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Christina Kühn - One of the best experts on this subject based on the ideXlab platform.
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What determines the composition of the phloem sap? Is there any selectivity filter for macromolecules entering the phloem Sieve Elements?
Plant physiology and biochemistry : PPB, 2020Co-Authors: Varsha Garg, Christina KühnAbstract:In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young Sieve Elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.
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Electrophysiological approach to determine kinetic parameters of sucrose uptake by single Sieve Elements or phloem parenchyma cells in intact Vicia faba plants.
Frontiers in plant science, 2013Co-Authors: Jens B. Hafke, Christina Kühn, Sabina-roxana Höll, Aart J. E. Van BelAbstract:Apart from a few using cut aphid stylets, no attempts have been made thus far to measure in vivo sucrose-uptake properties of Sieve Elements. We investigated the kinetics of sucrose uptake by single Sieve Elements and phloem parenchyma cells in Vicia faba plants. To this end, microelectrodes were inserted into free-lying phloem cells in the main vein of the youngest fully-expanded leaf, half-way along the stem, in the transition zone between the autotrophic and heterotrophic part of the stem, and in the root axis. A top-to-bottom membrane potential gradient of Sieve Elements was observed along the stem (-130 mV to -110 mV), while the membrane potential of the phloem parenchyma cells was stable (approx. -100 mV). In roots, the membrane potential of Sieve Elements dropped abruptly to -55 mV. Bathing solutions having various sucrose concentrations were administered and sucrose/H+-induced depolarisations were recorded. Data analysis by nonlinear least-square data fittings as well as by linear Eadie-Hofstee (EH) -transformations pointed at biphasic Michaelis-Menten kinetics (2 MM, EH: Km1 1.2-1.8 mM, Km2 6.6-9.0 mM) of sucrose uptake by Sieve Elements. However, Akaike’s Information Criterion (AIC) favoured single MM kinetics. Using single MM as the best-fitting model, Km values for sucrose uptake by Sieve Elements decreased along the plant axis from 1 to 7 mM. For phloem parenchyma cells, higher Km values (EH: Km1 10 mM, Km2 70 mM) as compared to Sieve Elements were found. In preliminary patch-clamp experiments with Sieve-element protoplasts, small sucrose-coupled proton currents (-0.1 to -0.3 pA/ pF) were detected in the whole-cell mode. In conclusion (a) Km values for sucrose uptake measured by electrophysiology are similar to those obtained with heterologous systems, (b) electrophysiology provides a useful tool for in-situ determination of Km values, (c) As yet, it remains unclear if one or two uptake systems are involved in sucrose uptake by Sieve Elements, (d) Affin
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Recycling of Solanum Sucrose Transporters Expressed in Yeast, Tobacco, and in Mature Phloem Sieve Elements
Molecular plant, 2010Co-Authors: Johannes Liesche, Alexander Schulz, Bernhard Grimm, Christina KühnAbstract:The plant sucrose transporter SUT1 (from Solanum tuberosum, S. lycopersicum, or Zea mays) exhibits redox-dependent dimerization and targeting if heterologously expressed in S. cerevisiae (Krugel et al., 2008). It was also shown that SUT1 is present in motile vesicles when expressed in tobacco cells and that its targeting to the plasma membrane is reversible. StSUT1 is internalized in the presence of brefeldin A (BFA) in yeast, plant cells, and in mature Sieve Elements as confirmed by immunolocalization. These results were confirmed here and the dynamics of intracellular SUT1 localization were further elucidated. Inhibitor studies revealed that vesicle movement of SUT1 is actin-dependent. BFA-mediated effects might indicate that anterograde vesicle movement is possible even in mature Sieve Elements, and could involve components of the cytoskeleton that were previously thought to be absent in SEs. Our results are in contradiction to this old dogma of plant physiology and the potential of mature Sieve Elements should therefore be re-evaluated. In addition, SUT1 internalization was found to be dependent on the plasma membrane lipid composition. SUT1 belongs to the detergent-resistant membrane (DRM) fraction in planta and is targeted to membrane raft-like microdomains when expressed in yeast (Krugel et al., 2008). Here, SUT1–GFP expression in different yeast mutants, which were unable to perform endocytosis and/or raft formation, revealed a strong link between SUT1 raft localization, the sterol composition and membrane potential of the yeast plasma membrane, and the capacity of the SUT1 protein to be internalized by endocytosis. The results provide new insight into the regulation of sucrose transport and the mechanism of endocytosis in plant cells.
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Dimerization and endocytosis of the sucrose transporter StSUT1 in mature Sieve Elements.
