Phloem-localized, Proton-coupled Sucrose Carrier ZmSUT1 Mediates Sucrose Efflux under the Control of the Sucrose Gradient and the Proton Motive Force

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 K (+) battery-regulating Arabidopsis K (+) channel AKT2 is under the control of multiple post-translational steps

Potassium (K (+) ) is an important nutrient for plants. It serves as a cofactor of various enzymes and as the major inorganic solute maintaining plant cell turgor. In a recent study, an as yet unknown role of K (+) in plant homeostasis was shown. It was demonstrated that K (+) gradients in vascular tissues can serve as an energy source for phloem (re)loading processes and that the voltage-gated K (+) channels of the AKT2-type play a unique role in this process. The AKT2 channel can be converted by phosphorylation of specific serine residues (S210 and S329) into a non-rectifying channel that allows a rapid efflux of K (+) from the sieve element/companion cells (SE/CC) complex. The energy of this flux is used by other transporters for phloem (re)loading processes. Nonetheless, the results do indicate that post-translational modifications at S210 and S329 alone cannot explain AKT2 regulation. Here, we discuss the existence of multiple post-translational modification steps that work in concert to convert AKT2 from an inward-rectifying into a non-rectifying K (+) channel.

Potassium (K+) gradients serve as a mobile energy source in plant vascular tissues

The essential mineral nutrient potassium (K(+)) is the most important inorganic cation for plants and is recognized as a limiting factor for crop yield and quality. Nonetheless, it is only partially understood how K(+) contributes to plant productivity. K(+) is used as a major active solute to maintain turgor and to drive irreversible and reversible changes in cell volume. K(+) also plays an important role in numerous metabolic processes, for example, by serving as an essential cofactor of enzymes. Here, we provide evidence for an additional, previously unrecognized role of K(+) in plant growth. By combining diverse experimental approaches with computational cell simulation, we show that K(+) circulating in the phloem serves as a decentralized energy storage that can be used to overcome local energy limitations. Posttranslational modification of the phloem-expressed Arabidopsis K(+) channel AKT2 taps this "potassium battery," which then efficiently assists the plasma membrane H(+)-ATPase in energizing the transmembrane phloem (re)loading processes.

Functional characterization and RNAi-mediated suppression reveals roles for hexose transporters in sugar accumulation by tomato fruit

Hexoses accumulate to high concentrations (∼ 200 mM) in storage parenchyma cells of tomato fruit. Hexoses are sourced from the fruit apoplasm as hydrolysis products of phloem-imported sucrose. Three hexose transporters (LeHT1, LeHT2, LeHT3), expressed in fruit storage parenchyma cells, may contribute to hexose uptake by these cells. An analysis of their full-length sequences demonstrated that all three transporters belong to the STP sub-family of monosaccharide transporters that localize to plasma membranes. Heterologous expression of LeHT1 (and previously LeHT2, Gear et al., 2000), but not LeHT3, rescued a hexose transport-impaired yeast mutant when raised on glucose or fructose as the sole carbon source. Biochemically, LeHT1, similarly to LeHT2, exhibited transport properties consistent with a high-affinity glucose/H(+) symporter. Significantly, LeHT1 and LeHT2 also functioned as low-affinity fructose/H(+) symporters with apparent K(m) values commensurate with those of fruit tissues. A substantial reduction (80-90%) in fruit expression levels of all LeHT genes by RNAi-mediated knockdown caused a 55% decrease in fruit hexose accumulation. In contrast, photoassimilate production by source leaves and phloem transport capacity to fruit were unaffected by transporter knockdown. Collectively, these findings demonstrate that LeHTs play key roles in driving accumulation of hexoses into storage parenchyma cells during tomato fruit development.

Structural and metabolic transitions of C4 leaf development and differentiation defined by microscopy and quantitative proteomics in maize

C(4) grasses, such as maize (Zea mays), have high photosynthetic efficiency through combined biochemical and structural adaptations. C(4) photosynthesis is established along the developmental axis of the leaf blade, leading from an undifferentiated leaf base just above the ligule into highly specialized mesophyll cells (MCs) and bundle sheath cells (BSCs) at the tip. To resolve the kinetics of maize leaf development and C(4) differentiation and to obtain a systems-level understanding of maize leaf formation, the accumulation profiles of proteomes of the leaf and the isolated BSCs with their vascular bundle along the developmental gradient were determined using large-scale mass spectrometry. This was complemented by extensive qualitative and quantitative microscopy analysis of structural features (e.g., Kranz anatomy, plasmodesmata, cell wall, and organelles). More than 4300 proteins were identified and functionally annotated. Developmental protein accumulation profiles and hierarchical cluster analysis then determined the kinetics of organelle biogenesis, formation of cellular structures, metabolism, and coexpression patterns. Two main expression clusters were observed, each divided in subclusters, suggesting that a limited number of developmental regulatory networks organize concerted protein accumulation along the leaf gradient. The coexpression with BSC and MC markers provided strong candidates for further analysis of C(4) specialization, in particular transporters and biogenesis factors. Based on the integrated information, we describe five developmental transitions that provide a conceptual and practical template for further analysis. An online protein expression viewer is provided through the Plant Proteome Database.

