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Papers by Christina Kuhn

Frontiers in Plant Science, May 15, 2019

International Journal of Molecular Sciences, Feb 1, 2021

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

BioMed Research International, 2013

Vitamin B 6 is one of the most versatile cofactors in plants and an essential phytonutrient in th... more Vitamin B 6 is one of the most versatile cofactors in plants and an essential phytonutrient in the human diet that benefits a variety of human health aspects. Although biosynthesis of the vitamin has been well resolved in recent years, the main research is currently based on Arabidopsis thaliana with very little work done on major crop plants. Here we provide the first report on interactions and expression profiles of PDX genes for vitamin B 6 biosynthesis in potato and how vitamin B 6 content varies in tubers of different genotypes. The results demonstrate that potato is an excellent resource for this vitamin and that strong natural variation in vitamin B 6 content among the tested cultivars indicates high potential to fortify vitamin B 6 nutrition in potato-based foods.

Acta Physiologiae Plantarum, Apr 16, 2022

The role of phloem-xylem interaction via sucrose exchanges in recovery of dehydration impacts, sp... more The role of phloem-xylem interaction via sucrose exchanges in recovery of dehydration impacts, specifically xylem embolism, has not been directly investigated thus far. Most previous studies were indirect approaches leading to suggestive conclusions. We hypothesized that a block in phloem loading and so no exchange of sucrose with xylem affect tolerance and recovery of tobacco plants (Nicotiana tabacum) during dehydration and after the rehydration phase. Transgenic N. tabacum (αNtSUT1-antisense) plants, which showed impaired phloem loading and high accumulation of soluble sugars in leaves, were compared to the wild-type (WT) plants. The water status, osmotic adjustments, leaf turgor, stomatal conductance, xylem cavitation, and stem xylem sucrose content were determined during dehydration and after the rehydration phases. Results showed that retention of sucrose outside phloem conduits highly improved water status, osmotic adjustment and turgidity of the source leaves in the transgenics during drought period. However, no impact occurred on stomata function and tolerance to xylem cavitation in αNtSUT1. After the rehydration period, WT plants with free phloem transport and phloem-xylem exchange of sucrose recovered better their water status, leaf turgidity, stomatal conductance and xylem functioning than αNtSUT1 plants. The accumulation of sucrose in leaves of transformants ameliorated their tolerance to water deficit by reinforcing the osmotic adjustment mechanism at the leaf level. However, lack of sucrose in phloem sieve resulted in impairment of hydraulic recovery of xylem from drought of αNtSUT1 after rehydration. This suggests a crucial role of the phloem-xylem exchange of sucrose in refilling of embolized xylem vessels.

Plants, Jan 27, 2020

Post-translational regulation of sucrose transporters represents one possibility to adapt transpo... more Post-translational regulation of sucrose transporters represents one possibility to adapt transporter activity in a very short time frame. This can occur either via phosphorylation/ dephosphorylation, oligomerization, protein-protein interactions, endocytosis/exocytosis, or degradation. It is also known that StSUT1 can change its compartmentalization at the plasma membrane and concentrate in membrane microdomains in response to changing redox conditions. A systematic screen for protein-protein-interactions of plant sucrose transporters revealed that the interactome of all three known sucrose transporters from the Solanaceous species Solanum tuberosum and Solanum lycopersicum represents a specific subset of interaction partners, suggesting different functions for the three different sucrose transporters. Here, we focus on factors that affect the subcellular distribution of the transporters. It was already known that sucrose transporters are able to form homo-as well as heterodimers. Here, we reveal the consequences of homo-and heterodimer formation and the fact that the responses of individual sucrose transporters will respond differently. Sucrose transporter SlSUT2 is mainly found in intracellular vesicles and several of its interaction partners are involved in vesicle traffic and subcellular targeting. The impact of interaction partners such as SNARE/VAMP proteins on the localization of SlSUT2 protein will be investigated, as well as the impact of inhibitors, excess of substrate, or divalent cations which are known to inhibit SUT1-mediated sucrose transport in yeast cells. Thereby we are able to identify factors regulating sucrose transporter activity via a change of their subcellular distribution.