Plant signaling & behavior, 2008Co-Authors: Johannes Liesche, Alexander Schulz, Bernhard Grimm, Undine Krügel, Christina KühnAbstract:The sucrose transporter StSUT1 from Solanum tuberosum was shown to be regulated post-translationally by redox reagents. Its activity is increased at least 10-fold in the presence of oxidizing agents if expressed in yeast. Oxidation has also an effect on plasma membrane targeting and dimerization of the protein. In response to oxidizing agents, StSUT1 is targeted to lipid raft-like microdomains and SUT1 protein is detectable in the detergent resistant membrane fraction of plant plasma membranes. Interestingly, StSUT1 treated with brefeldin A seems to aggregate in endocytic compartments in mature Sieve Elements.1 Further analysis of SUT1 targeting will certainly provide more information about the putative involvement of lipid raft-like microdomains in endocytic events. We provide here additional information on the dimerization and endocytosis of the SUT1 protein. The oligomerization of overexpressed SoSUT1 from Spinacia oleracea in transgenic potato plants was analyzed by two-dimensional gel electrophoresis and endocytosis of the StSUT1 protein was confirmed by immunogold labeling.
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The Sucrose Transporter StSUT1 Localizes to Sieve Elements in Potato Tuber Phloem and Influences Tuber Physiology and Development
Plant physiology, 2003Co-Authors: Christina Kühn, Brigitte Hirner, Mohammad-reza Hajirezaei, Alisdair R. Fernie, Ute Roessner-tunali, Tomasz Czechowski, Wolf B FrommerAbstract:The sucrose (Suc) H(+)-cotransporter StSUT1 from potato (Solanum tuberosum), which is essential for long-distance transport of Suc and assumed to play a role in phloem loading in mature leaves, was found to be expressed in sink tubers. To answer the question of whether SUT1 serves a function in phloem unloading in tubers, the promoter was fused to gusA and expression was analyzed in transgenic potato. SUT1 expression was unexpectedly detected not in tuber parenchyma but in the phloem of sink tubers. Immunolocalization demonstrated that StSUT1 protein was present only in Sieve Elements of sink tubers, cells normally involved in export of Suc from the phloem to supply developing tubers, raising the question of the role of SUT1 in tubers. SUT1 expression was inhibited by antisense in transgenic potato plants using a class I patatin promoter B33, which is primarily expressed in the phloem of developing tubers. Reduced SUT1 expression in tubers did not affect aboveground organs but led to reduced fresh weight accumulation during early stages of tuber development, indicating that in this phase SUT1 plays an important role for sugar transport. Changes in Suc- and starch-modifying enzyme activities and metabolite profiles are consistent with the developmental switch in unloading mechanisms. Altogether, the findings may suggest a role of SUT1 in retrieval of Suc from the apoplasm, thereby regulating the osmotic potential in the extracellular space, or a direct role in phloem unloading acting as a phloem exporter transferring Suc from the Sieve Elements into the apoplasm.
Wolf B Frommer - One of the best experts on this subject based on the ideXlab platform.
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The Sucrose Transporter StSUT1 Localizes to Sieve Elements in Potato Tuber Phloem and Influences Tuber Physiology and Development
Plant physiology, 2003Co-Authors: Christina Kühn, Brigitte Hirner, Mohammad-reza Hajirezaei, Alisdair R. Fernie, Ute Roessner-tunali, Tomasz Czechowski, Wolf B FrommerAbstract:The sucrose (Suc) H(+)-cotransporter StSUT1 from potato (Solanum tuberosum), which is essential for long-distance transport of Suc and assumed to play a role in phloem loading in mature leaves, was found to be expressed in sink tubers. To answer the question of whether SUT1 serves a function in phloem unloading in tubers, the promoter was fused to gusA and expression was analyzed in transgenic potato. SUT1 expression was unexpectedly detected not in tuber parenchyma but in the phloem of sink tubers. Immunolocalization demonstrated that StSUT1 protein was present only in Sieve Elements of sink tubers, cells normally involved in export of Suc from the phloem to supply developing tubers, raising the question of the role of SUT1 in tubers. SUT1 expression was inhibited by antisense in transgenic potato plants using a class I patatin promoter B33, which is primarily expressed in the phloem of developing tubers. Reduced SUT1 expression in tubers did not affect aboveground organs but led to reduced fresh weight accumulation during early stages of tuber development, indicating that in this phase SUT1 plays an important role for sugar transport. Changes in Suc- and starch-modifying enzyme activities and metabolite profiles are consistent with the developmental switch in unloading mechanisms. Altogether, the findings may suggest a role of SUT1 in retrieval of Suc from the apoplasm, thereby regulating the osmotic potential in the extracellular space, or a direct role in phloem unloading acting as a phloem exporter transferring Suc from the Sieve Elements into the apoplasm.
