Lipid rafts in higher plant cells: purification and characterization of Triton X-100-insoluble microdomains from tobacco plasma membrane

A vacuolar class III peroxidase and the metabolism of anticancer indole alkaloids in Catharanthus roseus: Can peroxidases, secondary metabolites and arabinogalactan proteins be partners in microcompartmentation of cellular reactions?

Plants possess a unique metabolic diversity commonly designated as secondary metabolism, of which the anticancer alkaloids from Catharanthus roseus are among the most studied. Recently, in a classical function-to-protein-to-gene approach, we have characterized the main class III peroxidase (Prx) expressed in C. roseus leaves, CrPrx1, implicated in a key biosynthetic step of the anticancer alkaloids. We have shown the vacuolar sorting determination of CrPrx1 using GFP fusions and we have obtained further evidence supporting the role of this enzyme in alkaloid biosynthesis, indicating the potential of CrPrx1 as a molecular tool for the manipulation of alkaloid metabolism. Here, we discuss how plant cells may regulate Prx reactions. In fact, Prxs form a large multigenic family whose members accept a broad range of substrates and, in their two subcellular localizations, the cell wall and the vacuole, Prxs co-locate with a large variety of secondary metabolites which can be accepted as substrates. How then, are Prx reactions regulated? Localization data obtained in our lab suggest that arabinogalactan proteins (AGPs) and Prxs may be associated in membrane microdomains, evocative of lipid rafts. Whether plasma membrane and/or tonoplast microcompartmentation involve AGPs and Prxs and whether this enables metabolic channeling determining Prx substrate selection are challenging questions ahead.

The secretory system of Arabidopsis

Over the past few years, a vast amount of research has illuminated the workings of the secretory system of eukaryotic cells. The bulk of this work has been focused on the yeast Saccharomyces cerevisiae, or on mammalian cells. At a superficial level, plants are typical eukaryotes with respect to the operation of the secretory system; however, important differences emerge in the function and appearance of endomembrane organelles. In particular, the plant secretory system has specialized in several ways to support the synthesis of many components of the complex cell wall, and specialized kinds of vacuole have taken on a protein storage role-a role that is intended to support the growing seedling, but has been co-opted to support human life in the seeds of many crop plants. In the past, most research on the plant secretory system has been guided by results in mammalian or fungal systems but recently plants have begun to stand on their own as models for understanding complex trafficking events within the eukaryotic endomembrane system.

Enemy at the gates: traffic at the plant cell pathogen interface

The plant apoplast constitutes a space for early recognition of potentially harmful non-self. Basal pathogen recognition operates via dynamic sensing of conserved microbial patterns by pattern recognition receptors or of elicitor-active molecules released from plant cell walls during infection. Recognition elicits defence reactions depending on cellular export via SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex-mediated vesicle fusion or plasma membrane transporter activity. Lipid rafts appear also involved in focusing immunity-associated proteins to the site of pathogen contact. Simultaneously, pathogen effectors target recognition, apoplastic host proteins and transport for cell wall-associated defence. This microreview highlights most recent reports on the arms race for plant disease and immunity at the cell surface.

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

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

Combining transcriptional datasets using the generalized singular value decomposition

Background

Both microarrays and quantitative real-time PCR are convenient tools for studying the transcriptional levels of genes. The former is preferable for large scale studies while the latter is a more targeted technique. Because of platform-dependent systematic effects, simple comparisons or merging of datasets obtained by these technologies are difficult, even though they may often be desirable. These difficulties are exacerbated if there is only partial overlap between the experimental conditions and genes probed in the two datasets.

Results

We show here that the generalized singular value decomposition provides a practical tool for merging a small, targeted dataset obtained by quantitative real-time PCR of specific genes with a much larger microarray dataset. The technique permits, for the first time, the identification of genes present in only one dataset co-expressed with a target gene present exclusively in the other dataset, even when experimental conditions for the two datasets are not identical. With the rapidly increasing number of publically available large scale microarray datasets the latter is frequently the case. The method enables us to discover putative candidate genes involved in the biosynthesis of the (1,3;1,4)-β-D-glucan polysaccharide found in plant cell walls.

Conclusion

We show that the generalized singular value decomposition provides a viable tool for a combined analysis of two gene expression datasets with only partial overlap of both gene sets and experimental conditions. We illustrate how the decomposition can be optimized self-consistently by using a judicious choice of genes to define it. The ability of the technique to seamlessly define a concept of "co-expression" across both datasets provides an avenue for meaningful data integration. We believe that it will prove to be particularly useful for exploiting large, publicly available, microarray datasets for species with unsequenced genomes by complementing them with more limited in-house expression measurements.

