Routes to the tonoplast: the sorting of tonoplast transporters in Arabidopsis mesophyll protoplasts

Vacuoles perform a multitude of functions in plant cells, including the storage of amino acids and sugars. Tonoplast-localized transporters catalyze the import and release of these molecules. The mechanisms determining the targeting of these transporters to the tonoplast are largely unknown. Using the paralogous Arabidopsis thaliana inositol transporters INT1 (tonoplast) and INT4 (plasma membrane), we performed domain swapping and mutational analyses and identified a C-terminal di-leucine motif responsible for the sorting of higher plant INT1-type transporters to the tonoplast in Arabidopsis mesophyll protoplasts. We demonstrate that this motif can reroute other proteins, such as INT4, SUCROSE TRANSPORTER2 (SUC2), or SWEET1, to the tonoplast and that the position of the motif relative to the transmembrane helix is critical. Rerouted INT4 is functionally active in the tonoplast and complements the growth phenotype of an int1 mutant. In Arabidopsis plants defective in the β-subunit of the AP-3 adaptor complex, INT1 is correctly localized to the tonoplast, while sorting of the vacuolar sucrose transporter SUC4 is blocked in cis-Golgi stacks. Moreover, we demonstrate that both INT1 and SUC4 trafficking to the tonoplast is sensitive to brefeldin A. Our data show that plants possess at least two different Golgi-dependent targeting mechanisms for newly synthesized transporters to the tonoplast.     

Proton-driven sucrose symport and antiport are provided by the vacuolar transporters SUC4 and TMT1/2

The vacuolar membrane is involved in solute uptake into and release from the vacuole, which is the largest plant organelle. In addition to inorganic ions and metabolites, large quantities of protons and sugars are shuttled across this membrane. Current models suggest that the proton gradient across the membrane drives the accumulation and/or release of sugars. Recent studies have associated AtSUC4 with the vacuolar membrane. Some members of the SUC family are plasma membrane proton/sucrose symporters. In addition, the sugar transporters TMT1 and TMT2, which are localized to the vacuolar membrane, have been suggested to function in proton-driven glucose antiport. Here we used the patch-clamp technique to monitor carrier-mediated sucrose transport by AtSUC4 and AtTMTs in intact Arabidopsis thaliana mesophyll vacuoles. In the whole-vacuole configuration with wild-type material, cytosolic sucrose-induced proton currents were associated with a proton/sucrose antiport mechanism. To identify the related transporter on one hand, and to enable the recording of symporter-mediated currents on the other hand, we electrophysiologically characterized vacuolar proteins recognized by Arabidopsis mutants of partially impaired sugar compartmentation. To our surprise, the intrinsic sucrose/proton antiporter activity was greatly reduced when vacuoles were isolated from plants lacking the monosaccharide transporter AtTMT1/TMT2. Transient expression of AtSUC4 in this mutant background resulted in proton/sucrose symport activity. From these studies, we conclude that, in the natural environment within the Arabidopsis cell, AtSUC4 most likely catalyses proton-coupled sucrose export from the vacuole. However, TMT1/2 probably represents a proton-coupled antiporter capable of high-capacity loading of glucose and sucrose into the vacuole.     

Identification of a vacuolar sucrose transporter in barley and Arabidopsis mesophyll cells by a tonoplast proteomic approach

The vacuole is the main cellular storage pool, where sucrose (Suc) accumulates to high concentrations. While a limited number of vacuolar membrane proteins, such as V-type H(+)-ATPases and H(+)-pyrophosphatases, are well characterized, the majority of vacuolar transporters are still unidentified, among them the transporter(s) responsible for vacuolar Suc uptake and release. In search of novel tonoplast transporters, we used a proteomic approach, analyzing the tonoplast fraction of highly purified mesophyll vacuoles of the crop plant barley (Hordeum vulgare). We identified 101 proteins, including 88 vacuolar and putative vacuolar proteins. The Suc transporter (SUT) HvSUT2 was discovered among the 40 vacuolar proteins, which were previously not reported in Arabidopsis (Arabidopsis thaliana) vacuolar proteomic studies. To confirm the tonoplast localization of this Suc transporter, we constructed and expressed green fluorescent protein (GFP) fusion proteins with HvSUT2 and its closest Arabidopsis homolog, AtSUT4. Transient expression of HvSUT2-GFP and AtSUT4-GFP in Arabidopsis leaves and onion (Allium cepa) epidermal cells resulted in green fluorescence at the tonoplast, indicating that these Suc transporters are indeed located at the vacuolar membrane. Using a microcapillary, we selected mesophyll protoplasts from a leaf protoplast preparation and demonstrated unequivocally that, in contrast to the companion cell-specific AtSUC2, HvSUT2 and AtSUT4 are expressed in mesophyll protoplasts, suggesting that HvSUT2 and AtSUT4 are involved in transport and vacuolar storage of photosynthetically derived Suc.     

