A UDP-Glucose:Monoterpenol Glucosyltransferase Adds to the Chemical Diversity of the Grapevine Metabolome

Changes in glycosylation patterns of monoterpenes during grape berry maturation in six cultivars of Vitis vinifera

Plants conjugate monoterpenoids to sugars, rendering them non-volatile. Hydrolysis of these glycosidic precursors frees the volatile aroma compounds. Here, we profile intact monoterpenyl glycosides in six Vitis vinifera grape berry cultivars. Relative concentrations of twenty-six monoterpenyl glycosides, including nine new putatively identified compounds, were analyzed by UHPLC-QTOF MS/MS at three times during grape maturation (pre-véraison, véraison, and post-véraison). Total glycoside content reached a maximum in Muscat cultivars post-véraison but remained relatively constant in all other cultivars. Three types of monoterpenyl glycosides predominated in all samples: malonylated monoterpenol glucosides, monoterpenol hexose-pentoses, and monoterpendiol hexose-pentoses. The two Muscat cultivars were not differentiated at the earlier developmental stages but could be differentiated post-véraison. In contrast, similarities between Chardonnay and Pinot noir glycoside profiles developed post-véraison. Overall monoterpene glycoconjugation patterns may align with underlying genetic relationships among cultivars. By understanding monoterpene glycoconjugation in wine grapes, scientists and winemakers can better understand grape and wine aromas.

Triterpenoid-biosynthetic UDP-glycosyltransferases from plants

Triterpenoid saponins are naturally occurring structurally diverse glycosides of triterpenes that are widely distributed among plant species. Great interest has been expressed by pharmaceutical and agriculture industries for the glycosylation of triterpenes. Such modifications alter their taste and bio-absorbability, affect their intra-/extracellular transport and storage in plants, and induce novel biological activities in the human body. Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze glycosylation using UDP sugar donors. These enzymes belong to a multigene family and recognize diverse natural products, including triterpenes, as the acceptor molecules. For this review, we collected and analyzed all of the UGT sequences found in Arabidopsis thaliana as well as 31 other species of triterpene-producing plants. To identify potential UGTs with novel functions in triterpene glycosylation, we screened and classified those candidates based on similarity with UGTs from Panax ginseng, Glycine max, Medicago truncatula, Saponaria vaccaria, and Barbarea vulgaris that are known to function in glycosylate triterpenes. We highlight recent findings on UGT inducibility by methyl jasmonate, tissue-specific expression, and subcellular localization, while also describing their catalytic activity in terms of regioselectivity for potential key UGTs dedicated to triterpene glycosylation in plants. Discovering these new UGTs expands our capacity to manipulate the biological and physicochemical properties of such valuable molecules.

Glucosylation of (Z)-3-hexenol informs intraspecies interactions in plants: A case study in Camellia sinensis

Plants emit a variety of volatiles in response to herbivore attack, and (Z)-3-hexenol and its glycosides have been shown to function as defence compounds. Although the ability to incorporate and convert (Z)-3-hexenol to glycosides is widely conserved in plants, the enzymes responsible for the glycosylation of (Z)-3-hexenol remained unknown until today. In this study, uridine-diphosphate-dependent glycosyltransferase (UGT) candidate genes were selected by correlation analysis and their response to airborne (Z)-3-hexenol, which has been shown to be taken up by the tea plant. The allelic proteins UGT85A53-1 and UGT85A53-2 showed the highest activity towards (Z)-3-hexenol and are distinct from UGT85A53-3, which displayed a similar catalytic efficiency for (Z)-3-hexenol and nerol. A single amino acid exchange E59D enhanced the activity towards (Z)-3-hexenol, whereas a L445M mutation reduced the catalytic activity towards all substrates tested. Transient overexpression of CsUGT85A53-1 in tobacco significantly increased the level of (Z)-3-hexenyl glucoside. The functional characterization of CsUGT85A53 as a (Z)-3-hexenol UGT not only provides the foundation for the biotechnological production of (Z)-3-hexenyl glucoside but also delivers insights for the development of novel insect pest control strategies in tea plant and might be generally applicable to other plants.

