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Aart J. E. Van Bel - One of the best experts on this subject based on the ideXlab platform.

  • Thermodynamic Battle for Photosynthate Acquisition between Sieve Tubes and Adjoining Parenchyma in Transport Phloem1
    2015
    Co-Authors: Jens B. Hafke, Jan-kees Van Amerongen, Frits Kelling, Ra C. U. Furch, Frank Gaupels, Aart J. E. Van Bel
    Abstract:

    In transport phloem, photoassimilates escaping from the Sieve Tubes are released into the apoplasmic space between Sieve element (SE)/companion cell (CC) complexes (SE/CCs) and phloem parenchyma cells (PPCs). For uptake respective retrieval, PPCs and SE/CCs make use of plasma membrane translocators energized by the proton motive force (PMF). Their mutual competitiveness, which essentially determines the amount of photoassimilates translocated through the Sieve Tubes, therefore depends on the respective PMFs.Wemeasured the components of thePMF,membranepotential andDpH, of SE/CCs andPPCs in transport phloem. Membrane potentials of SE/CCs and PPCs in tissue slices as well as in intact plants fell into two categories. In the first group including apoplasmically phloem-loading species (e.g. Vicia, Solanum), the membrane potentials of the SEs are more negative than those of the PPCs. In the second group including symplasmically phloem-loading species (e.g. Cucurbita, Ocimum),membranepotentials of SEs are equal to or slightlymore positive than those of PPCs. Pure Sieve Tube sap collected from cut aphid stylets wasmeasuredwithH1-selectivemicroelectrodes. Under our experimental conditions, pH of the Sieve Tube saps was around 7.5, which is comparable to the pH of cytoplasmic compartments in parenchymatous cells. In conclusion, only the membrane potential appears to be relevant for the PMF-determined competition between SE/CCs and PPCs. The findings may imply that the axial sinks along the pathway withdraw more photoassimilates from the Sieve Tubes in symplasmically loading species than in apoplasmically loading species. Sieve Tubes are not hermetically sealed pipes, bu

  • Aphid salivary proteases are capable of degrading Sieve-Tube proteins
    Journal of experimental botany, 2014
    Co-Authors: Alexandra C. U. Furch, Aart J. E. Van Bel, Torsten Will
    Abstract:

    Sieve Tubes serve as transport conduits for photo-assimilates and other resources in angiosperms and are profitable targets for piercing-sucking insects such as aphids. Sieve-Tube sap also contains significant amounts of proteins with diverse functions, for example in signalling, metabolism, and defence. The identification of salivary proteases in Acyrthosiphon pisum led to the hypothesis that aphids might be able to digest these proteins and by doing so suppress plant defence and access additional nitrogen sources. Here, the scarce knowledge of proteases in aphid saliva is briefly reviewed. In order to provide a better platform for discussion, we conducted a few tests on in vitro protease activity and degradation of Sieve-Tube sap proteins of Cucurbita maxima by watery saliva. Inhibition of protein degradation by EDTA indicates the presence of different types of proteases (e.g. metalloproteses) in saliva of A. pisum. Proteases in the watery saliva from Macrosiphum euphorbiae and A. pisum were able to degrade the most abundant phloem protein, which is phloem protein 1. Our results provide support for the breakdown of Sieve-element proteins by aphid saliva in order to suppress/neutralize the defence responses of the plant and to make proteins of Sieve-Tube sap accessible as a nitrogen source, as is discussed in detail. Finally, we discuss whether glycosylation of Sieve-element proteins and the presence of protease inhibitors may confer partial protection against the proteolytic activity of aphid saliva.

  • Phytoplasma-triggered Ca(2+) influx is involved in Sieve-Tube blockage.
    Molecular plant-microbe interactions : MPMI, 2013
    Co-Authors: Rita Musetti, Stefanie V. Buxa, Federica De Marco, Alberto Loschi, Rachele Polizzotto, Karl-heinz Kogel, Aart J. E. Van Bel
    Abstract:

    Phytoplasmas are obligate, phloem-restricted phytopathogens that are disseminated by phloem-sap-sucking insects. Phytoplasma infection severely impairs assimilate translocation in host plants and might be responsible for massive changes in phloem physiology. Methods to study phytoplasma- induced changes thus far provoked massive, native occlusion artifacts in Sieve Tubes. Hence, phytoplasma-phloem relationships were investigated here in intact Vicia faba host plants using a set of vital fluorescent probes and confocal laser-scanning microscopy. We focused on the effects of phytoplasma infection on phloem mass-flow performance and evaluated whether phytoplasmas induce Sieve-plate occlusion. Apparently, phytoplasma infection brings about Ca(2+) influx into Sieve Tubes, leading to Sieve-plate occlusion by callose deposition or protein plugging. In addition, Ca(2+) influx may confer cell wall thickening of conducting elements. In conclusion, phytoplasma effectors may cause gating of Sieve-element Ca(2+) channels leading to Sieve-Tube occlusion with presumptive dramatic effects on phytoplasma spread and photoassimilate distribution.