Arabidopsis INOSITOL TRANSPORTER4 mediates high-affinity H+ symport of myoinositol across the plasma membrane

Four genes of the Arabidopsis (Arabidopsis thaliana) monosaccharide transporter-like superfamily share significant homology with transporter genes previously identified in the common ice plant (Mesembryanthemum crystallinum), a model system for studies on salt tolerance of higher plants. These ice plant transporters had been discussed as tonoplast proteins catalyzing the inositol-dependent efflux of Na(+) ions from vacuoles. The subcellular localization and the physiological role of the homologous proteins in the glycophyte Arabidopsis were unclear. Here we describe Arabidopsis INOSITOL TRANSPORTER4 (AtINT4), the first member of this subgroup of Arabidopsis monosaccharide transporter-like transporters. Functional analyses of the protein in yeast (Saccharomyces cerevisiae) and Xenopus laevis oocytes characterize this protein as a highly specific H(+) symporter for myoinositol. These activities and analyses of the subcellular localization of an AtINT4 fusion protein in Arabidopsis and tobacco (Nicotiana tabacum) reveal that AtINT4 is located in the plasma membrane. AtINT4 promoter-reporter gene plants demonstrate that AtINT4 is strongly expressed in Arabidopsis pollen and phloem companion cells. The potential physiological role of AtINT4 is discussed.     

Arabidopsis SUC1 loads the phloem in suc2 mutants when expressed from the SUC2 promoter

Active loading of sucrose into phloem companion cells (CCs) is an essential process in apoplastic loaders, such as Arabidopsis or tobacco (Nicotiana sp.), and is even used by symplastic loaders such as melon (Cucumis melo) under certain stress conditions. Reduction of the amount or complete removal of the transporters catalysing this transport step results in severe developmental defects. Here we present analyses of two Arabidopsis lines, suc2-4 and suc2-5, that carry a null allele of the SUC2 gene which encodes the Arabidopsis phloem loader. These lines were complemented with constructs expressing either the Arabidopsis SUC1 or the Ustilago maydis srt1 cDNA from the SUC2 promoter. Both SUC1 and Srt1 are energy-dependent sucrose/H(+) symporters and differ in specific kinetic properties from the SUC2 protein. Transgene expression was confirmed by RT-PCRs, the subcellular localization of Srt1 in planta with an Srt1-RFP fusion, and the correct CC-specific localization of the recombinant proteins by immunolocalization with anti-Srt1 and anti-SUC1 antisera. The transport capacity of Srt1 was studied in Srt1-GFP expressing Arabidopsis protoplasts. Although both proteins were found exclusively in CCs, only SUC1 complemented the developmental defects of suc2-4 and suc2-5 mutants. As SUC1 and Srt1 are well characterized, this result provides an insight into the properties that are essential for sucrose transporters to load the phloem successfully.     

Functional characterization of the Arabidopsis AtSUC2 Sucrose/H+ symporter by tissue-specific complementation reveals an essential role in phloem loading but not in long-distance transport

AtSUC2 (At1g22710) encodes a phloem-localized sucrose (Suc)/H(+) symporter necessary for efficient Suc transport from source tissues to sink tissues in Arabidopsis (Arabidopsis thaliana). AtSUC2 is highly expressed in the collection phloem of mature leaves, and its function in phloem loading is well established. AtSUC2, however, is also expressed strongly in the transport phloem, where its role is more ambiguous, and it has been implicated in mediating both efflux and retrieval to and from flanking tissues via the apoplast. To characterize the role of AtSUC2 in controlling carbon partitioning along the phloem path, AtSUC2 cDNA was expressed from tissue-specific promoters in an Atsuc2 mutant background. Suc transport in this mutant is highly compromised, as indicated by stunted growth and the accumulation of large quantities of sugar and starch in vegetative tissues. Expression of AtSUC2 cDNA from the 2-kb AtSUC2 promoter was sufficient to restore growth and carbon partitioning to nearly wild-type levels. The GALACTINOL SYNTHASE promoter of Cucumis melo (CmGAS1p) confers expression only in the minor veins of mature leaves, not in the transport phloem of larger leaf veins and stems. Mutant plants expressing AtSUC2 cDNA from CmGAS1p had intermediate growth and accumulated sugar and starch, but otherwise they had normal morphology. These characteristics support a role for AtSUC2 in retrieval but not efflux along the transport phloem and show that the only vital function of AtSUC2 in photoassimilate distribution is phloem loading. In addition, Atsuc2 mutant plants, although debilitated, do grow, and AtSUC2-independent modes of phloem transport are discussed, including an entirely symplastic pathway from mesophyll cells to sink tissues.     