Functional Differentiation of Duplicated Flavonoid 3-O-Glycosyltransferases in the Flavonol and Anthocyanin Biosynthesis of Freesia hybrida

Flavonols and anthocyanins are two widely distributed groups of flavonoids that occurred apart during plant evolution and biosynthesized by shared specific enzymes involved in flavonoid metabolism. UDP-glucose, flavonoid 3-O-glycosyltransferase (UF3GT), is one of the common enzymes which could catalyze the glycosylation of both flavonol and anthocyanidin aglycons simultaneously in vitro. However, whether and how UF3GT paralogous genes function diversely at the biochemical and transcriptional levels are largely unknown. Recently, Fh3GT1 was identified to be a member of UF3GTs in Freesia hybrida. However, its expression patterns and enzymatic characteristics could not coincide well with flavonol accumulation. In an attempt to characterize other flavonoids, especially flavonol glycosyltransferase genes in Freesia, three closest candidate UFGT genes-Fh3GT2, Fh3GT3, and Fh3GT4-were mined from the Freesia transcriptomic database and isolated from the flowers of the widely distributed Freesia cultivar, Red River^®. Based on bioinformatic analysis and enzymatic assays, Fh3GT2 turned out to be another bona fide glycosyltransferase gene. Biochemical analysis further proved that Fh3GT2 preferentially glucosylated kaempferol while Fh3GT1 controlled the glucosylation of quercetin and anthocyanidins. In addition, transfection assays demonstrated that Fh3GT2 could be mainly activated by the flavonol regulator FhMYBF1 or the anthocyanin regulator FhPAP1, whereas Fh3GT1 could only be activated by FhPAP1. These findings suggested that Fh3GTs might have functionally diverged in flavonoid biosynthesis at both the biochemical and transcriptional levels.

Attractive but Toxic: Emerging Roles of Glycosidically Bound Volatiles and Glycosyltransferases Involved in Their Formation

Plants emit an overabundance of volatile compounds, which act in their producers either as appreciated attractants to lure beneficial animals or as repellent toxins to deter pests in a species-specific and concentration-dependent manner. Plants have evolved solutions to provide sufficient volatiles without poisoning themselves. Uridine-diphosphate sugar-dependent glycosyltransferases (UGTs) acting on volatiles is one important part of this sophisticated system, which balances the levels of bioactive metabolites and prepares them for cellular and long-distance transport and storage but enables the remobilization of disarmed toxins for the benefit of plant protection. This review provides an overview of the research history of glycosidically bound volatiles (GBVs), a relatively new group of plant secondary metabolites, and discusses the role of UGTs in the production of GBVs for plant protection.

Combining liquid chromatography and tandem mass spectrometry approaches to the study of monoterpene glycosides (aroma precursors) in wine grape

Monoterpene-glycosides are important aroma precursors that, undergo hydrolysis, confer intense floral notes to the wines. Therefore, the knowledge of the nature of the sugar residues and the structure of these molecules is of great interest. In present study, liquid chromatography (LC) separation coupled with different mass spectrometry (MS) experiments for the characterization of these compounds were explored. The LC parameters were tuned to optimize the resolution between the analytes present in grape sample extracts. Twenty principal peaks with a relative abundance >1% were selected and divided in 4 classes characterized by different molecular weight. In general, positive ionization of the studied compounds displayed the [M + NH₄ ]⁺ ion as base peak. On the contrary, a distribution between [M + Cl]^- and [M + HCOO]^- species was observed in negative ion mode. However, a clear differentiation between the studied compounds was only possible by combining both LC and tandem MS (MS/MS). Indeed, by applying a series of energy resolved MS/MS experiments and monitoring both positive and negative ions, a structural characterization of the analytes was achieved. The proposed LC-MS/MS approach provided the profile of monoterpenol-diglycosides and allowed the identification of a series of isobaric terpene-diglycosides in grape. The study of their MS/MS spectra indicated the structure of geranic and/or nerolic acid aglycones. To verify the interest of studied compounds, a preliminary evaluation of the intensity of signals of these glycosides were carried out. The obtained results showed a significant difference between the grape samples collected in two different vintages.

