Plasmodesmata in Arabidopsis thaliana suspension cells

Heavy Metal-Associated Isoprenylated Plant Proteins (HIPPs) at Plasmodesmata: Exploring the Link between Localization and Function

Heavy metal-associated isoprenylated plant proteins (HIPPs) are a metallochaperone-like protein family comprising a combination of structural features unique to vascular plants. HIPPs possess both one or two heavy metal-binding domains and an isoprenylation site, facilitating a posttranslational protein lipid modification. Recent work has characterized individual HIPPs across numerous different species and provided evidence for varied functionalities. Interestingly, a significant number of HIPPs have been identified in proteomes of plasmodesmata (PD)-nanochannels mediating symplastic connectivity within plant tissues that play pivotal roles in intercellular communication during plant development as well as responses to biotic and abiotic stress. As characterized functions of many HIPPs are linked to stress responses, plasmodesmal HIPP proteins are potentially interesting candidate components of signaling events at or for the regulation of PD. Here, we review what is known about PD-localized HIPP proteins specifically, and how the structure and function of HIPPs more generally could link to known properties and regulation of PD.

Enzymatic fingerprinting reveals specific xyloglucan and pectin signatures in the cell wall purified with primary plasmodesmata

Plasmodesmata (PD) pores connect neighbouring plant cells and enable direct transport across the cell wall. Understanding the molecular composition of these structures is essential to address their formation and later dynamic regulation. Here we provide a biochemical characterisation of the cell wall co-purified with primary PD of Arabidopsis thaliana cell cultures. To achieve this result we combined subcellular fractionation, polysaccharide analyses and enzymatic fingerprinting approaches. Relative to the rest of the cell wall, specific patterns were observed in the PD fraction. Most xyloglucans, although possibly not abundant as a group, were fucosylated. Homogalacturonans displayed short methylated stretches while rhamnogalacturonan I species were remarkably abundant. Full rhamnogalacturonan II forms, highly methyl-acetylated, were also present. We additionally showed that these domains, compared to the broad wall, are less affected by wall modifying activities during a time interval of days. Overall, the protocol and the data presented here open new opportunities for the study of wall polysaccharides associated with PD.

Isolation of Plasmodesmata Membranes for Lipidomic and Proteomic Analysis

Plasmodesmata (PD) are membranous intercellular nanochannels crossing the plant cell wall to connect adjacent cells in plants. Our understanding of PD function heavily relies on the identification of their molecular components, these being proteins or lipids. In that regard, proteomic and lipidomic analyses of purified PD represent a crucial strategy in the field. Here we describe a simple two-step purification procedure that allows isolation of pure PD-derived membranes from Arabidopsis suspension cells suitable for "omic" approaches. The first step of this procedure consists on isolating pure cell walls containing intact PD, followed by a second step which involves an enzymatic degradation of the wall matrix to release PD membranes. The PD-enriched fraction can then serve to identify the lipid and protein composition of PD using lipidomic and proteomic approaches, which we also describe in this method article.

Qualitative and quantitative analyses of the plasmodesmata that accompany cell fate changes during the somatic embryogenesis of Arabidopsis thaliana

Plasmodesmata (PD) are cytoplasmic and membrane-lined microchannels that enable symplasmic communication in plants, which is involved in the regulation of cell differentiation. The presented results emphasise the qualitative and quantitative analyses of PD, which are the basis of the symplasmic communication. The cells that initiate various development programmes create symplasmic domains that are characterised by different degrees of symplasmic communication. Changes in symplasmic communication are caused by the presence or absence of PD and/or the ability of signals to move through them. In the presented studies, somatic embryogenesis was used to describe the characteristics of the PD within and between the symplasmic domains in explants of the Arabidopsis thaliana (L.) Heynh ecotype Columbia-0 and 35S:BBM transgenic line. Transmission electron microscopy was used to describe the cells that regain totipotency/pluripotency during somatic embryogenesis, as well as the number and shape of the PD in the different symplasmic domains of the explants and somatic embryos. Array tomography was used to create a 3D reconstruction of the protodermal cells of the somatic embryos with particular emphasis on the PD distribution in the cell walls. The results showed that there were different frequencies of the PD within and between the symplasmic domain that emerges during somatic embryogenesis and between the Col-0 and 35S:BBM somatic embryos with regard to the differences in the shape of the PD.

Purification of Plasmodesmata-Enriched Fraction for Proteomic Analyses

In plants, plasmodesmata (PD) are plasmamembrane-lined pores that traverse the cell wall to establish cytoplasmic and endomembrane continuity between neighboring cells. As intercellular channels, PD play pivotal roles in plant growth and development, defense responses, and are also co-opted by viruses to spread cell-to-cell to establish systemic infection. Proteomic analyses of PD-enriched fractions may provide critical insights on plasmodesmal biology and PD-mediated virus-host interactions. However, it is difficult to isolate PD from plant tissues as they are firmly embedded in the cell wall. Here, we describe a protocol for the purification of PD from Nicotiana benthamiana leaves for proteomic analysis.