Increased activity of the vacuolar monosaccharide transporter TMT1 alters cellular sugar partitioning, sugar signaling, and seed yield in Arabidopsis

The extent to which vacuolar sugar transport activity affects molecular, cellular, and developmental processes in Arabidopsis (Arabidopsis thaliana) is unknown. Electrophysiological analysis revealed that overexpression of the tonoplast monosaccharide transporter TMT1 in a tmt1-2::tDNA mutant led to increased proton-coupled monosaccharide import into isolated mesophyll vacuoles in comparison with wild-type vacuoles. TMT1 overexpressor mutants grew faster than wild-type plants on soil and in high-glucose (Glc)-containing liquid medium. These effects were correlated with increased vacuolar monosaccharide compartmentation, as revealed by nonaqueous fractionation and by chlorophyll(ab)-binding protein1 and nitrate reductase1 gene expression studies. Soil-grown TMT1 overexpressor plants respired less Glc than wild-type plants and only about half the amount of Glc respired by tmt1-2::tDNA mutants. In sum, these data show that TMT activity in wild-type plants limits vacuolar monosaccharide loading. Remarkably, TMT1 overexpressor mutants produced larger seeds and greater total seed yield, which was associated with increased lipid and protein content. These changes in seed properties were correlated with slightly decreased nocturnal CO(2) release and increased sugar export rates from detached source leaves. The SUC2 gene, which codes for a sucrose transporter that may be critical for phloem loading in leaves, has been identified as Glc repressed. Thus, the observation that SUC2 mRNA increased slightly in TMT1 overexpressor leaves, characterized by lowered cytosolic Glc levels than wild-type leaves, provided further evidence of a stimulated source capacity. In summary, increased TMT activity in Arabidopsis induced modified subcellular sugar compartmentation, altered cellular sugar sensing, affected assimilate allocation, increased the biomass of Arabidopsis seeds, and accelerated early plant development.

Sucrose- and H-dependent charge movements associated with the gating of sucrose transporter ZmSUT1

Background

In contrast to man the majority of higher plants use sucrose as mobile carbohydrate. Accordingly proton-driven sucrose transporters are crucial for cell-to-cell and long-distance distribution within the plant body. Generally very negative plant membrane potentials and the ability to accumulate sucrose quantities of more than 1 M document that plants must have evolved transporters with unique structural and functional features.

Methodology/Principal Findings

To unravel the functional properties of one specific high capacity plasma membrane sucrose transporter in detail, we expressed the sucrose/H⁺ co-transporter from maize ZmSUT1 in Xenopus oocytes. Application of sucrose in an acidic pH environment elicited inward proton currents. Interestingly the sucrose-dependent H⁺ transport was associated with a decrease in membrane capacitance (C_m). In addition to sucrose C_m was modulated by the membrane potential and external protons. In order to explore the molecular mechanism underlying these C_m changes, presteady-state currents (I_pre) of ZmSUT1 transport were analyzed. Decay of I_pre could be best fitted by double exponentials. When plotted against the voltage the charge Q, associated to I_pre, was dependent on sucrose and protons. The mathematical derivative of the charge Q versus voltage was well in line with the observed C_m changes. Based on these parameters a turnover rate of 500 molecules sucrose/s was calculated. In contrast to gating currents of voltage dependent-potassium channels the analysis of ZmSUT1-derived presteady-state currents in the absence of sucrose (I = Q/τ) was sufficient to predict ZmSUT1 transport-associated currents.

Conclusions

Taken together our results indicate that in the absence of sucrose, ‘trapped’ protons move back and forth between an outer and an inner site within the transmembrane domains of ZmSUT1. This movement of protons in the electric field of the membrane gives rise to the presteady-state currents and in turn to C_m changes. Upon application of external sucrose, protons can pass the membrane turning presteady-state into transport currents.

Inverse pH regulation of plant and fungal sucrose transporters: a mechanism to regulate competition for sucrose at the host/pathogen interface?

BACKGROUND

Plant sucrose transporter activities were shown to respond to changes in the extracellular pH and redox status, and oxidizing compounds like glutathione (GSSG) or H(2)O(2) were reported to effect the subcellular targeting of these proteins. We hypothesized that changes in both parameters might be used to modulate the activities of competing sucrose transporters at a plant/pathogen interface. We, therefore, compared the effects of redox-active compounds and of extracellular pH on the sucrose transporters UmSRT1 and ZmSUT1 known to compete for extracellular sucrose in the Ustilago maydis (corn smut)/Zea mays (maize) pathosystem.

