Agnès Ageorges

Ectopic Expression of VvMybPA2 Promotes Proanthocyanidin Biosynthesis in Grapevine and Suggests Additional Targets in the Pathway

Grapevine (Vitis vinifera) proanthocyanidins contribute to plant defense mechanisms against biotic stress and also play a critical role in organoleptic properties of wine. In grapevine berry, these compounds are mainly accumulated in exocarps and seeds in the very early stages of development. A previous study has already identified VvMybPA1 as the first transcription factor involved in the regulation of the proanthocyanidin pathway during seed development in grapevine. A novel Myb factor, VvMybPA2, which is described in this study, is in contrast mainly expressed in the exocarp of young berries and in the leaves. This transcription factor shows very high protein sequence homology with other plant Myb factors, which regulate flavonoid biosynthesis. Ectopic expression of either VvMybPA1 or VvMybPA2 in grapevine hairy roots induced qualitative and quantitative changes of the proanthocyanidin profiles. High-throughput transcriptomic analyses of transformed grapevine organs identified a large set of putative targets of the VvMybPA1 and VvMybPA2 transcription factors. Both genes significantly activated enzymes of the flavonoid pathway, including anthocyanidin reductase and leucoanthocyanidin reductase 1, the specific terminal steps in the biosynthesis of epicatechin and catechin, respectively, but not leucoanthocyanidin reductase 2. The functional annotation of the genes whose expression was modified revealed putative new actors of the proanthocyanidin pathway, such as glucosyltransferases and transporters.

Grapevine MATE-Type Proteins Act as Vacuolar H+-Dependent Acylated Anthocyanin Transporters

In grapevine (Vitis vinifera), anthocyanins are responsible for most of the red, blue, and purple pigmentation found in the skin of berries. In cells, anthocyanins are synthesized in the cytoplasm and accumulated into the vacuole. However, little is known about the transport of these compounds through the tonoplast. Recently, the sequencing of the grapevine genome allowed us to identify genes encoding proteins with high sequence similarity to the Multidrug And Toxic Extrusion (MATE) family. Among them, we selected two genes as anthocyanin transporter candidates and named them anthoMATE1 (AM1) and AM3. The expression of both genes was mainly fruit specific and concomitant with the accumulation of anthocyanin pigment. Subcellular localization assays in grapevine hairy roots stably transformed with AM1∷ or AM3∷green fluorescent protein fusion protein revealed that AM1 and AM3 are primarily localized to the tonoplast. Yeast vesicles expressing anthoMATEs transported acylated anthocyanins in the presence of MgATP. Inhibitor studies demonstrated that AM1 and AM3 proteins act in vitro as vacuolar H+-dependent acylated anthocyanin transporters. By contrast, under our experimental conditions, anthoMATEs could not transport malvidin 3-O-glucoside or cyanidin 3-O-glucoside, suggesting that the acyl conjugation was essential for the uptake. Taken together, these results provide evidence that in vitro the two grapevine AM1 and AM3 proteins mediate specifically acylated anthocyanin transport.

Quantitative Genetic Bases of Anthocyanin Variation in Grape ( Vitis vinifera L. ssp. sativa ) Berry: A Quantitative Trait Locus to Quantitative Trait Nucleotide Integrated Study