Nanoparticles for protein delivery in planta

Delivery of proteins into walled plant cells remains a challenge with few tractable solutions. Recent advances in biomacromolecule delivery using nanotechnology may evince methods to be exploited for protein delivery. While protein delivery remains no small feat, even in mammalian systems, the ability for nanoparticles to penetrate the cell wall and be decorated with a plethora of functional moieties makes them ideal protein vehicles in plants. As advances in protein biotechnology accelerate, so does the need for commensurate delivery systems. However, the road to nanoparticle-mediated protein delivery is fraught with challenges in regard to cell wall penetration, intracellular delivery, endosomal escape, and nanoparticle chemistry and design. The dearth of literature surrounding protein delivery in walled plant cells hints at the challenge of this problem but also indicates vast opportunity for innovations in plant-tailored nanotechnology.

Extracellular Matrix Proteome: Isolation of ECM Proteins for Proteomics Studies

Understanding molecular mechanisms and cellular metabolism in varied plant processes necessitates knowledge of the expressed proteins and their subcellular distribution. Spatial partitioning of organelles generates an enclosed milieu for physiochemical reactions designed and tightly linked to a specific organelle function. Of which, extracellular matrix (ECM)/cell wall (CW) is a dynamic and chemically active compartment. The ECM proteins are organized into complex structural and functional networks involved in several metabolic processes, including carbon and nitrogen metabolism. Organellar proteomics aim for comprehensive identification of resident proteins that rely on the isolation of highly purified organelle free from contamination by other intracellular components. Extraction and isolation of plant ECM proteins features key caveats due to the lack of adjoining membrane, the presence of a polysaccharide-protein network that traps contaminants, and the existence of high phenolic content. Furthermore, due to diverse biochemical forces, including labile, weakly bound and strongly bound protein in the protein-polysaccharide matrix different elution procedures are required to enrich ECM proteins. Here, we describe a method that allows efficient fractionation of plant ECM, extraction of ECM proteins and protein profiling from variety of crop plants, including rice, chickpea and potato. This method can easily be adapted to other plant species for varied experimental conditions.

Multiple C2 domains and transmembrane region proteins (MCTPs) tether membranes at plasmodesmata

In eukaryotes, membrane contact sites (MCS) allow direct communication between organelles. Plants have evolved a unique type of MCS, inside intercellular pores, the plasmodesmata, where endoplasmic reticulum (ER)-plasma membrane (PM) contacts coincide with regulation of cell-to-cell signalling. The molecular mechanism and function of membrane tethering within plasmodesmata remain unknown. Here, we show that the multiple C2 domains and transmembrane region protein (MCTP) family, key regulators of cell-to-cell signalling in plants, act as ER-PM tethers specifically at plasmodesmata. We report that MCTPs are plasmodesmata proteins that insert into the ER via their transmembrane region while their C2 domains dock to the PM through interaction with anionic phospholipids. A Atmctp3/Atmctp4 loss of function mutant induces plant developmental defects, impaired plasmodesmata function and composition, while MCTP4 expression in a yeast Δtether mutant partially restores ER-PM tethering. Our data suggest that MCTPs are unique membrane tethers controlling both ER-PM contacts and cell-to-cell signalling.

Localization of calreticulin and calcium ions in mycorrhizal roots of Medicago truncatula in response to aluminum stress

Aluminum (Al) toxicity limits growth and symbiotic interactions of plants. Calcium plays essential roles in abiotic stresses and legume-Rhizobium symbiosis, but the sites and mechanism of Ca²⁺ mobilization during mycorrhizae have not been analyzed. In this study, the changes of cytoplasmic Ca²⁺ and calreticulin (CRT) in Medicago truncatula mycorrhizal (MR) and non-mycorrizal (NM) roots under short Al stress [50 μM AlCl₃ pH 4.3 for 3 h] were analyzed. Free Ca²⁺ ions were detected cytochemically by their reaction with potassium pyroantimonate and anti-CRT antibody was used to locate this protein in Medicago roots by immunocytochemical methods. In MR and NM roots, Al induced accumulation of CRT and free Ca²⁺. Similar calcium and CRT distribution in the MR were found at the surface of fungal structures (arbuscules and intercellular hyphae), cell wall and in plasmodesmata, and in plant and fungal intracellular compartments. Additionally, degenerated arbuscules were associated with intense Ca²⁺ and CRT accumulation. In NM roots, Ca²⁺ and CRT epitopes were observed in the stele, near wall of cortex and endodermis. The present study provides new insight into Ca²⁺ storage and mobilization in mycorrhizae symbiosis. The colocalization of CRT and Ca²⁺ suggests that CRT is essential for calcium mobilization for normal mycorrhiza development and response to Al stress.