Production of cecropin A in transgenic rice plants has an impact on host gene expression

Expression of the cecropin A gene in rice confers resistance to the rice blast fungus Magnaporthe oryzae. In this study, a polymerase chain reaction-based suppression subtractive hybridization approach was used to generate a cDNA macroarray from the elite japonica rice (Oryza sativa L.) cultivar 'Senia'. Gene expression studies revealed that the expression of components of the protein secretory and vesicular transport machinery is co-ordinately activated at the pre-invasive stage of infection of rice by the blast fungus. Comparisons of gene expression between wild-type and cecropin A plants revealed the over-expression of genes involved in protection against oxidative stress in transgenic plants in the absence of the pathogen, which correlated well with the tolerance of these plants to oxidative stress. A subcellular fractionation analysis suggested that cecropin A accumulates in the endoplasmic reticulum in cecropin A rice. Moreover, a large number of genes related to the processes of synthesis, folding and stabilization of proteins that enter into the secretory pathway are over-expressed in cecropin A rice, confirming that these plants constitutively express the unfolded protein response. Transgenic expression of cecropin A in rice has an effect on the transcriptional reprogramming that accompanies plant adaptation to fungal infection. Overall, this study provides evidence for transgene-induced changes in gene expression in cecropin A rice under both optimal growth conditions and stress conditions imposed by fungal infection. The data also indicate that resistance to blast in cecropin A rice may be the consequence of a combination of the antifungal activity of cecropin A and cecropin A-mediated over-expression of rice genes.

Grape berry plasma membrane proteome analysis and its differential expression during ripening

High purity berry plasma membranes (PMs) of Vitis vinifera L. cv. Cabernet Sauvignon were isolated by two-phase partitioning of microsome fractions at different stages of berry ripening. PM proteins resolvable by the detergent cocktail of CHAPS and ASB-14 were separated by two-dimensional electrophoresis. A total of 119 protein spots from pre-véraison berry PMs on 2-D gels detected with silver staining were subjected to MALDI-TOF mass spectrometry analysis. Sixty-two spots were identified as putative PM proteins, with 1-6 predicted transmembrane helices, including true PM proteins such as ATP synthase, ABC transporters, and GTP-binding proteins reported in plants. They were then grouped into eight functional categories, mainly involved in transport, metabolism, signal transduction, and protein synthesis. Another 11 spots were identified as proteins of unknown function. The véraison and post-véraison samples stained 98 and 86 spots on the gels, respectively. During the berry ripening process, total PM protein content gradually decreased. Among all identified proteins, 12 showed significant differences in terms of their relative abundance. Increasing ubiquitin proteolysis and cytoskeleton proteins were observed from pre-véraison to post-véraison. Zeatin O-glucosyltransferase peaked at véraison, while ubiquitin-conjugating enzyme E2-21 was down-regulated at this stage. This proteome research provides the first information on PM protein characterization during the grape berry ripening process.

Loss-of-function mutations and inducible RNAi suppression of Arabidopsis LCB2 genes reveal the critical role of sphingolipids in gametophytic and sporophytic cell viability

Serine palmitoyltransferase (SPT) catalyzes the first step in sphingolipid biosynthesis, and downregulation of this enzyme provides a means for exploring sphingolipid function in cells. We have previously demonstrated that Arabidopsis SPT requires LCB1 and LCB2 subunits for activity, as is the case in other eukaryotes. In this study, we show that Arabidopsis has two genes (AtLCB2a and AtLCB2b) that encode functional isoforms of the LCB2 subunit. No alterations in sphingolipid content or growth were observed in T-DNA mutants for either gene, but homozygous double mutants were not recoverable, suggesting that these genes are functionally redundant. Reciprocal crosses conducted with Atlcb2a and Atlcb2b mutants indicated that lethality is associated primarily with the inability to transmit the lcb2 null genotype through the haploid pollen. Consistent with this, approximately 50% of the pollen obtained from plants homozygous for a mutation in one gene and heterozygous for a mutation in the second gene arrested during transition from uni-nucleate microspore to bicellular pollen. Ultrastructural analyses revealed that these pollen grains contained aberrant endomembranes and lacked an intine layer. To examine sphingolipid function in sporophytic cells, Arabidopsis lines were generated that allowed inducible RNAi silencing of AtLCB2b in an Atlcb2a mutant background. Studies conducted with these lines demonstrated that sphingolipids are essential throughout plant development, and that lethality resulting from LCB2 silencing in seedlings could be partially rescued by supplying exogenous long-chain bases. Overall, these studies provide insights into the genetic and biochemical properties of SPT and sphingolipid function in Arabidopsis.

