Analysis of the Transport Activity of Barley Sucrose Transporter HvSUT1

PROCERA interacts with JACKDAW in gibberellin-enhanced source-sink sucrose partitioning in tomato

Proper regulation of the source-sink relationship is an effective way to increase crop yield. Gibberellin (GA) is an important regulator of plant growth and development, and physiological evidence has demonstrated that GA can promote source-sink sucrose partitioning. However, the underlying molecular mechanism remains unclear. Here, we used a combination of physiological and molecular approaches to identify the components involved in GA-enhanced source-sink sucrose partitioning in tomato (Solanum lycopersicum). GA treatment increased the sucrose export rate from source leaves and the sucrose level in young leaves (sink organ). GA-mediated enhancement of source-sink sucrose partitioning depended on SlPROCERA (SlPRO), the DELLA protein in tomato. Sucrose transporter 1 (SlSUT1) was involved in phloem sucrose loading. SlJACKDAW (SlJKD) was identified as an interaction partner of SlPRO. SlJKD negatively regulated the sucrose export rate from source leaves and could directly bind to the promoter of SlSUT1 and repress its expression, while SlPRO enhanced the transcription repression function of SlJKD. This study reveals the molecular mechanism by which GA promotes source-sink sucrose partitioning in tomato and provides potential targets for source-sink relationship optimization.

The high-affinity pineapple sucrose transporter AcSUT1B, regulated by AcCBF1, exhibited enhanced cold tolerance in transgenic Arabidopsis

Sucrose transporter (SUT) plays essential roles in plant growth and development, as well as responses to diverse abiotic stresses. However, limited information about the function of SUT was available in pineapple, an important tropical fruit crop with crassulacean acid metabolism. Here, four AcSUT genes were identified in pineapple genome, and divided into three clades according to the phylogenetic analysis. The expression profiles of AcSUTs were systemically examined, and they were all localized to plasma membrane. Transport activity assay by two-electrode voltage clamp of Xenopus oocytes showed that AcSUT1A and AcSUT1B were capable of transporting a range of glucosides, and they were exhibited high affinity for sucrose with Km value of 0.09 mM and 0.41 mM at pH 5.0, respectively. Overexpression of the cold-induced AcSUT1B conferred enhanced cold tolerance in transgenic Arabidopsis. DNA-protein interaction analysis further demonstrated that AcCBF1 directly binds the CRT/DRE element of the AcSUT1B promoter and activated its expression. Heterologous expression of AcCBF1 in Arabidopsis also increased cold tolerance. In this study, we investigated the transport activities of AcSUTs in pineapple and identified the AcCBF1-AcSUT1B module involved in cold stress, which provided new insights into the molecular mechanism of the cold response in pineapple.

The role of sugar transporters in the battle for carbon between plants and pathogens

In photosynthetic cells, plants convert carbon dioxide to sugars that can be moved between cellular compartments by transporters before being subsequently metabolized to support plant growth and development. Most pathogens cannot synthesize sugars directly but have evolved mechanisms to obtain plant-derived sugars as C resource for successful infection and colonization. The availability of sugars to pathogens can determine resistance or susceptibility. Here, we summarize current progress on the roles of sugar transporters in plant-pathogen interactions. We highlight how transporters are manipulated antagonistically by both host and pathogens in competing for sugars. We examine the potential application of this target in resistance breeding and discuss opportunities and challenges for the future.

Phaseolus vulgaris STP13.1 is an H+-coupled monosaccharide transporter, present in source leaves and seed coats, with higher substrate affinity at depolarized potentials

Sugar transport proteins (STPs) are high-affinity H+-coupled hexose symporters. Recently, the contribution of STP13 to bacterial and fungal pathogen resistance across multiple plant species has garnered significant interest. Quantitative PCR analysis of source leaves, developing embryos, and seed coats of Phaseolus vulgaris L. (common bean) revealed that PvSTP13.1 was expressed in source leaves and seed coats throughout seed development. In contrast, PvSTP13.1 transcripts were detected at exceedingly low levels in developing embryos. To characterize the transport mechanism, PvSTP13.1 was expressed in Xenopus laevis oocytes, and inward-directed currents were analyzed using two-electrode voltage clamping. PvSTP13.1 was shown to function as an H+-coupled monosaccharide symporter exhibiting a unique high affinity for hexoses and aldopentoses at depolarized membrane potentials. Specifically, of the 31 assessed substrates, which included aldohexoses, deoxyhexoses, fructose, 3-O-methyl-D-glucose, aldopentoses, polyols, glycosides, disaccharides, trisaccharides, and glucuronic acid, PvSTP13.1 displayed the highest affinity (K 0.5) for glucose (43 μM), mannose (92 μM), galactose (145 μM), fructose (224 μM), xylose (1.0 mM), and fucose (3.7 mM) at pH 5.6 at a depolarized membrane potential of -40 mV. The results presented here suggest PvSTP13.1 contributes to retrieval of hexoses from the apoplasmic space in source leaves and coats of developing seeds.

Structure and sucrose binding mechanism of the plant SUC1 sucrose transporter

Sucrose import from photosynthetic tissues into the phloem is mediated by transporters from the low-affinity sucrose transporter family (SUC/SUT family). Furthermore, sucrose redistribution to other tissues is driven by phloem sap movement, the product of high turgor pressure created by this import activity. Additionally, sink organs such as fruits, cereals and seeds that accumulate high concentrations of sugar also depend on this active transport of sucrose. Here we present the structure of the sucrose-proton symporter, Arabidopsis thaliana SUC1, in an outward open conformation at 2.7 Å resolution, together with molecular dynamics simulations and biochemical characterization. We identify the key acidic residue required for proton-driven sucrose uptake and describe how protonation and sucrose binding are strongly coupled. Sucrose binding is a two-step process, with initial recognition mediated by the glucosyl moiety binding directly to the key acidic residue in a stringent pH-dependent manner. Our results explain how low-affinity sucrose transport is achieved in plants, and pinpoint a range of SUC binders that help define selectivity. Our data demonstrate a new mode for proton-driven symport with links to cation-driven symport and provide a broad model for general low-affinity transport in highly enriched substrate environments.