Structural and Functional Analysis of UGT92G6 Suggests an Evolutionary Link Between Mono- and Disaccharide Glycoside-Forming Transferases

Glycosylation mediated by UDP-dependent glycosyltransferase (UGT) is one of the most common reactions for the biosynthesis of small molecule glycosides. As glycosides have various biological roles, we characterized UGT genes from grapevine (Vitis vinifera). In silico analysis of VvUGT genes that were highly expressed in leaves identified UGT92G6 which showed sequence similarity to both monosaccharide and disaccharide glucoside-forming transferases. The recombinant UGT92G6 glucosylated phenolics, among them caffeic acid, carvacrol, eugenol and raspberry ketone, and also accepted geranyl glucoside and citronellyl glucoside. Thus, UGT92G6 formed mono- and diglucosides in vitro from distinct compounds. The enzyme specificity constant Vmax/Km ratios indicated that UGT92G6 exhibited the highest specificity towards caffeic acid, producing almost equal amounts of the 3- and 4-O-glucoside. Transient overexpression of UGT92G6 in Nicotiana benthamiana leaves confirmed the production of caffeoyl glucoside; however, the level of geranyl diglucoside was not elevated upon overexpression of UGT92G6, even after co-expression of genes encoding geraniol synthase and geraniol UGT to provide sufficient precursor. Comparative sequence and 3-D structure analysis identified a sequence motif characteristic for monoglucoside-forming UGTs in UGT92G6, suggesting an evolutionary link between mono- and disaccharide glycoside UGTs. Thus, UGT92G6 functions as a mono- and diglucosyltransferase in vitro, but acts as a caffeoyl glucoside UGT in N. benthamiana.

Phenylpropenes: Occurrence, Distribution, and Biosynthesis in Fruit

Phenylpropenes such as eugenol, chavicol, estragole, and anethole contribute to the flavor and aroma of a number of important herbs and spices. They have been shown to function as floral attractants for pollinators and to have antifungal and antimicrobial activities. Phenylpropenes are also detected as free volatiles and sequestered glycosides in a range of economically important fresh fruit species including apple, strawberry, tomato, and grape. Although they contribute a relatively small percentage of total volatiles compared with esters, aldehydes, and alcohols, phenylpropenes have been shown to contribute spicy anise- and clove-like notes to fruit. Phenylpropenes are typically found in fruit throughout development and to reach maximum concentrations in ripe fruit. Genes involved in the biosynthesis of phenylpropenes have been characterized and manipulated in strawberry and apple, which has validated the importance of these compounds to fruit aroma and may help elucidate other functions for phenylpropenes in fruit.

Glucosylation of Smoke-Derived Volatiles in Grapevine (Vitis vinifera) is Catalyzed by a Promiscuous Resveratrol/Guaiacol Glucosyltransferase

Vinification of grapes (Vitis vinifera) exposed to forest fire smoke can yield unpalatable wine due to the presence of taint compounds from smoke and the release of smoke derived volatiles from their respective glycosides during the fermentation process or in-mouth during consumption. To identify glycosyltransferases (GTs) involved in the formation of glycosidically bound smoke-derived volatiles we performed gene expression analysis of candidate GTs in different grapevine tissues. Second, substrates derived from bushfire smoke or naturally occurring in grapes were screened with the candidate recombinant GTs. A resveratrol GT (UGT72B27) gene, highly expressed in grapevine leaves and berries was identified to be responsible for the production of the phenolic glucosides. UGT72B27 converted the stilbene trans-resveratrol mainly to the 3-O-glucoside. Kinetic analyses yielded specificity constants (k_cat/K_M) of 114, 17, 9, 8, and 2 mM^-1 s^-1 for guaiacol, trans-resveratrol, syringol, methylsyringol, and methylguaiacol, respectively. This knowledge will help to design strategies for managing the risk of producing smoke-affected wines.

Effects of sunlight exclusion on the profiles of monoterpene biosynthesis and accumulation in grape exocarp and mesocarp

Terpenes are important aroma compounds in table Muscat grape and wine, and their content in the berry can be affected by sunlight. The effects of sunlight exclusion on monoterpene profiles and relevant gene expression profiles in the exocarp and mesocarp of table Muscat grape 'Jingxiangyu' at different development stages were thoroughly surveyed by bagging pre-veraison clusters in special opaque boxes. The responses of monoterpenes to sunlight treatments varied in three types, representatively linalool, ocimene and geraniol. Linalool was the most sensitive compound to sunlight, whose biosynthesis was severely inhibited by sunlight exclusion and then was elevated by re-exposure. Ocimene and glycosylated geraniol showed a certain suppressive and stimulative responses to sunlight exclusion respectively. Further transcription analysis revealed that VvPNLinNer1, VvCSbOci, VvGT7 and VvGT14 genes were mainly responsible for monoterpene accumulation and sensitivity to sunlight. VvDXS2 and VvDXR genes were partially related to the differential accumulation of total terpenes under different sunlight treatments.