  • aphid watery saliva counteracts Sieve Tube occlusion a universal phenomenon
    The Journal of Experimental Biology, 2009
    Co-Authors: Torsten Will, Alexandra C. U. Furch, Fred W Tjallingii, Sarah R Kornemann, Aart J. E. Van Bel
    Abstract:

    Ca2+-binding proteins in the watery saliva of Megoura viciae counteract Ca2+-dependent occlusion of Sieve plates in Vicia faba and so prevent the shut-down of food supply in response to stylet penetration. The question arises whether this interaction between aphid saliva and Sieve-element proteins is a universal phenomenon as inferred by the coincidence between Sieve-Tube occlusion and salivation. For this purpose, leaf tips were burnt in a number of plant species from four different families to induce remote Sieve-plate occlusion. Resultant Sieve-plate occlusion in these plant species was counteracted by an abrupt switch of aphid behaviour. Each of the seven aphid species tested interrupted its feeding behaviour and started secreting watery saliva. The protein composition of watery saliva appeared strikingly different between aphid species with less than 50% overlap. Secretion of watery saliva seems to be a universal means to suppress Sieve-plate occlusion, although the protein composition of watery saliva seems to diverge between species.

  • Sieve element ca2 channels as relay stations between remote stimuli and Sieve Tube occlusion in vicia faba
    The Plant Cell, 2009
    Co-Authors: Alexandra C. U. Furch, Aart J. E. Van Bel, Mark D Fricker, Hubert H Felle, Maike Fuchs, Jens B. Hafke
    Abstract:

    Damage induces remote occlusion of Sieve Tubes in Vicia faba by forisome dispersion, triggered during the passage of an electropotential wave (EPW). This study addresses the role of Ca2+ channels and cytosolic Ca2+ elevation as a link between EPWs and forisome dispersion. Ca2+ channel antagonists affect the initial phase of the EPW as well as the prolonged plateau phase. Resting levels of Sieve Tube Ca2+ of ∼50 nM were independently estimated using Ca2+-selective electrodes and a Ca2+-sensitive dye. Transient changes in cytosolic Ca2+ were observed in phloem tissue in response to remote stimuli and showed profiles similar to those of EPWs. The measured elevation of Ca2+ in Sieve Tubes was below the threshold necessary for forisome dispersion. Therefore, forisomes need to be associated with Ca2+ release sites. We found an association between forisomes and endoplasmic reticulum (ER) at Sieve plates and pore-plasmodesma units where high-affinity binding of a fluorescent Ca2+ channel blocker mapped an increased density of Ca2+ channels. In conclusion, propagation of EPWs in response to remote stimuli is linked to forisome dispersion through transiently high levels of parietal Ca2+, release of which depends on both plasma membrane and ER Ca2+ channels.

William J Lucas - One of the best experts on this subject based on the ideXlab platform.

  • Proteomics and metabolomics analyses reveal the cucurbit Sieve Tube system as a complex metabolic space.
    Plant Journal, 2016
    Co-Authors: Chaoyang Hu, Hattem M. El-shabrawi, Danny Alexander, JOHN ANDREW RYALS, Dabing Zhang, William J Lucas
    Abstract:

    Summary The plant vascular system, and specifically the phloem, plays a pivotal role in allocation of fixed carbon to developing sink organs. Although the processes involved in loading and unloading of sugars and amino acids are well characterized, little information is available regarding the nature of other metabolites in the Sieve Tube system (STS) at specific sites along the pathway. Here, we elucidate spatial features of metabolite composition mapped with phloem enzymes along the cucurbit STS. Phloem sap (PS) was collected from the loading (source), unloading (apical sink region) and shoot–root junction regions of cucumber, watermelon and pumpkin. Our PS analyses revealed significant differences in the metabolic and proteomic profiles both along the source–sink pathway and between the STSs of these three cucurbits. In addition, metabolite profiles established for PS and vascular tissue indicated the presence of distinct compositions, consistent with the operation of the STS as a unique symplasmic domain. In this regard, at various locations along the STS we could map metabolites and their related enzymes to specific metabolic pathways. These findings are discussed with regard to the function of the STS as a unique and highly complex metabolic space within the plant vascular system.