Expression of the wheat multipathogen resistance hexose transporter Lr67res is associated with anion fluxes

Many disease resistance genes in wheat (Triticum aestivum L.) confer strong resistance to specific pathogen races or strains, and only a small number of genes confer multipathogen resistance. The Leaf rust resistance 67 (Lr67) gene fits into the latter category as it confers partial resistance to multiple biotrophic fungal pathogens in wheat and encodes a Sugar Transport Protein 13 (STP13) family hexose-proton symporter variant. Two mutations (G144R, V387L) in the resistant variant, Lr67res, differentiate it from the susceptible Lr67sus variant. The molecular function of the Lr67res protein is not understood, and this study aimed to broaden our knowledge on this topic. Biophysical analysis of the wheat Lr67sus and Lr67res protein variants was performed using Xenopus laevis oocytes as a heterologous expression system. Oocytes injected with Lr67sus displayed properties typically associated with proton-coupled sugar transport proteins-glucose-dependent inward currents, a Km of 110 ± 10 µM glucose, and a substrate selectivity permitting the transport of pentoses and hexoses. By contrast, Lr67res induced much larger sugar-independent inward currents in oocytes, implicating an alternative function. Since Lr67res is a mutated hexose-proton symporter, the possibility of protons underlying these currents was investigated but rejected. Instead, currents in Lr67res oocytes appeared to be dominated by anions. This conclusion was supported by electrophysiology and 36Cl- uptake studies and the similarities with oocytes expressing the known chloride channel from Torpedo marmorata, TmClC-0. This study provides insights into the function of an important disease resistance gene in wheat, which can be used to determine how this gene variant underpins disease resistance in planta.

Tomato sucrose transporter SlSUT4 participates in flowering regulation by modulating gibberellin biosynthesis

The functions of sucrose transporters (SUTs) differ among family members. The physiological function of SUT1 has been studied intensively, while that of SUT4 in various plant species including tomato (Solanum lycopersicum) is less well-understood. In this study, we characterized the function of tomato SlSUT4 in the regulation of flowering using a combination of molecular and physiological analyses. SlSUT4 displayed transport activity for sucrose when expressed in yeast (Saccharomyces cerevisiae), and it localized at both the plasma membrane and tonoplast. SlSUT4 interacted with SlSUT1, causing partial internalization of the latter, the main phloem loader of sucrose in tomato. Silencing of SlSUT4 promoted SlSUT1 localization to the plasma membrane, contributing to increased sucrose export and thus increased sucrose level in the shoot apex, which promoted flowering. Both silencing of SlSUT4 and spraying with sucrose suppressed gibberellin biosynthesis through repression of ent-kaurene oxidase and gibberellin 20-oxidase-1 (2 genes encoding key enzymes in gibberellin biosynthesis) expression by SlMYB76, which directly bound to their promoters. Silencing of SlMYB76 promoted gibberellin biosynthesis. Our results suggest that SlSUT4 is a functional SUT in tomato; downregulation of SlSUT4 expression enhances sucrose transport to the shoot apex, which promotes flowering by inhibiting gibberellin biosynthesis.

Evidence for conifer sucrose transporters' functioning in the light-dependent adjustment of sugar allocation

Sucrose is the central unit of carbon and energy in plants. Active intercellular transport of sucrose is mediated by sucrose transporters (SUTs), genes for which have been found in the genomes of all land plants. However, they have only been assigned functions in angiosperm species. Here, we cloned two types of SUTs from two gymnosperms, the conifers Cedrus deodara (Roxb. G. Don) and Pinus massoniana Lambert, and analyzed their sucrose transport activities. Uptake of the fluorescent sucrose-analog esculin into tobacco epidermis cells expressing the conifer SUT confirmed their transport ability. To determine their function in planta, we investigated their mRNA abundance in relation to photosynthesis and sugar levels in leaves, inner bark, wood and roots. Combined with measurements of protein abundance and immunolocalization of C. deodara SUTs, our results suggest a role for CdSUT1G and CdSUT2 in supporting phloem transport under varying environmental conditions. The implications of these findings regarding conifer physiology and SUT evolution are discussed.

Physiology and whole-plant carbon partitioning during stem sugar accumulation in sweet dwarf sorghum

A greater rate of phloem unloading and storage in the stem, not a higher rate of sugar production by photosynthesis or sugar export from leaves, is the main factor that results in sugar accumulation in sweet dwarf sorghum compared to grain sorghum. At maturity, the stem internodes of sweet sorghum varieties accumulate high concentrations of fermentable sugars and represent an efficient feedstock for bioethanol production. Although stem sugar accumulation is a heritable trait, additional factors that drive sugar accumulation in sorghum have not been identified. To identify the constraints on stem sugar accumulation in sweet sorghum, we used a combination of carbon-11 (¹¹C) radiotracer, physiological and biochemical approaches, and compared a grain sorghum and sweet dwarf sorghum line that have similar growth characteristics including height. Photosynthesis did not increase during development or differ between the sorghum lines. During the developmental transition to the reproductive stage, export of ¹¹C from leaves approximately doubled in both sorghum lines, but ¹¹C export in the sweet dwarf line did not exceed that of the grain sorghum. Defoliation to manipulate relative sink demand did not result in increased photosynthetic rates, indicating that the combined accumulation of C by all sink tissues was limited by the maximum photosynthetic capacity of source leaves. Nearly 3/4 of the ¹¹C exported from leaves was transported to the lower stem in sweet sorghum within 2 h, whereas in grain sorghum nearly 3/4 of the ¹¹C was in the panicle. Accordingly, the transcripts of several sucrose transporter (SUT) genes were more abundant in the stem internodes of the sweet dwarf line compared to the grain sorghum. Overall, these results indicate that sugar accumulation in sweet sorghum stems is influenced by the interplay of different sink tissues for the same sugars, but is likely driven by elevated sugar phloem unloading and uptake capacity in mature stem internodes.

Phaseolus vulgaris SUT1.1 is a high affinity sucrose-proton co-transporter

Plant sucrose transporters are required for phloem loading, and therefore are essential for plant growth and development. In common beans (Phaseolus vulgaris) there are only two sucrose transporters functionally characterized. Through a previous RNA-seq study, we identified a putative sucrose transporter in common bean, which we hypothesize to function in import of sucrose into plant cells. In silico analysis revealed that PvSUT1.1 is a putative sucrose-proton co-transporter distinct from other characterized sucrose transporters in common bean indicating that this is a previously undescribed transporter protein in beans. Further analysis revealed that PvSUT1.1 shares high protein sequence homology to the phloem loader Arabidopsis SUC2; both have 12 transmembrane domains, a typical characteristic of plant sucrose transporters. Heterologous expression in yeast further showed PvSUT1.1 to be functional and it imported sucrose into yeast cells with a Km of 0.7 mM sucrose. Import of sucrose through PvSUT1.1 is also pH-dependent with highest uptake at pH 4.0, and activity is lost in the presence of the uncoupler carbonyl cyanide 3-chlorophenylhydrazone. Consistent with identification of PvSUT1.1 as a Type I transporter, PvSUT1.1 also transports esculin. Finally, PvSUT1.1 showed expression in multiple tissues and the protein was localized to the plasma membrane. The results show that PvSUT1.1 is a sucrose transporter that is probably involved in the uptake of sucrose into source and sink cells. The potential role of PvSUT1.1 in leaf phloem loading of sucrose in common beans and its importance in heat tolerance of reproductive tissues are further discussed.