Drawing Links from Transcriptome to Metabolites: The Evolution of Aroma in the Ripening Berry of Moscato Bianco (Vitis vinifera L.)

Monoterpenes confer typical floral notes to "Muscat" grapevine varieties and, to a lesser extent, to other aromatic non-Muscat varieties. Previous studies have led to the identification and functional characterization of some enzymes and genes in this pathway. However, the underlying genetic map is still far from being complete. For example, the specific steps of monoterpene metabolism and its regulation are largely unknown. With the aim of identifying new candidates for the missing links, we applied an integrative functional genomics approach based on the targeted metabolic and genome-wide transcript profiling of Moscato Bianco ripening berries. In particular, gas chromatography-mass spectrometry analysis of free and bound terpenoid compounds was combined with microarray analysis in the skins of berries collected at five developmental stages from pre-veraison to over-ripening. Differentially expressed metabolites and probes were identified in the pairwise comparison between time points by using the early stage as a reference. Metabolic and transcriptomic data were integrated through pairwise correlation and clustering approaches to discover genes linked with particular metabolites or groups of metabolites. These candidate transcripts were further checked for co-localization with quantitative trait loci (QTLs) affecting aromatic compounds. Our findings provide insights into the biological networks of grapevine secondary metabolism, both at the catalytic and regulatory levels. Examples include a nudix hydrolase as component of a terpene synthase-independent pathway for monoterpene biosynthesis, genes potentially involved in monoterpene metabolism (cytochrome P450 hydroxylases, epoxide hydrolases, glucosyltransferases), transport (vesicle-associated proteins, ABCG transporters, glutathione S-transferases, amino acid permeases), and transcriptional control (transcription factors of the ERF, MYB and NAC families, intermediates in light- and circadian cycle-mediated regulation with supporting evidence from the literature and additional regulatory genes with a previously unreported association to monoterpene accumulation).

Monoterpenyl Glycosyltransferases Differentially Contribute to Production of Monoterpenyl Glycosides in Two Aromatic Vitis vinifera Varieties

HIGHLIGHTS A similar trend on accumulation of glycosidically bound monoterpenes was observed in both varietiesTwo VvGT7 alleles mutations occurred at key sites in Muscat blanc à PetitVvGT14 exerted a major role in production of monoterpenyl glycosides in both varieties Terpenoids are the major aroma components and generally exist as both free and glycosidically-bound forms, of which nonvolatile glycosides account for a large fraction in grape berries. Our previous study has indicated that differential accumulation of monoterpenyl glycosides in Vitis vinifera "Muscat blanc à Petit" between two regions is closely correlated to monoterpenyl glucosyltransferase (VvGT14, XM_002285734.2) transcript abundance. However, it has not been determined yet whether this correlation also exists in other Vitis vinifera varieties. This study investigated the evolution of free and glycosidically bound monoterpenes in two Vitis vinifera variety "Muscat blanc à Petit" and "Gewurztraminer" under two vintages, and further assessed the relation between the accumulation of bound monoterpenes and two monoterpenyl glycosyltransferase transcript levels. Results showed that free monoterpenes exhibited three evolution patterns in both varieties during berry development of two vintages, whereas glycosidically bound monoterpenes showed a concentration elevation with berry maturation. The Cis-rose oxide and geraniol were major components contributing to the aroma odors of "Gewürztraminer" grapes while linalool was major aroma contributor to the "Muscat blanc à Petit grain" grapes. The accumulation of glycosidically bound monoterpenes in both varieties was accompanied with the high expression of VvGT7 (XM_002276510.2) and VvGT14. Only one allele of VvGT7 was found in the variety "Gewürztraminer" and no mutation was observed in its enzyme active sites. XB-VvGT7-4 and XB-VvGT7-5 were two alleles of VvGT7 detected in "Muscat blanc à Petit grain." The mutation on its enzyme active site inhibited the activity of XB-VvGT7-4, whereas VvGT7-5 exhibited an alteration on enzyme activity due to the insertion mutation at the position 443. Only one VvGT14 allele was found in both varieties, and the VvGT14 allele in both varieties showed the similarity on amino acid sequence. No mutation occurred in active sites of VvGT14 allele. These indicated that VvGT7 and VvGT14 differentially contributed to the production of monoterpenyl glycosides in these Vitis Vinifera varieties.