  • The angiosperm phloem Sieve Tube system: a role in mediating traits important to modern agriculture
    Journal of Experimental Botany, 2013
    Co-Authors: William J Lucas
    Abstract:

    The plant vascular system serves a vital function by distributing water, nutrients and hormones essential for growth and development to the various organs of the plant. In this review, attention is focused on the role played by the phloem as the conduit for delivery of both photosynthate and information macromolecules, especially from the context of its mediation in traits that are important to modern agriculture. Resource allocation of sugars and amino acids, by the phloem, to specific sink tissues is of importance to crop yield and global food security. Current findings are discussed in the context of a hierarchical control network that operates to integrate resource allocation to competing sinks. The role of plasmodesmata that connect companion cells to neighbouring Sieve elements and phloem parenchyma cells is evaluated in terms of their function as valves, connecting the Sieve Tube pressure manifold system to the various plant tissues. Recent studies have also revealed that plasmodesmata and the phloem Sieve Tube system function cooperatively to mediate the long-distance delivery of proteins and a diverse array of RNA species. Delivery of these information macromolecules is discussed in terms of their roles in control over the vegetative-to-floral transition, Tuberization in potato, stress-related signalling involving miRNAs, and genetic reprogramming through the delivery of 24-nucleotide small RNAs that function in transcriptional gene silencing in recipient sink organs. Finally, we discuss important future research areas that could contribute to developing agricultural crops with engineered performance characteristics for enhance yield potential.

  • Pumpkin eIF5A isoforms interact with components of the translational machinery in the cucurbit Sieve Tube system.
    The Plant journal : for cell and molecular biology, 2010
    Co-Authors: Eriko Miura, Byung-kook Ham, Hao-wen Cheng, Young Jin Lee, William J Lucas
    Abstract:

    Summary In yeast, eIF5A, in combination with eEF2, functions at the translation step, during the protein elongation cycle. This result is of significance with respect to functioning of the enucleate Sieve Tube system, as eIF5A was recently detected in Cucurbita maxima (pumpkin) phloem sap. In the present study, we further characterized four CmeIF5A isoforms, encoding three proteins, all of which were present in the phloem sap. Although hypusination of CmeIF5A was not necessary for entry into the Sieve elements, this unique post-translational modification was necessary for RNA binding. The two enzymes required for hypusination were detected in pumpkin phloem sap, where presumably this modification takes place. A combination of gel-filtration chromatography and protein overlay assays demonstrated that, as in yeast, CmeIF5A interacts with phloem proteins, like eEF2, known to be involved in protein synthesis. These findings are discussed in terms of a potential role for eIF5A in regulating protein synthesis within the enucleate Sieve Tube system of the angiosperms.

  • analysis of the pumpkin phloem proteome provides insights into angiosperm Sieve Tube function
    Molecular & Cellular Proteomics, 2009
    Co-Authors: Mingkuem Lin, Young Jin Lee, Tony James Lough, Brett S Phinney, William J Lucas
    Abstract:

    Increasing evidence suggests that proteins present in the angiosperm Sieve Tube system play an important role in the long distance signaling system of plants. To identify the nature of these putatively non-cell-autonomous proteins, we adopted a large scale proteomics approach to analyze pumpkin phloem exudates. Phloem proteins were fractionated by fast protein liquid chromatography using both anion and cation exchange columns and then either in-solution or in-gel digested following further separation by SDS-PAGE. A total of 345 LC-MS/MS data sets were analyzed using a combination of Mascot and X!Tandem against the NCBI non-redundant green plant database and an extensive Cucurbit maxima expressed sequence tag database. In this analysis, 1,209 different consensi were obtained of which 1,121 could be annotated from GenBank and BLAST search analyses against three plant species, Arabidopsis thaliana, rice (Oryza sativa), and poplar (Populus trichocarpa). Gene ontology (GO) enrichment analyses identified sets of phloem proteins that function in RNA binding, mRNA translation, ubiquitin-mediated proteolysis, and macromolecular and vesicle trafficking. Our findings indicate that protein synthesis and turnover, processes that were thought to be absent in enucleate Sieve elements, likely occur within the angiosperm phloem translocation stream. In addition, our GO analysis identified a set of phloem proteins that are associated with the GO term "embryonic development ending in seed dormancy"; this finding raises the intriguing question as to whether the phloem may exert some level of control over seed development. The universal significance of the phloem proteome was highlighted by conservation of the phloem proteome in species as diverse as monocots (rice), eudicots (Arabidopsis and pumpkin), and trees (poplar). These results are discussed from the perspective of the role played by the phloem proteome as an integral component of the whole plant communication system.