METHODOLOGY/PRINCIPAL FINDINGS

We present functional analyses of the U. maydis sucrose transporter UmSRT1 and of the plant sucrose transporters ZmSUT1 and StSUT1 in Saccharomyces cerevisiae or in Xenopus laevis oocytes in the presence of different extracellular pH-values and redox systems, and study the possible effects of these treatments on the subcellular targeting. We observed an inverse regulation of host and pathogen sucrose transporters by changes in the apoplastic pH. Under none of the conditions analyzed, we could confirm the reported effects of redox-active compounds.

CONCLUSIONS/SIGNIFICANCE

Our data suggest that changes in the extracellular pH but not of the extracellular redox status might be used to oppositely adjust the transport activities of plant and fungal sucrose transporters at the host/pathogen interface.

The phloem pathway: new issues and old debates

The phloem is a central actor in plant development and nutrition, providing nutrients and energy to sink organs and integrating interorgan communication. A comprehensive picture of the molecules trafficking in phloem sap is being made available, with recent surveys of proteins, RNAs, sugars, and other metabolites, some of which are potentially acting as signals. In this review, we focus on recent breakthroughs on phloem transport and signalling. A case study was phloem loading of sucrose, acting both as a nutrient and as a signal, whose activity was shown to be tightly regulated. Recent advances also described actors of macromolecular trafficking in sieve elements, including chaperones and RNA binding proteins, involved potentially in the formation of ribonucleoprotein complexes. Likewise, long distance signalling appeared to integrate electrical potential waves, calcium bursts and potentially the generation of reactive oxygen species. The ubiquitin-proteasome system was also proposed to be on action in sieve elements for signalling and protein turnover. Surprisingly, several basic processes of phloem physiology are still under debate. Hence, the absence in phloem sap of reducing sugar species, such as hexoses, was recently challenged with observations based on an analysis of the sap from Ranunculaceae and Papaveraceae. The possibility that protein synthesis might occur in sieve elements was again questioned with the identification of components of the translational machinery in Pumpkin phloem sap. Altogether, these new findings strengthen the idea that phloem is playing a central role in interorgan nutrient exchanges and communication and demonstrate that the ways by which this is achieved can obey various patterns among species.

Seasonal and cell type specific expression of sulfate transporters in the phloem of Populus reveals tree specific characteristics for SO(4)(2-) storage and mobilization

The storage and mobilization of nutrients in wood and bark tissues is a typical feature of trees. Sulfur can be stored as sulfate, which is transported from source to sink tissues through the phloem. In the present study two transcripts encoding sulfate transporters (SULTR) were identified in the phloem of grey poplar (Populus tremula x P. alba). Their cell-specific expression was analyzed throughout poplar in source tissues, such as mature leaves, and in sink tissues, such as the wood and bark of the stem, roots and the shoot apex. PtaSULTR1;1 mRNA was detected in companion cells of the transport phloem, in the phloem of high-order leaf veins and in fine roots. PtaSULTR3;3a mRNA was found exclusively in the transport phloem and here in both, companion cells and sieve elements. Both sulfate transporter transcripts were located in xylem parenchyma cells indicating a role for PtaSULTR1;1 and PtaSULTR3;3a in xylem unloading. Changes in mRNA abundance of these and of the sulfate transporters PtaSULTR4;1 and PtaSULTR4;2 were analyzed over an entire growing season. The expression of PtaSULTR3;3a and of the putative vacuolar efflux transporter PtaSULTR4;2 correlated negatively with the sulfate content in the bark. Furthermore, the expression pattern of both PtaSULTR3;3a and PtaSULTR4;2 correlated significantly with temperature and day length. Thus both SULTRs seem to be involved in mobilization of sulfate during spring: PtaSULTR4;2 mediating efflux from the vacuole and PtaSULTR3;3a mediating loading into the transport phloem. In contrast, the abundance of PtaSULTR1;1 and PtaSULTR4;1 transcripts was not affected by environmental changes throughout the whole season. The transcript abundance of all tested sulfate transporters in leaves was independent of weather conditions. However, PtaSULTR1;1 abundance correlated negatively with sulfate content in leaves, supporting its function in phloem loading. Taken together, these findings indicate a transcriptional regulation of sulfate distribution in poplar trees.

