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.

Eutypa dieback is a devastating disease of Vitis vinifera L. caused by the fungal pathogen Eutypa lata. This wood-inhabiting fungus degrades tissues in the trunk and cordons of infected vines and induces symptoms in the foliage. These symptoms have been attributed to the production of toxic metabolites by the pathogen, in particular eutypine. Recently, we have isolated polypeptide compounds secreted by the fungus in artificial culture. The aims of this study were to examine the effects induced in leaves by applying polypeptides and eutypine to detached canes and to compare this to the changes in leaf structure induced by E. lata in the vineyard. In leaves taken from vines infected with E. lata, the changes in mesophyll cells indicate that the fungus has an effect on tissue remote from the infected area. The size of mesophyll cells decreased by more than half, starch content was reduced and tannins were abundant. Plastids, mitochondria and cell walls were highly modified. In leaves taken from healthy canes treated with polypeptides of E. lata, the structure of mesophyll cells was also modified. The cell size did not change, but the tannin content increased and modifications in plastids and mitochondria were similar to those observed in leaves taken from infected vines. The major effect was the complete disorganisation of cell walls. Eutypine had less effect on organelle structure and did not modify the cell wall. In canes treated with polypeptides, vessel-associated cells (VACs) were also damaged. Abundant tannins occurred in the vacuoles of VACs and marked changes were noted in mitochondria, plastids and the protective layer, in particular in the pit at the vessel interface. In these pits, the protective layer, the primary wall and the middle lamella were all highly modified. In contrast, treatment with eutypine induced the development of a large transfer apparatus bordering the unmodified pectocellulose wall. These results illustrate that treatment with polypeptides produced by E. lata may cause changes in mesophyll cells in leaves and VACs in canes, that resemble changes observed in naturally infected vines. Comparatively, the differences with eutypine action were stressed. Both types of toxins may co-operate in vivo to produce the degeneration observed during the disease.