  • analysis of the pumpkin phloem proteome provides insights into angiosperm Sieve Tube function
    Molecular & Cellular Proteomics, 2009
    Co-Authors: Tony James Lough, Brett S Phinney, William J Lucas
    Abstract:

    : Increasing evidence suggests that proteins present in the angiosperm Sieve Tube system play an important role in the long distance signaling system of plants. To identify the nature of these putatively non-cell-autonomous proteins, we adopted a large scale proteomics approach to analyze pumpkin phloem exudates. Phloem proteins were fractionated by fast protein liquid chromatography using both anion and cation exchange columns and then either in-solution or in-gel digested following further separation by SDS-PAGE. A total of 345 LC-MS/MS data sets were analyzed using a combination of Mascot and X!Tandem against the NCBI non-redundant green plant database and an extensive Cucurbit maxima expressed sequence tag database. In this analysis, 1,209 different consensi were obtained of which 1,121 could be annotated from GenBank and BLAST search analyses against three plant species, Arabidopsis thaliana, rice (Oryza sativa), and poplar (Populus trichocarpa). Gene ontology (GO) enrichment analyses identified sets of phloem proteins that function in RNA binding, mRNA translation, ubiquitin-mediated proteolysis, and macromolecular and vesicle trafficking. Our findings indicate that protein synthesis and turnover, processes that were thought to be absent in enucleate Sieve elements, likely occur within the angiosperm phloem translocation stream. In addition, our GO analysis identified a set of phloem proteins that are associated with the GO term "embryonic development ending in seed dormancy"; this finding raises the intriguing question as to whether the phloem may exert some level of control over seed development. The universal significance of the phloem proteome was highlighted by conservation of the phloem proteome in species as diverse as monocots (rice), eudicots (Arabidopsis and pumpkin), and trees (poplar). These results are discussed from the perspective of the role played by the phloem proteome as an integral component of the whole plant communication system.

Alexandra C. U. Furch - One of the best experts on this subject based on the ideXlab platform.

  • Aphid salivary proteases are capable of degrading Sieve-Tube proteins
    Journal of experimental botany, 2014
    Co-Authors: Alexandra C. U. Furch, Aart J. E. Van Bel, Torsten Will
    Abstract:

    Sieve Tubes serve as transport conduits for photo-assimilates and other resources in angiosperms and are profitable targets for piercing-sucking insects such as aphids. Sieve-Tube sap also contains significant amounts of proteins with diverse functions, for example in signalling, metabolism, and defence. The identification of salivary proteases in Acyrthosiphon pisum led to the hypothesis that aphids might be able to digest these proteins and by doing so suppress plant defence and access additional nitrogen sources. Here, the scarce knowledge of proteases in aphid saliva is briefly reviewed. In order to provide a better platform for discussion, we conducted a few tests on in vitro protease activity and degradation of Sieve-Tube sap proteins of Cucurbita maxima by watery saliva. Inhibition of protein degradation by EDTA indicates the presence of different types of proteases (e.g. metalloproteses) in saliva of A. pisum. Proteases in the watery saliva from Macrosiphum euphorbiae and A. pisum were able to degrade the most abundant phloem protein, which is phloem protein 1. Our results provide support for the breakdown of Sieve-element proteins by aphid saliva in order to suppress/neutralize the defence responses of the plant and to make proteins of Sieve-Tube sap accessible as a nitrogen source, as is discussed in detail. Finally, we discuss whether glycosylation of Sieve-element proteins and the presence of protease inhibitors may confer partial protection against the proteolytic activity of aphid saliva.

  • aphid watery saliva counteracts Sieve Tube occlusion a universal phenomenon
    The Journal of Experimental Biology, 2009
    Co-Authors: Torsten Will, Alexandra C. U. Furch, Fred W Tjallingii, Sarah R Kornemann, Aart J. E. Van Bel
    Abstract:

    Ca2+-binding proteins in the watery saliva of Megoura viciae counteract Ca2+-dependent occlusion of Sieve plates in Vicia faba and so prevent the shut-down of food supply in response to stylet penetration. The question arises whether this interaction between aphid saliva and Sieve-element proteins is a universal phenomenon as inferred by the coincidence between Sieve-Tube occlusion and salivation. For this purpose, leaf tips were burnt in a number of plant species from four different families to induce remote Sieve-plate occlusion. Resultant Sieve-plate occlusion in these plant species was counteracted by an abrupt switch of aphid behaviour. Each of the seven aphid species tested interrupted its feeding behaviour and started secreting watery saliva. The protein composition of watery saliva appeared strikingly different between aphid species with less than 50% overlap. Secretion of watery saliva seems to be a universal means to suppress Sieve-plate occlusion, although the protein composition of watery saliva seems to diverge between species.

