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C. E. J. Botha - One of the best experts on this subject based on the ideXlab platform.
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A tale of two neglected systems—structure and function of the thin- and thick-walled Sieve Tubes in monocotyledonous leaves
Frontiers in plant science, 2013Co-Authors: C. E. J. BothaAbstract:There is a large body of information relating to the ontogeny, development and the vasculature of eudicotyledonous leaves. However there is less information available concerning the vascular anatomy of monocotyledonous leaves. This is surprising, given that there are two uniquely different phloem systems present in large groups such as grasses and sedges. Monocotyledonous leaves contain marginal, large, intermediate and small longitudinal veins that are interconnected by numerous transverse veins. The longitudinal veins contain two metaphloem Sieve tube types, which, based upon their ontogeny and position within the phloem, are termed early (thin-walled) and late (thick-walled) Sieve Tubes. Early metaphloem comprises Sieve Tubes, companion cells and vascular parenchyma cells, whilst the late metaphloem, contains thick-walled Sieve Tubes that lack companion cells. Thick-walled Sieve Tubes are generally adjacent to, or no more than one cell removed from the metaxylem. Unlike thin-walled Sieve tube-companion cell complexes, thick-walled Sieve Tubes are connected to parenchyma by pore-plasmodesma units and are generally symplasmically isolated from the thin walled Sieve Tubes. This paper addresses key structural and functional differences between thin- and thick-walled Sieve Tubes and explores the unique advantages of alternate transport strategies that this 5 to 7 million year old dual system may offer. It would seem that these two systems may enhance, add to, or play a significant role in increasing the efficiency of solute retrieval as well as of assimilate transfer.
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a tale of two neglected systems structure and function of the thin and thick walled Sieve Tubes in monocotyledonous leaves
Frontiers in Plant Science, 2013Co-Authors: C. E. J. BothaAbstract:There is a large body of information relating to the ontogeny, development and the vasculature of eudicotyledonous leaves. However there is less information available concerning the vascular anatomy of monocotyledonous leaves. This is surprising, given that there are two uniquely different phloem systems present in large groups such as grasses and sedges. Monocotyledonous leaves contain marginal, large, intermediate and small longitudinal veins that are interconnected by numerous transverse veins. The longitudinal veins contain two metaphloem Sieve tube types, which, based upon their ontogeny and position within the phloem, are termed early (thin-walled) and late (thick-walled) Sieve Tubes. Early metaphloem comprises Sieve Tubes, companion cells and vascular parenchyma cells, whilst the late metaphloem, contains thick-walled Sieve Tubes that lack companion cells. Thick-walled Sieve Tubes are generally adjacent to, or no more than one cell removed from the metaxylem. Unlike thin-walled Sieve tube-companion cell complexes, thick-walled Sieve Tubes are connected to parenchyma by pore-plasmodesma units and are generally symplasmically isolated from the thin walled Sieve Tubes. This paper addresses key structural and functional differences between thin- and thick-walled Sieve Tubes and explores the unique advantages of alternate transport strategies that this 5 to 7 million year old dual system may offer. It would seem that these two systems may enhance, add to, or play a significant role in increasing the efficiency of solute retrieval as well as of assimilate transfer.
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Interaction of Phloem and Xylem During Phloem Loading: Functional Symplasmic Roles for Thin- and Thick-Walled Sieve Tubes in Monocotyledons
Vascular Transport in Plants, 2005Co-Authors: C. E. J. BothaAbstract:Publisher Summary This chapter focuses on structure–function relationships that influence phloem loading in monocotyledonous leaves. These major issues are considered: (1) What are the structural implications associated with the coexistence of two distinct Sieve tube types? (2) Do companion cells function in the same manner as those associated with Sieve elements in dicotyledonous plants or are the companion cell's functions taken over by parenchymatous elements associated with the thick-walled Sieve elements? (3) Is the vascular parenchyma implicated in solute retrieval from the xylem and does retrieved solute end up in both thin- and thick-walled Sieve Tubes? The use of fluorescent dyes to visualize symplasmic pathways for solute movement in monocotyledonous leaves is reviewed with emphasis on understanding the functional roles of the two types of Sieve Tubes, as well as the existence of a solute retrieval pathway between the xylem and the phloem.