Calreticulin localizes to plant intra/extracellular peripheries of highly specialized cells involved in pollen-pistil interactions

Calcium (Ca²⁺) plays essential roles in generative reproduction of angiosperms, but the sites and mechanisms of Ca²⁺ storage and mobilization during pollen-pistil interactions have not been fully defined. Both external and internal Ca²⁺ stores are likely important during male gametophyte communication with the sporophytic and gametophytic cells within the pistil. Given that calreticulin (CRT), a Ca²⁺-buffering protein, is able to bind Ca²⁺ reversibly, it can serve as a mobile store of easily releasable Ca²⁺ (so called an exchangeable Ca²⁺) in eukaryotic cells. CRT has typical endoplasmic reticulum (ER) targeting and retention signals and resides primarily in the ER. However, localization of this protein outside the ER has also been revealed in both animal and plant cells, including Golgi/dictyosomes, nucleus, plasma membrane/cell surface, plasmodesmata, and even extracellular matrix. These findings indicate that CRT may function in a variety of different cell compartments and specialized structures. We have recently shown that CRT is highly expressed and accumulated in the ER of plant cells involved in pollen-pistil interactions in Petunia, and we proposed an essential role for CRT in intracellular Ca²⁺ storage and mobilization during the key reproductive events. Here, we demonstrate that both CRT and exchangeable Ca²⁺ are localized in the intra/extracellular peripheries of highly specialized plant cells, such as the pistil transmitting tract cells, pollen tubes, nucellus cells surrounding the embryo sac, and synergids. Based on our present results, we propose that extracellularly located CRT is also involved in Ca²⁺ storage and mobilization during sexual reproduction of angiosperms.

Proteomic Analysis of Plasmodesmata From Populus Cell Suspension Cultures in Relation With Callose Biosynthesis

Plasmodesmata are channels that link adjacent cells in plant tissues through which molecular exchanges take place. They are involved in multiple processes vital to plant cells, such as responses to hormonal signaling or environmental challenges including osmotic stress, wounding and pathogen attack. Despite the importance of plasmodesmata, their proteome is not well-defined. Here, we have isolated fractions enriched in plasmodesmata from cell suspension cultures of Populus trichocarpa and identified 201 proteins that are enriched in these fractions, thereby providing further insight on the multiple functions of plasmodesmata. Proteomics analysis revealed an enrichment of proteins specifically involved in responses to stress, transport, metabolism and signal transduction. Consistent with the role of callose deposition and turnover in the closure and aperture of the plasmodesmata and our proteomic analysis, we demonstrate the enrichment of callose synthase activity in the plasmodesmata represented by several gene products. A new form of calcium-independent callose synthase activity was detected, in addition to the typical calcium-dependent enzyme activity, suggesting a role of calcium in the regulation of plasmodesmata through two forms of callose synthase activities. Our report provides the first proteomic investigation of the plasmodesmata from a tree species and the direct biochemical evidence for the occurrence of several forms of active callose synthases in these structures. Data are available via ProteomeXchange with identifier PXD010692.

Comparative proteomic profiling of the choline transporter-like1 (CHER1) mutant provides insights into plasmodesmata composition of fully developed Arabidopsis thaliana leaves

In plants, intercellular communication and exchange are highly dependent on cell wall bridging structures between adhering cells, so-called plasmodesmata (PD). In our previous genetic screen for PD-deficient Arabidopsis mutants, we described choline transporter-like 1 (CHER1) being important for PD genesis and maturation. Leaves of cher1 mutant plants have up to 10 times less PD, which do not develop to complex structures. Here we utilize the T-DNA insertion mutant cher1-4 and report a deep comparative proteomic workflow for the identification of cell-wall-embedded PD-associated proteins. Analyzing triplicates of cell-wall-enriched fractions in depth by fractionation and quantitative high-resolution mass spectrometry, we compared > 5000 proteins obtained from fully developed leaves. Comparative data analysis and subsequent filtering generated a list of 61 proteins being significantly more abundant in Col-0. This list was enriched for previously described PD-associated proteins. To validate PD association of so far uncharacterized proteins, subcellular localization analyses were carried out by confocal laser-scanning microscopy. This study confirmed the association of PD for three out of four selected candidates, indicating that the comparative approach indeed allowed identification of so far undescribed PD-associated proteins. Performing comparative cell wall proteomics of Nicotiana benthamiana tissue, we observed an increase in abundance of these three selected candidates during sink to source transition. Taken together, our comparative proteomic approach revealed a valuable data set of potential PD-associated proteins, which can be used as a resource to unravel the molecular composition of complex PD and to investigate their function in cell-to-cell communication.

Architecture and permeability of post-cytokinesis plasmodesmata lacking cytoplasmic sleeves

Plasmodesmata are remarkable cellular machines responsible for the controlled exchange of proteins, small RNAs and signalling molecules between cells. They are lined by the plasma membrane (PM), contain a strand of tubular endoplasmic reticulum (ER), and the space between these two membranes is thought to control plasmodesmata permeability. Here, we have reconstructed plasmodesmata three-dimensional (3D) ultrastructure with an unprecedented level of 3D information using electron tomography. We show that within plasmodesmata, ER-PM contact sites undergo substantial remodelling events during cell differentiation. Instead of being open pores, post-cytokinesis plasmodesmata present such intimate ER-PM contact along the entire length of the pores that no intermembrane gap is visible. Later on, during cell expansion, the plasmodesmata pore widens and the two membranes separate, leaving a cytosolic sleeve spanned by tethers whose presence correlates with the appearance of the intermembrane gap. Surprisingly, the post-cytokinesis plasmodesmata allow diffusion of macromolecules despite the apparent lack of an open cytoplasmic sleeve, forcing the reassessment of the mechanisms that control plant cell-cell communication.