Comparative phosphoproteomic analysis of developing maize seeds suggests a pivotal role for enolase in promoting starch synthesis

Maize (Zea mays) seeds are the major source of starch all over the world and the excellent model for researching starch synthesis. Seed starch content is a typical quantitative phenotype and many reports revealed that the glycolytic enzymes are involved in regulating starch synthesis, however the regulatory mechanism is still unclear. Here, we present a comparative phosphoproteomic study of three maize inbred lines with different seed starch content. It reveals that abundances of 62 proteins and 63 phosphoproteins were regulated during maize seed development. Dynamics of 17 enzymes related to glycolysis and starch synthesis were used to construct a phosphorylation regulatory network of starch synthesis. It shows that starch synthesis and glycolysis in maize seeds utilize the same hexose phosphates pool coming from sorbitol and sucrose as carbon source, and phosphorylation of ZmENO1 are suggested to contribute to increase starch content, because it is positively related to seed starch content in different developmental stages and different lines, and the phosphor-mimic mutant (ZmENO1^S43D) damaged its enzyme activity which is vital in glycolysis. Our results provide a new sight into regulatory process of seed starch synthesis and can be used in maize breeding for high starch content.

Protoplast-Esculin Assay as a New Method to Assay Plant Sucrose Transporters: Characterization of AtSUC6 and AtSUC7 Sucrose Uptake Activity in Arabidopsis Col-0 Ecotype

The best characterized function of sucrose transporters of the SUC family in plants is the uptake of sucrose into the phloem for long-distance transport of photoassimilates. This important step is usually performed by one specific SUC in every species. However, plants possess small families of several different SUCs which are less well understood. Here, we report on the characterization of AtSUC6 and AtSUC7, two members of the SUC family in Arabidopsis thaliana. Heterologous expression in yeast (Saccharomyces cerevisiae) revealed that AtSUC6_Col-0 is a high-affinity H⁺-symporter that mediates the uptake of sucrose and maltose across the plasma membrane at exceptionally low pH values. Reporter gene analyses revealed a strong expression of AtSUC6_Col-0 in reproductive tissues, where the protein product might contribute to sugar uptake into pollen tubes and synergid cells. A knockout of AtSUC6 did not interfere with vegetative development or reproduction, which points toward physiological redundancy of AtSUC6_Col-0 with other sugar transporters. Reporter gene analyses showed that AtSUC7_Col-0 is expressed in roots and pollen tubes and that this sink specific expression of AtSUC7_Col-0 is regulated by intragenic regions. Transport activity of AtSUC7_Col-0 could not be analyzed in baker's yeast or Xenopus oocytes because the protein was not correctly targeted to the plasma membrane in both heterologous expression systems. Therefore, a novel approach to analyze sucrose transporters in planta was developed. Plasma membrane localized SUCs including AtSUC6_Col-0 and also sucrose specific SWEETs were able to mediate transport of the fluorescent sucrose analog esculin in transformed mesophyll protoplasts. In contrast, AtSUC7_Col-0 is not able to mediate esculin transport across the plasma membrane which implicates that AtSUC7_Col-0 might be a non-functional pseudogene. The novel protoplast assay provides a useful tool for the quick and quantitative analysis of sucrose transporters in an in planta expression system.

An analysis of the role of the ShSUT1 sucrose transporter in sugarcane using RNAi suppression

The role of ShSUT1 in sucrose mobilisation and storage in sugarcane was investigated by employing RNAi technology to reduce the expression of this gene. Transcript profiling in non-transformed plants showed an alignment between expression and sucrose concentration, with strongest expression in source leaves and increasing expression through the daylight period of a diurnal cycle. Five transgenic plant lines were produced with reduced ShSUT1 expression ranging from 52 to 92% lower than control plants. Differential suppression of ShSUT1 sequence variants in the highly polyploid sugarcane genome were also investigated. Amplicon sequencing of the ShSUT1 variants within the transgenic lines and controls showed no preferential suppression with only minor differences in the proportional expression of the variants. A range of altered sugar, fibre and moisture contents were measured in mature leaf and internode samples, but no phenotype was consistently exhibited by all five transgenic lines. Phenotypes observed indicate that ShSUT1 does not play a direct role in phloem loading. ShSUT1 is likely involved with retrieving sucrose from intercellular spaces for transport and storage.

In vivo transport of three radioactive [18F]-fluorinated deoxysucrose analogs by the maize sucrose transporter ZmSUT1

Sucrose transporter (SUT) proteins translocate sucrose across cell membranes; however, mechanistic aspects of sucrose binding by SUTs are not well resolved. Specific hydroxyl groups in sucrose participate in hydrogen bonding with SUT proteins. We previously reported that substituting a radioactive fluorine-18 [¹⁸F] at the C-6' position within the fructosyl moiety of sucrose did not affect sucrose transport by the maize (Zea mays) ZmSUT1 protein. To determine how ¹⁸F substitution of hydroxyl groups at two other positions within sucrose, the C-1' in the fructosyl moiety or the C-6 in the glucosyl moiety, impact sucrose transport, we synthesized 1'-[F¹⁸]fluoro-1'-deoxysucrose and 6-[F¹⁸]fluoro-6-deoxysucrose ([¹⁸F]FDS) analogs. Each [¹⁸F]FDS derivative was independently introduced into wild-type or sut1 mutant plants, which are defective in sucrose phloem loading. All three (1'-, 6'-, and 6-) [¹⁸F]FDS derivatives were efficiently and equally translocated, similarly to carbon-14 [¹⁴C]-labeled sucrose. Hence, individually replacing the hydroxyl groups at these positions within sucrose does not interfere with substrate recognition, binding, or membrane transport processes, and hydroxyl groups at these three positions are not essential for hydrogen bonding between sucrose and ZmSUT1. [¹⁸F]FDS imaging afforded several advantages compared to [¹⁴C]-sucrose detection. We calculated that 1'-[¹⁸F]FDS was transported at approximately a rate of 0.90 ± 0.15 m.h-1 in wild-type leaves, and at 0.68 ± 0.25 m.h-1 in sut1 mutant leaves. Collectively, our data indicated that [¹⁸F]FDS analogs are valuable tools to probe sucrose-SUT interactions and to monitor sucrose transport in plants.