The combination of QTL mapping studies of synthetic lines and association mapping studies of natural diversity represents an opportunity to throw light on the genetically based variation of quantitative traits. With the positional information provided through quantitative trait locus (QTL) mapping, which often leads to wide intervals encompassing numerous genes, it is now feasible to directly target candidate genes that are likely to be responsible for the observed variation in completely sequenced genomes and to test their effects through association genetics. This approach was performed in grape, a newly sequenced genome, to decipher the genetic architecture of anthocyanin content. Grapes may be either white or colored, ranging from the lightest pink to the darkest purple tones according to the amount of anthocyanin accumulated in the berry skin, which is a crucial trait for both wine quality and human nutrition. Although the determinism of the white phenotype has been fully identified, the genetic bases of the quantitative variation of anthocyanin content in berry skin remain unclear. A single QTL responsible for up to 62% of the variation in the anthocyanin content was mapped on a Syrah × Grenache F1 pseudo-testcross. Among the 68 unigenes identified in the grape genome within the QTL interval, a cluster of four Myb-type genes was selected on the basis of physiological evidence (VvMybA1, VvMybA2, VvMybA3, and VvMybA4). From a core collection of natural resources (141 individuals), 32 polymorphisms revealed significant association, and extended linkage disequilibrium was observed. Using a multivariate regression method, we demonstrated that five polymorphisms in VvMybA genes except VvMybA4 (one retrotransposon, three single nucleotide polymorphisms and one 2-bp insertion/deletion) accounted for 84% of the observed variation. All these polymorphisms led to either structural changes in the MYB proteins or differences in the VvMybAs promoters. We concluded that the continuous variation in anthocyanin content in grape was explained mainly by a single gene cluster of three VvMybA genes. The use of natural diversity helped to reduce one QTL to a set of five quantitative trait nucleotides and gave a clear picture of how isogenes combined their effects to shape grape color. Such analysis also illustrates how isogenes combine their effect to shape a complex quantitative trait and enables the definition of markers directly targeted for upcoming breeding programs.

In vivo grapevine anthocyanin transport involves vesicle‐mediated trafficking and the contribution of anthoMATE transporters and GST

In cells, anthocyanin pigments are synthesized at the cytoplasmic surface of the endoplasmic reticulum, and are then transported and finally accumulated inside the vacuole. In Vitis vinifera (grapevine), two kinds of molecular actors are putatively associated with the vacuolar sequestration of anthocyanins: a glutathione-S-transferase (GST) and two MATE-type transporters, named anthoMATEs. However, the sequence of events by which anthocyanins are imported into the vacuole remains unclear. We used MYBA1 transformed hairy roots as a grapevine model tissue producing anthocyanins, and took advantage of the unique autofluorescence of anthocyanins to study their cellular trafficking. In these tissues, anthocyanins were not only visible in the largest vacuoles, but were also present at higher concentrations in several vesicles of different sizes. In the cell, small vesicles actively moved alongside the tonoplast, suggesting a vesicular trafficking to the vacuole. Subcellular localization assays revealed that anthoMATE transporters were closely related with these small vesicles, whereas GST was localized in the cytoplasm around the nucleus, suggesting an association with the endoplasmic reticulum. Furthermore, cells in hairy roots expressing anthoMATE antisense did not display small vesicles filled with anthocyanins, whereas in hairy roots expressing GST antisense, anthocyanins were accumulated in vesicles but not in the vacuole. This suggests that in grapevine, anthoMATE transporters and GST are involved in different anthocyanin transport mechanisms.

ABCC1, an ATP Binding Cassette Protein from Grape Berry, Transports Anthocyanidin 3-O-Glucosides

This work provides biochemical evidence that ABCC transporters are directly involved in anthocyanin transport into plant vacuoles. The presence of reduced glutathione is a prerequisite for the transport. Our data support that anthocyanins and glutathione are cotransported but that no glutathione anthocyanin conjugate is formed. Accumulation of anthocyanins in the exocarp of red grapevine (Vitis vinifera) cultivars is one of several events that characterize the onset of grape berry ripening (véraison). Despite our thorough understanding of anthocyanin biosynthesis and regulation, little is known about the molecular aspects of their transport. The participation of ATP binding cassette (ABC) proteins in vacuolar anthocyanin transport has long been a matter of debate. Here, we present biochemical evidence that an ABC protein, ABCC1, localizes to the tonoplast and is involved in the transport of glucosylated anthocyanidins. ABCC1 is expressed in the exocarp throughout berry development and ripening, with a significant increase at véraison (i.e., the onset of ripening). Transport experiments using microsomes isolated from ABCC1-expressing yeast cells showed that ABCC1 transports malvidin 3-O-glucoside. The transport strictly depends on the presence of GSH, which is cotransported with the anthocyanins and is sensitive to inhibitors of ABC proteins. By exposing anthocyanin-producing grapevine root cultures to buthionine sulphoximine, which reduced GSH levels, a decrease in anthocyanin concentration is observed. In conclusion, we provide evidence that ABCC1 acts as an anthocyanin transporter that depends on GSH without the formation of an anthocyanin-GSH conjugate.