A grapevine cytochrome P450 generates the precursor of wine lactone, a key odorant in wine

Monoterpenes are important constituents of the aromas of food and beverages, including wine. Among monoterpenes in wines, wine lactone has the most potent odor. It was proposed to form via acid-catalyzed cyclization of (E)-8-carboxylinalool during wine maturation. It only reaches very low concentrations in wine but its extremely low odor detection threshold makes it an important aroma compound. Using LC-MS/MS, we show here that the (E)-8-carboxylinalool content in wines correlates with their wine lactone content and estimate the kinetic constant for the very slow formation of wine lactone from (E)-8-carboxylinalool. We show that (E)-8-carboxylinalool is accumulated as a glycoside in grape (Vitis vinifera) berries and that one of the cytochrome P450 enzymes most highly expressed in maturing berries, CYP76F14, efficiently oxidizes linalool to (E)-8-carboxylinalool. Our analysis of (E)-8-carboxylinalool in Riesling × Gewurztraminer grapevine progeny established that the CYP76F14 gene co-locates with a quantitative trait locus for (E)-8-carboxylinalool content in grape berries. Our data support the role of CYP76F14 as the major (E)-8-carboxylinalool synthase in grape berries and the role of (E)-8-carboxylinalool as a precursor to wine lactone in wine, providing new insights into wine and grape aroma metabolism, and new methods for food and aroma research and production.

Genome-Wide Identification, Expression Patterns, and Functional Analysis of UDP Glycosyltransferase Family in Peach (Prunus persica L. Batsch)

Peach (Prunus persica L. Batsch) is a commercial grown fruit trees, important because of its essential nutrients and flavor promoting secondary metabolites. The glycosylation processes mediated by UDP-glycosyltransferases (UGTs) play an important role in regulating secondary metabolites availability. Identification and characterization of peach UGTs is therefore a research priority. A total of 168 peach UGT genes that distributed unevenly across chromosomes were identified based on their conserved PSPG motifs. Phylogenetic analysis of these genes with plant UGTs clustered them into 16 groups (A-P). Comparison of the patterns of intron-extron and their positions within genes revealed one highly conserved intron insertion event in peach UGTs. Tissue specificity, temporal expression patterns in peach fruit during development and ripening, and in response to abiotic stress UV-B irradiation was investigated using RNA-seq strategy. The relationship between UGTs transcript levels and concentrations of glycosylated volatiles was examined to select candidates for functional analysis. Heterologous expressing these candidate genes in Escherichia coli identified UGTs that were involved in the in vitro volatile glycosylation. Our results provide an important source for the identification of functional UGT genes to potential manipulate secondary biosynthesis in peach.

Plant secondary metabolism linked glycosyltransferases: An update on expanding knowledge and scopes

The multigene family of enzymes known as glycosyltransferases or popularly known as GTs catalyze the addition of carbohydrate moiety to a variety of synthetic as well as natural compounds. Glycosylation of plant secondary metabolites is an emerging area of research in drug designing and development. The unsurpassing complexity and diversity among natural products arising due to glycosylation type of alterations including glycodiversification and glycorandomization are emerging as the promising approaches in pharmacological studies. While, some GTs with broad spectrum of substrate specificity are promising candidates for glycoengineering while others with stringent specificity pose limitations in accepting molecules and performing catalysis. With the rising trends in diseases and the efficacy/potential of natural products in their treatment, glycosylation of plant secondary metabolites constitutes a key mechanism in biogeneration of their glycoconjugates possessing medicinal properties. The present review highlights the role of glycosyltransferases in plant secondary metabolism with an overview of their identification strategies, catalytic mechanism and structural studies on plant GTs. Furthermore, the article discusses the biotechnological and biomedical application of GTs ranging from detoxification of xenobiotics and hormone homeostasis to the synthesis of glycoconjugates and crop engineering. The future directions in glycosyltransferase research should focus on the synthesis of bioactive glycoconjugates via metabolic engineering and manipulation of enzyme's active site leading to improved/desirable catalytic properties. The multiple advantages of glycosylation in plant secondary metabolomics highlight the increasing significance of the GTs, and in near future, the enzyme superfamily may serve as promising path for progress in expanding drug targets for pharmacophore discovery and development.