Trends in Plant Science, Jul 1, 2012

Dimerization and endocytosis of the sucrose transporter StSUT1 in mature sieve elements

Plant Signaling & Behavior, Dec 1, 2008

The main objective of this study was to assess responses of mid-early (Spunta) and mid-late (Bell... more The main objective of this study was to assess responses of mid-early (Spunta) and mid-late (Bellini) potato cultivars to different temperature regimes during subsequent stages of potato growth. The impact of high temperature (25/22°C day/night), low temperature (18/16°C day/night) and intermediate temperature (20/18°C day/night) was evaluated for different growth stages. Data were obtained for photosynthesis, carbohydrates in leaves, stems and tubers as well as production parameters. Enzyme activities were determined for sucrose-phosphate synthase in leaves, acid invertase in stems and acid and neutral invertases in tubers. Gene expression levels of relevant sugar metabolizing enzymes was quantified.A detailed correlation analysis revealed a strong impact of the expression level of sugar metabolizing enzymes in leaves on the final number of tubers per plant.Whereas total tuber yield increases with temperature, the number of tubers per plant was highest under low temperature conditi...

Biochimie, Nov 1, 2013

Plant sucrose transporters (SUTs) are functional as sucrose-proton-cotransporters with an optimal... more Plant sucrose transporters (SUTs) are functional as sucrose-proton-cotransporters with an optimal transport activity in the acidic pH range. Recently, the pH optimum of the Solanum tuberosum sucrose transporter StSUT1 was experimentally determined to range at an unexpectedly low pH of 3 or even below. Various research groups have confirmed these surprising findings independently and in different organisms. Here we provide further experimental evidence for a pH optimum at physiological extrema. Site directed mutagenesis provides information about functional amino acids, which are highly conserved and responsible for this extraordinary increase in transport capacity under extreme pH conditions. Redox-dependent dimerization of the StSUT1 protein was described earlier. Here the ability of StSUT1 to form homodimers was demonstrated by heterologous expression in Lactococcus lactis and Xenopus leavis using Western blots, and in plants by bimolecular fluorescence complementation. Mutagenesis of highly conserved cysteine residues revealed their importance in protein stability. The accessibility of regulatory amino acid residues in the light of StSUT1's compartmentalization in membrane microdomains is discussed.

Signals, Sensing, and Plant Primary Metabolism

Journal of Plant Growth Regulation, Dec 1, 2006

The Third Symposium Signals, Sensing and Plant Primary Metabolism, organized by the DFG Sonderfor... more The Third Symposium Signals, Sensing and Plant Primary Metabolism, organized by the DFG Sonderforschungsbereich 429 (‘‘Molecular Physiology, Energetics, and Regulation of Primary Metabolism in Plants’’) was held on the beautiful premises of the World Heritage Site, Park Sanssouci, Potsdam, Germany, from April 26th to 29th 2006. The program covered regulatory aspects of plant primary metabolism and fascinated attendees by integrating physiological, structural, and biophysical data. Numerous national and international speakers followed the invitation to the symposium and presented their most recent data.

Molecular Plant, Nov 1, 2010

The plant sucrose transporter SUT1 (from Solanum tuberosum, S. lycopersicum, or Zea mays) exhibit... more The plant sucrose transporter SUT1 (from Solanum tuberosum, S. lycopersicum, or Zea mays) exhibits redoxdependent dimerization and targeting if heterologously expressed in S. cerevisiae (Krü gel 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 (Krü gel 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.

Journal of Plant Physiology, Jul 1, 2011

Arbuscular mycorrhizal fungi enhance CO 2 assimilation of their hosts which ensure the demand for... more Arbuscular mycorrhizal fungi enhance CO 2 assimilation of their hosts which ensure the demand for carbohydrates of these obligate biotrophic microorganisms. Photosynthetic parameters were measured in tomato colonised or not by the arbuscular mycorrhizal fungus Glomus mosseae. In addition, carbohydrate contents and mRNA accumulation of three sucrose transporter genes were analysed. Mycorrhizal plants showed increased opening of stomata and assimilated significant more CO 2. A higher proportion of the absorbed light was used for photochemical processes, while non-photochemical quenching and the content of photoprotective pigments were lower. Analysis of sugar contents showed no significant differences in leaves but enhanced levels of sucrose and fructose in roots, while glucose amounts stayed constant. The three sucrose transporter encoding genes of tomato SlSUT1, SlSUT2 and SlSUT4 were upregulated providing transport capacities to transfer sucrose into the roots. It is proposed that a significant proportion of sugars is used by the mycorrhizal fungus, because only amounts of fructose were increased, while levels of glucose, which is mainly transferred towards the fungus, were nearly constant.