Changes in acidity and in proton transport at the tonoplast of grape berries during development.

Abstract. As in many fruits, the induction of grape berry (Vitis vinifera L.) ripening results in intense breakdown of malic acid. Using membrane fractions, we tested the hypothesis that changes in acidity resulted from malate vacuolar decompartmentation. The hydrolytic activities of the two primary vacuolar pumps inorganic pyrophosphatase (V-PPase; EC 3.6.1.1) and vacuolar ATPase (V-ATPase; EC 3.6.1.3) increased throughout development with an acceleration during ripening, as confirmed by Western blotting and analysis of transcript expression. The ratio of V-PPase activity to V-ATPase activity was always in favour of V-PPase and reached its maximum value at véraison. The rate of anion transport strongly increased during ripening. Before ripening, tonoplast passive permeability was low, but rose during ripening. Our data indicate that tonoplast leakage dramatically increased during ripening. This leakage is probably the prime cause of malate decompartmentation, amplified by the incapacity of oxidative phosphorylation to face increased energy demand.

A novel cation-dependent O-methyltransferase involved in anthocyanin methylation in grapevine

Anthocyanins are major pigments in colored grape (Vitis vinifera) berries, and most of them are monomethoxylated or dimethoxylated. We report here the functional characterization of an anthocyanin O-methyltransferase (AOMT) from grapevine. The expression pattern in two cultivars with different anthocyanin methylation profiles (Syrah and Nebbiolo) showed a peak at start ripening (véraison), when the concentrations of all methylated anthocyanins begin to increase. The purified recombinant AOMT protein was active on both anthocyanins and flavonols in vitro, with Km in the micromolar range, and was dependent on divalent cations for activity. AOMT showed a preference for 3′,5′ methylation when a 3′,4′,5′ hydroxylated anthocyanin substrate was tested. In order to assess its in planta activity, we performed transient expression of AOMT in tobacco (Nicotiana benthamiana) leaves expressing the Production of Anthocyanin Pigment1 (PAP1) transcription factor from Arabidopsis (Arabidopsis thaliana). PAP1 expression in leaves induced the accumulation of the nonmethylated anthocyanin delphinidin 3-rutinoside. The coexpression of PAP1 and AOMT resulted in an accumulation of malvidin 3-rutinoside. We also showed that AOMT localized exclusively in the cytoplasm of tobacco leaf cells. These results demonstrate the ability of this enzyme to methylate anthocyanins both in vitro and in vivo, indicating that AOMT plays a major role in anthocyanin biosynthesis in grape berries.

Identification of genes associated with flesh morphogenesis during grapevine fruit development

Fruit morphogenesis is a process unique to the angiosperms, and yet little is known about its developmental control. Following fertilization, fruits typically undergo a dramatic enlargement that is accompanied by differentiation of numerous distinct cell types. To identify genes putatively involved in the early development of grapevine fruit, we used the fleshless berry mutant (Vitis vinifera L. cv Ugni Blanc) that has dramatically reduced fruit size due to a lack of pericarp development. Using oligo-specific arrays, 53 and 50 genes were identified as being down- and up-regulated, respectively, in the mutant. In parallel, Suppression Subtractive Hybridization performed between the mutant and the wild type (WT) allowed the identification of new transcripts differentially expressed during the first stages of mutant and WT pericarp development. From this data, the picture emerged that the mutation promotes the expression of several genes related to ripening and/or to stress and impairs the expression of several regulatory genes. Among those, five genes encoding proteins previously reported to be associated with, or involved in, developmental processes in other species (a specific tissue protein 2, ATHB13, a BURP domain protein, PISTILLATA, and YABBY2), were identified and investigated further using real-time PCR and in situ hybridization. Expression in the pericarp was confirmed, specific spatial and/or temporal patterns were detected and differences were observed between the WT and the mutant during fruit development. Expression of these genes appeared to be affected during young fruit development in the mutant, suggesting that they may play a role in grape berry morphogenesis.