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Aphid (Sitobion yakini) investigation suggests thin-walled Sieve Tubes in barley (Hordeum vulgare) to be more functional than thick-walled Sieve Tubes.
Physiologia plantarum, 2002Co-Authors: B. Matsiliza, C. E. J. BothaAbstract:Barley, like most other grasses that have been studied, contains two kinds of Sieve tube. The first formed are called thin-walled Sieve Tubes because of their thin wall compared to the late-formed, and are associated with companion cells. The late-formed are thick-walled Sieve Tubes, which differentiate next to the metaxylem vessels and lack companion cells. Aphid (Sitobion yakini (Eastop) feeding was studied using light microscopy to determine if they preferentially feed from thin- or thick-walled Sieve Tubes in the barley leaf. Penetration of the stylets through the leaf epidermis and mesophyll was largely intercellular, becoming partly intercellular and, partly, intracellular inside the vascular bundle. Sixteen of 19 pairs of stylets (84%), and 293 of 317 (92%) stylet tracks terminated at the thin-walled Sieve Tubes, suggesting that Sitobion yakini feeds preferentially on the thin-walled Sieve Tubes which seem to be more attractive to the aphid. These thin-walled Sieve Tubes are thus probably the most functional in terms of phloem loading and transport.
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Plasmodesmatal frequency in relation to short-distance transport and phloem loading in leaves of barley (Hordeum vulgare).Phloem is not loaded directly from the symplast
Physiologia Plantarum, 1997Co-Authors: C. E. J. Botha, R. H. M. CrossAbstract:We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin-walled Sieve Tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin- and thick-walled Sieve Tubes within this C3 species. Lack of connections between vascular parenchyma and Sieve Tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore-plasmodesmatal connections between companion cells and Sieve Tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell-Sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles. The degree of cellular connectivity and the potential isolation of the companion cell-Sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell-thin-walled Sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s-1) longitudinal and radial transport in the bundle sheath-vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the Sieve Tubes to the vascular parenchyma. Injection of the companion cell-Sieve tube complex, did not demonstrate backward connectivity to the bundle sheath. We conclude that the low plasmodesmatal frequencies, coupled with a two-domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.
Michael Knoblauch - One of the best experts on this subject based on the ideXlab platform.
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Sieve elements rapidly develop ‘nacreous walls’ following injury − a common wounding response?
The Plant journal : for cell and molecular biology, 2020Co-Authors: Jan Knoblauch, Michael Knoblauch, Viktoriya V. Vasina, Winfried S PetersAbstract:Thick glistening cell walls occur in Sieve Tubes of all major land plant taxa. Historically, these ‘nacreous walls’ have been considered a diagnostic feature of Sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure–flow theory as they severely constrict Sieve Tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in Sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional Sieve Tubes that occur when prepared for microscopic observation. Over 90% of Sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem‐mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross‐sections instead, an appropriate structure for Munch‐type mass flow of the cell contents. Puncturing of transporting Sieve elements with micropipettes triggered the rapid (
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REVIEW PAPER SEORious business: structural proteins in Sieve Tubes and their involvement in Sieve element occlusion
2016Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S PetersAbstract: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
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their involvement in Sieve element occlusion
2016Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S PetersAbstract:SEORious business: structural proteins in Sieve Tubes an
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In situ microscopy reveals reversible cell wall swelling in kelp Sieve Tubes: one mechanism for turgor generation and flow control?
Plant Cell and Environment, 2016Co-Authors: Jan Knoblauch, Sarah Tepler Drobnitch, Winfried S Peters, Michael KnoblauchAbstract: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.
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SEORious business: structural proteins in Sieve Tubes and their involvement in Sieve element occlusion
Journal of Experimental Botany, 2014Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S PetersAbstract: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.
Aart J. E. Van Bel - One of the best experts on this subject based on the ideXlab platform.