Journal of Plant Growth Regulation, May 21, 2007

The Plant Cell, Jul 1, 2002

Suc represents the major transport form for carbohydrates in plants. Suc is loaded actively again... more Suc represents the major transport form for carbohydrates in plants. Suc is loaded actively against a concentration gradient into sieve elements, which constitute the conduit for assimilate export out of leaves. Three members of the Suc transporter family with different properties were identified: SUT1, a high-affinity Suc proton cotransporter; SUT4, a low-affinity transporter; and SUT2, which in yeast is only weakly active and shows features similar to those of the yeast sugar sensors RGT2 and SNF3. Immunolocalization demonstrated that all three SUT proteins are localized in the same enucleate sieve element. Thus, the potential of Suc transporters to form homooligomers was tested by the yeast-based split-ubiquitin system. The results show that both SUT1 and SUT2 have the potential to form homooligomers. Moreover, all three Suc transporters have the potential to interact with each other. As controls, a potassium channel and a monosaccharide transporter, expressed in the plasma membrane, did not interact with the SUTs. The in vivo interaction between the functionally different Suc transporters indicates that the membrane proteins are capable of forming oligomeric structures that, like mammalian Glc transporter complexes, might be of functional significance for the regulation of transport.

Plant Physiology, Dec 14, 2007

Sucrose (Suc) transporters belong to a large gene family. The physiological role of SUT1 proteins... more Sucrose (Suc) transporters belong to a large gene family. The physiological role of SUT1 proteins has been intensively investigated in higher plants, whereas that of SUT4 proteins is so far unknown. All three known Suc transporters from potato (Solanum tuberosum), SUT1, SUT2, and SUT4, are colocalized and their RNA levels not only follow a diurnal rhythm, but also oscillate in constant light. Here, we examined the physiological effects of transgenic potato plants on RNA interference (RNAi)inactivated StSUT4 expression. The phenotype of StSUT4-RNAi plants includes early flowering, higher tuber production, and reduced sensitivity toward light enriched in far-red wavelength (i.e. in canopy shade). Inhibition of StSUT4 led to tuber production of the strict photoperiodic potato subsp. andigena even under noninductive long-day conditions. Accumulation of soluble sugars and Suc efflux from leaves of transgenic plants are modified in StSUT4-RNAi plants, leading to modified Suc levels in sink organs. StSUT4 expression of wild-type plants is induced by gibberellins and ethephon, and external supply of gibberellic acid leads to even more pronounced differences between wild-type and StSUT4-RNAi plants regarding tuber yield and internode elongation, indicating a reciprocal regulation of StSUT4 and gibberellins.

Molecular Plant, 2012

Organization of proteins into complexes is crucial for many cellular functions. Recently, the SUT... more Organization of proteins into complexes is crucial for many cellular functions. Recently, the SUT1 protein was shown to form homodimeric complexes, to be associated with lipid raft-like microdomains in yeast as well as in plants and to undergo endocytosis in response to brefeldin A. We therefore aimed to identify SUT1-interacting proteins that might be involved in dimerization, endocytosis, or targeting of SUT1 to raft-like microdomains. Therefore, we identified potato membrane proteins, which are associated with the detergent-resistant membrane (DRM) fraction. Among the proteins identified, we clearly confirmed StSUT1 as part of DRM in potato source leaves. We used the yeast two-hybrid split ubiquitin system (SUS) to systematically screen for interaction between the sucrose transporter StSUT1 and other membraneassociated or soluble proteins in vivo. The SUS screen was followed by immunoprecipitation using affinity-purified StSUT1-specific peptide antibodies and mass spectrometric analysis of co-precipitated proteins. A large overlap was observed between the StSUT1-interacting proteins identified in the co-immunoprecipitation and the detergent-resistant membrane fraction. One of the SUT1-interacting proteins, a protein disulfide isomerase (PDI), interacts also with other sucrose transporter proteins. A potential role of the PDI as escort protein is discussed.