A MYB transcription factor regulates very-long-chain fatty acid biosynthesis for activation of the hypersensitive cell death response in Arabidopsis

Plant immune responses to pathogen attack include the hypersensitive response (HR), a form of programmed cell death occurring at invasion sites. We previously reported on Arabidopsis thaliana MYB30, a transcription factor that acts as a positive regulator of a cell death pathway conditioning the HR. Here, we show by microarray analyses of Arabidopsis plants misexpressing MYB30 that the genes encoding the four enzymes forming the acyl-coA elongase complex are putative MYB30 targets. The acyl-coA elongase complex synthesizes very-long-chain fatty acids (VLCFAs), and the accumulation of extracellular VLCFA-derived metabolites (leaf epidermal wax components) was affected in MYB30 knockout mutant and overexpressing lines. In the same lines, a lipid extraction procedure allowing high recovery of sphingolipids revealed changes in VLCFA contents that were amplified in response to inoculation. Finally, the exacerbated HR phenotype of MYB30-overexpressing lines was altered by the loss of function of the acyl-ACP thioesterase FATB, which causes severe defects in the supply of fatty acids for VLCFA biosynthesis. Based on these findings, we propose a model in which MYB30 modulates HR via VLCFAs by themselves, or VLCFA derivatives, as cell death messengers in plants.

Multidrug transporters CaCdr1p and CaMdr1p of Candida albicans display different lipid specificities: both ergosterol and sphingolipids are essential for targeting of CaCdr1p to membrane rafts

In this study, we compared the effects of altered membrane lipid composition on the localization of two membrane drug transporters from different superfamilies of the pathogenic yeast Candida albicans. We demonstrated that in comparison to the major facilitator superfamily multidrug transporter CaMdr1p, ATP-binding cassette transporter CaCdr1p of C. albicans is preferentially localized within detergent-resistant membrane (DRM) microdomains called 'rafts.' Both CaCdr1p and CaMdr1p were overexpressed as green fluorescent protein (GFP)-tagged proteins in a heterologous host Saccharomyces cerevisiae, wherein either sphingolipid (Deltasur4 or Deltafen1 or Deltaipt1) or ergosterol (Deltaerg24 or Deltaerg6 or Deltaerg4) biosynthesis was compromised. CaCdr1p-GFP, when expressed in the above mutant backgrounds, was not correctly targeted to plasma membranes (PM), which also resulted in severely impaired drug resistance. In contrast, CaMdr1p-GFP displayed no sorting defect in the mutant background and remained properly surface localized and displayed no change in drug resistance. Our data clearly show that CaCdr1p is selectively recruited, over CaMdr1p, to the DRM microdomains of the yeast PM and that any imbalance in the raft lipid constituents results in missorting of CaCdr1p.

Transport and secretion in plant-microbe interactions

Microbial elicitors and effectors, as well as plant receptors and defence compounds, traffic at the interface of plants and microbes in pathogenic or mutualistic interactions. Net exocytosis appears to be required for surface enlargement of plasma membrane during accommodation of microbes in intact plant cells. By contrast, ligand-induced endocytosis of surface receptors operates in basal defence. The first layer of plant defence appears to depend on polarized transport of small molecules and on local secretion of defence proteins. In return, pathogen effectors target plasma membrane bound and intracellular proteins to inhibit extracellular host defences.

A High Content in Lipid-modified Peripheral Proteins and Integral Receptor Kinases Features in the Arabidopsis Plasma Membrane Proteome

The proteomics of plasma membrane has brought to date only scarce and partial information on the actual protein repertoire. In this work, the plant plasma membrane proteome of Arabidopsis thaliana was investigated. A highly purified plasma membrane fraction was washed by NaCl and Na2CO3 salts, and the insoluble fractions were further analyzed by nano-LC-MS/MS. With 446 proteins identified, we hereby describe the largest plasma membrane proteome diversity reported so far. Half of the proteins were predicted to display transmembrane domains and/or to be anchored to the membrane, validating a posteriori the pertinence of the approach. A fine analysis highlighted two main specific and novel features. First, the main functional category is represented by a majority of as yet unreported signaling proteins, including 11% receptor-like kinases. Second, 16% of the identified proteins are predicted to be lipid-modified, specifically involving double lipid linkage through N-terminal myristoylation, S-palmitoylation, C-terminal prenylation, or glycosylphosphatidylinositol anchors. Thus, our approach led for the first time to the identification of a large number of peripheral proteins as part of the plasma membrane and allowed the functionality of the plasma membrane in the cell context to be reconsidered.

SNARE-Ware: The Role of SNARE-Domain Proteins in Plant Biology

In yeast and animal cells, members of the superfamily of N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE)-domain-containing proteins are key players in vesicle-associated membrane fusion events during transport processes between individual compartments of the endomembrane system, including exocytosis and endocytosis. Compared with genomes of other eukaryotes, genomes of monocotyledonous and dicotyledonous plants encode a surprisingly high number of SNARE proteins, suggesting vital roles for this protein class in higher plant species. Although to date it remains elusive whether plant SNARE proteins function like their yeast and animal counterparts, genetic screens have recently begun to unravel the variety of biological tasks in which plant SNAREs are involved. These duties involve fundamental processes such as cytokinesis, shoot gravitropism, pathogen defense, symbiosis, and abiotic stress responses, suggesting that SNAREs contribute essentially to many facets of plant biology.