Fungal carbohydrate support in the ectomycorrhizal symbiosis: a review

Ectomycorrhizal (ECM) symbiosis is a mutualistic interaction between certain soil fungi and fine roots of perennial plants, mainly forest trees, by which both partners become capable of efficiently colonising nutrient-limited environments. The success of this interaction is reflected in the dominance of ECM forest ecosystems in the Northern hemisphere. Apart from their economic importance (wood production), forest ecosystems are essential for large-scale carbon sequestration, leading to substantial reductions in anthropogenic CO(2) release. The biological function of ECM symbiosis is the exchange of fungus-derived mineral nutrients for plant-derived carbohydrates. Improved plant nutrition as a result of this interaction, however, has a price. Together with their fungal partner, root systems of ECM plants can receive about half of the photosynthetically fixed carbon. To enable such a strong carbohydrate sink, the monosaccharide uptake capacity and carbohydrate flux through glycolysis and intermediate carbohydrate storage pools (trehalose and/or mannitol) of mycorrhizal fungi is strongly increased at the plant-fungus interface. Apart from their function as a carbohydrate store, trehalose/mannitol are additionally considered to be involved in carbon allocation within the fungal colony. Dependent on the fungal species involved in the symbiosis, regulation and fine-tuning of fungal carbohydrate uptake and metabolism seems to be controlled either by developmental mechanisms or by the apoplastic sugar content. As a consequence of the increased carbohydrate demand in symbiosis, trees increase their photosynthetic capacity. In addition, host plants control and restrict carbohydrate flux towards their partner to avoid fungal parasitism. The mechanisms behind this phenomenon are still largely unknown but rates of local sucrose hydrolysis and hexose uptake by rhizodermal cells are thought to restrict fungal carbohydrate nutrition under certain conditions (e.g., reduced fungal nutrient export).

A novel high-affinity sucrose transporter is required for virulence of the plant pathogen Ustilago maydis

Plant pathogenic fungi cause massive yield losses and affect both quality and safety of food and feed produced from infected plants. The main objective of plant pathogenic fungi is to get access to the organic carbon sources of their carbon-autotrophic hosts. However, the chemical nature of the carbon source(s) and the mode of uptake are largely unknown. Here, we present a novel, plasma membrane-localized sucrose transporter (Srt1) from the corn smut fungus Ustilago maydis and its characterization as a fungal virulence factor. Srt1 has an unusually high substrate affinity, is absolutely sucrose specific, and allows the direct utilization of sucrose at the plant/fungal interface without extracellular hydrolysis and, thus, without the production of extracellular monosaccharides known to elicit plant immune responses. srt1 is expressed exclusively during infection, and its deletion strongly reduces fungal virulence. This emphasizes the central role of this protein both for efficient carbon supply and for avoidance of apoplastic signals potentially recognized by the host.

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.

RNA interference of LIN5 in tomato confirms its role in controlling Brix content, uncovers the influence of sugars on the levels of fruit hormones, and demonstrates the importance of sucrose cleavage for normal fruit development and fertility

It has been previously demonstrated, utilizing intraspecific introgression lines, that Lycopersicum Invertase5 (LIN5), which encodes a cell wall invertase, controls total soluble solids content in tomato (Solanum lycopersicum). The physiological role of this protein, however, has not yet been directly studied, since evaluation of data obtained from the introgression lines is complicated by the fact that they additionally harbor many other wild species alleles. To allow a more precise comparison, we generated transgenic tomato in which we silenced the expression of LIN5 using the RNA interference approach. The transformants were characterized by an altered flower and fruit morphology, displaying increased numbers of petals and sepals per flower, an increased rate of fruit abortion, and a reduction in fruit size. Evaluation of the mature fruit revealed that the transformants were characterized by a reduction of seed number per plant. Furthermore, detailed physiological analysis revealed that the transformants displayed aberrant pollen morphology and a reduction in the rate of pollen tube elongation. Metabolite profiling of ovaries and green and red fruit revealed that metabolic changes in the transformants were largely confined to sugar metabolism, whereas transcript and hormone profiling revealed broad changes both in the hormones themselves and in transcripts encoding their biosynthetic enzymes and response elements. These results are discussed in the context of current understanding of the role of sugar during the development of tomato fruit, with particular focus given to its impact on hormone levels and organ morphology.

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.

Conversion of the 2 Cl(-)/1 H+ antiporter ClC-5 in a NO3(-)/H+ antiporter by a single point mutation

Several members of the CLC family are secondary active anion/proton exchangers, and not passive chloride channels. Among the exchangers, the endosomal ClC-5 protein that is mutated in Dent's disease shows an extreme outward rectification that precludes a precise determination of its transport stoichiometry from measurements of the reversal potential. We developed a novel imaging method to determine the absolute proton flux in Xenopus oocytes from the extracellular proton gradient. We determined a transport stoichiometry of 2 Cl(-)/1 H+. Nitrate uncoupled proton transport but mutating the highly conserved serine 168 to proline, as found in the plant NO3(-)/H+ antiporter atClCa, led to coupled NO3(-)/H+ exchange. Among several amino acids tested at position 168, S168P was unique in mediating highly coupled NO3(-)/H+ exchange. We further found that ClC-5 is strongly stimulated by intracellular protons in an allosteric manner with an apparent pK of approximately 7.2. A 2:1 stoichiometry appears to be a general property of CLC anion/proton exchangers. Serine 168 has an important function in determining anionic specificity of the exchange mechanism.

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.