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Thermodynamic Battle for Photosynthate Acquisition between Sieve Tubes and Adjoining Parenchyma in Transport Phloem1
2015Co-Authors: Jens B. Hafke, Jan-kees Van Amerongen, Frits Kelling, Ra C. U. Furch, Frank Gaupels, Aart J. E. Van BelAbstract: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
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molecular sabotage of plant defense by aphid saliva
Proceedings of the National Academy of Sciences of the United States of America, 2007Co-Authors: Torsten Will, Fred W Tjallingii, Alexandra Thonnessen, Aart J. E. Van BelAbstract:Aphids, which constitute one of the most important groups of agricultural pests, ingest nutrients from Sieve Tubes, the photoassimilate transport conduits in plants. Aphids are able to successfully puncture Sieve Tubes with their piercing mouthparts (stylets) and ingest phloem sap without eliciting the Sieve Tubes' normal occlusion response to injury. Occlusion mechanisms are calcium-triggered and may be prevented by chemical constituents in aphid saliva injected into Sieve Tubes before and during feeding. We recorded aphid feeding behavior with the electrical penetration graph (EPG) technique and then experimentally induced Sieve tube plugging. Initiation of Sieve tube occlusion caused a change in aphid behavior from phloem sap ingestion to secretion of watery saliva. Direct proof of "unplugging" properties of aphid saliva was provided by the effect of aphid saliva on forisomes. Forisomes are proteinaceous inclusions in Sieve Tubes of legumes that show calcium-regulated changes in conformation between a contracted state (below calcium threshold) that does not occlude the Sieve Tubes and a dispersed state (above calcium threshold) that occludes the Sieve Tubes. We demonstrated in vitro that aphid saliva induces dispersed forisomes to revert back to the nonplugging contracted state. Labeling Western-blotted saliva proteins with 45Ca2+ or ruthenium red inferred the presence of calcium-binding domains. These results demonstrate that aphid saliva has the ability to prevent Sieve tube plugging by molecular interactions between salivary proteins and calcium. This provides aphids with access to a continuous flow of phloem sap and is a critical adaptation instrumental in the evolutionary success of aphids.
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Thermodynamic Battle for Photosynthate Acquisition between Sieve Tubes and Adjoining Parenchyma in Transport Phloem
Plant physiology, 2005Co-Authors: Jens B. Hafke, Jan-kees Van Amerongen, Frits Kelling, Frank Gaupels, Alexandra C. U. Furch, Aart J. E. Van BelAbstract: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. We measured the components of the PMF, membrane potential and ΔpH, of SE/CCs and PPCs 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), membrane potentials of SEs are equal to or slightly more positive than those of PPCs. Pure Sieve tube sap collected from cut aphid stylets was measured with H+-selective microelectrodes. 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.
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Reversible Calcium-Regulated Stopcocks in Legume Sieve Tubes
The Plant cell, 2001Co-Authors: Michael Knoblauch, Winfried S Peters, Katrin Ehlers, Aart J. E. Van BelAbstract:Sieve Tubes of legumes (Fabaceae) contain characteristic P-protein crystalloids with controversial function. We studied their behavior by conventional light, electron, and confocal laser scanning microscopy. In situ, crystalloids are able to undergo rapid (
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reversible calcium regulated stopcocks in legume Sieve Tubes
The Plant Cell, 2001Co-Authors: Michael Knoblauch, Winfried S Peters, Katrin Ehlers, Aart J. E. Van BelAbstract:Sieve Tubes of legumes (Fabaceae) contain characteristic P-protein crystalloids with controversial function. We studied their behavior by conventional light, electron, and confocal laser scanning microscopy. In situ, crystalloids are able to undergo rapid (<1 sec) and reversible conversions from the condensed resting state into a dispersed state, in which they occlude the Sieve Tubes. Crystalloid dispersal is triggered by plasma membrane leakage induced by mechanical injury or permeabilizing substances. Similarly, abrupt turgor changes imposed by osmotic shock cause crystalloid dispersal. Because chelators generally prevent the response, divalent cations appear to be the decisive factor in crystalloid expansion. Cycling between dispersal and condensation can be induced in opened cells by repetitive exchange of bathing media containing either Ca(2)+ or chelators. Sr(2)+ and Ba(2)+, but not Mg(2)+, are equally active. In conclusion, the fabacean P-protein crystalloids represent a novel class of mechanically active proteinaceous structures, which provide an efficient mechanism with which to control Sieve tube conductivity.
Winfried S Peters - One of the best experts on this subject based on the ideXlab platform.
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Sieve elements rapidly develop ‘nacreous walls’ following injury − a common wounding response?