Plant lipid rafts: fluctuat nec mergitur

Lipid rafts in plasma membranes are hypothesized to play key roles in many cellular processes including signal transduction, membrane trafficking and entry of pathogens. We recently documented the biochemical characterization of lipid rafts, isolated as detergent-insoluble membranes, from Medicago truncatula root plasma membranes. We evidenced that the plant-specific lipid steryl-conjugates are among the main lipids of rafts together with free sterols and sphingolipids. An extensive proteomic analysis showed the presence of a specific set of proteins common to other lipid rafts, plus the presence of a redox system around a cytochrome b(561) not previously identified in lipid rafts of either plants or animals. Here, we discuss the similarities and differences between the lipids and proteins of plant and animal lipid rafts. Moreover we describe the potential biochemical functioning of the M. truncatula root lipid raft redox proteins and question whether they may play a physiological role in legume-symbiont interactions.

Selective targeting of plasma membrane and tonoplast traffic by inhibitory (dominant-negative) SNARE fragments

Vesicle traffic underpins cell homeostasis, growth and development in plants, and is facilitated by a superfamily of proteins known as SNAREs [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptors] that interact to draw vesicle and target membrane surfaces together for fusion. Structural homologies, biochemical and genetic analyses have yielded information about the localization and possible roles of these proteins. However, remarkably little evidence is yet available that speaks directly to the functional specificities of these proteins in selected trafficking pathways in vivo. Previously, we found that expressing a cytosolic (so-called Sp2) fragment of one plasma membrane SNARE from tobacco and Arabidopsis had severe effects on growth, tissue development and secretory traffic to the plasma membrane. We have explored this dominant-negative approach further to examine the specificity and overlaps in Sp2 activity by generating a toolbox of truncated SNARE constructs and antibodies for transient expression and analysis. Using a quantitative ratiometric approach with secreted green fluorescent protein (secGFP), we report here that traffic to the plasma membrane is suppressed selectively by Sp2 fragments of plasma membrane SNAREs AtSYP121 and AtSYP122, but not of the closely related SNARE AtSYP111 nor of the SNARE AtSYP21 that resides at the pre-vacuolar compartment (PVC). By contrast, traffic of the YFP-tagged aquaporin fusion protein TIP1;1-YFP to the tonoplast was blocked (leading to its accumulation in the PVC) when co-expressed with the Sp2 fragment of AtSYP21, but not when co-expressed with that of AtSYP121. Export of secGFP was also sensitive to the Sp2 fragment of the novel, plant-specific SNARE AtSYP71 that was recently found to be present in detergent-resistant, plasma membrane fractions. Co-incubation analyses of the plasma membrane SNAREs with the regulatory subdomain included within the Sp2 fragments showed activity in destabilizing protein complexes, but only with the complementary SNAREs. We conclude that the Sp2 fragment action accurately reflects the known specificity and targeting of these SNAREs, implies functional overlaps that are of potential physiological interest, and underscores the use of a dominant-negative strategy in functional studies of a major subfamily of SNAREs in plants.

A higher plant delta8 sphingolipid desaturase with a preference for (Z)-isomer formation confers aluminum tolerance to yeast and plants

Three plant cDNA libraries were expressed in yeast (Saccharomyces cerevisiae) and screened on agar plates containing toxic concentrations of aluminum. Nine cDNAs were isolated that enhanced the aluminum tolerance of yeast. These cDNAs were constitutively expressed in Arabidopsis (Arabidopsis thaliana) and one cDNA from the roots of Stylosanthes hamata, designated S851, conferred greater aluminum tolerance to the transgenic seedlings. The protein predicted to be encoded by S851 showed an equally high similarity to Delta6 fatty acyl lipid desaturases and Delta8 sphingolipid desaturases. We expressed other known Delta6 desaturase and Delta8 desaturase genes in yeast and showed that a Delta6 fatty acyl desaturase from Echium plantagineum did not confer aluminum tolerance, whereas a Delta8 sphingobase desaturase from Arabidopsis did confer aluminum tolerance. Analysis of the fatty acids and sphingobases of the transgenic yeast and plant cells demonstrated that S851 encodes a Delta8 sphingobase desaturase, which leads to the accumulation of 8(Z/E)-C(18)-phytosphingenine and 8(Z/E)-C(20)-phytopshingenine in yeast and to the accumulation of 8(Z/E)-C(18)-phytosphingenine in the leaves and roots of Arabidopsis plants. The newly formed 8(Z/E)-C(18)-phytosphingenine in transgenic yeast accounted for 3 mol% of the total sphingobases with a 8(Z):8(E)-isomer ratio of approximately 4:1. The accumulation of 8(Z)-C(18)-phytosphingenine in transgenic Arabidopsis shifted the ratio of the 8(Z):8(E) isomers from 1:4 in wild-type plants to 1:1 in transgenic plants. These results indicate that S851 encodes the first Delta8 sphingolipid desaturase to be identified in higher plants with a preference for the 8(Z)-isomer. They further demonstrate that changes in the sphingolipid composition of cell membranes can protect plants from aluminum stress.

