransferase and/or rhamnosyltransferase. Biochemical analyses revealed that a broad range of flavonoid substrates could be glucosylated at their 3- and/or 7-hydrogen sites by the recombinant enzyme, including hesperetin, naringenin, diosmetin, quercetin, and kaempferol. Furthermore, overexpression of the gene could significantly increase the accumulations of quercetin 7-O-rhamnoside, quercetin 7-O-glucoside, and kaempferol 7-O-glucoside, implying that the enzyme has flavonoid 7-O-glucosyltransferase and 7-O-rhamnosyltransferase activities in vivo.
Highlights
Flavonoids, such as flavanones, flavones, and flavonols, are among the most widespread groups of plant secondary metabolites and show a broad diversity of biological functions (Wang et al, 2007, 2011)
Most flavonoid glycosylation reactions depend on uridine diphosphate (UDP)-sugar dependent glycosyltransferases (UGTs), which use UDP sugars as sugar donors (Dixon and Steele, 1999; Bowles, 2002; Bowles et al, 2005; Lairson et al, 2008) and contain a 44-amino-acid conserved sequence in their C-terminal regions that is responsible for combining the UDP sugars (Gachon et al, 2005)
The results demonstrated that the recombinant UGT was able to glucosylate quercetin at the 3- or 7-hydrogen position (Figure 5D), but using kaempferol as substrate, FIGURE 4 | Relative expression of CsUGT76F1 in sweet orange peels (A) and pulp (B) during fruit development
Summary
Flavonoids, such as flavanones, flavones, and flavonols, are among the most widespread groups of plant secondary metabolites and show a broad diversity of biological functions (Wang et al, 2007, 2011). Flavonoids are supposed to be beneficial to human health because of their antioxidant, anti-allergenic, and anti-inflammatory properties (Pietta, 2000; Ross et al, 2001). Many flavonoid aglycones are glycosylated with pentoses and hexoses, resulting in the functional and structural diversity of flavonoids (Vogt and Jones, 2000; Jones and Vogt, 2001). The glycan moieties present in flavonoid glycosides may play key roles in increasing the aqueous solubility of these glycosides, improving their bioavailability in humans (Hollman et al, 1995; Hollman and Katan, 1999). Most flavonoid glycosylation reactions depend on UDP-sugar dependent glycosyltransferases (UGTs), which use UDP sugars as sugar donors (Dixon and Steele, 1999; Bowles, 2002; Bowles et al, 2005; Lairson et al, 2008) and contain a 44-amino-acid conserved sequence (the “PSPG motif ”) in their C-terminal regions that is responsible for combining the UDP sugars (Gachon et al, 2005)
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