Glucosylation of 4-Hydroxy-2,5-Dimethyl-3(2H)-Furanone, the Key Strawberry Flavor Compound in Strawberry Fruit.

Strawberries emit hundreds of different volatiles, but only a dozen, including the key compound HDMF [4-hydroxy-2,5-dimethyl-3(2H)-furanone] contribute to the flavor of the fruit. However, during ripening, a considerable amount of HDMF is metabolized to the flavorless HDMF ß-d-glucoside. Here, we functionally characterize nine ripening-related UGTs (UDP-glucosyltransferases) in Fragaria that function in the glucosylation of volatile metabolites by comprehensive biochemical analyses. Some UGTs showed a rather broad substrate tolerance and glucosylated a range of aroma compounds in vitro, whereas others had a more limited substrate spectrum. The allelic UGT71K3a and b proteins and to a lesser extent UGT73B24, UGT71W2, and UGT73B23 catalyzed the glucosylation of HDMF and its structural homolog 2(or 5)-ethyl-4-hydroxy-5(or 2)-methyl-3(2H)-furanone. Site-directed mutagenesis to introduce single K458R, D445E, D343E, and V383A mutations and a double G433A/I434V mutation led to enhanced HDMF glucosylation activity compared to the wild-type enzymes. In contrast, a single mutation in the center of the plant secondary product glycosyltransferase box (A389V) reduced the enzymatic activity. Down-regulation of UGT71K3 transcript expression in strawberry receptacles led to a significant reduction in the level of HDMF-glucoside and a smaller decline in HDMF-glucoside-malonate compared with the level in control fruits. These results provide the foundation for improvement of strawberry flavor and the biotechnological production of HDMF-glucoside.

A UDP-glucosyltransferase functions in both acylphloroglucinol glucoside and anthocyanin biosynthesis in strawberry (Fragaria × ananassa).

Physiologically active acylphloroglucinol (APG) glucosides were recently found in strawberry (Fragaria sp.) fruit. Although the formation of the APG aglycones has been clarified, little is known about APG glycosylation in plants. In this study we functionally characterized ripening-related glucosyltransferase genes in Fragaria by comprehensive biochemical analyses of the encoded proteins and by a RNA interference (RNAi) approach in vivo. The allelic proteins UGT71K3a/b catalyzed the glucosylation of diverse hydroxycoumarins, naphthols and flavonoids as well as phloroglucinols, enzymatically synthesized APG aglycones and pelargonidin. Total enzymatic synthesis of APG glucosides was achieved by co-incubation of recombinant dual functional chalcone/valerophenone synthase and UGT71K3 proteins with essential coenzyme A esters and UDP-glucose. An APG glucoside was identified in strawberry fruit which has not yet been reported in other plants. Suppression of UGT71K3 activity in transient RNAi-silenced fruits led to a loss of pigmentation and a substantial decrease of the levels of various APG glucosides and an anthocyanin. Metabolite analyses of transgenic fruits confirmed UGT71K3 as a UDP-glucose:APG glucosyltransferase in planta. These results provide the foundation for the breeding of fruits with improved health benefits and for the biotechnological production of bioactive natural products.

Formation of ß-glucogallin, the precursor of ellagic acid in strawberry and raspberry.

Ellagic acid/ellagitannins are plant polyphenolic antioxidants that are synthesized from gallic acid and have been associated with a reduced risk of cancer and cardiovascular diseases. Here, we report the identification and characterization of five glycosyltransferases (GTs) from two genera of the Rosaceae family (Fragaria and Rubus; F. × ananassa FaGT2*, FaGT2, FaGT5, F. vesca FvGT2, and R. idaeus RiGT2) that catalyze the formation of 1-O-galloyl-ß-D-glucopyranose (ß-glucogallin) the precursor of ellagitannin biosynthesis. The enzymes showed substrate promiscuity as they formed glucose esters of a variety of (hydroxyl)benzoic and (hydroxyl)cinnamic acids. Determination of kinetic values and site-directed mutagenesis revealed amino acids that affected substrate preference and catalytic activity. Green immature strawberry fruits were identified as the main source of gallic acid, ß-glucogallin, and ellagic acid in accordance with the highest GT2 gene expression levels. Injection of isotopically labeled gallic acid into green fruits of stable transgenic antisense FaGT2 strawberry plants clearly confirmed the in planta function. Our results indicate that GT2 enzymes might contribute to the production of ellagic acid/ellagitannins in strawberry and raspberry, and are useful to develop strawberry fruit with additional health benefits and for the biotechnological production of bioactive polyphenols.

Functional Characterization and Substrate Promiscuity of UGT71 Glycosyltransferases from Strawberry (Fragaria × ananassa).

Glycosylation determines the complexity and diversity of plant natural products. To characterize fruit ripening-related UDP-dependent glycosyltransferases (UGTs) functionally in strawberry, we mined the publicly available Fragaria vesca genome sequence and found 199 putative UGT genes. Candidate UGTs whose expression levels were strongly up-regulated during fruit ripening were cloned from F.×ananassa and six were successfully expressed in Escherichia coli and biochemically characterized. UGT75T1 showed very strict substrate specificity and glucosylated only galangin out of 33 compounds. The other recombinant enzymes exhibited broad substrate tolerance, accepting numerous flavonoids, hydroxycoumarins, naphthols and the plant hormone, (+)-S-abscisic acid (ABA). UGT71W2 showed the highest activity towards 1-naphthol, while UGT71A33, UGT71A34a/b and UGT71A35 preferred 3-hydroxycoumarin and formed 3- and 7-O-glucosides as well as a diglucoside from flavonols. Screening of a strawberry physiological aglycone library identified kaempferol, quercetin, ABA and three unknown natural compounds as putative in planta substrates of UGT71A33, UGT71A34a and UGT71W2. Metabolite analyses of RNA interference (RNAi)-mediated silenced fruits demonstrated that UGT71W2 contributes to the glycosylation of flavonols, xenobiotics and, to a minor extent, of ABA, in planta. The study showed that both specialist and generalist UGTs were expressed during strawberry fruit ripening and the latter were probably not restricted to only one function in plants.