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 β-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.

Formation of β-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-β-D-glucopyranose (β-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, β-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.

Understanding the Constitutive and Induced Biosynthesis of Mono- and Sesquiterpenes in Grapes (Vitis vinifera): A Key to Unlocking the Biochemical Secrets of Unique Grape Aroma Profiles

The present review integrates current knowledge on mono- and sesquiterpenes in grapes with a special focus on biochemical and physiological aspects. Recent research has impressively shown the prominence of terpenoid metabolism in grapevine (Vitis sp). The 69 putatively functional mono- and sesquiterpene synthases that were identified by the analysis of the updated 12-fold sequencing and assembly of the grapevine genome deliver the scaffolds for structural diversity and display a surprising expansion of the terpene synthase (TPS) gene family in grapevine when compared to other plants like Arabidopsis thaliana (32 TPS). While monoterpenes occur as highly functionalized compounds and are stored as their corresponding glycoconjugates in berry tissues, sesquiterpenes are mainly present as unsaturated hydrocarbons and accumulate in the epicuticular wax layer of intact berries. Interestingly, both groups of terpenes appear to be involved as volatile organic compounds in plant defense and their biosynthesis is enhanced via the jasmonic acid signaling pathway. These novel aspects will help to understand how environmental cues affect the genes and enzymes of various metabolic pathways of relevant wine aroma compounds with numerous links to enology and wine flavor chemistry.

Using the combined analysis of transcripts and metabolites to propose key genes for differential terpene accumulation across two regions

BACKGROUND

Terpenes are of great interest to winemakers because of their extremely low perception thresholds and pleasant floral odors. Even for the same variety, terpene profile can be substantially different for grapevine growing environments. Recently a series of genes required for terpene biosynthesis were biochemically characterized in grape berries. However, the genes that dominate the differential terpene accumulation of grape berries between regions have yet to be identified.

METHODS

Free and glycosidically-bound terpenes were identified and quantified using gas chromatography-mass spectrometry (GC-MS) technique. The transcription expression profiling of the genes was obtained by RNA sequencing and part of the results were verified by quantitative real time PCR (QPCR). The gene co-expression networks were constructed with the Cytoscape software v 2.8.2 ( www.cytoscape.org).

RESULTS

'Muscat Blanc a Petits Grains' berries were collected from two wine-producing regions with strikingly different climates, Gaotai (GT) in Gansu Province and Changli (CL) in Hebei Province in China, at four developmental stages for two consecutive years. GC-MS analysis demonstrated that both free and glycosidically bound terpenes accumulated primarily after veraison and that mature grape berries from CL contained significantly higher concentrations of free and glycosidically bound terpenes than berries from GT. Transcriptome analysis revealed that some key genes involved in terpene biosynthesis were markedly up-regulated in the CL region. Particularly in the MEP pathway, the expression of VviHDR (1-hydroxy-2-methyl-2-butenyl 4-diphosphate reductase) paralleled with the accumulation of terpenes, which can promote the flow of isopentenyl diphosphate (IPP) into the terpene synthetic pathway. The glycosidically bound monoterpenes accumulated differentially along with maturation in both regions, which is synchronous with the expression of a monoterpene glucosyltransferase gene (VviUGT85A2L4 (VviGT14)). Other genes were also found to be related to the differential accumulation of terpenes and monoterpene glycosides in the grapes between regions. Transcription factors that could regulate terpene synthesis were predicted through gene co-expression network analysis. Additionally, the genes involved in abscisic acid (ABA) and ethylene signal responses were expressed at high levels earlier in GT grapes than in CL grapes.

CONCLUSIONS

Differential production of free and glycosidically-bound terpenes in grape berries across GT and CL regions should be related at least to the expression of both VviHDR and VviUGT85A2L4 (VviGT14). Considering the expression patterns of both transcription factors and mature-related genes, we infer that less rainfall and stronger sunshine in the GT region could initiate the earlier expression of ripening-related genes and accelerate the berry maturation, eventually limiting the production of terpene volatiles.