Mutation of the Arabidopsis NRT1.5 nitrate transporter causes defective root-to-shoot nitrate transport

Little is known about the molecular and regulatory mechanisms of long-distance nitrate transport in higher plants. NRT1.5 is one of the 53 Arabidopsis thaliana nitrate transporter NRT1 (Peptide Transporter PTR) genes, of which two members, NRT1.1 (CHL1 for Chlorate resistant 1) and NRT1.2, have been shown to be involved in nitrate uptake. Functional analysis of cRNA-injected Xenopus laevis oocytes showed that NRT1.5 is a low-affinity, pH-dependent bidirectional nitrate transporter. Subcellular localization in plant protoplasts and in planta promoter-beta-glucuronidase analysis, as well as in situ hybridization, showed that NRT1.5 is located in the plasma membrane and is expressed in root pericycle cells close to the xylem. Knockdown or knockout mutations of NRT1.5 reduced the amount of nitrate transported from the root to the shoot, suggesting that NRT1.5 participates in root xylem loading of nitrate. However, root-to-shoot nitrate transport was not completely eliminated in the NRT1.5 knockout mutant, and reduction of NRT1.5 in the nrt1.1 background did not affect root-to-shoot nitrate transport. These data suggest that, in addition to that involving NRT1.5, another mechanism is responsible for xylem loading of nitrate. Further analyses of the nrt1.5 mutants revealed a regulatory loop between nitrate and potassium at the xylem transport step.

Transport and sorting of the solanum tuberosum sucrose transporter SUT1 is affected by posttranslational modification

The plant sucrose transporter SUT1 from Solanum tuberosum revealed a dramatic redox-dependent increase in sucrose transport activity when heterologously expressed in Saccharomyces cerevisiae. Plant plasma membrane vesicles do not show any change in proton flux across the plasma membrane in the presence of redox reagents, indicating a SUT1-specific effect of redox reagents. Redox-dependent sucrose transport activity was confirmed electrophysiologically in Xenopus laevis oocytes with SUT1 from maize (Zea mays). Localization studies of green fluorescent protein fusion constructs showed that an oxidative environment increased the targeting of SUT1 to the plasma membrane where the protein concentrates in 200- to 300-nm raft-like microdomains. Using plant plasma membranes, St SUT1 can be detected in the detergent-resistant membrane fraction. Importantly, in yeast and in plants, oxidative reagents induced a shift in the monomer to dimer equilibrium of the St SUT1 protein and increased the fraction of dimer. Biochemical methods confirmed the capacity of SUT1 to form a dimer in plants and yeast cells in a redox-dependent manner. Blue native PAGE, chemical cross-linking, and immunoprecipitation, as well as the analysis of transgenic plants with reduced expression of St SUT1, confirmed the dimerization of St SUT1 and Sl SUT1 (from Solanum lycopersicum) in planta. The ability to form homodimers in plant cells was analyzed by the split yellow fluorescent protein technique in transiently transformed tobacco (Nicotiana tabacum) leaves and protoplasts. Oligomerization seems to be cell type specific since under native-like conditions, a phloem-specific reduction of the dimeric form of the St SUT1 protein was detectable in SUT1 antisense plants, whereas constitutively inhibited antisense plants showed reduction only of the monomeric form. The role of redox control of sucrose transport in plants is discussed.

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.

Review: Nutrient loading of developing seeds

Interest in nutrient loading of seeds is fuelled by its central importance to plant reproductive success and human nutrition. Rates of nutrient loading, imported through the phloem, are regulated by transport and transfer processes located in sources (leaves, stems, reproductive structures), phloem pathway and seed sinks. During the early phases of seed development, most control is likely to be imposed by a low conductive pathway of differentiating phloem cells serving developing seeds. Following the onset of storage product accumulation by seeds, and, depending on nutrient species, dominance of path control gives way to regulation by processes located in sources (nitrogen, sulfur, minor minerals), phloem path (transition elements) or seed sinks (sugars and major mineral elements, such as potassium). Nutrients and accompanying water are imported into maternal seed tissues and unloaded from the conducting sieve elements into an extensive post-phloem symplasmic domain. Nutrients are released from this symplasmic domain into the seed apoplasm by poorly understood membrane transport mechanisms. As seed development progresses, increasing volumes of imported phloem water are recycled back to the parent plant by process(es) yet to be discovered. However, aquaporins concentrated in vascular and surrounding parenchyma cells of legume seed coats could provide a gated pathway of water movement in these tissues. Filial cells, abutting the maternal tissues, take up nutrients from the seed apoplasm by membrane proteins that include sucrose and amino acid/H⁺ symporters functioning in parallel with non-selective cation channels. Filial demand for nutrients, that comprise the major osmotic species, is integrated with their release and phloem import by a turgor-homeostat mechanism located in maternal seed tissues. It is speculated that turgors of maternal unloading cells are sensed by the cytoskeleton and transduced by calcium signalling cascades.