AtSUC8 and AtSUC9 encode functional sucrose transporters, but the closely related AtSUC6 and AtSUC7 genes encode aberrant proteins in different Arabidopsis ecotypes

Three members of the Arabidopsis sucrose transporter gene family, AtSUC6-AtSUC8 (At5g43610; At1g66570; At2g14670), share a high degree of sequence homology in their coding regions and even in their introns and in their 5'- and 3'-flanking regions. A fourth sucrose transporter gene, AtSUC9 (At5g06170), which is on the same branch of the AtSUC-phylogenetic tree, shows only slightly less sequence homology. Here we present data demonstrating that two genes from this subgroup, AtSUC6 and AtSUC7, encode aberrant proteins and seem to represent sucrose transporter pseudogenes, whereas AtSUC8 and AtSUC9 encode functional sucrose transporters. These results are based on analyses of splice patterns and polymorphic sites between these genes in different Arabidopsis ecotypes, as well as on functional analyses by cDNA expression in baker's yeast. For one of these genes, AtSUC7 (At1g66570), different, ecotype-specific splice patterns were observed in Wassilewskija (Ws), C24, Columbia wild type (Col-0) and Landsberg erecta (Ler). No incorrect splicing and no sequence polymorphism were detected in the cDNAs of AtSUC8 and AtSUC9, which encode functional sucrose transporters and are expressed in floral tissue. Finally, promoter-reporter gene plants and T-DNA insertion lines were analyzed for AtSUC8 and AtSUC9.     

Wounding enhances expression of AtSUC3, a sucrose transporter from Arabidopsis sieve elements and sink tissues

The Arabidopsis AtSUC3 gene encodes a sucrose (Suc) transporter that differs in size and intron number from all other Arabidopsis Suc transport proteins. Each plant species analyzed so far possesses one transporter of this special type, and several functions have been discussed for these proteins, including the catalysis of transmembrane Suc transport, and also Suc sensing and regulation of other Suc transporters. Here, we show that the AtSUC3 protein is localized in the sieve elements of the Arabidopsis phloem and is not colocalized with the companion cell-specific AtSUC2 phloem loader. Even stronger AtSUC3 expression is observed in numerous sink cells and tissues, such as guard cells, trichomes, germinating pollen, root tips, the developing seed coat, or stipules. Moreover, AtSUC3 expression is strongly induced upon wounding of Arabidopsis tissue. The physiological role of AtSUC3 in these different cells and tissues is discussed.     

The AtSUC1 sucrose carrier may represent the osmotic driving force for anther dehiscence and pollen tube growth in Arabidopsis

The Arabidopsis AtSUC1 protein has previously been characterized as a plasma membrane H+-sucrose symporter. This paper describes the sites of AtSUC1 gene expression and AtSUC1 protein localization and assigns specific functions to this sucrose transporter in anther development and pollen tube growth. RNase protection assays revealed AtSUC1 expression exclusively in floral tissue, which was confirmed by analyses of AtSUC1 promoter-beta-glucuronidase (GUS) plants. In situ hybridizations identified AtSUC1 expression in anther connective tissue, in funiculi and in fully developed pollen grains. Indirect immuno-fluorescence analyses with anti-AtSUC1 antiserum confirmed AtSUC1 protein localization in the connective tissue and funiculi. In mature pollen grains, however, despite high AtSUC1 mRNA levels no AtSUC1 protein was found. Only after pollination of stylar papillae was AtSUC1 protein detected inside the pollen and later inside the growing pollen tubes, suggesting a translation of pre-existing AtSUC1 mRNA after pollination. Pollen germination analyses underlined the important role of sucrose for pollen tube growth. The data presented suggest a role of AtSUC1 in the controlled dehiscence of Arabidopsis anthers. It is postulated that an important function of AtSUC1 is the cell-specific modulation of water potentials.     