The Plant Cellular Systems for Plant Virus Movement

Plasmodesmata (PDs) are specialized intercellular channels that facilitate the exchange of various molecules, including sugars, ribonucleoprotein complexes, transcription factors, and mRNA. Their diameters, estimated to be 2.5 nm in the neck region, are too small to transfer viruses or viral genomes. Tobacco mosaic virus and Potexviruses are the most extensively studied viruses. In viruses, the movement protein (MP) is responsible for the PD gating that allows the intercellular movement of viral genomes. Various host factors interact with MP to regulate complicated mechanisms related to PD gating. Virus replication and assembly occur in viral replication complex (VRC) with membrane association, especially in the endoplasmic reticulum. VRC have a highly organized structure and are highly regulated by interactions among the various host factors, proteins encoded by the viral genome, and the viral genome. Virus trafficking requires host machineries, such as the cytoskeleton and the secretory systems. MP facilitates the virus replication and movement process. Despite the current level of understanding of virus movement, there are still many unknown and complex interactions between virus replication and virus movement. While numerous studies have been conducted to understand plant viruses with regards to cell-to-cell movement and replication, there are still many knowledge gaps. To study these interactions, adequate research tools must be used such as molecular, and biochemical techniques. Without such tools, virologists will not be able to gain an accurate or detailed understanding of the virus infection process.

Photoperiod- and temperature-mediated control of phenology in trees - a molecular perspective

Contents 511 I. 511 II. 512 III. 513 IV. 513 V. 517 VI. 517 VII. 521 VIII. 521 Acknowledgements 521 References 521 SUMMARY: Trees growing in boreal and temperate regions synchronize their growth with seasonal climatic changes in adaptive responses that are essential for their survival. These trees cease growth before the winter and establish a dormant state during which growth cessation is maintained by repression of responses to growth-promotive signals. Reactivation of growth in the spring follows the release from dormancy promoted by prolonged exposure to low temperature during the winter. The timing of the key events and regulation of the molecular programs associated with the key stages of the annual growth cycle are controlled by two main environmental cues: photoperiod and temperature. Recently, key components mediating photoperiodic control of growth cessation and bud set have been identified, and striking similarities have been observed in signaling pathways controlling growth cessation in trees and floral transition in Arabidopsis. Although less well understood, the regulation of bud dormancy and bud burst may involve cell-cell communication and chromatin remodeling. Here, we discuss current knowledge of the molecular-level regulation of the annual growth cycle of woody trees in temperate and boreal regions, and identify key questions that need to be addressed in the future.

Isolation of Plasmodesmata

Plasmodesmata (PD) are plasma membrane lined pores that cross the plant cell wall and connect adjacent cells. Plasmodesmata are composed of elements of the endoplasmic reticulum, plasma membrane, cytosol, and cell wall and thus, as multicomposite structures that are embedded in the cell wall, they are notoriously difficult to isolate from whole plant tissue. However, understanding PD structure, function, and regulation necessitates identification of their molecular components and therefore proteomic and lipidomic analyses of PD fractions are an essential strategy for plasmodesmal biology. Here we outline a simple two-step purification procedure that allows isolation of PD-derived membranes from Arabidopsis suspension cells. The method involves isolation of purified cell wall fragments containing intact PD which is followed by enzymatic degradation of the cell wall to release the PD. This membrane-rich fraction can be subjected to protein and lipid extraction for molecular characterization of PD components. The first step of this procedure involves the isolation of cell wall fragments containing intact PD, free from contamination from other cellular compartments. Purified PD membranes are then released from the cell wall matrix by enzymatic degradation. Isolated PD membranes provide a suitable starting material for the analysis of PD-associated proteins and lipids.

Late progamic phase and fertilization affect calreticulin expression in the Hyacinthus orientalis female gametophyte

Calreticulin expression is upregulated during sexual reproduction of Hyacinthus orientalis, and the protein is localized both in the cytoplasm and a highly specialized cell wall within the female gametophyte. Several evidences indicate calreticulin (CRT) as an important calcium (Ca(2+))-binding protein that is involved in the generative reproduction of higher plants, including both pre-fertilization and post-fertilization events. Because CRT is able to bind and sequester exchangeable Ca(2+), it can serve as a mobile intracellular store of easily releasable Ca(2+) and control its local cytosolic concentrations in the embryo sac. This phenomenon seems to be essential during the late progamic phase, gamete fusion, and early embryogenesis. In this report, we demonstrate the differential expression of CRT within Hyacinthus female gametophyte cells before and during anthesis, during the late progamic phase when the pollen tube enters the embryo sac, and at the moment of fertilization and zygote/early endosperm activation. CRT mRNA and the protein localize mainly to the endoplasmic reticulum (ER) and Golgi compartments of the cells, which are involved in sexual reproduction events, such as those in sister synergids, the egg cell, the central cell, zygote and the developing endosperm. Additionally, immunogold research demonstrates selective CRT distribution in the filiform apparatus (FA), a highly specific component of the synergid cell wall. In the light of our previous data showing the total transcriptional activity of the Hyacinthus female gametophyte and the results presented here, we discuss the possible functions of CRT with respect to the critical role of Ca(2+) homeostasis during key events of sexual plant reproduction. Moreover, we propose that the elevated expression of CRT within the female gametophyte is a universal phenomenon in the cells involved in double fertilization in higher plants.

