VvHT1 encodes a monosaccharide transporter expressed in the conducting complex of the grape berry phloem

Abstract

The accumulation of sugars in grape berries requires the co-ordinate expression of sucrose transporters, invertases, and monosaccharide transporters. A monosaccharide transporter homologue (VvHT1, Vitis vinifera hexose transporter 1) has previously been isolated from grape berries at the veraison stage, and its expression was shown to be regulated by sugars and abscisic acid. The present work investigates the function and localization of VvHT1. Heterologous expression in yeast indicates that VvHT1 encodes a monosaccharide transporter with maximal activity at acidic pH (pH 4.5) and high affinity for glucose (K(m)=70 muM). Fructose, mannose, sorbitol, and mannitol are not transported by VvHT1. In situ hybridization shows that VvHT1 transcripts are primarily found in the phloem region of the conducting bundles. Immunofluorescence and immunogold labelling experiments localized VvHT1 in the plasma membrane of the sieve element/companion cell interface and of the flesh cells. The expression and functional properties of VvHT1 suggests that it retrieves the monosaccharides needed to provide the energy necessary for cell division and cell growth at an early stage of berry development.

Involvement of Abscisic Acid in the Coordinated Regulation of a Stress-Inducible Hexose Transporter (VvHT5) and a Cell Wall Invertase in Grapevine in Response to Biotrophic Fungal Infection

“…In contrast, SUREs are absent from the VvHT5 promoter region and VvHT5 transcription has been shown to be insensitive to sugar regulation in grape suspension cells (Conde et al, 2006). This further reinforces the hypothesis that, unlike VvHT1 and VvHT3, which appear to be predominantly involved in hexose transport in developing leaves and berries (Vignault et al, 2005;Hayes et al, 2007), VvHT5 appears to have a specific role in enhancing sink strength under stress conditions and that this is controlled through ABA.…”

“…This indicates that the role of VvHT5 is not to specifically increase hexose transport across the plasma membrane of powdery mildew-infected epidermal cells but rather to enhance the supply of sugar from the phloem to the surrounding tissues during the transition from source to sink. Vignault et al (2005) previously showed VvHT1 to be localized to the plasma membrane of the sieve element/companion cell interface in developing grape berries, and the location of VvHT5 promoter-directed expression in Arabidopsis appears to be similar. However, further work will be required to confirm the exact cellular location within the vascular complex of grapevine leaves.…”

“…Uptake studies with VvHT1 and VvHT5 reveal that while there is little difference in their affinity for Glc, VvHT5 is capable of binding Fru whereas VvHT1 is not (Vignault et al, 2005;Hayes et al, 2007). Interestingly, VvHT5 is most closely related to the Arabidopsis HT AtSTP13 (83% amino acid identity compared with only 48%-58% identity for the other 13 AtSTPs), which has also been demonstrated to facilitate Fru uptake into Xenopus oocytes (Norholm et al, 2006).…”

“…Functional Analysis of the VvHT5 Promoter Vignault et al (2005) have previously shown VvHT1 expression to be localized to the phloem region of the vascular bundles in grapevine leaves and berries. In order to determine the localization of VvHT5 induction under stress conditions, a VvHT5-2K promoter:: GUS construct was transformed into Arabidopsis using the VvHT5 promoter sequence amplified from grapevine cv Cabernet Sauvignon genomic DNA (GenBank accession no.…”

A Sugar-Inducible Protein Kinase, VvSK1, Regulates Hexose Transport and Sugar Accumulation in Grapevine Cells

“…Seven plasma membrane monosaccharide transporter homologs (VvHTs) and a plastidic Glc transporter (pGLT) have been cloned from grape berries (Vignault et al, 2005;Hayes et al, 2007). Specific primers were designed for the sequences of VvHT1 (accession no.…”