Functional and physiological characterization of Arabidopsis INOSITOL TRANSPORTER1, a novel tonoplast-localized transporter for myo-inositol

Arabidopsis thaliana INOSITOL TRANSPORTER1 (INT1) is a member of a small gene family with only three more genes (INT2 to INT4). INT2 and INT4 were shown to encode plasma membrane-localized transporters for different inositol epimers, and INT3 was characterized as a pseudogene. Here, we present the functional and physiological characterization of the INT1 protein, analyses of the tissue-specific expression of the INT1 gene, and analyses of phenotypic differences observed between wild-type plants and mutant lines carrying the int1.1 and int1.2 alleles. INT1 is a ubiquitously expressed gene, and Arabidopsis lines with T-DNA insertions in INT1 showed increased intracellular myo-inositol concentrations and reduced root growth. In Arabidopsis, tobacco (Nicotiana tabacum), and Saccharomyces cerevisiae, fusions of the green fluorescent protein to the C terminus of INT1 were targeted to the tonoplast membranes. Finally, patch-clamp analyses were performed on vacuoles from wild-type plants and from both int1 mutant lines to study the transport properties of INT1 at the tonoplast. In summary, the presented molecular, physiological, and functional studies demonstrate that INT1 is a tonoplast-localized H(+)/inositol symporter that mediates the efflux of inositol that is generated during the degradation of inositol-containing compounds in the vacuolar lumen.     

Sucrose supply to nematode-induced syncytia depends on the apoplasmic and symplasmic pathways

The plant parasitic nematode Heterodera schachtii induces syncytial feeding structures in the roots of host plants. Nematode-induced syncytia become strong sink tissues in the plant solute circulation system as the parasites start withdrawing nutrients. In the present work, the expression pattern of the phloem-specific sucrose transporter AtSUC4 (also described as AtSUT4) is analysed in syncytia induced by H. schachtii and it is compared with that of AtSUC2, another phloem-specific sucrose transporter, which is expressed in syncytia. The temporal expression pattern was monitored by GUS-tests and real-time RT-PCR, while the localization within the syncytia was performed using in situ RT-PCR. In this context, the concentration of sucrose in infection sites was also analysed and, in fact, an increase in response to syncytium development was found. Silencing of the AtSUC4 gene finally resulted in a significant reduction of female nematode development, thus demonstrating a function for this gene for the first time. It is therefore concluded that AtSUC4 plays a significant role in the early phase of syncytium differentiation when functional plasmodesmata to the phloem are not yet established. It is further concluded that, during syncytium establishment, transporters are responsible for sucrose supply and, at a later stage, when a connection to the phloem is established via plasmodesmata, transporters are required for sucrose retrieval.     

The CsSUT1 promoter from Citrus sinensis confers sink-specific expression of a downstream reporter gene in transgenic Arabidopsis

Membrane-localized H⁺-symporting sucrose transporters (SUC or SUT proteins) are involved in sucrose loading into the phloem of source tissues and sucrose uptake into sink tissues, which are essential events in the growth and development of higher plants. While many of these sucrose transporters are localized in the phloem, others function in sink tissues. In an attempt to gain insight into which class the CsSUT1 gene from Citrus sinensis falls, we isolated a 1537-bp upstream region of this gene (CsSUT1p), inserted it upstream of the ??-glucuronidase (GUS) reporter gene and transformed the resulting vector into Arabidopsis thaliana. Histochemical and semi-quantitative RT-PCR analyses indicated that the CsSUT1p conferred GUS expression in floral tissues and the roots of young seedlings, but not above ground vegetative tissues. In flowers, GUS expression was noted in young floral buds, as well as immature stamens and carpels. Deletion analyses indicated that a ?1052 to ?1 fragment (relative to the translational start codon at +1) of the CsSUT1p, but not a ?496 to ?1 fragment, was able to drive the same pattern of expression of a downstream reporter gene in transgenic Arabidopsis. Taken together, these results suggest that the CsSUT1 gene, like numerous SUC/SUT genes from other plant species, may play a role in the uptake of sucrose into sink tissues.     