The Qb-SNARE Memb11 interacts specifically with Arf1 in the Golgi apparatus of Arabidopsis thaliana

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are critical for the function of the secretory pathway. The SNARE Memb11 is involved in membrane trafficking at the ER-Golgi interface. The aim of the work was to decipher molecular mechanisms acting in Memb11-mediated ER-Golgi traffic. In mammalian cells, the orthologue of Memb11 (membrin) is potentially involved in the recruitment of the GTPase Arf1 at the Golgi membrane. However molecular mechanisms associated to Memb11 remain unknown in plants. Memb11 was detected mainly at the cis-Golgi and co-immunoprecipitated with Arf1, suggesting that Arf1 may interact with Memb11. This interaction of Memb11 with Arf1 at the Golgi was confirmed by in vivo BiFC (Bimolecular Fluorescence Complementation) experiments. This interaction was found to be specific to Memb11 as compared to either Memb12 or Sec22. Using a structural bioinformatic approach, several sequences in the N-ter part of Memb11 were hypothesized to be critical for this interaction and were tested by BiFC on corresponding mutants. Finally, by using both in vitro and in vivo approaches, we determined that only the GDP-bound form of Arf1 interacts with Memb11. Together, our results indicate that Memb11 interacts with the GDP-bound form of Arf1 in the Golgi apparatus.

Collagenase as a useful tool for the analysis of plant cellular peripheries

A technique for the selective loosening of the cell wall structure and the isolation of proteins permanently knotted in the cell walls was elaborated. Following treatment with collagenase, some proteins, such as calreticulin (CRT) and auxin binding protein 1 (ABP1) were released from purified cell walls, most probably through destruction of respective interacting proteins. The results were confirmed by the immunolocalization of the ABP1 and CRT with confocal and electron microscopy. On the other hand, potential substrates of collagenase, among them annexin 1 have been recognized. Mass spectra of annexin 1 obtained after collagenase digestion and results from analysis of potential cleavage sites suggested that the mechanism of enzyme cleavage might not depend on the amino acid sequence. Summarizing, collagenase was found to be a very useful tool for exploring molecules involved in the functioning of cellular peripheries.

Specific membrane lipid composition is important for plasmodesmata function in Arabidopsis

Plasmodesmata (PD) are nano-sized membrane-lined channels controlling intercellular communication in plants. Although progress has been made in identifying PD proteins, the role played by major membrane constituents, such as the lipids, in defining specialized membrane domains in PD remains unknown. Through a rigorous isolation of "native" PD membrane fractions and comparative mass spectrometry-based analysis, we demonstrate that lipids are laterally segregated along the plasma membrane (PM) at the PD cell-to-cell junction in Arabidopsis thaliana. Remarkably, our results show that PD membranes display enrichment in sterols and sphingolipids with very long chain saturated fatty acids when compared with the bulk of the PM. Intriguingly, this lipid profile is reminiscent of detergent-insoluble membrane microdomains, although our approach is valuably detergent-free. Modulation of the overall sterol composition of young dividing cells reversibly impaired the PD localization of the glycosylphosphatidylinositol-anchored proteins Plasmodesmata Callose Binding 1 and the β-1,3-glucanase PdBG2 and altered callose-mediated PD permeability. Altogether, this study not only provides a comprehensive analysis of the lipid constituents of PD but also identifies a role for sterols in modulating cell-to-cell connectivity, possibly by establishing and maintaining the positional specificity of callose-modifying glycosylphosphatidylinositol proteins at PD. Our work emphasizes the importance of lipids in defining PD membranes.

Isolation of plasmodesmata from Arabidopsis suspension culture cells

Due to their position firmly anchored within the plant cell wall, plasmodesmata (PD) are notoriously difficult to isolate from plant tissue. Yet, getting access to isolated PD represents the most straightforward strategy for the identification of their molecular components. Proteomic and lipidomic analyses of such PD fractions have provided and will continue to provide critical information on the functional and structural elements that define these membranous nano-pores. Here, we describe a two-step simple purification procedure that allows isolation of pure PD-derived membranes from Arabidopsis suspension cells. The first step of this procedure consists in isolating cell wall fragments containing intact PD while free of contamination from other cellular compartments. The second step relies on an enzymatic degradation of the wall matrix and the subsequent release of "free" PD. Isolated PD membranes provide a suitable starting material for the analysis of PD-associated proteins and lipids.

