Weakly bound cell wall proteins of Arabidopsis thaliana were identified using a proteomic and bioinformatic approach. An efficient protocol of extraction based on vacuum-infiltration of the tissues was developed. Several salts and a chelating agent were compared for their ability to extract cell wall proteins without releasing cytoplasmic contaminants. Of the 93 proteins that were identified, a large proportion (60%) was released by calcium chloride. From bioinformatics analysis, it may be predicted that most of them (87 out of 93) had a signal peptide, whereas only six originated from the cytoplasm. Among the putative apoplastic proteins, a high proportion (67 out of 87) had a basic pI. Numerous glycoside hydrolases and proteins with interacting domains were identified, in agreement with the expected role of the extracellular matrix in polysaccharide metabolism and recognition phenomena. Ten proteinases were also found as well as six proteins with unknown functions. Comparison of the cell wall proteome of rosettes with the previously published cell wall proteome of cell suspension cultures showed a high level of cell specificity, especially for the different members of several large multigenic families.

The plant extracellular matrix contains typical polysaccharides such as cellulose, hemicelluloses, and pectins that interact to form dense interwoven networks. Plant cell walls play crucial roles during development and constitute the first barrier of defense against invading pathogens. Cell wall proteomics has greatly contributed to the description of the protein content of a compartment specific to plants. Around 400 cell wall proteins (CWPs) of Arabidopsis, representing about one fourth of its estimated cell wall proteome, have been described. The main points to note are that: (i) the diversity of enzymes acting on polysaccharides suggests a great plasticity of cell walls; (ii) CWPs such as proteases, polysaccharide hydrolytic enzymes, and lipases may contribute to the generation of signals; (iii) proteins of unknown functions were identified, suggesting new roles for cell walls. Recently, the characterization of PTMs such as N- and O-glycosylations improved our knowledge of CWP structure. The presence of many glycoside hydrolases and proteases suggests a complex regulation of CWPs involving various types of post-translational events. The first 3-D structures to be resolved gave clues about the interactions between CWPs, or between CWPs and polysaccharides. Future work should include: extracting and identifying CWPs still recalcitrant to proteomics, describing the cell wall interactome, improving quantification, and unraveling the roles of each of the CWPs.

Cell wall proteins (CWP) are essential constituents of plant cell walls involved in modifications of cell wall components, wall structure, signaling, and interactions with plasma membrane proteins at the cell surface. The application of proteomic approaches to the cell wall compartment raises important questions: Are there technical problems specific to cell wall proteomics? What kinds of proteins can be found in Arabidopsis walls? Are some of them unexpected? What sort of post-translational modifications have so far been characterized in CWP? The purpose of this review is to discuss the experimental results obtained so far using protomics, as well as some of the new questions challenging future research. Glossary AGP (arabinogalactan proteins):cell wall highly glycosylated HRGP that contain repetitive motifs such as (Ser, Thr, Ala)-Hyp-(Ser, Thr, Ala)-Hyp or (Ser, Thr, Ala)-Hyp-Hyp. AG (arabinogalactan)-peptides: AGP that have a predicted mature protein backbone of 10 to 13 amino acid residues. BBE: berberine-bridge (S)-reticulin:oxygen oxidoreductases. CWP: cell wall proteins. Extensins: cell wall structural HRGP, with numerous Ser-Hyp n (n≥3) motifs separated by Tyr-, Lys-, His-and Val-rich regions. GAP (glycosylphosphatidylinositol anchored proteins): proteins that are lipid-anchored to the external phase of the plasma membrane. GH (glycoside hydrolases): enzymes that hydrolyze the glycosidic bond between two carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. HRGP: hydroxyproline-rich glycoproteins, i.e. extensins and AGP. Lectins: structurally diverse proteins that bind to specific carbohydrates. LRR (leucine-rich repeat): short sequence motifs having diverse functions and cellular locations, usually involved in protein-protein interactions. PME (pectin methylesterases): enzymes that catalyze the de-esterification of pectin into pectate and methanol. Proteome: the complete profile of proteins expressed, at a given time and environmental conditions, in a given organ, tissue, or cell. PRP (proline-rich proteins): structural CWP rich in proline residues. Transcriptome: the complete collection of transcribed elements of the genome.

Background: Cell elongation in plants requires addition and re-arrangements of cell wall components. Even if some protein families have been shown to play roles in these events, a global picture of proteins present in cell walls of elongating cells is still missing. A proteomic study was performed on etiolated hypocotyls of Arabidopsis used as model of cells undergoing elongation followed by growth arrest within a short time.

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The complete sequencing of the Arabidopsis thaliana genome allows the use of the recently developed mass spectrometry techniques to identify the cell wall proteins (CWPs). Most proteomic approaches depend on the quality of sample preparation. Extraction of CWPs is particularly complex since the proteins may be free in the apoplast or are embedded in a polysaccharide matrix where they are retained by Van der Waals interactions, hydrogen bonds, hydrophobic or ionic interactions, or cross-linked by covalent bonds. Specific and sequential extraction procedures thus need to be developed. We report on the sequential extraction of loosely bound CWPs from living A. thaliana cells in culture. Different salts and chelating agents were used for releasing the proteins from the wall. Their effects on the extraction of CWPs and on the integrity of the plasma membrane were evaluated. Bioinformatic software was used to identify proteins and to predict their sub-cellular localization. The obtained data show that the plasma membrane of cells in culture was easily damaged by some steps of the extraction procedure, leading to the release of increasing amounts of intracellular proteins. Nevertheless, we identified fifty CWPs among which thirteen were new proteins for the cell wall. In addition, 76% of these CWPs were basic proteins not resolved in two-dimensional (2-D) gel electrophoresis. The existence of two hypothetical proteins was confirmed. The structure of three proteins could be confirmed using mass spectrometry data.

N-glycosylated proteins were isolated from Arabidopsis thaliana mature stems using affinity chromatography on Concanavalin A Sepharose, separated by 2D-electrophoresis and identified using nanoHPLC-MS/MS and MALDI-TOF MS. 102 glycoproteins were identified. 94% of these proteins were predicted by bioinformatics to be targeted to the secretory pathway and 87% of them were predicted to be localized in the cell wall or at the plasma membrane. 30% of these proteins belong to glycoside hydrolase (GH) families with some of them possibly involved in the hydrolysis of cell wall polysaccharides. The second major class of identified proteins comprises aspartyl and serine proteases. Other proteins are predicted to be oxido-reductases, contain interacting domains, are potentially involved in signalling or have an unknown function. This is, to our knowledge, the first survey of plant cell wall N-glycosylated proteins.

Extracellular vesicles (EV) are membrane particles released by cells into their environment and are considered to be key players in intercellular communication. EV are produced by all domains of life but limited knowledge about EV in plants is available, although their implication in plant defense has been suggested. We have characterized sunflower EV and tested whether they could interact with fungal cells. EV were isolated from extracellular fluids of seedlings and characterized by transmission electron microscopy and proteomic analysis. These nanovesicles appeared to be enriched in cell wall remodeling enzymes and defense proteins. Membrane-labeled EV were prepared and their uptake by the phytopathogenic fungus Sclerotinia sclerotiorum was verified. Functional tests further evaluated the ability of EV to affect fungal growth. Spores treated with plant EV showed growth inhibition, morphological changes, and cell death. Conclusive evidence on the existence of plant EV is presented and we demonstrate their ability to interact with and kill fungal cells. Our results introduce the concept of cell-to-cell communication through EV in plants.