  • Sieve element ca2 channels as relay stations between remote stimuli and Sieve Tube occlusion in vicia faba
    The Plant Cell, 2009
    Co-Authors: Alexandra C. U. Furch, Aart J. E. Van Bel, Mark D Fricker, Hubert H Felle, Maike Fuchs, Jens B. Hafke
    Abstract:

    Damage induces remote occlusion of Sieve Tubes in Vicia faba by forisome dispersion, triggered during the passage of an electropotential wave (EPW). This study addresses the role of Ca2+ channels and cytosolic Ca2+ elevation as a link between EPWs and forisome dispersion. Ca2+ channel antagonists affect the initial phase of the EPW as well as the prolonged plateau phase. Resting levels of Sieve Tube Ca2+ of ∼50 nM were independently estimated using Ca2+-selective electrodes and a Ca2+-sensitive dye. Transient changes in cytosolic Ca2+ were observed in phloem tissue in response to remote stimuli and showed profiles similar to those of EPWs. The measured elevation of Ca2+ in Sieve Tubes was below the threshold necessary for forisome dispersion. Therefore, forisomes need to be associated with Ca2+ release sites. We found an association between forisomes and endoplasmic reticulum (ER) at Sieve plates and pore-plasmodesma units where high-affinity binding of a fluorescent Ca2+ channel blocker mapped an increased density of Ca2+ channels. In conclusion, propagation of EPWs in response to remote stimuli is linked to forisome dispersion through transiently high levels of parietal Ca2+, release of which depends on both plasma membrane and ER Ca2+ channels.

  • plant and stimulus specific variations in remote controlled Sieve Tube occlusion
    Plant Signaling & Behavior, 2008
    Co-Authors: Alexandra C. U. Furch, Jens B. Hafke, Aart J. E. Van Bel
    Abstract:

    Phloem injury triggers local Sieve-plate occlusion including callose-mediated constriction and protein plugging of Sieve pores. In intact plants, reversible Sieve-plate occlusion is induced by electric potential waves (EPWs)—accompanied by Ca2+-influx—as result of distant burning. Here, we present additional results which pertain to (a) the variability of EPW-profiles in relation to forisome conformation in intact Vicia faba plants and (b) the differential occlusion reactions to burning and cutting in various plant species. A correlation between stimulus perception and mode of phloem loading is discussed.

  • ca2 mediated remote control of reversible Sieve Tube occlusion in vicia faba
    Journal of Experimental Botany, 2007
    Co-Authors: Alexandra C. U. Furch, Jens B. Hafke, Alexander Schulz, Aart J. E. Van Bel
    Abstract:

    According to an established concept, injury of the phloem triggers local Sieve plate occlusion including callose-mediated constriction and, possibly, protein plugging of the Sieve pores. Sieve plate occlusion can also be achieved by distant stimuli, depends on the passage of electropotential waves (EPWs), and is reversible in intact plants. The time-course of the wound response was studied in Sieve elements of main veins of intact Vicia faba plants using confocal and multiphoton microscopy. Only 15-45 s after burning a leaf tip, forisomes (giant protein bodies specific for legume Sieve Tubes) suddenly dispersed, as observed at 3-4 cm from the stimulus site. The dispersion was reversible; the forisomes had fully re-contracted 7-15 min after burning. Meanwhile, callose appeared at the Sieve pores in response to the heat shock. Callose production reached a maximum after ∼20 min and was also reversible; callose degraded over the subsequent 1-2 h. The heat induction of both modes of occlusion coincided with the passage of an EPW visualized by electrophysiology or the potential-sensitive dye RH-414. In contrast to burning, cutting of the leaf tip induced neither an EPW nor callose deposition. The data are consistent with a remote-controlled occlusion of Sieve plates depending on the longitudinal propagation of an EPW releasing Ca 2+ into the Sieve element lumen. It is hypothesized that forisome plugs and callose constriction are removed once the cytosolic calcium level has returned to the initial level in those Sieve Tubes.

Michael Knoblauch - One of the best experts on this subject based on the ideXlab platform.

  • non dispersive phloem protein bodies npbs of populus trichocarpa consist of a seor protein and do not respond to cell wounding and ca2
    PeerJ, 2018
    Co-Authors: Daniel L Mullendore, Winfried S Peters, Timothy J Rosselliott, Yan Liu, Hanjo Hellmann, Eric H Roalson, Michael Knoblauch
    Abstract:

    Differentiating Sieve elements in the phloem of angiosperms produce abundant phloem-specific proteins before their protein synthesis machinery is degraded. These P-proteins initially form dense bodies, which disperse into individual filaments when the Sieve element matures. In some cases, however, the dense protein agglomerations remain intact and are visible in functional Sieve Tubes as non-dispersive P-protein bodies, or NPBs. Species exhibiting NPBs are distributed across the entire angiosperm clade. We found that NPBs in the model tree, Populus trichocarpa, resemble the protein bodies described from other species of the order Malpighiales as they all consist of coaligned tubular fibrils bundled in hexagonal symmetry. NPBs of all Malpighiales tested proved unresponsive to Sieve Tube wounding and Ca2+. The P. trichocarpa NPBs consisted of a protein encoded by a gene that in the genome database of this species had been annotated as a homolog of SEOR1 (Sieve element occlusion-related 1) in Arabidopsis. Sequencing of the gene in our plants corroborated this interpretation, and we named the gene PtSEOR1. Previously characterized SEOR proteins form irregular masses of P-protein slime in functional Sieve Tubes. We conclude that a subgroup of these proteins is involved in the formation of NPBs at least in the Malpighiales, and that these protein bodies have no role in rapid wound responses of the Sieve Tube network.