Alternate Modes of Photosynthate Transport in the Alternating Generations of Physcomitrella patens

Physcomitrella patens has emerged as a model moss system to investigate the evolution of various plant characters in early land plant lineages. Yet, there is merely a disparate body of ultrastructural and physiological evidence from other mosses to draw inferences about the modes of photosynthate transport in the alternating generations of Physcomitrella. We performed a series of ultrastructural, fluorescent tracing, physiological, and immunohistochemical experiments to elucidate a coherent model of photosynthate transport in this moss. Our ultrastructural observations revealed that Physcomitrella is an endohydric moss with water-conducting and putative food-conducting cells in the gametophytic stem and leaves. Movement of fluorescent tracer 5(6)-carboxyfluorescein diacetate revealed that the mode of transport in the gametophytic generation is symplasmic and is mediated by plasmodesmata, while there is a diffusion barrier composed of transfer cells that separates the photoautotrophic gametophyte from the nutritionally dependent heterotrophic sporophyte. We posited that, analogous to what is found in apoplasmically phloem loading higher plants, the primary photosynthate sucrose, is actively imported into the transfer cells by sucrose/H⁺ symporters (SUTs) that are, in turn, powered by P-type ATPases, and that the transfer cells harbor an ATP-conserving Sucrose Synthase (SUS) pathway. Supporting our hypothesis was the finding that a protonophore (2,4-dinitrophenol) and a SUT-specific inhibitor (diethyl pyrocarbonate) reduced the uptake of radiolabeled sucrose into the sporangia. In situ immunolocalization of P-type ATPase, Sucrose Synthase, and Proton Pyrophosphatase - all key components of the SUS pathway - showed that these proteins were prominently localized in the transfer cells, providing further evidence consistent with our argument.

Genome-Wide Analysis and Expression Profiling of the SUC and SWEET Gene Families of Sucrose Transporters in Oilseed Rape (Brassica napus L.)

Sucrose is the principal transported product of photosynthesis from source leaves to sink organs. SUTs/SUCs (sucrose transporters or sucrose carriers) and SWEETs (Sugars Will Eventually be Exported Transporters) play significant central roles in phloem loading and unloading. SUTs/SUCs and SWEETs are key players in sucrose translocation and are associated with crop yields. The SUT/SUC and SWEET genes have been characterized in several plant species, but a comprehensive analysis of these two gene families in oilseed rape has not yet been reported. In our study, 22 and 68 members of the SUT/SUCs and SWEET gene families, respectively, were identified in the oilseed rape (Brassica napus) genome through homology searches. An analysis of the chromosomal distribution, phylogenetic relationships, gene structures, motifs and the cis-acting regulatory elements in the promoters of BnSUC and BnSWEET genes were analyzed. Furthermore, we examined the expression of the 18 BnSUC and 16 BnSWEET genes in different tissues of "ZS11" and the expression of 9 BnSUC and 7 BnSWEET genes in "ZS11" under various conditions, including biotic stress (Sclerotinia sclerotiorum), abiotic stresses (drought, salt and heat), and hormone treatments (abscisic acid, auxin, cytokinin, brassinolide, gibberellin, and salicylic acid). In conclusion, our study provides the first comprehensive analysis of the oilseed rape SUC and SWEET gene families. Information regarding the phylogenetic relationships, gene structure and expression profiles of the SUC and SWEET genes in the different tissues of oilseed rape helps to identify candidates with potential roles in specific developmental processes. Our study advances our understanding of the important roles of sucrose transport in oilseed rape.

Expression of peach sucrose transporters in heterologous systems points out their different physiological role

Sucrose is the major phloem-translocated component in a number of economically important plant species. The comprehension of the mechanisms involved in sucrose transport in peach fruit appears particularly relevant, since the accumulation of this sugar, during ripening, is crucial for the growth and quality of the fruit. Here, we report the functional characterisation and subcellular localisation of three sucrose transporters (PpSUT1, PpSUT2, PpSUT4) in peach, and we formulate novel hypotheses about their role in accumulation of sugar. We provide evidence, about the capability of both PpSUT1 and PpSUT4, expressed in mutant yeast strains to transport sucrose. The functionality of PpSUT1 at the plasma membrane, and of PpSUT4 at the tonoplast, has been demonstrated. On the other hand, the functionality of PpSUT2 was not confirmed: this protein is unable to complement two sucrose uptake-deficient mutant yeast strains. Our results corroborate the hypotheses that PpSUT1 partakes in phloem loading in leaves, and PpSUT4 sustains cell metabolism by regulating sucrose efflux from the vacuole.

Transport Function of Rice Amino Acid Permeases (AAPs)

The transport function of four rice (Oryza sativa) amino acid permeases (AAPs), OsAAP1 (Os07g04180), OsAAP3 (Os06g36180), OsAAP7 (Os05g34980) and OsAAP16 (Os12g08090), was analyzed by expression in Xenopus laevis oocytes and electrophysiology. OsAAP1, OsAAP7 and OsAAP16 functioned, similarly to Arabidopsis AAPs, as general amino acid permeases. OsAAP3 had a distinct substrate specificity compared with other rice or Arabidopsis AAPs. OsAAP3 transported the basic amino acids lysine and arginine well but selected against aromatic amino acids. The transport of basic amino acids was further analyzed for OsAAP1 and OsAAP3, and the results support the transport of both neutral and positively charged forms of basic amino acids by the rice AAPs. Cellular localization using the tandem enhanced green fluorescent protein (EGFP)-red fluorescent protein (RFP) reporter pHusion showed that OsAAP1 and OsAAP3 localized to the plasma membrane after transient expression in onion epidermal cells or stable expression in Arabidopsis.

Multispectral phloem-mobile probes: properties and applications

Using Arabidopsis (Arabidopsis thaliana) seedlings, we identified a range of small fluorescent probes that entered the translocation stream and were unloaded at the root tip. These probes had absorbance/emission maxima ranging from 367/454 to 546/576 nm and represent a versatile toolbox for studying phloem transport. Of the probes that we tested, naturally occurring fluorescent coumarin glucosides (esculin and fraxin) were phloem loaded and transported in oocytes by the sucrose transporter, AtSUC2. Arabidopsis plants in which AtSUC2 was replaced with barley (Hordeum vulgare) sucrose transporter (HvSUT1), which does not transport esculin in oocytes, failed to load esculin into the phloem. In wild-type plants, the fluorescence of esculin decayed to background levels about 2 h after phloem unloading, making it a suitable tracer for pulse-labeling studies of phloem transport. We identified additional probes, such as carboxytetraethylrhodamine, a red fluorescent probe that, unlike esculin, was stable for several hours after phloem unloading and could be used to study phloem transport in Arabidopsis lines expressing green fluorescent protein.

