Three putative sucrose transporters are differentially expressed in grapevine tissues

Abstract

A total of three full-length cDNA clones (VvSUC11, VvSUC12, VvSUC27) encoding proteins with sequence similarity to membrane located solute transporters have been isolated from grape (Vitis vinifera L.) berries. Sequence analysis indicated that these deduced proteins are most closely related to plant, plasma membrane located, sucrose-H⁺ symporters. Northern blot analysis showed that these clones are differentially expressed in different tissues. Expression of VvSUC11 and VvSUC12 was upregulated in berries when hexose accumulation commenced in the vacuole while the expression of VvSUC27 declined during ripening. These results indicate a possible role for sucrose transporters in the ripening-associated accumulation of hexoses in the berry vacuole.

Introduction

Grape berries are non-climacteric fruit with a double sigmoidal growth curve. In the second phase of rapid growth berries undergo ripening, during which they soften, become less acidic due to a decrease in the levels of organic acids and accumulate hexoses, aroma and flavour compounds and, in the case of red varieties, anthocyanins in the skin [1]. In viticultural terms the commencement of ripening is known as ‘veraison’. The transport of sugars is an important part of the ripening process in grapes. Photosynthate is transported to the fruit via the phloem as sucrose [2], and is subsequently cleaved in the berry where glucose and fructose accumulate in the cell vacuole in roughly equal amounts after veraison [3]. Previous work [4] has shown that two putative vacuolar invertase genes are expressed early in berry development but their expression diminishes rapidly after veraison. The mRNA accumulation patterns parallel the increase in soluble invertase activity. Most, if not all, of the invertase activity in grape berries is present in the soluble fraction [5], [6], [7]. Levels of another sucrose cleaving enzyme, sucrose synthase, do increase slightly after veraison but are still approximately 100-fold lower than the level of invertase activity [8]. It was therefore concluded that although these invertases probably control the composition of sugars in the berry vacuole they are not likely to trigger the onset of hexose accumulation during ripening. The simplest interpretation of these results is that sucrose is imported into the vacuole where it is cleaved by the vacuolar invertases to hexoses. There are however, a number of proposed models for the unloading and import of sugars into plant cells and it is possible that more than one route may be possible in any given cell depending on the tissue type and the stage of development. Whatever the route taken, the unloading of sucrose from the phloem and the subsequent transport of sugars (either as sucrose or hexoses) into the vacuole are likely to be key control points in the storage pathway in grapes.

Membrane located transporter proteins have been shown to conduct the active transport of sugars in a wide range of organisms. Both hexose and sucrose transporters have been identified in plants [9], [10]. These proteins contain 12 membrane spanning regions and form part of a larger group of transporters with similar structures found in both eucaryotes and procaryotes, known as the major facilitator superfamily (MFS) [11].

To investigate the role of transporters in sugar accumulation in grapes during ripening we have isolated putative sucrose transporter cDNAs. Analysis of their sequences and determination of their patterns of expression suggest that they comprise two groups which are likely to differ in their roles in sugar accumulation in grape berries.