  • REVIEW PAPER SEORious business: structural proteins in Sieve Tubes and their involvement in Sieve element occlusion
    2016
    Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S Peters
    Abstract:

    The phloem provides a network of Sieve Tubes for long-distance translocation of photosynthates. For over a century, structural proteins in Sieve Tubes have presented a conundrum since they presumably increase the hydraulic resist-ance of the Tubes while no potential function other than Sieve Tube or wound sealing in the case of injury has been suggested. Here we summarize and critically evaluate current speculations regarding the roles of these proteins. Our understanding suffers from the suggestive power of images; what looks like a Sieve Tube plug on micrographs may not actually impede translocation very much. Recent reports of an involvement of SEOR (Sieve element occlusion-related) proteins, a class of P-proteins, in the sealing of injured Sieve Tubes are inconclusive; various lines of evidence suggest that, in neither intact nor injured plants, are SEORs determinative of translocation stoppage. Similarly, the popular notion that P-proteins serve in the defence against phloem sap-feeding insects is unsupported by empirical facts; it is conceivable that in functional Sieve Tubes, aphids actually could benefit from inducing a plug. The idea that rising cytosolic Ca2+ generally triggers Sieve Tube blockage by P-proteins appears widely accepted, despite lacking experimental support. Even in forisomes, P-protein assemblages restricted to one single plant family and the only Ca2+-responsive P-proteins known, the available evidence does not unequivocally suggest that plug formation is the cause rather than a consequence of translocation stoppage. We conclude that the physiological roles of structura

  • In situ microscopy reveals reversible cell wall swelling in kelp Sieve Tubes: one mechanism for turgor generation and flow control?
    Plant Cell and Environment, 2016
    Co-Authors: Jan Knoblauch, Sarah Tepler Drobnitch, Winfried S Peters, Michael Knoblauch
    Abstract:

    Kelps, brown algae (Phaeophyceae) of the order Laminariales, possess Sieve Tubes for the symplasmic long-distance transport of photoassimilates that are evolutionarily unrelated but structurally similar to the Tubes in the phloem of vascular plants. We visualized Sieve Tube structure and wound responses in fully functional, intact Bull Kelp (Nereocystis luetkeana [K. Mertens] Postels & Ruprecht 1840). In injured Tubes, apparent slime plugs formed but were unlikely to cause Sieve Tube occlusion as they assembled at the downstream side of Sieve plates. Cell walls expanded massively in the radial direction, reducing the volume of the wounded Sieve elements by up to 90%. Ultrastructural examination showed that a layer of the immediate cell wall characterized by circumferential cellulose fibrils was responsible for swelling, and suggested that alginates, abundant gelatinous polymers of the cell wall matrix, were involved. Wall swelling was rapid, reversible, and depended on intracellular pressure, as demonstrated by pressure-injection of silicon oil. Our results revive the concept of turgor generation and buffering by swelling cell walls, which had fallen into oblivion over the last century. Because Sieve Tube transport is pressure-driven and controlled physically by Tube diameter, a regulatory role of wall swelling in photoassimilate distribution is implied in kelps.

  • SEORious business: structural proteins in Sieve Tubes and their involvement in Sieve element occlusion
    Journal of Experimental Botany, 2014
    Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S Peters
    Abstract:

    The phloem provides a network of Sieve Tubes for long-distance translocation of photosynthates. For over a century, structural proteins in Sieve Tubes have presented a conundrum since they presumably increase the hydraulic resistance of the Tubes while no potential function other than Sieve Tube or wound sealing in the case of injury has been suggested. Here we summarize and critically evaluate current speculations regarding the roles of these proteins. Our understanding suffers from the suggestive power of images; what looks like a Sieve Tube plug on micrographs may not actually impede translocation very much. Recent reports of an involvement of SEOR (Sieve element occlusionrelated) proteins, a class of P-proteins, in the sealing of injured Sieve Tubes are inconclusive; various lines of evidence suggest that, in neither intact nor injured plants, are SEORs determinative of translocation stoppage. Similarly, the popular notion that P-proteins serve in the defence against phloem sap-feeding insects is unsupported by empirical facts; it is conceivable that in functional Sieve Tubes, aphids actually could benefit from inducing a plug. The idea that rising cytosolic Ca 2+ generally triggers Sieve Tube blockage by P-proteins appears widely accepted, despite lacking experimental support. Even in forisomes, P-protein assemblages restricted to one single plant family and the only Ca 2+ -responsive P-proteins known, the available evidence does not unequivocally suggest that plug formation is the cause rather than a consequence of translocation stoppage. We conclude that the physiological roles of structural P-proteins remain elusive, and that in vivo studies of their dynamics in continuous Sieve Tube networks combined with flow velocity measurements will be required to (hopefully) resolve this scientific roadblock.