Photoassimilation, assimilate translocation and plasmodesmal biogenesis in the source leaves of Arabidopsis thaliana grown under an increased atmospheric CO2 concentration

Using 18-day-old Arabidopsis thaliana seedlings grown under increased (780 p.p.m., experimental plants) or ambient (390 p.p.m., control plants) CO2 conditions, we evaluated (14)CO2 photoassimilation in and translocation from representative source leaves. The total (14)CO2 photoassimilation amounts increased in the third leaves of the experimental plants in comparison with that found for the third leaves of the control plants, but the rates were comparable for the first leaves of the two groups. In contrast, translocation of labeled assimilates doubled in the first leaves of the experimental group, whereas translocation was, at best, passively enhanced even though photoassimilation increased in their third leaves. The transcript levels of the companion cell-specific sucrose:H(+) symporter gene SUC2 were not significantly affected in the two groups of plants, whereas those of the sucrose effluxer gene SWEET12 and the sieve element-targeted sucrose:H(+) symporter gene SUT4 were up-regulated in the experimental plants, suggesting up-regulation of SUT4-dependent apoplastic phloem loading. Compared with SUC2, SUT4 is a minor component that is expressed in companion cells but functions in sieve elements after transfer through plasmodesmata. The number of aniline blue-stained spots for plasmodesma-associated callose in the midrib wall increased in the first leaf of the experimental plants but was comparable in the third leaf between the experimental and control plants. These results suggest that A. thaliana responds to greater than normal concentrations of CO2 differentially in the first and third leaves in regards to photoassimilation, assimilate translocation and plasmodesmal biogenesis.

Plasmodesmata without callose and calreticulin in higher plants - open channels for fast symplastic transport?

Plasmodesmata (PD) represent membrane-lined channels that link adjacent plant cells across the cell wall. PD of higher plants contain a central tube of endoplasmic reticulum (ER) called desmotubule. Membrane and lumen proteins seem to be able to move through the desmotubule, but most transport processes through PD occur through the cytoplasmic annulus (Brunkard etal., 2013). Calreticulin (CRT), a highly conserved Ca(2+)-binding protein found in all multicellular eukaryotes, predominantly located in the ER, was shown to localize to PD, though not all PD accumulate CRT. In nitrogen-fixing actinorhizal root nodules of the Australian tree Casuarina glauca, the primary walls of infected cells containing the microsymbiont become lignified upon infection. TEM analysis of these nodules showed that during the differentiation of infected cells, PD connecting infected cells, and connecting infected and adjacent uninfected cells, were reduced in number as well as diameter (Schubert etal., 2013). In contrast with PD connecting young infected cells, and most PD connecting mature infected and adjacent uninfected cells, PD connecting mature infected cells did not accumulate CRT. Furthermore, as shown here, these PD were not associated with callose, and based on their diameter, they probably had lost their desmotubules. We speculate that either this is a slow path to PD degradation, or that the loss of callose accumulation and presumably also desmotubules leads to the PD becoming open channels and improves metabolite exchange between cells.

Quantitative proteomics reveals that plasma membrane microdomains from poplar cell suspension cultures are enriched in markers of signal transduction, molecular transport, and callose biosynthesis

The plasma membrane (PM) is a highly dynamic interface that contains detergent-resistant microdomains (DRMs). The aim of this work was to determine the main functions of such microdomains in poplar through a proteomic analysis using gel-based and solution (iTRAQ) approaches. A total of 80 proteins from a limited number of functional classes were found to be significantly enriched in DRM relative to PM. The enriched proteins are markers of signal transduction, molecular transport at the PM, or cell wall biosynthesis. Their intrinsic properties are presented and discussed together with the biological significance of their enrichment in DRM. Of particular importance is the significant and specific enrichment of several callose [(1 → 3)-β-glucan] synthase isoforms, whose catalytic activity represents a final response to stress, leading to the deposition of callose plugs at the surface of the PM. An integrated functional model that connects all DRM-enriched proteins identified is proposed. This report is the only quantitative analysis available to date of the protein composition of membrane microdomains from a tree species.

Dissecting plasmodesmata molecular composition by mass spectrometry-based proteomics

In plants, the intercellular communication through the membranous channels called plasmodesmata (PD; singular plasmodesma) plays pivotal roles in the orchestration of development, defence responses, and viral propagation. PD are dynamic structures embedded in the plant cell wall that are defined by specialized domains of the endoplasmic reticulum (ER) and the plasma membrane (PM). PD structure and unique functions are guaranteed by their particular molecular composition. Yet, up to recent years and despite numerous approaches such as mutant screens, immunolocalization, or screening of random cDNAs, only few PD proteins had been conclusively identified and characterized. A clear breakthrough in the search of PD constituents came from mass-spectrometry-based proteomic approaches coupled with subcellular fractionation strategies. Due to their position, firmly anchored in the extracellular matrix, PD are notoriously difficult to isolate for biochemical analysis. Proteomic-based approaches have therefore first relied on the use of cell wall fractions containing embedded PD then on "free" PD fractions whereby PD membranes were released from the walls by enzymatic degradation. To discriminate between likely contaminants and PD protein candidates, bioinformatics tools have often been used in combination with proteomic approaches. GFP fusion proteins of selected candidates have confirmed the PD association of several protein families. Here we review the accomplishments and limitations of the proteomic-based strategies to unravel the functional and structural complexity of PD. We also discuss the role of the identified PD-associated proteins.