The Plant Cell, Sep 1, 2008

The plant sucrose transporter SUT1 from Solanum tuberosum revealed a dramatic redox-dependent inc... more 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.

Plant Physiology, Sep 1, 1998

In many species translocation of sucrose from the mesophyll to the phloem is carrier mediated. A ... more In many species translocation of sucrose from the mesophyll to the phloem is carrier mediated. A sucrose/H ؉-symporter cDNA, NtSUT1, was isolated from tobacco (Nicotiana tabacum) and shown to be highly expressed in mature leaves and at low levels in other tissues, including floral organs. To study the in vivo function of NtSUT1, tobacco plants were transformed with a SUT1 antisense construct under control of the cauliflower mosaic virus 35S promoter. Upon maturation, leaves of transformants expressing reduced amounts of SUT1 mRNA curled downward, and strongly affected plants developed chloroses and necroses that led to death. The leaves exhibited impaired ability to export recently fixed 14 CO 2 and were unable to export transient starch during extended periods of darkness. As a consequence, soluble carbohydrates accumulated and photosynthesis was reduced. Autoradiographs of leaves show a heterogenous pattern of CO 2 fixation even after a 24-h chase. The 14 C pattern does not change with time, suggesting that movement of photosynthate between mesophyll cells may also be impaired. The affected lines show a reduction in the development of the root system and delayed or impaired flowering. Taken together, the effects observed in a seed plant (tobacco) demonstrate the importance of SUT1 for sucrose loading into the phloem via an apoplastic route and possibly for intermesophyll transport as well.

The Plant Cell, Aug 1, 2000

A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (S... more A new subfamily of sucrose transporters from Arabidopsis (AtSUT4), tomato (LeSUT4), and potato (StSUT4) was isolated, demonstrating only 47% similarity to the previously characterized SUT1. SUT4 from two plant species conferred sucrose uptake activity when expressed in yeast. The K m for sucrose uptake by AtSUT4 of 11.6 ؎ 0.6 mM was ‫ف‬ 10-fold greater than for all other plant sucrose transporters characterized to date. An ortholog from potato had similar kinetic properties. Thus, SUT4 corresponds to the low-affinity/high-capacity saturable component of sucrose uptake found in leaves. In contrast to SUT1, SUT4 is expressed predominantly in minor veins in source leaves, where high-capacity sucrose transport is needed for phloem loading. In potato and tomato, SUT4 was immunolocalized specifically to enucleate sieve elements, indicating that like SUT1, macromolecular trafficking is required to transport the mRNA or the protein from companion cells through plasmodesmata into the sieve elements.

Frontiers in Plant Science, 2013

Several recent publications reported different subcellular localization of the sucrose transporte... more Several recent publications reported different subcellular localization of the sucrose transporters belonging to the SUT4 subfamily. The physiological function of the SUT4 sucrose transporters requires clarification, because down-regulation of the members of the SUT4 clade had different effects in rice, poplar, and potato. Here, we provide new data for the localization and function of the Solanaceous StSUT4 protein, further elucidating involvement in the onset of flowering, tuberization and in the shade avoidance syndrome of potato plants. Induction of an early flowering and a tuberization in the SUT4-inhibited potato plants correlates with increased sucrose export from leaves and increased sucrose and starch accumulation in terminal sink organs, such as developing tubers. SUT4 affects expression of the enzymes involved in gibberellin and ethylene biosynthesis, as well as the rate of ethylene biosynthesis in potato. In the SUT4-inhibited plants, the ethylene production no longer follows a diurnal rhythm. Thus it was concluded that StSUT4 controls circadian gene expression, potentially by regulating sucrose export from leaves. Furthermore, SUT4 expression affects clock-regulated genes such as StFT, StSOC1, and StCO, which might be also involved in a photoperiod-dependent tuberization. A model is proposed in which StSUT4 controls a phloem-mobile signaling molecule generated in leaves, which together with enhanced sucrose export affects developmental switches in apical meristems. SUT4 seems to link photoreceptor-perceived information about the light quality and day length with phytohormone biosynthesis and the expression of circadian-regulated genes.