Cytoskeleton and cell wall function in penetration resistance

Plants successfully repel the vast majority of potential pathogens that arrive on their surface, with most microorganisms failing to breach the outer epidermal wall. Resistance to penetration at the epidermis is a key component of basal defence against disease and critically depends on fortification of the cell wall at the site of attempted penetration through the development of specialised cell wall appositions rich in antimicrobial compounds. Formation of cell wall appositions is achieved by rapid reorganisation of actin microfilaments, actin-dependent transport of secretory products to the infection site and local activation of callose synthesis. Plants are finely tuned to detect the presence of pathogens on their surface, perceiving both chemical and physical signals of pathogen origin. In the on-going evolution of interaction strategies, plants must continually monitor and out manoeuvre pathogen avoidance or suppression of plant defences in order to preserve the effectiveness of penetration resistance.

Abscisic Acid Triggers the Endocytosis of the Arabidopsis KAT1 K+ Channel and Its Recycling to the Plasma Membrane

Membrane vesicle traffic to and from the plasma membrane is essential for cellular homeostasis in all eukaryotes. In plants, constitutive traffic to and from the plasma membrane has been implicated in maintaining the population of integral plasma-membrane proteins and its adjustment to a variety of hormonal and environmental stimuli. However, direct evidence for evoked and selective traffic has been lacking. Here, we report that the hormone abscisic acid (ABA), which controls ion transport and transpiration in plants under water stress, triggers the selective endocytosis of the KAT1 K+ channel protein in epidermal and guard cells. Endocytosis of the K+ channel from the plasma membrane initiates in concert with changes in K+ channel activities evoked by ABA and leads to sequestration of the K+ channel within an endosomal membrane pool that recycles back to the plasma membrane over a period of hours. Selective K+ channel endocytosis, sequestration, and recycling demonstrates a tight and dynamic control of the population of K+ channels at the plasma membrane as part of a key plant signaling and response mechanism, and the observations point to a role for channel traffic in adaptive changes in the capacity for osmotic solute flux of stomatal guard cells.

Distinct fluorescent pattern of KAT1::GFP in the plasma membrane of Vicia faba guard cells

The organisation of membrane proteins into certain domains of the plasma membrane (PM) has been proposed to be important for signalling in yeast and animal cells. Here we describe the formation of a very distinct pattern of the K(+) channel KAT1 fused to the green fluorescent protein (KAT1::GFP) when transiently expressed in guard cells of Vicia faba. Using confocal laser scanning microscopy we observed a radially striped pattern of KAT1::GFP fluorescence in the PM in about 70% of all transfected guard cells. This characteristic pattern was found to be cell type and protein specific and independent of the stomatal aperture and the cytoskeleton. Staining of the cell wall of guard cells with Calcofluor White revealed a great similarity between the arrangement of cellulose microfibrils and the KAT1::GFP pattern. Furthermore, the radial pattern of KAT1::GFP immediately disappeared when turgor pressure was strongly decreased by changing from hypotonic to hypertonic conditions. The pattern reappeared within 15 min upon reestablishment of high turgor pressure in hypotonic solution. Evaluation of the staining pattern by a mathematical algorithm further confirmed this reversible abolishment of the radial pattern during hypertonic treatment. We therefore conclude that the radial organisation of KAT1::GFP depends on the close contact between the PM and cell wall in turgid guard cells. These results offer the first indication for a role of the cell wall in the localisation of ion channels. We propose a model in which KAT1 is located in the cellulose fibrils intermediate areas of the PM and discuss the physiological role of this phenomenon.

Regulation of reactive oxygen species production by a 14-3-3 protein in elicited tobacco cells

The regulation of the system responsible for the production of reactive oxygen species (ROS) during plant-micro-organism interaction is still largely unknown. The protein NtrbohD has been recently demonstrated as the plasma membrane oxidase responsible for ROS production in elicited tobacco cells. Here, its C-terminus part was used as a bait in a two-hybrid screen in order to identify putative regulators of this system. This led to the isolation of a cDNA coding for a member of the 14-3-3 protein family. The corresponding transcript was induced after infiltration of tobacco leaves with the fungal elicitor cryptogein. Tobacco cells transformed with an antisense construct of this 14-3-3 no longer accumulated ROS, which constitutes a functional validation of the two-hybrid screen. This work provides new insights to the understanding of the regulation of ROS production in a signalling context and gives a new light to the possible role of 14-3-3 proteins in plant-micro-organisms interactions.

Characterization of lipid rafts from Medicago truncatula root plasma membranes: a proteomic study reveals the presence of a raft-associated redox system

Several studies have provided new insights into the role of sphingolipid/sterol-rich domains so-called lipid rafts of the plasma membrane (PM) from mammalian cells, and more recently from leaves, cell cultures, and seedlings of higher plants. Here we show that lipid raft domains, defined as Triton X-100-insoluble membranes, can also be prepared from Medicago truncatula root PMs. These domains have been extensively characterized by ultrastructural studies as well as by analysis of their content in lipids and proteins. M. truncatula lipid domains are shown to be enriched in sphingolipids and Delta(7)-sterols, with spinasterol as the major compound, but also in steryl glycosides and acyl-steryl glycosides. A large number of proteins (i.e. 270) have been identified. Among them, receptor kinases and proteins related to signaling, cellular trafficking, and cell wall functioning were well represented whereas those involved in transport and metabolism were poorly represented. Evidence is also given for the presence of a complete PM redox system in the lipid rafts.