Immunolocalization of the PmSUC1 sucrose transporter in Plantago major flowers and reporter-gene analyses of the PmSUC1 promoter suggest a role in sucrose release from the inner integument

This paper presents a detailed analysis of the PmSUC1 gene from plantago major, of its promoter activity in Arabidopsis, and of the tissue specific localization of the encoded protein in Plantago. PmSUC1 promoter activity was detected in the innermost layer of the inner integument (the endothel) of Arabidopsis plants expressing the gene of the green fluorescent protein (GFP) under the control of the PmSUC1 promoter. This promoter activity was confirmed with a PmSUC1-specific antiserum that identified the PmSUC1 protein in the endothel of Plantago and of Arabidopsis plants expressing the PmSUC1 gene under the control of its own promoter. PmSUC1 promoter activity and PmSUC1 protein were also detected in pollen grains during maturation inside the anthers and in pollen tubes during and after germination. These results demonstrate that PmSUC1 is involved in sucrose partitioning to the young embryo and to the developing pollen and growing pollen tube. In the innermost cell layer of the inner integument, a tissue that delivers nutrients to the endosperm and the embryo, PmSUC1 may catalyze the release of sucrose into the apoplast.

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.

Fluxomics with ratiometric metabolite dyes

Today's major excitement in biology centers on signaling: How can a cell or organism measure the myriad of environmental cues, integrate it, and acclimate to the new conditions? Hormonal signals and second messengers are in the focus of most of these studies, e.g., regulation of glucose transporter GLUT4 cycling by insulin, or regulation of plant growth by auxin or brassinosteroids.1-3 In comparison, we generally assume that we know almost everything about basic metabolism since it has been studied for many decades; for example we know since the early 80s that allosteric regulation by fructose-2,6-bisphophate plays an important role in regulating glycolysis in plants and animals.4 This may be the reason why studies of metabolism appear to be a bit out of fashion. But if we look to other organisms such as E. coli or yeast, we rapidly realize that metabolism is controlled by complex interconnected signaling networks, and that we understand little of these signaling networks in humans and plants.5,6 As it turns out, the cell registers many metabolites, and flux through the pathways is regulated using complex signaling networks that involve calcium as well as hormones.

A suite of sucrose transporters expressed in coats of developing legume seeds includes novel pH-independent facilitators

A suite of newly discovered sucrose transporter genes, PsSUF1, PsSUF4, PvSUT1 and PvSUF1, were isolated from the coats of developing pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) seeds. Sequence analysis indicated that deduced proteins encoded by PsSUF1, PvSUT1 and PvSUF1 clustered in a separate sub-group under sucrose transporter Clade I, whereas the deduced protein encoded by PsSUF4 clustered in Clade II. When expressed in yeast, these genes were shown to encode sucrose transporters with apparent Michaelis Menten constant (Km) values ranging from 8.9 to 99.8 mm. PvSUT1 exhibited functional characteristics of a sucrose/H+ symporter. In contrast, PsSUF1, PvSUF1 and PsSUF4 supported the pH- and energy independent transport of sucrose that was shown to be bi-directional. These transport properties, together with that of counter transport, indicated that PsSUF1, PvSUF1 and PsSUF4 function as carriers that support the facilitated diffusion of sucrose. Carrier function was unaffected by diethylpyrocarbonate and by maltose competition, suggesting that the sucrose binding sites of these transporters differed from those of known sucrose/H+ symporters. All sucrose transporters were expressed throughout the plant and, of greatest interest, were co-expressed in cells considered responsible for sucrose efflux from seed coats. The possible roles played by the novel facilitators in sucrose efflux from seed coats are discussed.

Isolation, functional characterization, and expression analysis of grapevine (Vitis vinifera L.) hexose transporters: differential roles in sink and source tissues

Three hexose transporters (VvHT3, VvHT4, and VvHT5) were cloned from Vitis vinifera L. and functionally characterized in the hexose transport-impaired Saccharomyces cerevisiae mutant EBY.VW4000. Both VvHT4 and VvHT5 facilitated glucose uptake, with K(m)s of 137 muM and 89 muM, respectively. VvHT3 was not functional in the yeast system but a VvHT3:GFP (green fluorescent protein) fusion protein was targeted to the plasma membrane in plant cells. In young 'sink' leaves, transcript levels of all five VvHTs and a cell wall invertase (VvcwINV) were low. In mature leaves, there were increased levels of VvHT1, VvHT3, VvHT5, and VvcwINV transcripts, suggesting that mature leaves may have an increased capacity for apoplastic sucrose hydrolysis and hexose retrieval. In grape berries, VvHT1, VvHT2, and VvHT3 transcript levels were found to be significantly higher than those of VvHT4 and VvHT5. VvHT1 was most highly expressed early in berry development but decreased during the period of rapid sugar accumulation, while VvHT2 and VvHT3 expression remained high during this accumulation phase. VvcwINV expression occurred throughout berry development but peaked just prior to veraison. It is clear that the machinery to transport the hexose molecules produced through the cleavage of sucrose, by cell wall invertase, is present in the berry. This agrees with the suggestion that hexose accumulation to high levels during the ripening phase occurs through an apoplastic pathway. Interestingly, there is no direct relationship between VvHT gene expression and hexose accumulation, which suggests either that transcription is not the main determinant of transport activity or that other transport pathways are also active.