The Phenylpropanoid Pathway Is Controlled at Different Branches by a Set of R2R3-MYB C2 Repressors in Grapevine

A set of transcriptional repressors negatively regulates the expression of genes involved in different branches of the phenylpropanoid pathway. Because of the vast range of functions that phenylpropanoids possess, their synthesis requires precise spatiotemporal coordination throughout plant development and in response to the environment. The accumulation of these secondary metabolites is transcriptionally controlled by positive and negative regulators from the MYB and basic helix-loop-helix protein families. We characterized four grapevine (Vitis vinifera) R2R3-MYB proteins from the C2 repressor motif clade, all of which harbor the ethylene response factor-associated amphiphilic repression domain but differ in the presence of an additional TLLLFR repression motif found in the strong flavonoid repressor Arabidopsis (Arabidopsis thaliana) AtMYBL2. Constitutive expression of VvMYB4a and VvMYB4b in petunia (Petunia hybrida) repressed general phenylpropanoid biosynthetic genes and selectively reduced the amount of small-weight phenolic compounds. Conversely, transgenic petunia lines expressing VvMYBC2-L1 and VvMYBC2-L3 showed a severe reduction in petal anthocyanins and seed proanthocyanidins together with a higher pH of crude petal extracts. The distinct function of these regulators was further confirmed by transient expression in tobacco (Nicotiana benthamiana) leaves and grapevine plantlets. Finally, VvMYBC2-L3 was ectopically expressed in grapevine hairy roots, showing a reduction in proanthocyanidin content together with the down-regulation of structural and regulatory genes of the flavonoid pathway as revealed by a transcriptomic analysis. The physiological role of these repressors was inferred by combining the results of the functional analyses and their expression patterns in grapevine during development and in response to ultraviolet B radiation. Our results indicate that VvMYB4a and VvMYB4b may play a key role in negatively regulating the synthesis of small-weight phenolic compounds, whereas VvMYBC2-L1 and VvMYBC2-L3 may additionally fine tune flavonoid levels, balancing the inductive effects of transcriptional activators.

Analysis of cell wall neutral sugar composition, β-galactosidase activity and a related cDNA clone throughout the development of Vitis vinifera grape berries.

Abstract Fruit softening during ripening is accompanied by changes in cell wall composition due to the action of cell wall modifying enzymes. Moreover, the cell walls of grape berries form a barrier to the diffusion of aromatic and polyphenolic compounds which are important for wine quality. Samples of grape berries ( Vitis vinifera L., cv Ugni blanc) were harvested in 1996 and 1997 at twelve different developmental stages. The development of berries was characterized by physical, chemical and biochemical analysis. Isolated cell walls were analysed for their neutral sugar contents. The main changes during grape berry development were a large decrease in galactose parallel with glucose accumulation, while other neutral sugars (arabinose, rhamnose, xylose, fucose and mannose) showed no significant variations. For individual berries, galactose loss seemed to be softening-related, while galactose removal per mg of cell wall material was involved in a more general ripening process. β-Galactosidase (EC 3.2.1.23) activity was temporally associated with the loss of cell wall-linked galactosyl residues. A 545-base long partial cDNA ( ϐ-gal 10, accession No. AF159124, GenBank) was isolated from first strand cDNA, and shared significant similarities with several β-gals in data banks. The pattern of transcript expression showed that β-gal 10 was only detectable in the early stages of development, suggesting that β-gal 10 may encode for a β-galactosidase active on cell walls during the early development of grape berries. Relationships between galactose content of the cell wall, β-galactosidase activity and expression of the corresponding transcripts, and their possible involvement in grape berry softening and ripening are discussed.