Characterization and expression analysis of genes encoding alpha and beta carbonic anhydrases in Arabidopsis

Carbonic anhydrases (CAs) are Zn-containing metalloenzymes that catalyse the reversible hydration of CO(2). We investigated the alphaCA and betaCA families in Arabidopsis, which contain eight alphaCA (At alphaCA1-8) and six betaCA genes (At betaCA1-6). Analyses of expressed sequence tags (ESTs) from The Arabidopsis Information Resource (TAIR) database indicate that all the betaCA encoding sequences, but only three of the At alphaCA, are expressed. Using semi-quantitative PCR experiments, functional CA genes were more strongly expressed in green tissue, but strong expression was also found in roots for betaCA3, betaCA6 and alphaCA2. Two alphaCA genes were shown to respond to the CO(2) environment, while the others were unresponsive. Using the green fluorescent reporter protein gene fused with cDNA sequences coding for betaCAs, we provided evidence that betaCAs were targeted to specific subcellular compartments: betaCA1 and betaCA5 were targeted to the chloroplast, betaCA2 and betaCA3 to the cytosol, betaCA4 to the plasma membrane and betaCA6 to the mitochondria. The targeting and the pattern of gene expression suggest that CA isoforms play specific roles in subcellular compartments, tissues and organs. The data indicate that other CA isoforms than the well-characterized betaCA1 may contribute to the CO(2) transfer in the cell to the catalytic site of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco).

Increases in the number of SNARE genes parallels the rise of multicellularity among the green plants

The green plant lineage is the second major multicellular expansion among the eukaryotes, arising from unicellular ancestors to produce the incredible diversity of morphologies and habitats observed today. In the unicellular ancestors, secretion of material through the endomembrane system was the major mechanism for interacting and shaping the external environment. In a multicellular organism, the external environment can be made of other cells, some of which may have vastly different developmental fates, or be part of different tissues or organs. In this context, a given cell must find ways to organize its secretory pathway at a level beyond that of the unicellular ancestor. Recently, sequence information from many green plants have become available, allowing an examination of the genomes for the machinery involved in the secretory pathway. In this work, the SNARE proteins of several green plants have been identified. While little increase in gene number was seen in the SNAREs of the early secretory system, many new SNARE genes and gene families have appeared in the multicellular green plants with respect to the unicellular plants, suggesting that this increase in the number of SNARE genes may have some relation to the rise of multicellularity in green plants.

Plant aquaporins: novel functions and regulation properties

Aquaporins are water channel proteins of intracellular and plasma membranes that play a crucial role in plant water relations. The present review focuses on the most recent findings concerning the molecular and cellular properties of plant aquaporins. The mechanisms of transport selectivity and gating (i.e. pore opening and closing) have recently been described, based on aquaporin structures at atomic resolution. Novel dynamic aspects of aquaporin subcellular localisation have been uncovered. Also, some aquaporin isoforms can transport, besides water, physiologically important molecules such as CO(2), H(2)O(2), boron or silicon. Thus, aquaporins are involved in many great functions of plants, including nutrient acquisition, carbon fixation, cell signalling and stress responses.

Plant sterols in “rafts”: a better way to regulate membrane thermal shocks

Specialized lipid domains (rafts) that are generally enriched in sterols and sphingolipids, are most likely present in cell membranes of animals, plants and fungi. While cholesterol and ergosterol are predominant in vertebrates and fungi, plants possess complex sterol profiles, dominated by sitosterol and stigmasterol in Arabidopsis thaliana. Fully hydrated model membranes of composition approaching those found in rafts of mammals, fungi and plants were investigated by means of solid-state 2H-NMR, using deuterated dipalmitoylphosphatidylcholine (2H(62)-DPPC). The dynamics of such membranes was determined through measuring of membrane ordering or disordering properties. The presence of the liquid-ordered, lo, phase, which may be an indicator of rigid sterol-sphingolipid domains, was detected in all binary or ternary mixtures of all sterols investigated. Of great interest, the dynamics of ternary mixtures mimicking rafts in plants (phytosterol/glucosylcerebroside/DPPC), showed a lesser temperature sensitivity to thermal shocks, on comparing to systems mimicking rafts in mammals and fungi. This effect was particularly marked with sitosterol. The presence of an ethyl group branched on the alkyl chain of sitosterol and stigmasterol is proposed as reinforcing the membrane cohesion by additional attractive van der Waals interactions with the alkyl chains of sphingolipids and phospholipids. As a side result, the elevated resolution of NMR spectra in the presence of sitosterol also suggests domains of smaller size than with other sterols. Finally, the role of phytosterols in maintaining plant membranes in a state of dynamics less sensitive to temperature shocks is discussed.