Conformational changes represent the rate-limiting step in the transport cycle of maize sucrose transporter1

Proton-driven Suc transporters allow phloem cells of higher plants to accumulate Suc to more than 1 M, which is up to ~1000-fold higher than in the surrounding extracellular space. The carrier protein can accomplish this task only because proton and Suc transport are tightly coupled. This study provides insights into this coupling by resolving the first step in the transport cycle of the Suc transporter SUT1 from maize (Zea mays). Voltage clamp fluorometry measurements combining electrophysiological techniques with fluorescence-based methods enable the visualization of conformational changes of SUT1 expressed in Xenopus laevis oocytes. Using the Suc derivate sucralose, binding of which hinders conformational changes of SUT1, the association of protons to the carrier could be dissected from transport-associated movements of the protein. These combined approaches enabled us to resolve the binding of protons to the carrier and its interrelationship with the alternating movement of the protein. The data indicate that the rate-limiting step of the reaction cycle is determined by the accessibility of the proton binding site. This, in turn, is determined by the conformational change of the SUT1 protein, alternately exposing the binding pockets to the inward and to the outward face of the membrane.

Starch biosynthesis, its regulation and biotechnological approaches to improve crop yields

Structurally composed of the glucose homopolymers amylose and amylopectin, starch is the main storage carbohydrate in vascular plants, and is synthesized in the plastids of both photosynthetic and non-photosynthetic cells. Its abundance as a naturally occurring organic compound is surpassed only by cellulose, and represents both a cornerstone for human and animal nutrition and a feedstock for many non-food industrial applications including production of adhesives, biodegradable materials, and first-generation bioethanol. This review provides an update on the different proposed pathways of starch biosynthesis occurring in both autotrophic and heterotrophic organs, and provides emerging information about the networks regulating them and their interactions with the environment. Special emphasis is given to recent findings showing that volatile compounds emitted by microorganisms promote both growth and the accumulation of exceptionally high levels of starch in mono- and dicotyledonous plants. We also review how plant biotechnologists have attempted to use basic knowledge on starch metabolism for the rational design of genetic engineering traits aimed at increasing starch in annual crop species. Finally we present some potential biotechnological strategies for enhancing starch content.

Arg188 in rice sucrose transporter OsSUT1 is crucial for substrate transport

Background

Plant sucrose uptake transporters (SUTs) are H⁺/sucrose symporters related to the major facilitator superfamily (MFS). SUTs are essential for plant growth but little is known about their transport mechanism. Recent work identified several conserved, charged amino acids within transmembrane spans (TMS) in SUTs that are essential for transport activity. Here we further evaluated the role of one of these positions, R188 in the fourth TMS of OsSUT1, a type II SUT.

Results

The OsSUT1(R188K) mutant, studied by expression in plants, yeast, and Xenopus oocytes, did not transport sucrose but showed a H⁺ leak that was blocked by sucrose. The H⁺ leak was also blocked by β-phenyl glucoside which is not translocated by OsSUT1. Replacing the corresponding Arg in type I and type III SUTs, AtSUC1(R163K) and LjSUT4(R169K), respectively, also resulted in loss of sucrose transport activity. Fluorination at the glucosyl 3 and 4 positions of α-phenyl glucoside greatly decreased transport by wild type OsSUT1 but did not affect the ability to block H⁺ leak in the R188K mutant.

Conclusion

OsSUT1 R188 appears to be essential for sucrose translocation but not for substrate interaction that blocks H⁺ leak. Therefore, we propose that an additional binding site functions in the initial recognition of substrates. The corresponding Arg in type I and III SUTs are equally important. We propose that R188 interacts with glucosyl 3-OH and 4-OH during translocation.

Identification of amino acids important for substrate specificity in sucrose transporters using gene shuffling

Plant sucrose transporters (SUTs) are H(+)-coupled uptake transporters. Type I and II (SUTs) are phylogenetically related but have different substrate specificities. Type I SUTs transport sucrose, maltose, and a wide range of natural and synthetic α- and β-glucosides. Type II SUTs are more selective for sucrose and maltose. Here, we investigated the structural basis for this difference in substrate specificity. We used a novel gene shuffling method called synthetic template shuffling to introduce 62 differentially conserved amino acid residues from type I SUTs into OsSUT1, a type II SUT from rice. The OsSUT1 variants were tested for their ability to transport the fluorescent coumarin β-glucoside esculin when expressed in yeast. Fluorescent yeast cells were selected using fluorescence-activated cell sorting (FACS). Substitution of five amino acids present in type I SUTs in OsSUT1 was found to be sufficient to confer esculin uptake activity. The changes clustered in two areas of the OsSUT1 protein: in the first loop and the top of TMS2 (T80L and A86K) and in TMS5 (S220A, S221A, and T224Y). The substrate specificity of this OsSUT1 variant was almost identical to that of type I SUTs. Corresponding changes in the sugarcane type II transporter ShSUT1 also changed substrate specificity, indicating that these residues contribute to substrate specificity in type II SUTs in general.

SUT Sucrose and MST Monosaccharide Transporter Inventory of the Selaginella Genome

Most metazoa use hexose transporters to acquire hexoses from their diet and as a transport form for distributing carbon and energy within their bodies; insects use trehalose, and plants use sucrose as their major form for translocation. Plant genomes contain at least three families of mono- and disaccharide transporters: monosaccharide/polyol transporters that are evolutionary closely related to the yeast and human glucose transporters, sucrose transporters of the SUT family, which similar to the hexose transporters belong to the major facilitator superfamily, but share only minimal amino acid sequence homology with the hexose transporters, and the family of SWEET sugar transporters conserved between animals and plants. Recently, the genome sequence of the spikemoss Selaginella has been determined. In order to study the evolution of sugar transport in plants, we carefully annotated of the complement of sugar transporters in Selaginella. We review the current knowledge regarding sugar transport in spikemoss and provide phylogenetic analyses of the complement of MST and SUT homologs in Selaginella (and Physcomitrella).