Conversion of a Putative Agrobacterium Sugar-binding Protein into a FRET Sensor with High Selectivity for Sucrose

Glucose is the main sugar transport form in animals, whereas plants use sucrose to supply non-photosynthetic organs with carbon skeletons and energy. Many aspects of sucrose transport, metabolism, and signaling are not well understood, including the route of sucrose efflux from leaf mesophyll cells and transport across vacuolar membranes. Tools that can detect sucrose with high spatial and temporal resolution in intact organs may help elucidate the players involved. Here, FRET sensors were generated by fusing putative sucrose-binding proteins to green fluorescent protein variants. Plant-associated bacteria such as Rhizobium and Agrobacterium can use sucrose as a nutrient source; sugar-binding proteins were, thus, used as scaffolds for developing sucrose nanosensors. Among a set of putative sucrose-binding protein genes cloned in between eCFP and eYFP and tested for sugar-dependent FRET changes, an Agrobacterium sugar-binding protein bound sucrose with 4 mum affinity. This FLIPsuc-4mu protein also recognized other sugars including maltose, trehalose, and turanose and, with lower efficiency, glucose and palatinose. Homology modeling enabled the prediction of binding pocket mutations to modulate the relative affinity of FLIPsuc-4mu for sucrose, maltose, and glucose. Mutant nanosensors showed up to 50- and 11-fold increases in specificity for sucrose over maltose and glucose, respectively, and the sucrose binding affinity was simultaneously decreased to allow detection in the physiological range. In addition, the signal-to-noise ratio of the sucrose nanosensor was improved by linker engineering. This novel reagent complements FLIPs for glucose, maltose, ribose, glutamate, and phosphate and will be used for analysis of sucrose-derived carbon flux in bacterial, fungal, plant, and animal cells.

Quantification of plasmodesmatal endoplasmic reticulum coupling between sieve elements and companion cells using fluorescence redistribution after photobleaching

Transgenic tobacco (Nicotiana tabacum) was studied to localize the activity of phloem loading during development and to establish whether the endoplasmic reticulum (ER) of the companion cell (CC) and the sieve element (SE) reticulum is continuous by using a SUC2 promoter-green fluorescent protein (GFP) construct targeted to the CC-ER. Expression of GFP marked the collection phloem in source leaves and cotyledons as expected, but also the transport phloem in stems, petioles, midveins of sink leaves, nonphotosynthetic flower parts, roots, and newly germinated seedlings, suggesting that sucrose retrieval along the pathway is an integral component of phloem function. GFP fluorescence was limited to CCs where it was visualized as a well-developed ER network in close proximity to the plasma membrane. ER coupling between CC and SEs was tested in wild-type tobacco using an ER-specific fluorochrome and fluorescence redistribution after photobleaching (FRAP), and showed that the ER is continuous via pore-plasmodesma units. ER coupling between CC and SE was quantified by determining the mobile fraction and half-life of fluorescence redistribution and compared with that of other cell types. In all tissues, fluorescence recovered slowly when it was rate limited by plasmodesmata, contrasting with fast intracellular FRAP. FRAP was unaffected by treatment with cytochalasin D. The highest degree of ER coupling was measured between CC and SE. Intimate ER coupling is consistent with a possible role for ER in membrane protein and signal exchange between CC and SE. However, a complete lack of GFP transfer between CC and SE indicated that the intraluminal pore-plasmodesma contact has a size exclusion limit below 27 kD.

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.

30-year progress of membrane transport in plants

In the past 30 years enormous progress was made in plant membrane biology and transport physiology, a fact reflected in the appearance of textbooks. The first book dedicated to 'Membrane Transport in Plants' was published on the occasion of the 'International Workshop on Membrane Transport in Plants' held at the Nuclear Research Center, Jülich, Germany [Zimmermann and Dainty (eds) 1974] and was followed in 1976 by a related volume 'Transport in plants II' in the 'Encyclopedia of plant physiology' [Lüttge and Pitman (eds) 1976]. A broad spectrum of topics including thermodynamics of transport processes, water relations, primary reactions of photosynthesis, as well as more conventional aspects of membrane transport was presented. The aim of the editors of the first book was to bring advanced thermodynamical concepts to the attention of biologists and to show physical chemists and biophysicist what the more complex biological systems were like. To bundle known data on membrane transport in plants and relevant fields for mutual understanding, interdisciplinary research and clarification of problems were considered highly important for further progress in this scientific area of plant physiology. The present review will critically evaluate the progress in research in membrane transport in plants that was achieved during the past. How did 'Membrane Transport in Plants' progress within the 30 years between the publication of the first book about this topic (Zimmermann and Dainty 1974), a recent one with the same title (Blatt 2004), and today?

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.