The elongation factor 1A: a novel regulator in the DNA replication/repair protein network in wheat cells?

Proliferating cell nuclear antigen (PCNA) is a DNA sliding clamp interacting with multiple partners in DNA transactions such as DNA replication/repair and recombination as well as chromatin assembly. We previously detected and purified by chromatographic procedures a 31 kDa PCNA from cultured wheat cells (Triticum monococcum L). Here we report the complete sequence of the wheat 31 kDa PCNA showing a very high aminoacid identity with its plant counterparts (maize and rice). This recombinant PCNA has been used as a bait in an affinity chromatography procedure, in order to capture PCNA interacting proteins. We detected by liquid chromatography, tandem mass spectrometry and search in plant protein databases, several specific bands from wheat cell lysates in fractions bound to wheat PCNA-affinity column. One of them is the wheat elongation factor 1A. Its putative regulatory role in DNA replication/repair is discussed.

Plasma membrane lipid alterations induced by cold acclimation and abscisic acid treatment of winter wheat seedlings differing in frost resistance

Cold acclimation of plants affects many aspects of metabolism. Changes in plasma membrane lipids have always been considered to be important for development of frost resistance and survival at subzero temperatures. We studied different cultivars of winter wheat (Triticum aestivum L.) that differed in frost resistance induced either by cold acclimation or treatment with the plant hormone abscisic acid (ABA). Plasma membranes were isolated from non-acclimated and cold- as well as from ABA-acclimated plants, and were subjected to detailed lipid analysis. Cold acclimation affected virtually all plasma membrane lipid components and their constituents, resulting in both increases and decreases, which varied between the three groups of plants investigated. Including the cold-induced variations observed in the few plant species studied in detail previously, cerebrosides were the only components reduced by cold acclimation in all plants. In wheat, more uniform and consistent patterns were obtained when considering colligative parameters such as total free sterols, phospholipids or glycolipids, either as the proportion of total lipids or based on plasma membrane protein. The parameter which changed most significantly in parallel to the increase of inducible frost resistance in the three groups of plants was the ratio of free sterols/glycolipids, which increased. ABA treatment resulted in qualitatively similar effects in only one cultivar, but in general these changes were less pronounced. Compared to changes in transcription rates of several cold-induced genes and in the concentration of various compatible solutes reported for other plants, the observed changes in plasma membrane lipids are minor ones. This may indicate that acclimation-induced changes can be accomplished by posttranscriptional regulation of enzymatic activities, which is in agreement with the failure to detect significant changes in transcription of the corresponding genes during cold induction.

Purification and physico-kinetic characterization of 3beta-hydroxy specific sterol glucosyltransferase from Withania somnifera (L) and its stress response

Sterol glycosyltransferases catalyze the synthesis of diverse glycosteroids in plants, leading to a change in their participation in cellular metabolism. Withania somnifera is a medically important plant, known for a variety of pharmacologically important withanolides and their glycosides. In this study, a cytosolic sterol glucosyltransferase was purified 3406 fold to near homogeneity from W. somnifera leaves and studied for its biochemical and kinetic properties. The purified enzyme was active with UDP-glucose but not with UDP-galactose as sugar donor. It exhibited broad sterol specificity by glucosylating a variety of sterols and phytosterols with 3beta-OH group. It showed a low level of activity with flavonoids and isoflavonoids. The enzyme gave maximum K(cat)/K(m) value (0.957) for 24-methylenecholesterol that resembles aglycone structure of pharmacologically important sitoindosides VII and VIII from W. somnifera. The enzyme follows ordered sequential bisubstrate mechanism of reaction, in which UDP-glucose and sterol are the first and second binding substrates. This is the first detailed kinetic study on purified plant cytosolic sterol glucosyltransferases. Results on peptide mass fingerprinting and substrate specificity suggested that the enzyme belongs to the family of secondary metabolite glucosylating glucosyltransferases. The enzyme activity exhibited a rapid in vivo response to high temperature and salicylic acid treatment of plants, suggesting its physiological role in abiotic and biotic stress.

Cell wall-associated mechanisms of disease resistance and susceptibility

The plant cuticle and cell wall separate microbial pathogens from the products of plant metabolism. While microbial pathogens try to breach these barriers for colonization, plants respond to attempted penetration by a battery of wall-associated defense reactions. Successful pathogens circumvent or suppress plant nonself recognition and basal defense during penetration and during microbial reproduction. Additionally, accommodation of fungal infection structures within intact cells requires host reprogramming. Recent data highlight that both early plant defense to fungal penetration and host reprogramming for susceptibility can function at the host cell periphery. Genetic evidence has also widened our understanding of how fungal pathogens are restricted during penetration at the plant cell wall. This review summarizes the current view of how plants monitor and model their cell periphery during interaction with microbial invaders.