  • phloem ultrastructure and pressure flow Sieve element occlusion related agglomerations do not affect translocation
    The Plant Cell, 2011
    Co-Authors: Daniel R. Froelich, Daniel L Mullendore, Kare Hartvig Jensen, Timothy J Rosselliott, James A Anstead, Gary A Thompson, Helene C Pelissier, Michael Knoblauch
    Abstract:

    Since the first ultrastructural investigations of Sieve Tubes in the early 1960s, their structure has been a matter of debate. Because Sieve Tube structure defines frictional interactions in the Tube system, the presence of P protein obstructions shown in many transmission electron micrographs led to a discussion about the mode of phloem transport. At present, it is generally agreed that P protein agglomerations are preparation artifacts due to injury, the lumen of Sieve Tubes is free of obstructions, and phloem flow is driven by an osmotically generated pressure differential according to Munch’s classical hypothesis. Here, we show that the phloem contains a distinctive network of protein filaments. Stable transgenic lines expressing Arabidopsis thaliana Sieve-Element-Occlusion-Related1 (SEOR1)–yellow fluorescent protein fusions show that At SEOR1 meshworks at the margins and clots in the lumen are a general feature of living Sieve Tubes. Live imaging of phloem flow and flow velocity measurements in individual Tubes indicate that At SEOR1 agglomerations do not markedly affect or alter flow. A transmission electron microscopy preparation protocol has been generated showing Sieve Tube ultrastructure of unprecedented quality. A reconstruction of Sieve Tube ultrastructure served as basis for Tube resistance calculations. The impact of agglomerations on phloem flow is discussed.

Jens B. Hafke - One of the best experts on this subject based on the ideXlab platform.

  • Thermodynamic Battle for Photosynthate Acquisition between Sieve Tubes and Adjoining Parenchyma in Transport Phloem1
    2015
    Co-Authors: Jens B. Hafke, Jan-kees Van Amerongen, Frits Kelling, Ra C. U. Furch, Frank Gaupels, Aart J. E. Van Bel
    Abstract:

    In transport phloem, photoassimilates escaping from the Sieve Tubes are released into the apoplasmic space between Sieve element (SE)/companion cell (CC) complexes (SE/CCs) and phloem parenchyma cells (PPCs). For uptake respective retrieval, PPCs and SE/CCs make use of plasma membrane translocators energized by the proton motive force (PMF). Their mutual competitiveness, which essentially determines the amount of photoassimilates translocated through the Sieve Tubes, therefore depends on the respective PMFs.Wemeasured the components of thePMF,membranepotential andDpH, of SE/CCs andPPCs in transport phloem. Membrane potentials of SE/CCs and PPCs in tissue slices as well as in intact plants fell into two categories. In the first group including apoplasmically phloem-loading species (e.g. Vicia, Solanum), the membrane potentials of the SEs are more negative than those of the PPCs. In the second group including symplasmically phloem-loading species (e.g. Cucurbita, Ocimum),membranepotentials of SEs are equal to or slightlymore positive than those of PPCs. Pure Sieve Tube sap collected from cut aphid stylets wasmeasuredwithH1-selectivemicroelectrodes. Under our experimental conditions, pH of the Sieve Tube saps was around 7.5, which is comparable to the pH of cytoplasmic compartments in parenchymatous cells. In conclusion, only the membrane potential appears to be relevant for the PMF-determined competition between SE/CCs and PPCs. The findings may imply that the axial sinks along the pathway withdraw more photoassimilates from the Sieve Tubes in symplasmically loading species than in apoplasmically loading species. Sieve Tubes are not hermetically sealed pipes, bu

  • Sieve element ca2 channels as relay stations between remote stimuli and Sieve Tube occlusion in vicia faba
    The Plant Cell, 2009
    Co-Authors: Alexandra C. U. Furch, Aart J. E. Van Bel, Mark D Fricker, Hubert H Felle, Maike Fuchs, Jens B. Hafke
    Abstract:

    Damage induces remote occlusion of Sieve Tubes in Vicia faba by forisome dispersion, triggered during the passage of an electropotential wave (EPW). This study addresses the role of Ca2+ channels and cytosolic Ca2+ elevation as a link between EPWs and forisome dispersion. Ca2+ channel antagonists affect the initial phase of the EPW as well as the prolonged plateau phase. Resting levels of Sieve Tube Ca2+ of ∼50 nM were independently estimated using Ca2+-selective electrodes and a Ca2+-sensitive dye. Transient changes in cytosolic Ca2+ were observed in phloem tissue in response to remote stimuli and showed profiles similar to those of EPWs. The measured elevation of Ca2+ in Sieve Tubes was below the threshold necessary for forisome dispersion. Therefore, forisomes need to be associated with Ca2+ release sites. We found an association between forisomes and endoplasmic reticulum (ER) at Sieve plates and pore-plasmodesma units where high-affinity binding of a fluorescent Ca2+ channel blocker mapped an increased density of Ca2+ channels. In conclusion, propagation of EPWs in response to remote stimuli is linked to forisome dispersion through transiently high levels of parietal Ca2+, release of which depends on both plasma membrane and ER Ca2+ channels.

  • plant and stimulus specific variations in remote controlled Sieve Tube occlusion
    Plant Signaling & Behavior, 2008
    Co-Authors: Alexandra C. U. Furch, Jens B. Hafke, Aart J. E. Van Bel
    Abstract:

    Phloem injury triggers local Sieve-plate occlusion including callose-mediated constriction and protein plugging of Sieve pores. In intact plants, reversible Sieve-plate occlusion is induced by electric potential waves (EPWs)—accompanied by Ca2+-influx—as result of distant burning. Here, we present additional results which pertain to (a) the variability of EPW-profiles in relation to forisome conformation in intact Vicia faba plants and (b) the differential occlusion reactions to burning and cutting in various plant species. A correlation between stimulus perception and mode of phloem loading is discussed.

  • ca2 mediated remote control of reversible Sieve Tube occlusion in vicia faba
    Journal of Experimental Botany, 2007
    Co-Authors: Alexandra C. U. Furch, Jens B. Hafke, Alexander Schulz, Aart J. E. Van Bel
    Abstract:

    According to an established concept, injury of the phloem triggers local Sieve plate occlusion including callose-mediated constriction and, possibly, protein plugging of the Sieve pores. Sieve plate occlusion can also be achieved by distant stimuli, depends on the passage of electropotential waves (EPWs), and is reversible in intact plants. The time-course of the wound response was studied in Sieve elements of main veins of intact Vicia faba plants using confocal and multiphoton microscopy. Only 15-45 s after burning a leaf tip, forisomes (giant protein bodies specific for legume Sieve Tubes) suddenly dispersed, as observed at 3-4 cm from the stimulus site. The dispersion was reversible; the forisomes had fully re-contracted 7-15 min after burning. Meanwhile, callose appeared at the Sieve pores in response to the heat shock. Callose production reached a maximum after ∼20 min and was also reversible; callose degraded over the subsequent 1-2 h. The heat induction of both modes of occlusion coincided with the passage of an EPW visualized by electrophysiology or the potential-sensitive dye RH-414. In contrast to burning, cutting of the leaf tip induced neither an EPW nor callose deposition. The data are consistent with a remote-controlled occlusion of Sieve plates depending on the longitudinal propagation of an EPW releasing Ca 2+ into the Sieve element lumen. It is hypothesized that forisome plugs and callose constriction are removed once the cytosolic calcium level has returned to the initial level in those Sieve Tubes.

  • ca2 mediated remote control of reversible Sieve Tube occlusion in vicia faba
    Journal of Experimental Botany, 2007
    Co-Authors: Alexandra C. U. Furch, Jens B. Hafke, Alexander Schulz, Aart J. E. Van Bel
    Abstract:

    According to an established concept, injury of the phloem triggers local Sieve plate occlusion including callose-mediated constriction and, possibly, protein plugging of the Sieve pores. Sieve plate occlusion can also be achieved by distant stimuli, depends on the passage of electropotential waves (EPWs), and is reversible in intact plants. The time-course of the wound response was studied in Sieve elements of main veins of intact Vicia faba plants using confocal and multiphoton microscopy. Only 15-45 s after burning a leaf tip, forisomes (giant protein bodies specific for legume Sieve Tubes) suddenly dispersed, as observed at 3-4 cm from the stimulus site. The dispersion was reversible; the forisomes had fully re-contracted 7-15 min after burning. Meanwhile, callose appeared at the Sieve pores in response to the heat shock. Callose production reached a maximum after approximately 20 min and was also reversible; callose degraded over the subsequent 1-2 h. The heat induction of both modes of occlusion coincided with the passage of an EPW visualized by electrophysiology or the potential-sensitive dye RH-414. In contrast to burning, cutting of the leaf tip induced neither an EPW nor callose deposition. The data are consistent with a remote-controlled occlusion of Sieve plates depending on the longitudinal propagation of an EPW releasing Ca(2+) into the Sieve element lumen. It is hypothesized that forisome plugs and callose constriction are removed once the cytosolic calcium level has returned to the initial level in those Sieve Tubes.