Sucrose transporter LeSUT1 and LeSUT2 inhibition affects tomato fruit development in different ways

Sucrose transporters of higher plants belong to a large gene family. At least four different sucrose transporters are known in Solanaceous plants, although their function remains to be elucidated in detail. The isolation of LeSUT1 and LeSUT2from Lycopersicon esculentum has been described earlier. Whereas SUT1 is supposed to be the main phloem loader of sucrose in Solanaceae, the role of SUT2 remains a matter of debate. A transgenic approach was taken to evaluate the potential functions of SUT2/SUC3 proteins in sucrose transport or sensing. Expression of LeSUT1 and LeSUT2 was inhibited independently in transgenic tomato plants, using the antisense technique, in order to analyse their specific functions. Although the phloem-specific inhibition of LeSUT1 antisense plants showed a phenotype consistent with an essential role in phloem loading, constitutive LeSUT2 antisense inhibition exclusively affected tomato fruit and seed development. Neither LeSUT1, nor the LeSUT2 antisense plants were able to produce normal tomato fruits; however, it is likely that independent mechanisms underlie these phenomena. While phloem loading was blocked in LeSUT1 antisense plants, the fertility of fruits was reduced in LeSUT2 antisense plants. A detailed physiological analysis of these plants established a role for SUT2 in pollen tube growth and thus assigned a physiological role for SUT2.

Polar-localised poplar K+ channel capable of controlling electrical properties of wood-forming cells

In previous studies, we have shown that annual expression profiles of cambial and wood tissue with respect to the Shaker K+ channel PTORK correlate with cambial activity. To follow PTORK-gene activity on the cellular level, we isolated the respective promoter regions and generated transgenic Arabidopsis plants expressing the GUS gene under the control of the PTORK promoter. Cross-sections of petioles showed PTORK-driven signals predominantly in the xylem parenchyma surrounding the vessels and in the phloem. Antibodies raised against a unique N-terminal region of PTORK in histo-immunochemical analyses recognised this K+-release channel in growth-active poplar plants only. PTORK labelling was found in differentiating xylem cells (young fibres) and mature xylem (vessel-associated cells of the ray parenchyma). Patch-clamp measurements on fibre cell protoplasts, derived from young poplar twigs, identified outward-rectifying K+ channels as the major K+ conductance of this cell type, which resembled the biophysical properties of PTORK when expressed in Xenopus oocytes.

Analysis of the Transport Activity of Barley Sucrose Transporter HvSUT1

Localization studies indicate that barley (Hordeum vulgare) sucrose transporter HvSUT1 functions in sucrose uptake into seeds during grain filling. To further understand the physiological function of HvSUT1, we have expressed the HvSUT1 cDNA in Xenopus laevis oocytes and analyzed the transport activity by two-electrode voltage clamping. Consistent with a H(+)-coupled transport mechanism, sucrose induced large inward currents in HvSUT1-expressing oocytes with a K (0.5) of 3.8 mM at pH 5.0 and a membrane potential of -157 mV. Of 21 other sugars tested, four glucosides were also transported by HvSUT1. These glucosides were maltose, salicin (2-(hydroxymethyl) phenyl beta-D-glucoside), alpha-phenylglucoside and alpha-paranitrophenylglucoside. Kinetic analysis of transport of these substrates by HvSUT1 was performed and K (0.5) values were measured. The apparent affinity for all substrates was dependent on membrane potential and pH with lower K (0.5) values at lower external pH and more negative membrane potentials. HvSUT1 was more selective for alpha-glucosides over beta-glucosides than the Arabidopsis sucrose transporter AtSUC2. Several substrates transported by AtSUC2 (beta-phenylglucoside, beta-paranitrophenylglucoside, alpha-methylglucoside, turanose, and arbutin (hydroquinone beta-D-glucoside)) showed low or undetectable transport by HvSUT1. Of these, beta-paranitrophenylglucoside inhibited sucrose transport by HvSUT1 indicating that it interacts with the transporter while arbutin and alpha-methyl glucoside did not inhibit. The results demonstrate significant differences in substrate specificity between HvSUT1 and AtSUC2.

In the light of stomatal opening: new insights into 'the Watergate'

Stomata can be regarded as hydraulically driven valves in the leaf surface, which open to allow CO2 uptake and close to prevent excessive loss of water. Movement of these 'Watergates' is regulated by environmental conditions, such as light, CO2 and humidity. Guard cells can sense environmental conditions and function as motor cells within the stomatal complex. Stomatal movement results from the transport of K+ salts across the guard cell membranes. In this review, we discuss the biophysical principles and mechanisms of stomatal movement and relate these to ion transport at the plasma membrane and vacuolar membrane. Studies with isolated guard cells, combined with recordings on single guard cells in intact plants, revealed that light stimulates stomatal opening via blue light-specific and photosynthetic-active radiation-dependent pathways. In addition, guard cells sense changes in air humidity and the water status of distant tissues via the stress hormone abscisic acid (ABA). Guard cells thus provide an excellent system to study cross-talk, as multiple signaling pathways induce both short- and long-term responses in these sensory cells.