Sugar-induced increase in cytosolic Ca(2+) in Arabidopsis thaliana whole plants.

Using Ca(2+)-dependent photoprotein aequorin-transformed Arabidopsis thaliana, sugar-induced increase in cytosolic free Ca(2+ )concentration ([Ca(2+)](cyt))( )was investigated by luminescence imaging technique. When 0.1 M sucrose was fed to roots of autotrophically grown intact whole plants whose roots had been incubated overnight with coelenterazine to reconstitute aequorin systemically, strong and transient (within 20 s) luminescence was observed in the roots; that luminescence was followed by weak luminescence moving from the lower leaves to the upper leaves. The moving rate of luminescence was roughly comparable to that of [(14)C]sucrose. Application of 0.1 M glucose or fructose induced transient luminescence in excised leaves. No such luminescence was observed in heterotrophically grown (with sucrose) whole plants or in excised tissues. mRNA levels of sucrose-H(+) symporter genes AtSUC1 and AtSUC2 were higher in autotrophic plants than in heterotrophic plants. These results indicate that influx of transported sucrose together with H(+) into the mesophyll cells of autotrophic plants may depolarize the membrane potential, and subsequently activate a voltage-gated Ca(2+) channel on the plasma membrane, resulting in a [Ca(2+)](cyt) increase. The [Ca(2+)](cyt) increase might initiate Ca(2+ )signaling leading to the expression of genes related to biosynthesis of storage carbohydrates. Hexoses, when applied, might also be involved in the [Ca(2+)](cyt) increase mediated by monosaccharide-H(+) co-transporters.     

Plant sucrose-H+ symporters mediate the transport of vitamin H

A cDNA coding for a vitamin H (biotin) transport protein from Arabidopsis was identified by genetic complementation of a biotin uptake-deficient yeast mutant. Vitamin H transport by this protein was sensitive to the SH-group inhibitor p-chloromercuribenzene sulfonic acid (PCMBS) and to the uncoupler carbonyl cyanide-m-chlorophenylhydrazone (CCCP), suggesting an energy-dependent biotin-H+ symport mechanism. The transport activity could contribute to the so-far uncharacterized plant sucrose-H+ symporter AtSUC5 which mediates the energy-dependent transport of biotin and sucrose, and restores growth of the biotin transport-deficient yeast mutant on medium with low biotin concentrations. Functional comparison of the AtSUC5 transporter with previously characterized plant sucrose or monosaccharide transporters revealed that biotin transport may be a general and specific property of all plant sucrose transporters (sucrose/biotin-H+ symporters). This first report on a transporter with dual substrate specificity for two structurally unrelated molecules has a major impact on general thinking concerning the specificity of membrane transporters. The physiological relevance of this finding is discussed.     

Carbon partitioning and export in transgenic Arabidopsis thaliana with altered capacity for sucrose synthesis grown at low temperature: a role for metabolite transporters

We investigated the role of metabolite transporters in cold acclimation by comparing the responses of wild-type (WT) Arabidopsis thaliana (Heynh.) with that of transgenic plants over-expressing sucrose-phosphate synthase (SPSox) or with that of antisense repression of cytosolic fructose-1,6-bisphosphatase (FBPas). Plants were grown at 23 degrees C and then shifted to 5 degrees C. We compared the leaves shifted to 5 degrees C for 3 and 10 d with new leaves that developed at 5 degrees C with control leaves on plants at 23 degrees C. At 23 degrees C, ectopic expression of SPS resulted in 30% more carbon being fixed per day and an increase in sucrose export from source leaves. This increase in fixation and export was supported by increased expression of the plastidic triose-phosphate transporter AtTPT and, to a lesser extent, the high-affinity Suc transporter AtSUC1. The improved photosynthetic performance of the SPSox plants was maintained after they were shifted to 5 degrees C and this was associated with further increases in AtSUC1 expression but with a strong repression of AtTPT mRNA abundance. Similar responses were shown by WT plants during acclimation to low temperature and this response was attenuated in the low sucrose producing FBPas plants. These data suggest that a key element in recovering flux through carbohydrate metabolism in the cold is to control the partitioning of metabolites between the chloroplast and the cytosol, and Arabidopsis modulates the expression of AtTPT to maintain balanced carbon flow. Arabidopsis also up-regulates the expression of AtSUC1, and to lesser extent AtSUC2, as down-stream components facilitate sucrose transport in leaves that develop at low temperatures.     

