A new subfamily of sucrose transporters from Arabidopsis ( AtSUT4 ), tomato ( LeSUT4 ), and potato ( StSUT4 ) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The K m for sucrose uptake by AtSUT4 of 11.6 ؎ 0.6 mM was ‫ف‬ 10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate sieve elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the sieve elements. INTRODUCTIONThe reduced carbon produced through photosynthesis in mature leaves is distributed by the vascular system, mainly in the form of sucrose, to support the growth of heterotrophic (sink) tissues such as developing leaves, the shoot apex, roots, and reproductive organs. Within the vascular tissue, the sieve elements in the phloem form the conduits for long-distance transport. Sieve elements are highly specialized, lacking many organelles (including a nucleus and vacuole) at maturity, and hence depend on tightly associated companion cells for metabolic support (Sjölund, 1997). The loading of sucrose into the sieve element/companion cell (SE/CC) complex in many plants requires the active uptake of sucrose from the extracellular space. Because of variability in the rate of photosynthesis according to changes in environmental conditions, and because sink demands change depending on development and external factors, we can reasonably assume that the rate of phloem loading of sucrose is regulated. In fact, the phenotype of transgenic plants overexpressing pyruvate decarboxylase indicates that sugar export from potato leaves can be upregulated by as much as 10-fold (Tadege et al., 1998). The increase in sucrose transport activity caused by modification of a conserved histidine in the first external loop (Lu and Bush, 1998) indicates that sucrose transporters may be directly regulated at the protein level. In addition, the amounts of mRNA for sucrose transporter SUT1 from potato are developmentally controlled and hormonally regulated (Riesmeier et al., 1993;Harms et al., 1994).Clearly, multiple kinetic components of sucrose uptake are present in leaves (Delrot and Bonnemain, 1981;Maynard and Lucas, 1982). As demonstrated by autoradiography, 14 C-sucrose, externally applied to source leaves of Vicia faba or Beta vulgaris , is taken up by mesophyll cells and phloem (Fondy and Geiger, 1977;Giaquinta, 1977;Delrot, 1981). The overall K m for sucrose uptake into leaves is pH dependent, with greater affinity being measured at ...

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In leaves, sucrose uptake kinetics involve high-and low-affinity components. A family of low-and high-affinity sucrose transporters (SUT) was identified. SUT1 serves as a high-affinity transporter essential for phloem loading and long-distance transport in solanaceous species. SUT4 is a low-affinity transporter with an expression pattern overlapping that of SUT1. Both SUT1 and SUT4 localize to enucleate sieve elements of tomato. New sucrose transporter-like proteins, named SUT2, from tomato and Arabidopsis contain extended cytoplasmic domains, thus structurally resembling the yeast sugar sensors SNF3 and RGT2. Features common to these sensors are low codon bias, environment of the start codon, low expression, and lack of detectable transport activity. In contrast to LeSUT1 , which is induced during the sink-to-source transition of leaves, SUT2 is more highly expressed in sink than in source leaves and is inducible by sucrose. LeSUT2 protein colocalizes with the low-and high-affinity sucrose transporters in sieve elements of tomato petioles, indicating that multiple SUT mRNAs or proteins travel from companion cells to enucleate sieve elements. The SUT2 gene maps on chromosome V of potato and is linked to a major quantitative trait locus for tuber starch content and yield. Thus, the putative sugar sensor identified colocalizes with two other sucrose transporters, differs from them in kinetic properties, and potentially regulates the relative activity of low-and high-affinity sucrose transport into sieve elements. INTRODUCTIONSucrose, the major product of photosynthesis in mature leaves, is loaded into the vascular tissue for translocation to heterotrophic tissues to support their growth. In solanaceous plants, SUT1 is essential for phloem loading into sieve elements (Riesmeier et al., 1994;Kühn et al., 1996;Bürkle et al., 1998). SUT1 serves as a high-affinity transporter for sucrose ( K m ‫ف‬ 1 mM; Riesmeier et al., 1993), whereas SUT4, with a K m of ‫ف‬ 11 mM, is a low-affinity sucrose transporter (Weise et al., 2000). Both proteins colocalize in sieve elements (Kühn et al., 1997;Weise et al., 2000). Localization of SUT1 protein in sieve elements and SUT1 mRNA at the orifices of plasmodesmata interconnecting companion cells and sieve elements, together with the high turnover of both SUT1 mRNA and protein, indicate that trafficking of mRNA or protein occurs from companion cells into enucleate sieve elements by way of plasmodesmata (Kühn et al., 1997).Sugar transport is highly regulated, and sucrose-specific signaling pathways are involved in controlling transport activity (Chiou and Bush, 1998), potentially by using protein phosphorylation (Roblin et al., 1998). Overexpression of pyruvate decarboxylase in potato leads to a 10-fold increase in sugar export, demonstrating the capacity to regulate sugar export from leaves within a wide dynamic range (Tadege et al., 1998). This poses the question of how regulation is coordinated between sieve elements that contain the transporters and companion cells in which transcri...

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