Evolution of plant sucrose uptake transporters

In angiosperms, sucrose uptake transporters (SUTs) have important functions especially in vascular tissue. Here we explore the evolutionary origins of SUTs by analysis of angiosperm SUTs and homologous transporters in a vascular early land plant, Selaginella moellendorffii, and a non-vascular plant, the bryophyte Physcomitrella patens, the charophyte algae Chlorokybus atmosphyticus, several red algae and fission yeast, Schizosaccharomyces pombe. Plant SUTs cluster into three types by phylogenetic analysis. Previous studies using angiosperms had shown that types I and II are localized to the plasma membrane while type III SUTs are associated with vacuolar membrane. SUT homologs were not found in the chlorophyte algae Chlamydomonas reinhardtii and Volvox carterii. However, the characean algae Chlorokybus atmosphyticus contains a SUT homolog (CaSUT1) and phylogenetic analysis indicated that it is basal to all other streptophyte SUTs analyzed. SUTs are present in both red algae and S. pombe but they are less related to plant SUTs than CaSUT1. Both Selaginella and Physcomitrella encode type II and III SUTs suggesting that both plasma membrane and vacuolar sucrose transporter activities were present in early land plants. It is likely that SUT transporters are important for scavenging sucrose from the environment and intracellular compartments in charophyte and non-vascular plants. Type I SUTs were only found in eudicots and we conclude that they evolved from type III SUTs, possibly through loss of a vacuolar targeting sequence. Eudicots utilize type I SUTs for phloem (vascular tissue) loading while monocots use type II SUTs for phloem loading. We show that HvSUT1 from barley, a type II SUT, reverted the growth defect of the Arabidopsis atsuc2 (type I) mutant. This indicates that type I and II SUTs evolved similar (and interchangeable) phloem loading transporter capabilities independently.

Dynamic ¹³C/¹ H NMR imaging uncovers sugar allocation in the living seed

Seed growth and accumulation of storage products relies on the delivery of sucrose from the maternal to the filial tissues. The transport route is hidden inside the seed and has never been visualized in vivo. Our approach, based on high-field nuclear magnetic resonance and a custom made (13)C/(1) H double resonant coil, allows the non-invasive imaging and monitoring of sucrose allocation within the seed. The new technique visualizes the main stream of sucrose and determines its velocity during the grain filling in barley (Hordeum vulgare L.). Quantifiable dynamic images are provided, which allow observing movement of (13)C-sucrose at a sub-millimetre level of resolution. The analysis of genetically modified barley grains (Jekyll transgenic lines, seg8 and Risø13 mutants) demonstrated that sucrose release via the nucellar projection towards the endosperm provides an essential mean for the control of seed growth by maternal organism. The sucrose allocation was further determined by structural and metabolic features of endosperm. Sucrose monitoring was integrated with an in silico flux balance analysis, representing a powerful platform for non-invasive study of seed filling in crops.

Characterization of Sucrose transporter alleles and their association with seed yield-related traits in Brassica napus L

BACKGROUND

Sucrose is the primary photosynthesis product and the principal translocating form within higher plants. Sucrose transporters (SUC/SUT) play a critical role in phloem loading and unloading. Photoassimilate transport is a major limiting factor for seed yield. Our previous research demonstrated that SUT co-localizes with yield-related quantitative trait loci. This paper reports the isolation of BnA7.SUT1 alleles and their promoters and their association with yield-related traits.

RESULTS

Two novel BnA7.SUT1 genes were isolated from B. napus lines 'Eagle' and 'S-1300' and designated as BnA7.SUT1.a and BnA7.SUT1.b, respectively. The BnA7.SUT1 protein exhibited typical SUT features and showed high amino acid homology with related species. Promoters of BnA7.SUT1.a and BnA7.SUT1.b were also isolated and classified as pBnA7.SUT1.a and pBnA7.SUT1.b, respectively. Four dominant sequence-characterized amplified region markers were developed to distinguish BnA7.SUT1.a and BnA7.SUT1.b. The two genes were estimated as alleles with two segregating populations (F2 and BC1) obtained by crossing '3715'×'3769'. BnA7.SUT1 was mapped to the A7 linkage group of the TN doubled haploid population. In silico analysis of 55 segmental BnA7.SUT1 alleles resulted three BnA7.SUT1 clusters: pBnA7.SUT1.a- BnA7.SUT1.a (type I), pBnA7.SUT1.b- BnA7.SUT1.a (type II), and pBnA7.SUT1.b- BnA7.SUT1.b (type III). Association analysis with a diverse panel of 55 rapeseed lines identified single nucleotide polymorphisms (SNPs) in promoter and coding domain sequences of BnA7.SUT1 that were significantly associated with one of three yield-related traits: number of effective first branches (EFB), siliques per plant (SP), and seed weight (n = 1000) (TSW) across all four environments examined. SNPs at other BnA7.SUT1 sites were also significantly associated with at least one of six yield-related traits: EFB, SP, number of seeds per silique, seed yield per plant, block yield, and TSW. Expression levels varied over various tissue/organs at the seed-filling stage, and BnA7.SUT1 expression positively correlated with EFB and TSW.

CONCLUSIONS

Sequence, mapping, association, and expression analyses collectively showed significant diversity between the two BnA7.SUT1 alleles, which control some of the phenotypic variation for branch number and seed weight in B. napus consistent with expression levels. The associations between allelic variation and yield-related traits may facilitate selection of better genotypes in breeding.

Membrane-transport systems for sucrose in relation to whole-plant carbon partitioning

Sucrose is the principal product of photosynthesis used for the distribution of assimilated carbon in plants. Transport mechanisms and efficiency influence photosynthetic productivity by relieving product inhibition and contribute to plant vigor by controlling source/sink relationships and biomass partitioning. Sucrose is synthesized in the cytoplasm and may move cell to cell through plasmodesmata or may cross membranes to be compartmentalized or exported to the apoplasm for uptake into adjacent cells. As a relatively large polar compound, sucrose requires proteins to facilitate efficient membrane transport. Transport across the tonoplast by facilitated diffusion, antiport with protons, and symport with protons have been proposed; for transport across plasma membranes, symport with protons and a mechanism resembling facilitated diffusion are evident. Despite decades of research, only symport with protons is well established at the molecular level. This review aims to integrate recent and older studies on sucrose flux across membranes with principles of whole-plant carbon partitioning.

Plant sucrose transporters from a biophysical point of view

The majority of higher plants use sucrose as their main mobile carbohydrate. Proton-driven sucrose transporters play a crucial role in cell-to-cell and long-distance distribution of sucrose throughout the plant. A very negative plant membrane potential and the ability of sucrose transporters to accumulate sucrose concentrations of more than 1 M indicate that plants evolved transporters with unique structural and functional features. The knowledge about the transport mechanism and structural/functional domains of these nano-machines is, however, still fragmentary. In this review, the current knowledge about the biophysical properties of plant sucrose transporters is summarized and discussed.