Vacuolar sulfate transporters are essential determinants controlling internal distribution of sulfate in Arabidopsis

Uptake of external sulfate from the environment and use of internal vacuolar sulfate pools are two important aspects of the acquisition of sulfur for metabolism. In this study, we demonstrated that the vacuolar SULTR4-type sulfate transporter facilitates the efflux of sulfate from the vacuoles and plays critical roles in optimizing the internal distribution of sulfate in Arabidopsis thaliana. SULTR4;1-green fluorescent protein (GFP) and SULTR4;2-GFP fusion proteins were expressed under the control of their own promoters in transgenic Arabidopsis. The fusion proteins were accumulated specifically in the tonoplast membranes and were localized predominantly in the pericycle and xylem parenchyma cells of roots and hypocotyls. In roots, SULTR4;1 was constantly accumulated regardless of the changes of sulfur conditions, whereas SULTR4;2 became abundant by sulfur limitation. In shoots, both transporters were accumulated by sulfur limitation. Vacuoles isolated from callus of the sultr4;1 sultr4;2 double knockout showed excess accumulation of sulfate, which was substantially decreased by overexpression of SULTR4;1-GFP. In seedlings, the supplied [(35)S]sulfate was retained in the root tissue of the sultr4;1 sultr4;2 double knockout mutant. Comparison of the double and single knockouts suggested that SULTR4;1 plays a major role and SULTR4;2 has a supplementary function. Overexpression of SULTR4;1-GFP significantly decreased accumulation of [(35)S]sulfate in the root tissue, complementing the phenotype of the double mutant. These results suggested that SULTR4-type transporters, particularly SULTR4;1, actively mediate the efflux of sulfate from the vacuole lumen into the cytoplasm and influence the capacity for vacuolar storage of sulfate in the root tissue. The efflux function will promote rapid turnover of sulfate from the vacuoles particularly in the vasculature under conditions of low-sulfur supply, which will optimize the symplastic (cytoplasmic) flux of sulfate channeled toward the xylem vessels.     

AtSUC3, a gene encoding a new Arabidopsis sucrose transporter, is expressed in cells adjacent to the vascular tissue and in a carpel cell layer

The cDNA corresponding to the open reading frame T17M13.3 from Arabidopsis chromosome II was isolated and the encoded protein was characterized as a member of a subgroup of higher plant sucrose transporters. The AtSUC3 (Arabidopsis thaliana sucrose transporter 3) open reading frame encodes a protein with 594 amino acid residues, being 81 and 82 residues longer than the previously described Arabidopsis sucrose carriers AtSUC1 and AtSUC2. About 50 of these additional amino acids are part of an extended cytoplasmic loop separating the N-terminal from the C-terminal half of the protein. For functional characterization the AtSUC3 cDNA was expressed in baker's yeast. Substrate specificities, energy dependence and K(m) values of the recombinant protein were determined. Removal of the enlarged cytoplasmic loop and expression of the truncated cDNA caused no detectable change in the kinetic properties of the protein, suggesting a transport-independent function for this cytoplasmic domain. Immunolocalization with an AtSUC3-specific antiserum identified the protein in a cell layer separating the phloem from the mesophyll and in a single, subepidermal cell layer of the carpels that is important for pod dehiscence. These localizations suggest a possible role of AtSUC3 in the funnelling of sucrose from the mesophyll towards the phloem, and possibly in pod shatter.     

Arabidopsis sucrose transporter AtSUC1 is important for pollen germination and sucrose-induced anthocyanin accumulation

The Arabidopsis (Arabidopsis thaliana) sucrose transporter AtSUC1 (At1g71880) is highly expressed in pollen; however, its function has remained unknown. Here, we show that suc1 mutant pollen is defective in vivo, as evidenced by segregation distortion, and also has low rates of germination in vitro. AtSUC1-green fluorescent protein was localized to the plasma membrane in pollen tubes. AtSUC1 is also expressed in roots and external application of sucrose increased AtSUC1 expression in roots. AtSUC1 is important for sucrose-dependent signaling leading to anthocyanin accumulation in seedlings. suc1 mutants accumulated less anthocyanins in response to exogenous sucrose or maltose and microarray analysis revealed reduced expression of many genes important for anthocyanin biosynthesis. The results indicate that AtSUC1 is important for sugar signaling in vegetative tissue and for normal male gametophyte function.