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a new subfamily of sucrose transporters sut4 with low affinity high capacity localized in enucleate Sieve Elements of plants
The Plant Cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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A New Subfamily of Sucrose Transporters, SUT4, with Low Affinity/High Capacity Localized in Enucleate Sieve Elements of Plants
The Plant cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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sut2 a putative sucrose sensor in Sieve Elements
The Plant Cell, 2000Co-Authors: Laurence Barker, Christina Kühn, Alexander Schulz, Andreas Weise, Christiane Gebhardt, Brigitte Hirner, Hanjo Hellmann, Waltraud X Schulze, John M Ward, Wolf B FrommerAbstract:In leaves, sucrose uptake kinetics involve high- and low-affinity components. A family of low- and high-affinity sucrose transporters (SUT) was identified. SUT1 serves as a high-affinity transporter essential for phloem loading and long-distance transport in solanaceous species. SUT4 is a low-affinity transporter with an expression pattern overlapping that of SUT1. Both SUT1 and SUT4 localize to enucleate Sieve Elements of tomato. New sucrose transporter–like proteins, named SUT2, from tomato and Arabidopsis contain extended cytoplasmic domains, thus structurally resembling the yeast sugar sensors SNF3 and RGT2. Features common to these sensors are low codon bias, environment of the start codon, low expression, and lack of detectable transport activity. In contrast to LeSUT1, which is induced during the sink-to-source transition of leaves, SUT2 is more highly expressed in sink than in source leaves and is inducible by sucrose. LeSUT2 protein colocalizes with the low- and high-affinity sucrose transporters in Sieve Elements of tomato petioles, indicating that multiple SUT mRNAs or proteins travel from companion cells to enucleate Sieve Elements. The SUT2 gene maps on chromosome V of potato and is linked to a major quantitative trait locus for tuber starch content and yield. Thus, the putative sugar sensor identified colocalizes with two other sucrose transporters, differs from them in kinetic properties, and potentially regulates the relative activity of low- and high-affinity sucrose transport into Sieve Elements.
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macromolecular trafficking indicated by localization and turnover of sucrose transporters in enucleate Sieve Elements
Science, 1997Co-Authors: Christina Kühn, Alexander Schulz, Vincent R. Franceschi, Remi Lemoine, Wolf B FrommerAbstract:The leaf sucrose transporter SUT1 is essential for phloem loading and long-distance transport of assimilates. Both SUT1 messenger RNA (mRNA) and protein were shown to be diurnally regulated and to have high turnover rates. SUT1 protein was detected by immunolocalization in plasma membranes of enucleate Sieve Elements (SEs) in tobacco, potato, and tomato. Analysis by in situ hybridization showed that SUT1 mRNA localizes mainly to the SE and is preferentially associated with plasmodesmata. Antisense inhibition of SUT1 expression under control of a companion cell (CC)-specific promoter indicated synthesis of SUT1 mRNA in the CC. These results provide evidence for targeting of plant endogenous mRNA and potentially SUT1 protein through phloem plasmodesmata and for sucrose loading at the plasma membrane of SE.
John M Ward - One of the best experts on this subject based on the ideXlab platform.
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A New Subfamily of Sucrose Transporters, SUT4, with Low Affinity/High Capacity Localized in Enucleate Sieve Elements of Plants
The Plant cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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a new subfamily of sucrose transporters sut4 with low affinity high capacity localized in enucleate Sieve Elements of plants
The Plant Cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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sut2 a putative sucrose sensor in Sieve Elements
The Plant Cell, 2000Co-Authors: Laurence Barker, Christina Kühn, Alexander Schulz, Andreas Weise, Christiane Gebhardt, Brigitte Hirner, Hanjo Hellmann, Waltraud X Schulze, John M Ward, Wolf B FrommerAbstract:In leaves, sucrose uptake kinetics involve high- and low-affinity components. A family of low- and high-affinity sucrose transporters (SUT) was identified. SUT1 serves as a high-affinity transporter essential for phloem loading and long-distance transport in solanaceous species. SUT4 is a low-affinity transporter with an expression pattern overlapping that of SUT1. Both SUT1 and SUT4 localize to enucleate Sieve Elements of tomato. New sucrose transporter–like proteins, named SUT2, from tomato and Arabidopsis contain extended cytoplasmic domains, thus structurally resembling the yeast sugar sensors SNF3 and RGT2. Features common to these sensors are low codon bias, environment of the start codon, low expression, and lack of detectable transport activity. In contrast to LeSUT1, which is induced during the sink-to-source transition of leaves, SUT2 is more highly expressed in sink than in source leaves and is inducible by sucrose. LeSUT2 protein colocalizes with the low- and high-affinity sucrose transporters in Sieve Elements of tomato petioles, indicating that multiple SUT mRNAs or proteins travel from companion cells to enucleate Sieve Elements. The SUT2 gene maps on chromosome V of potato and is linked to a major quantitative trait locus for tuber starch content and yield. Thus, the putative sugar sensor identified colocalizes with two other sucrose transporters, differs from them in kinetic properties, and potentially regulates the relative activity of low- and high-affinity sucrose transport into Sieve Elements.