The role of alpha-glucosidase in germinating barley grains

The importance of α-glucosidase in the endosperm starch metabolism of barley (Hordeum vulgare) seedlings is poorly understood. The enzyme converts maltose to glucose (Glc), but in vitro studies indicate that it can also attack starch granules. To discover its role in vivo, we took complementary chemical-genetic and reverse-genetic approaches. We identified iminosugar inhibitors of a recombinant form of an α-glucosidase previously discovered in barley endosperm (ALPHA-GLUCOSIDASE97 [HvAGL97]), and applied four of them to germinating grains. All four decreased the Glc-to-maltose ratio in the endosperm 10 d after imbibition, implying inhibition of maltase activity. Three of the four inhibitors also reduced starch degradation and seedling growth, but the fourth did not affect these parameters. Inhibition of starch degradation was apparently not due to inhibition of amylases. Inhibition of seedling growth was primarily a direct effect of the inhibitors on roots and coleoptiles rather than an indirect effect of the inhibition of endosperm metabolism. It may reflect inhibition of glycoprotein-processing glucosidases in these organs. In transgenic seedlings carrying an RNA interference silencing cassette for HvAgl97, α-glucosidase activity was reduced by up to 50%. There was a large decrease in the Glc-to-maltose ratio in these lines but no effect on starch degradation or seedling growth. Our results suggest that the α-glucosidase HvAGL97 is the major endosperm enzyme catalyzing the conversion of maltose to Glc but is not required for starch degradation. However, the effects of three glucosidase inhibitors on starch degradation in the endosperm indicate the existence of unidentified glucosidase(s) required for this process.

Molecular characterization and functional analysis of elite genes in wheat and its related species

The tribe Triticeae includes major cereal crops (bread wheat, durum wheat, triticale, barley and rye), as well as abundant forage and lawn grasses. Wheat and its wild related species possess numerous favourable genes for yield improvement, grain quality enhancement, biotic and abiotic stress resistance, and constitute a giant gene pool for wheat improvement. In recent years, significant progress on molecular characterization and functional analysis of elite genes in wheat and its related species have been achieved. In this paper, we review the cloned functional genes correlated with grain quality, biotic and abiotic stress resistance, photosystem and nutrition utilization in wheat and its related species.

Two novel disaccharides, rutinose and methylrutinose, are involved in carbon metabolism in Datisca glomerata

Datisca glomerata forms nitrogen-fixing root nodules in symbiosis with soil actinomycetes from the genus Frankia. Analysis of sugars in roots, nodules and leaves of D. glomerata revealed the presence of two novel compounds that were identified as alpha-L-rhamnopyranoside-(1 --> 6)-D-glucose (rutinose) and alpha-L-rhamnopyranoside-(1 --> 6)-1-O-beta-D-methylglucose (methylrutinose). Rutinose has been found previously as a/the glycoside part of several flavonoid glycosides, e.g. rutin, also of datiscin, the main flavonoid of Datisca cannabina, but had not been reported as free sugar. Time course analyses suggest that both rutinose and methylrutinose might play a role in transient carbon storage in sink organs and, to a lesser extent, in source leaves. Their concentrations show that they can accumulate in the vacuole. Rutinose, but not methylrutinose, was accepted as a substrate by the tonoplast disaccharide transporter SUT4 from Arabidopsis. In vivo (14)C-labeling and the study of uptake of exogenous sucrose and rutinose from the leaf apoplast showed that neither rutinose nor methylrutinose appreciably participate in phloem translocation of carbon from source to sink organs, despite rutinose being found in the apoplast at significant levels. A model for sugar metabolism in D. glomerata is presented.

Transport activity of rice sucrose transporters OsSUT1 and OsSUT5

Expression in Xenopus oocytes and electrophysiology was used to test for transport activity of the five sucrose transporter (SUT) homologs from rice. Expression of OsSUT1 and OsSUT5 resulted in sucrose-dependent currents that were analyzed by two-electrode voltage clamping. We examined the transport kinetics, substrate specificity and pH dependence of sucrose transport and K(0.5) for sucrose. OsSUT1 showed similar features to those of other type II SUTs from monocots examined previously, with a K(0.5) value of 7.50 mM at pH 5.6. In contrast, OsSUT5 had a higher substrate affinity (K(0.5) = 2.32 mM at pH 5.6), less substrate specificity and less pH dependence compared with all type II SUTs tested to date. Regulation of the rice SUTs, as well as ZmSUT1 from maize and HvSUT1 from barley, by reduced (GSH) and oxidized (GSSG) forms of glutathione was tested. GSSG and GSH were found to have no significant effect on the activity of sucrose transporters when expressed in Xenopus oocytes. In conclusion, differences in transport activity between OsSUT1 and OsSUT5 indicate that type II SUTs have a range of transport activities that are tuned to their function in the plant.

Activation of sucrose transport in defoliated Lolium perenne L.: an example of apoplastic phloem loading plasticity

The pathway of carbon phloem loading was examined in leaf tissues of the forage grass Lolium perenne. The effect of defoliation (leaf blade removal) on sucrose transport capacity was assessed in leaf sheaths as the major carbon source for regrowth. The pathway of carbon transport was assessed via a combination of electron microscopy, plasmolysis experiments and plasma membrane vesicles (PMVs) purified by aqueous two-phase partitioning from the microsomal fraction. Results support an apoplastic phloem loading mechanism. Imposition of an artificial proton-motive force to PMVs from leaf sheaths energized an active, transient and saturable uptake of sucrose (Suc). The affinity of Suc carriers for Suc was 580 microM in leaf sheaths of undefoliated plants. Defoliation induced a decrease of K(m) followed by an increase of V(max). A transporter was isolated from stubble (including leaf sheaths) cDNA libraries and functionally expressed in yeast. The level of L.perenne SUcrose Transporter 1 (LpSUT1) expression increased in leaf sheaths in response to defoliation. Taken together, the results indicate that Suc transport capacity increased in leaf sheaths of L. perenne in response to leaf blade removal. This increase might imply de novo synthesis of Suc transporters, including LpSUT1, and may represent one of the mechanisms contributing to rapid refoliation.