The Plant journal : for cell and molecular biology, 2020Co-Authors: Jan Knoblauch, Michael Knoblauch, Viktoriya V. Vasina, Winfried S PetersAbstract:Thick glistening cell walls occur in Sieve Tubes of all major land plant taxa. Historically, these ‘nacreous walls’ have been considered a diagnostic feature of Sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure–flow theory as they severely constrict Sieve Tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in Sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional Sieve Tubes that occur when prepared for microscopic observation. Over 90% of Sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem‐mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross‐sections instead, an appropriate structure for Munch‐type mass flow of the cell contents. Puncturing of transporting Sieve elements with micropipettes triggered the rapid (
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REVIEW PAPER SEORious business: structural proteins in Sieve Tubes and their involvement in Sieve element occlusion
2016Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S PetersAbstract: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
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their involvement in Sieve element occlusion
2016Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S PetersAbstract:SEORious business: structural proteins in Sieve Tubes an
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In situ microscopy reveals reversible cell wall swelling in kelp Sieve Tubes: one mechanism for turgor generation and flow control?
Plant Cell and Environment, 2016Co-Authors: Jan Knoblauch, Sarah Tepler Drobnitch, Winfried S Peters, Michael KnoblauchAbstract: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.
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SEORious business: structural proteins in Sieve Tubes and their involvement in Sieve element occlusion
Journal of Experimental Botany, 2014Co-Authors: Michael Knoblauch, Daniel R. Froelich, William F. Pickard, Winfried S PetersAbstract: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.
Fernando García-arenal - One of the best experts on this subject based on the ideXlab platform.
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Potential involvement of a cucumber homolog of phloem protein 1 in the long-distance movement of Cucumber mosaic virus particles.
Molecular Plant-microbe Interactions, 2006Co-Authors: Alberto Requena, Laureano Simon-buela, G. Salcedo, Fernando García-arenalAbstract:The systemic movement of Cucumber mosaic virus (CMV) in cucumber plants was analyzed. The structure that is translocated and its putative interactions with phloem components were analyzed in phloem exudate (PE) samples, which reflect Sieve Tubes stream composition. Rate zonal centrifugation and electron-microscopy analyses of PE from CMV-infected plants showed that CMV moves through Sieve Tubes as virus particles. Gel overlay assays revealed that CMV particles interact with a PE protein, p48. The amino-acid sequence of several tryptic peptides of p48 was determined. Partial amino-acid sequence of p48 showed it was a cucumber homolog of phloem protein 1 (PP1) from pumpkin, with which p48 also shares several chemical properties. PP1 from pumpkin has plasmodesmata-gating ability and translocates in Sieve Tubes. Encapsidated CMV RNA in PE samples from infected plants was less accessible to digestion by RNase A than RNA in purified CMV particles, a property that was reconstituted by the in vitro interaction of purified CMV particles and protein p48. These results indicate that the interaction with p48 modifies CMV particle structure and suggest that CMV particles interact with the cucumber homolog of PP1 during translocation in the Sieve Tubes.
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Potential involvement of a cucumber homolog of phloem protein 1 in the long-distance movement of Cucumber mosaic virus particles.
Molecular Plant-microbe Interactions, 2006Co-Authors: Alberto Requena, Laureano Simon-buela, G. Salcedo, Fernando García-arenalAbstract:The systemic movement of Cucumber mosaic virus (CMV) in cucumber plants was analyzed. The structure that is translocated and its putative interactions with phloem components were analyzed in phloem exudate (PE) samples, which reflect Sieve Tubes stream composition. Rate zonal centrifugation and electron-microscopy analyses of PE from CMV-infected plants showed that CMV moves through Sieve Tubes as virus particles. Gel overlay assays revealed that CMV particles interact with a PE protein, p48. The amino-acid sequence of several tryptic peptides of p48 was determined. Partial amino-acid sequence of p48 showed it was a cucumber homolog of phloem protein 1 (PP1) from pumpkin, with which p48 also shares several chemical properties. PP1 from pumpkin has plasmodesmata-gating ability and translocates in Sieve Tubes. Encapsidated CMV RNA in PE samples from infected plants was less accessible to digestion by RNase A than RNA in purified CMV particles, a property that was reconstituted by the in vitro interaction of purified CMV particles and protein p48. These results indicate that the interaction with p48 modifies CMV particle structure and suggest that CMV particles interact with the cucumber homolog of PP1 during translocation in the Sieve Tubes.