Andreas Weise - One of the best experts on this subject based on the ideXlab platform.
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A New Subfamily of Sucrose Transporters, SUT4, with Low Affinity/High Capacity Localized in Enucleate Sieve Elements of Plants
The Plant cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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a new subfamily of sucrose transporters sut4 with low affinity high capacity localized in enucleate Sieve Elements of plants
The Plant Cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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sut2 a putative sucrose sensor in Sieve Elements
The Plant Cell, 2000Co-Authors: Laurence Barker, Christina Kühn, Alexander Schulz, Andreas Weise, Christiane Gebhardt, Brigitte Hirner, Hanjo Hellmann, Waltraud X Schulze, John M Ward, Wolf B FrommerAbstract:In leaves, sucrose uptake kinetics involve high- and low-affinity components. A family of low- and high-affinity sucrose transporters (SUT) was identified. SUT1 serves as a high-affinity transporter essential for phloem loading and long-distance transport in solanaceous species. SUT4 is a low-affinity transporter with an expression pattern overlapping that of SUT1. Both SUT1 and SUT4 localize to enucleate Sieve Elements of tomato. New sucrose transporter–like proteins, named SUT2, from tomato and Arabidopsis contain extended cytoplasmic domains, thus structurally resembling the yeast sugar sensors SNF3 and RGT2. Features common to these sensors are low codon bias, environment of the start codon, low expression, and lack of detectable transport activity. In contrast to LeSUT1, which is induced during the sink-to-source transition of leaves, SUT2 is more highly expressed in sink than in source leaves and is inducible by sucrose. LeSUT2 protein colocalizes with the low- and high-affinity sucrose transporters in Sieve Elements of tomato petioles, indicating that multiple SUT mRNAs or proteins travel from companion cells to enucleate Sieve Elements. The SUT2 gene maps on chromosome V of potato and is linked to a major quantitative trait locus for tuber starch content and yield. Thus, the putative sugar sensor identified colocalizes with two other sucrose transporters, differs from them in kinetic properties, and potentially regulates the relative activity of low- and high-affinity sucrose transport into Sieve Elements.
Laurence Barker - One of the best experts on this subject based on the ideXlab platform.
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A New Subfamily of Sucrose Transporters, SUT4, with Low Affinity/High Capacity Localized in Enucleate Sieve Elements of Plants
The Plant cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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a new subfamily of sucrose transporters sut4 with low affinity high capacity localized in enucleate Sieve Elements of plants
The Plant Cell, 2000Co-Authors: Andreas Weise, Christina Kühn, Laurence Barker, Wolf B Frommer, Sylvie Lalonde, Henrik Buschmann, John M WardAbstract:A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The Km for sucrose uptake by AtSUT4 of 11.6 ± 0.6 mM was ∼10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate Sieve Elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the Sieve Elements.
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sut2 a putative sucrose sensor in Sieve Elements
The Plant Cell, 2000Co-Authors: Laurence Barker, Christina Kühn, Alexander Schulz, Andreas Weise, Christiane Gebhardt, Brigitte Hirner, Hanjo Hellmann, Waltraud X Schulze, John M Ward, Wolf B FrommerAbstract:In leaves, sucrose uptake kinetics involve high- and low-affinity components. A family of low- and high-affinity sucrose transporters (SUT) was identified. SUT1 serves as a high-affinity transporter essential for phloem loading and long-distance transport in solanaceous species. SUT4 is a low-affinity transporter with an expression pattern overlapping that of SUT1. Both SUT1 and SUT4 localize to enucleate Sieve Elements of tomato. New sucrose transporter–like proteins, named SUT2, from tomato and Arabidopsis contain extended cytoplasmic domains, thus structurally resembling the yeast sugar sensors SNF3 and RGT2. Features common to these sensors are low codon bias, environment of the start codon, low expression, and lack of detectable transport activity. In contrast to LeSUT1, which is induced during the sink-to-source transition of leaves, SUT2 is more highly expressed in sink than in source leaves and is inducible by sucrose. LeSUT2 protein colocalizes with the low- and high-affinity sucrose transporters in Sieve Elements of tomato petioles, indicating that multiple SUT mRNAs or proteins travel from companion cells to enucleate Sieve Elements. The SUT2 gene maps on chromosome V of potato and is linked to a major quantitative trait locus for tuber starch content and yield. Thus, the putative sugar sensor identified colocalizes with two other sucrose transporters, differs from them in kinetic properties, and potentially regulates the relative activity of low- and high-affinity sucrose transport into Sieve Elements.