In plants, SoSUT1 (Spinacia oleracea Sucrose Transporter 1) was the first disaccharide transporter (DST) functionally characterized in a yeast mutant deleted for invertase and expressing sucrose synthase (Riesmeier et al. 1992). DSTs genes, which belong to small multigenic families with 9 members in Arabidopsis and 4 in tomato for instance, encode for a 55 kD polypeptide (Arabidopsis Genome Initiative 2000, Delrot et al. 2001, Sauer et al. 2004, Hackel et al. 2006). Plant DSTs can be clustered into 4 groups regarding their protein sequence homologies: group 1 and group 2 are exclusively composed by monocots and dicots transporter proteins, respectively, whereas groups 3 and 4 are mixed. Group 3 clusters proteins possess longer N-termini and central cytoplasmic loops whereas group 4 sequences display shorter C-termini (Barth et al. 2003).
Group 3 transporters have been reported to localize all along the sieve element and have been proposed to sense the sucrose flux through the plasma membrane (Schulze et al. 2000). Group 4 sucrose transporters are low affinity high capacity transporters (LAHC) localized in the membranes of minor veins of source leaves (Weise et al. 2000). However, their subcellular localization is controversial since, although correctly directed to the yeast plasma membrane, some of these proteins may be specific to the tonoplast in plants (Endler et al.
2006). Up to date, three DST cDNAs have been cloned from Shiraz and Cabernet Sauvignon (VvSUCU; AF021808, also identified as VvSUT1 AF182445; VvSUC12 AF021809; VvSUC27, AF021810) and characterised as proton-dependent sucrose transporters, whereas 9 DSTs sequences are present in Arabidopsis genome (Sauer et al. 2004). VvSUC11 and VvSUC12 are intermediate affinity sucrose transporters (Km, 0.9 mM and 1.4 mM, respectively) (Ageorges et al. 2000, Manning et al. 2001), whereas VvSUC27 has a Km of about 10 mM and is thus a low affinity sucrose transporter (Zhang et al. 2008). The grape genome sequence recently released (Jaillon et al. 2007, Velasco et al.
2007) suggests that sucrose transporter genes would constitute a small multi-genic family of 4 members in this species (Fig. 3).
VvSUC11 is expressed in flowers and fruits whereas VvSUC12 expression is restricted to berries and young leaves. In addition, VvSUC11 is expressed in both young and expanded leaves. VvSUC27 expression is closely related to sink activity since its transcripts are strongly accumulated in flowers and unripe berries, roots and tendrils but poorly present in mature leaves (Davies et al. 1999). VvSUC27 expression is associated with the early stages of berry development, VvSUC11 and VvSUC12 transcription concomitantly increases with post-veraison sugar accumulation, which suggests a direct pathway for sucrose acquisition by berry cells (Davies et al. 1999). However, information regarding sucrose uptake in berry along ripening is scarce. Sucrose uptake activity has been demonstrated in berry slices (Conde et al. unpublished results) but further investigation, such as sucrose transporters localization in berry flesh is needed (Hayes et al. 2007).
Indeed, even if most of the sucrose transporters yet characterised are responsible for the loading of phloem conducting cells, the involvement of sucrose carriers into the uptake by the storage cells of sugars unloaded at the end of the phloem path has been described [e.g. DcSUT2 in carrot (Shakya and Sturm, 1998), StSUT1 in potato (Kühn et al. 1997), CiSTU2 in Citrus (Li et al. 2003), AtSUC3 in A. thaliana (Meyer et al. 2004) and LeSUT1 in tomato (Hackel et al. 2006)]. The presence of SUT transporters in sucrose releasing
tissues of cereals (Bagnall et al. 2000, Aoki et al. 2006) and the demonstration that AtSUC2 loss-of-function and StSUT1 sink-specific antisense repression affect phloem unloading in Arabidopsis and potato, respectively (Gottwald et al. 2000, Kühn et al. 2003) suggest that some of the SUT proteins can be directly involved in phloem unloading and thus may function as reversible transporters. Indeed, Carpaneto and co-workers expressed ZmSUT1 (orthologue of AtSUC2) in Xenopus oocytes and succeeded in inversing its transport mode (Carpaneto et al. 2005).
The involvement of a classical sucrose/proton symporter in the proton-independent sucrose efflux activity observed in potato plasma membrane vesicles has also been suggested. This protein would then act in a different way under certain conditions, as it has been reported for the CkHUP1 glucose transporter (Komor and Tanner 1974, Kühn et al. 1997). Since sucrose unloading occurs down the sucrose gradient, sucrose efflux involves biochemical mechanisms that differ from those driving its uptake into sucrose accumulating tissues and the functional asymmetry of these transporters would be driven by specific membrane potential and gradients (Carpaneto et al. 2005). Furthermore, the roles of a putative sucrose/proton antiporter and of a non-selective channel has also been suggested in seed coats of broad bean and pea, respectively (De Jong et al. 1996, Walker et al. 1995, 2000). Eventually, the participation of DSTs as retrievers of unloaded sucrose into the apoplasm, or as direct modulators of sucrose concentration and sink strength is well demonstrated in sink tissues (Lalonde et al. 2003).
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