Glycerolipid transfer for the building of membranes in plant cells

Membranes of plant organelles have specific glycerolipid compositions. Selective distribution of lipids at the levels of subcellular organelles, membrane leaflets and membrane domains reflects a complex and finely tuned lipid homeostasis. Glycerolipid neosynthesis occurs mainly in plastid envelope and endoplasmic reticulum membranes. Since most lipids are not only present in the membranes where they are synthesized, one cannot explain membrane specific lipid distribution by metabolic processes confined in each membrane compartment. In this review, we present our current understanding of glycerolipid trafficking in plant cells. We examine the potential mechanisms involved in lipid transport inside bilayers and from one membrane to another. We survey lipid transfers going through vesicular membrane flow and those dependent on lipid transfer proteins at membrane contact sites. By introducing recently described membrane lipid reorganization during phosphate deprivation and recent developments issued from mutant analyses, we detail the specific lipid transfers towards or outwards the chloroplast envelope.

Rapid measurement of sphingolipids from Arabidopsis thaliana by reversed-phase high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry

Changes in sphingolipids have been associated with profound effects in cell fate and development in both plants and animals. Sphingolipids as a group consist of a large number of different compound classes of which numerous individual species may vary in response to environmental stimuli to affect cellular responses. The ability to measure all sphingolipids simultaneously is, therefore, essential to an understanding of the biochemical regulation of sphingolipid metabolism and signaling molecules derived from it. In the model plant Arabidopsis thaliana, the major sphingolipid classes are glycosylinositolphosphoceramides, glucosylceramides, hydroxyceramides and ceramides. Other minor but potentially important sphingolipids are free long-chain bases and their phosphorylated derivates. By using a single solvent system with reversed-phase high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry detection we have been able to separate and measure 168 sphingolipids from a crude sample. This greatly speeds up and simplifies the analysis of plant sphingolipids and should pave the way for a better understanding of their role in plant performance.

Annexins: putative linkers in dynamic membrane-cytoskeleton interactions in plant cells

The plasma membrane, the most external cellular structure, is at the forefront between the plant cell and its environment. Hence, it is naturally adapted to function in detection of external signals, their transduction throughout the cell, and finally, in cell reactions. Membrane lipids and the cytoskeleton, once regarded as simple and static structures, have recently been recognized as significant players in signal transduction. Proteins involved in signal detection and transduction are organised in specific domains at the plasma membrane. Their aggregation allows to bring together and orient the downstream and upstream members of signalling pathways. The cortical cytoskeleton provides a structural framework for rapid signal transduction from the cell periphery into the nucleus. It leads to intracellular reorganisation and wide-scale modulation of cellular metabolism which results in accumulation of newly synthesised proteins and/or secondary metabolites which, in turn, have to be distributed to the appropriate cell compartments. And again, in plant cells, the secretory vesicles that govern polar cellular transport are delivered to their target membranes by interaction with actin microfilaments. In search for factors that could govern subsequent steps of the cell response delineated above we focused on an evolutionary conserved protein family, the annexins, that bind in a calcium-dependent manner to membrane phospholipids. Annexins were proposed to regulate dynamic changes in membrane architecture and to organise the interface between secretory vesicles and the membrane. Certain proteins from this family were also identified as actin binding, making them ideal mediators in cell membrane and cytoskeleton interactions.

Insights into the role of specific lipids in the formation and delivery of lipid microdomains to the plasma membrane of plant cells

The existence of sphingolipid- and sterol-enriched microdomains, known as lipid rafts, in the plasma membrane (PM) of eukaryotic cells is well documented. To obtain more insight into the lipid molecular species required for the formation of microdomains in plants, we have isolated detergent (Triton X-100)-resistant membranes (DRMs) from the PM of Arabidopsis (Arabidopsis thaliana) and leek (Allium porrum) seedlings as well as from Arabidopsis cell cultures. Here, we show that all DRM preparations are enriched in sterols, sterylglucosides, and glucosylceramides (GluCer) and depleted in glycerophospholipids. The GluCer of DRMs from leek seedlings contain hydroxypalmitic acid. We investigated the role of sterols in DRM formation along the secretory pathway in leek seedlings. We present evidence for the presence of DRMs in both the PM and the Golgi apparatus but not in the endoplasmic reticulum. In leek seedlings treated with fenpropimorph, a sterol biosynthesis inhibitor, the usual Delta(5)-sterols are replaced by 9beta,19-cyclopropylsterols. In these plants, sterols and hydroxypalmitic acid-containing GluCer do not reach the PM, and most DRMs are recovered from the Golgi apparatus, indicating that Delta(5)-sterols and GluCer play a crucial role in lipid microdomain formation and delivery to the PM. In addition, DRM formation in Arabidopsis cells is shown to depend on the unsaturation degree of fatty acyl chains as evidenced by the dramatic decrease in the amount of DRMs prepared from the Arabidopsis mutants, fad2 and Fad3+, affected in their fatty acid desaturases.