Using the combined analysis of transcripts and metabolites to propose key genes for differential terpene accumulation across two regions

BACKGROUND

Terpenes are of great interest to winemakers because of their extremely low perception thresholds and pleasant floral odors. Even for the same variety, terpene profile can be substantially different for grapevine growing environments. Recently a series of genes required for terpene biosynthesis were biochemically characterized in grape berries. However, the genes that dominate the differential terpene accumulation of grape berries between regions have yet to be identified.

METHODS

Free and glycosidically-bound terpenes were identified and quantified using gas chromatography-mass spectrometry (GC-MS) technique. The transcription expression profiling of the genes was obtained by RNA sequencing and part of the results were verified by quantitative real time PCR (QPCR). The gene co-expression networks were constructed with the Cytoscape software v 2.8.2 ( www.cytoscape.org).

RESULTS

'Muscat Blanc a Petits Grains' berries were collected from two wine-producing regions with strikingly different climates, Gaotai (GT) in Gansu Province and Changli (CL) in Hebei Province in China, at four developmental stages for two consecutive years. GC-MS analysis demonstrated that both free and glycosidically bound terpenes accumulated primarily after veraison and that mature grape berries from CL contained significantly higher concentrations of free and glycosidically bound terpenes than berries from GT. Transcriptome analysis revealed that some key genes involved in terpene biosynthesis were markedly up-regulated in the CL region. Particularly in the MEP pathway, the expression of VviHDR (1-hydroxy-2-methyl-2-butenyl 4-diphosphate reductase) paralleled with the accumulation of terpenes, which can promote the flow of isopentenyl diphosphate (IPP) into the terpene synthetic pathway. The glycosidically bound monoterpenes accumulated differentially along with maturation in both regions, which is synchronous with the expression of a monoterpene glucosyltransferase gene (VviUGT85A2L4 (VviGT14)). Other genes were also found to be related to the differential accumulation of terpenes and monoterpene glycosides in the grapes between regions. Transcription factors that could regulate terpene synthesis were predicted through gene co-expression network analysis. Additionally, the genes involved in abscisic acid (ABA) and ethylene signal responses were expressed at high levels earlier in GT grapes than in CL grapes.

CONCLUSIONS

Differential production of free and glycosidically-bound terpenes in grape berries across GT and CL regions should be related at least to the expression of both VviHDR and VviUGT85A2L4 (VviGT14). Considering the expression patterns of both transcription factors and mature-related genes, we infer that less rainfall and stronger sunshine in the GT region could initiate the earlier expression of ripening-related genes and accelerate the berry maturation, eventually limiting the production of terpene volatiles.

Profiling monoterpenol glycoconjugation in Vitis vinifera L. cv. Muscat of Alexandria using a novel putative compound database approach, high resolution mass spectrometry and collision induced dissociation fragmentation analysis

In this work we present a novel approach for the identification of plant metabolites using ultrahigh performance liquid chromatography coupled to accurate mass time-of-flight mass spectrometry. The workflow involves developing an in-house compound database consisting of exact masses of previously identified as well as putative compounds. The database is used to screen accurate mass spectrometry (MS) data to identify possible compound matches. Subsequent tandem MS data is acquired for possible matches and used for structural elucidation. The methodology is applied to profile monoterpene glycosides in Vitis vinifera cv. Muscat of Alexandria grape berries over three developmental stages. Monoterpenes are a subclass of terpenes, the largest class of plant secondary metabolites, and are found in two major forms in the plant, "bound" to one or more sugar moieties or "free" of said sugar moieties. In the free form, monoterpenes are noted for their fragrance and play important roles in plant defense and as attractants for pollinators. However, glycoconjugation renders these compounds odorless, and it is this form that the plant uses for monoterpene storage. In order to gain insight into monoterpene biochemistry and their fate in the plant an analysis of intact glycosides is essential. Eighteen monoterpene glycosides were identified including a monoterpene trisaccharide glycoside, which is tentatively identified here for this first time in any plant. Additionally, while previous studies have identified monoterpene malonylated glucosides in other grapevine tissue, we tentatively identify them for the first time in grape berries. This analytical approach can be readily applied to other plants and the workflow approach can also be used for other classes of compounds. This approach, in general, provides researchers with data to support the identification of putative compounds, which is especially useful when no standard is available.