Sucrose transporter1 functions in phloem loading in maize leaves

In most plants, sucrose is exported from source leaves to carbon-importing sink tissues to sustain their growth and metabolism. Apoplastic phloem-loading species require sucrose transporters (SUTs) to transport sucrose into the phloem. In many dicot plants, genetic and biochemical evidence has established that SUT1-type proteins function in phloem loading. However, the role of SUT1 in phloem loading in monocot plants is not clear since the rice (Oryza sativa) and sugarcane (Saccharum hybrid) SUT1 orthologues do not appear to function in phloem loading of sucrose. A SUT1 gene was previously cloned from maize (Zea mays) and shown to have expression and biochemical activity consistent with a hypothesized role in phloem loading. To determine the biological function of SUT1 in maize, a sut1 mutant was isolated and characterized. sut1 mutant plants hyperaccumulate carbohydrates in mature leaves and display leaf chlorosis with premature senescence. In addition, sut1 mutants have greatly reduced stature, altered biomass partitioning, delayed flowering, and stunted tassel development. Cold-girdling wild-type leaves to block phloem transport phenocopied the sut1 mutants, supporting a role for maize SUT1 in sucrose export. Furthermore, application of (14)C-sucrose to abraded sut1 mutant and wild-type leaves showed that sucrose export was greatly diminished in sut1 mutants compared with wild type. Collectively, these data demonstrate that SUT1 is crucial for efficient phloem loading of sucrose in maize leaves.

Functional analysis of LjSUT4, a vacuolar sucrose transporter from Lotus japonicus

Sucrose transporters in the SUT family are important for phloem loading and sucrose uptake into sink tissues. The recent localization of type III SUTs AtSUT4 and HvSUT2 to the vacuole membrane suggests that SUTs also function in vacuolar sucrose transport. The transport mechanism of type III SUTs has not been analyzed in detail. LjSUT4, a type III sucrose transporter homolog from Lotus japonicus, is expressed in nodules and its transport activity has not been previously investigated. In this report, LjSUT4 was expressed in Xenopus oocytes and its transport activity assayed by two-electrode voltage clamping. LjSUT4 transported a range of glucosides including sucrose, salicin, helicin, maltose, sucralose and both alpha- and beta-linked synthetic phenyl glucosides. In contrast to other sucrose transporters, LjSUT4 did not transport the plant glucosides arbutin, fraxin and esculin. LjSUT4 showed a low affinity for sucrose (K(0.5)=16 mM at pH 5.3). In addition to inward currents induced by sucrose, other evidence also indicated that LjSUT4 is a proton-coupled symporter: (14)C-sucrose uptake into LjSUT4-expressing oocytes was inhibited by CCCP and sucrose induced membrane depolarization in LjSUT4-expressing oocytes. A GFP-fusion of LjSUT4 localized to the vacuole membrane in Arabidopsis thaliana and in the roots and nodules of Medicago truncatula. Based on these results we propose that LjSUT4 functions in the proton-coupled uptake of sucrose and possibly other glucosides into the cytoplasm from the vacuole.

Molecular physiology of higher plant sucrose transporters

Sucrose is the primary product of photosynthetic CO(2) fixation that is used for the distribution of assimilated carbon within higher plants. Its partitioning from the site of synthesis to different sites of storage, conversion into other storage compounds or metabolic degradation involves various steps of cell-to-cell movement and transport. Many of these steps occur within symplastic domains, i.e. sucrose moves passively cell-to-cell through plasmodesmata. Some essential steps, however, occur between symplastically isolated cells or tissues. In these cases, sucrose is transiently released into the apoplast and its cell-to-cell transport depends on the activity of plasma membrane-localized, energy dependent, H(+)-symporting carrier proteins. This paper reviews the current knowledge of sucrose transporter physiology and molecular biology.

Sugarcane ShSUT1: analysis of sucrose transport activity and inhibition by sucralose

Plant sucrose transporters (SUTs) are members of the glycoside-pentoside-hexuronide (GPH) cation symporter family (TC2.A.2) that is part of the major facilitator superfamily (MFS). All plant SUTs characterized to date function as proton-coupled symporters and catalyze the cellular uptake of sucrose. SUTs are involved in loading sucrose into the phloem and sink tissues, such as seeds, roots and flowers. Because monocots are agriculturally important, SUTs from cereals have been the focus of recent research. Here we present a functional analysis of the SUT ShSUT1 from sugarcane, an important crop species grown for its ability to accumulate high amounts of sucrose in the stem. ShSUT1 was previously shown to be expressed in maturing stems and plays an important role in the accumulation of sucrose in this tissue. Using two-electrode voltage clamping in Xenopus oocytes expressing ShSUT1, we found that ShSUT1 is highly selective for sucrose, but has a relatively low affinity for sucrose (K(0.5) = 8.26 mM at pH 5.6 and a membrane potential of -137 mV). We also found that the sucrose analog sucralose (4,1',6'-trichloro-4,1',6'-trideoxy-galacto-sucrose) is a competitive inhibitor of ShSUT1 with an inhibition coefficient (K(i)) of 16.5 mM. The presented data contribute to our understanding of sucrose transport in plants in general and in monocots in particular.

Pathway of sugar transport in germinating wheat seeds

Three homeologous genes encoding a sucrose (Suc) transporter (SUT) in hexaploid wheat (Triticum aestivum), TaSUT1A, 1B, and 1D, were expressed in germinating seeds, where their function is unknown. All three TaSUT1 proteins were confirmed to be capable of transporting both Suc and maltose by complementation tests with the SUSY7/ura3 yeast (Saccharomyces cerevisiae) mutant strain. The role of Suc transporters in germinating grain was examined by combining in situ hybridization, immunolocalization, fluorescent dye tracer movement, and metabolite assays. TaSUT1 transcript and SUT protein were detected in cells of the aleurone layer, scutellar epidermis, scutellar ground cells, and sieve element-companion cell complexes located in the scutellum, shoot, and root. Ester loading of the membrane-impermeable fluorescent dye carboxyfluorescein into the scutellum epidermal cells of germinating seeds showed that a symplasmic pathway connects the scutellum to the shoot and root via the phloem. However, the scutellar epidermis provides an apoplasmic barrier to solute movement from endosperm tissue. Measurements of sugars in the root, shoot, endosperm, and scutellum suggest that, following degradation of endosperm starch, the resulting hexoses are converted to Suc in the scutellum. Suc was found to be the major sugar present in the endosperm early in germination, whereas maltose and glucose predominate during the later stage. It is proposed that loading the scutellar phloem in germinating wheat seeds can proceed by symplasmic and apoplasmic pathways, the latter facilitated by SUT activity. In addition, SUTs may function to transport Suc into the scutellum from the endosperm early in germination and later transport maltose.