Fast screening of highly glycosylated plant sphingolipids by tandem mass spectrometry

The structural characterization of Glycosyl-Inositol-Phospho-Ceramides (GIPCs), which are the main sphingolipids of plant tissues, is a critical step towards the understanding of their physiological function. After optimization of their extraction, numerous plant GIPCs have been characterized by mass spectrometry. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) full scan analysis of negative ions provides a quick overview of GIPC distribution. Clear differences were observed for the two plant models studied: six GIPC series bearing from two to seven saccharide units were detected in tobacco BY-2 cell extracts, whereas GIPCs extracted from A. thaliana cell cultures and leaves were less diverse, with a dominance of species containing only two saccharide units. The number of GIPC species was around 50 in A. thaliana and 120 in tobacco BY-2 cells. MALDI-MS/MS spectra gave access to detailed structural information relative to the ceramide moiety, the polar head, as well as the number and types of saccharide units. Once released from GIPCs, fatty acid chains and long-chain bases were analyzed by GC/MS to verify that all GIPC series were taken into account by the MALDI-MS/MS approach. ESI-MS/MS provided complementary information for the identification of isobaric species and fatty acid chains. Such a methodology, mostly relying on MALDI-MS/MS, should open new avenues to determine structure-function relationships between glycosphingolipids and membrane organization.

Arabidopsis plasmodesmal proteome

The multicellular nature of plants requires that cells should communicate in order to coordinate essential functions. This is achieved in part by molecular flux through pores in the cell wall, called plasmodesmata. We describe the proteomic analysis of plasmodesmata purified from the walls of Arabidopsis suspension cells. Isolated plasmodesmata were seen as membrane-rich structures largely devoid of immunoreactive markers for the plasma membrane, endoplasmic reticulum and cytoplasmic components. Using nano-liquid chromatography and an Orbitrap ion-trap tandem mass spectrometer, 1341 proteins were identified. We refer to this list as the plasmodesmata- or PD-proteome. Relative to other cell wall proteomes, the PD-proteome is depleted in wall proteins and enriched for membrane proteins, but still has a significant number (35%) of putative cytoplasmic contaminants, probably reflecting the sensitivity of the proteomic detection system. To validate the PD-proteome we searched for known plasmodesmal proteins and used molecular and cell biological techniques to identify novel putative plasmodesmal proteins from a small subset of candidates. The PD-proteome contained known plasmodesmal proteins and some inferred plasmodesmal proteins, based upon sequence or functional homology with examples identified in different plant systems. Many of these had a membrane association reflecting the membranous nature of isolated structures. Exploiting this connection we analysed a sample of the abundant receptor-like class of membrane proteins and a small random selection of other membrane proteins for their ability to target plasmodesmata as fluorescently-tagged fusion proteins. From 15 candidates we identified three receptor-like kinases, a tetraspanin and a protein of unknown function as novel potential plasmodesmal proteins. Together with published work, these data suggest that the membranous elements in plasmodesmata may be rich in receptor-like functions, and they validate the content of the PD-proteome as a valuable resource for the further uncovering of the structure and function of plasmodesmata as key components in cell-to-cell communication in plants.

Opportunities and successes in the search for plasmodesmal proteins

The proteinaceous composition of plasmodesmata (PDs) is a puzzle for which pieces have proven particularly difficult to find. This review describes the numerous approaches that have been undertaken in the search for PD-associated proteins and what each has contributed to our understanding of PD structure and function. These approaches include immunolocalisation of known proteins, proteomic characterisation of PD-enriched tissue fractions, high-throughput screens of random cDNAs and mutant screens. In addition to components of the cytoskeleton, novel proteins with predicted or unknown functions have been identified. Many of these have properties that relate to the symplastic and/or apoplastic faces of the plasma membrane. Mutant screens have identified proteins involved in previously unconnected cell pathways such as ROS signalling, implicating ROS in PD formation and regulation. Proteins associated with callose synthesis and degradation have also been identified and characterised, providing considerable weight to the hypothesis that callose deposition around the neck of the PD pore is one mechanism by which the PD aperture is regulated. The techniques described in this review have been developed such that it is to be expected that a considerable number of new PD proteins will be identified in coming years to fill in further detail of the structure and functional mechanisms of these dynamic pores.

Lights at the end of the tunnel: new views of plasmodesmal structure and function

Plasmodesmata (PDs), tiny channels connecting neighboring plant cells, play big roles in the transport of metabolites, viral movement, cell fate specification and development. Many recent studies are opening our eyes to the composition and formation of PDs, as well as the function and regulation of trafficking through them. Both proteomic and genetic approaches have revealed the central importance of callose in modulating PD connectivity. Moreover, many new developmental regulators, including transcription factors as well as small RNAs (sRNAs), have been found to be mobile and essential for specifying cell fate and tissue patterning.