Volatile Glycosylation in Tea Plants: Sequential Glycosylations for the Biosynthesis of Aroma β-Primeverosides Are Catalyzed by Two Camellia sinensis Glycosyltransferases

Tea plants (Camellia sinensis) store volatile organic compounds (VOCs; monoterpene, aromatic, and aliphatic alcohols) in the leaves in the form of water-soluble diglycosides, primarily as β-primeverosides (6-O-β-D-xylopyranosyl-β-D-glucopyranosides). These VOCs play a critical role in plant defenses and tea aroma quality, yet little is known about their biosynthesis and physiological roles in planta. Here, we identified two UDP-glycosyltransferases (UGTs) from C. sinensis, UGT85K11 (CsGT1) and UGT94P1 (CsGT2), converting VOCs into β-primeverosides by sequential glucosylation and xylosylation, respectively. CsGT1 exhibits a broad substrate specificity toward monoterpene, aromatic, and aliphatic alcohols to produce the respective glucosides. On the other hand, CsGT2 specifically catalyzes the xylosylation of the 6'-hydroxy group of the sugar moiety of geranyl β-D-glucopyranoside, producing geranyl β-primeveroside. Homology modeling, followed by site-directed mutagenesis of CsGT2, identified a unique isoleucine-141 residue playing a crucial role in sugar donor specificity toward UDP-xylose. The transcripts of both CsGTs were mainly expressed in young leaves, along with β-primeverosidase encoding a diglycoside-specific glycosidase. In conclusion, our findings reveal the mechanism of aroma β-primeveroside biosynthesis in C. sinensis. This information can be used to preserve tea aroma better during the manufacturing process and to investigate the mechanism of plant chemical defenses.

Potential applications of glucosyltransferases in terpene glucoside production: impacts on the use of aroma and fragrance

The detection of glucoconjugated forms of monoterpene alcohols in rose petals in the late 1960s opened the new field of nonvolatile aroma precursors in flavor research. It is now well established that odorless glycosides represent a significant pool of aroma precursors in plants where they act as preformed but inactivated defense or attractive chemicals. Technical improvements in the separation and identification of plant secondary metabolites have provided a multitude of chemical structures, but functional characterization of glycosyltransferases that catalyze their formation lags behind. As technical efforts and costs for DNA sequencing dramatically dropped during the last decade, the number of plant genome sequences increased significantly, thus providing opportunities to functionally characterize the glycosyltransferase gene families in plants. These studies yielded the first glycosyltransferase genes that encode efficient biocatalysts for the production of monoterpene glucosides. They have applications in the food, feed, chemical, cosmetic, and pharmaceutical industries as slow release aroma chemicals.

Activity-based profiling of a physiologic aglycone library reveals sugar acceptor promiscuity of family 1 UDP-glucosyltransferases from grape

Monoterpenols serve various biological functions and accumulate in grape (Vitis vinifera), where a major fraction occurs as nonvolatile glycosides. We have screened the grape genome for sequences with similarity to terpene URIDINE DIPHOSPHATE GLYCOSYLTRANSFERASES (UGTs) from Arabidopsis (Arabidopsis thaliana). A ripening-related expression pattern was shown for three candidates by spatial and temporal expression analyses in five grape cultivars. Transcript accumulation correlated with the production of monoterpenyl β-d-glucosides in grape exocarp during ripening and was low in vegetative tissue. Targeted functional screening of the recombinant UGTs for their biological substrates was performed by activity-based metabolite profiling (ABMP) employing a physiologic library of aglycones built from glycosides isolated from grape. This approach led to the identification of two UDP-glucose:monoterpenol β-d-glucosyltransferases. Whereas VvGT14a glucosylated geraniol, R,S-citronellol, and nerol with similar efficiency, the three allelic forms VvGT15a, VvGT15b, and VvGT15c preferred geraniol over nerol. Kinetic resolution of R,S-citronellol and R,S-linalool was shown for VvGT15a and VvGT14a, respectively. ABMP revealed geraniol as the major biological substrate but also disclosed that these UGTs may add to the production of further glycoconjugates in planta. ABMP of aglycone libraries provides a versatile tool to uncover novel biologically relevant substrates of small-molecule glycosyltransferases that often show broad sugar acceptor promiscuity.