Calreticulin expression and localization in plant cells during pollen-pistil interactions

In this report, the distributions of calreticulin (CRT) and its transcripts in Haemanthus pollen, pollen tubes, and somatic cells of the hollow pistil were studied. Immunoblot analysis of protein extracts from mature anthers, dry and germinated pollen, growing pollen tubes, and unpollinated/pollinated pistils revealed a strong expression of CRT. Both in vitro and in situ studies confirmed the presence of CRT mRNA and protein in pollen/pollen tubes and somatic cells of the pistil transmitting tract. The co-localization of these molecules in ER of these cells suggests that the rough ER is a site of CRT translation. In the pistil, accumulation of the protein in pollen tubes, transmitting tract epidermis (tte), and micropylar cells of the ovule (mc) was correlated with the increased level of exchangeable calcium. Therefore, CRT as a Ca(2+)-binding/buffering protein, may be involved in mechanism of regulation calcium homeostasis in these cells. The functional role of the protein in pollen-pistil interactions, apart from its postulated function in cellular Ca(2+) homeostasis, is discussed.

Regulation of callose synthase activity in situ in alamethicin-permeabilized Arabidopsis and tobacco suspension cells

BACKGROUND

The cell wall component callose is mainly synthesized at certain developmental stages and after wounding or pathogen attack. Callose synthases are membrane-bound enzymes that have been relatively well characterized in vitro using isolated membrane fractions or purified enzyme. However, little is known about their functional properties in situ, under conditions when the cell wall is intact. To allow in situ investigations of the regulation of callose synthesis, cell suspensions of Arabidopsis thaliana (Col-0), and tobacco (BY-2), were permeabilized with the channel-forming peptide alamethicin.

RESULTS

Nucleic acid-binding dyes and marker enzymes demonstrated alamethicin permeabilization of plasma membrane, mitochondria and plastids, also allowing callose synthase measurements. In the presence of alamethicin, Ca2+ addition was required for callose synthase activity, and the activity was further stimulated by Mg2+ Cells pretreated with oryzalin to destabilize the microtubules prior to alamethicin permeabilization showed significantly lower callose synthase activity as compared to non-treated cells. As judged by aniline blue staining, the callose formed was deposited both at the cell walls joining adjacent cells and at discrete punctate locations earlier described as half plasmodesmata on the outer walls. This pattern was unaffected by oryzalin pretreatment, showing a quantitative rather than a qualitative effect of polymerized tubulin on callose synthase activity. No callose was deposited unless alamethicin, Ca2+ and UDP-glucose were present. Tubulin and callose synthase were furthermore part of the same plasma membrane protein complex, as judged by two-dimensional blue native SDS-PAGE.

CONCLUSION

Alamethicin permeabilization allowed determination of callose synthase regulation and tubulin interaction in the natural crowded cellular environment and under conditions where contacts between the cell wall, the plasma membrane and cytoskeletal macromolecules remained. The results also suggest that alamethicin permeabilization induces a defense response mimicking the natural physical separation of cells (for example when intercellulars are formed), during which plasmodesmata are transiently left open.

Control of Arabidopsis meristem development by thioredoxin-dependent regulation of intercellular transport

Cell-to-cell transport in plants occurs through cytoplasmic channels called "plasmodesmata" and is regulated by developmental and environmental factors. Callose deposition modulates plasmodesmal transport in vivo, but little is known about the mechanisms that regulate this process. Here we report a genetic approach to identify mutants affecting plasmodesmal transport. We isolated 5 mutants, named gfp arrested trafficking (gat), affected in GFP unloading from the phloem into the meristem. gat1 mutants were seedling lethal and carried lesions in an m-type thioredoxin that is expressed in non-green plastids of meristems and organ primordia. Callose and hydrogen peroxide accumulated in gat1 mutants, and WT plants subjected to oxidative conditions phenocopied the gat1 trafficking defects. Ectopic expression of GAT1 in mature leaves increased plasmodesmal permeability and led to a delay in senescence and flowering time. We propose a role for the GAT1 thioredoxin in the redox regulation of callose deposition and symplastic permeability that is essential for meristem maintenance in Arabidopsis.

Inhibitors of protein glycosylation or secretion change the pattern of extracellular proteins in suspension-cultured cells of Arabidopsis thaliana

Cell walls are essential for plant development and morphogenesis. The majority of wall proteins are glycosylated, either as N- or O-glycans. Various inhibitors of glycosylation and secretion are used to determine the importance of wall proteins for the functioning of the walls. Tunicamycin is an inhibitor of the first enzyme in the N-glycosylation pathway, 3,4-dehydroproline inhibits peptidyl proline hydroxylation, and Brefeldin A is an inhibitor of vesicle trafficking, disrupting the delivery of wall polymers to the apoplast. In inhibitor-treated suspension-cultured Arabidopsis thaliana cells, qualitative and quantitative differences in the extracellular proteome were observed for both proteins secreted into medium or ionically-bound in the walls. Lack of O-glycosylation resulted in the selective protein loss from the extracellular compartments. Following tunicamycin treatment the secretion of additional proteins as well as ER-resident chaperones from the Hsp70 and Hsp90 families outside the protoplasts was noted. Moreover, changes in the proteolytic degradation pattern of culture filtrate proteins were also observed. Application of Brefeldin A resulted in transient and selective loss of individual proteins from the extracellular compartments of A. thaliana cell suspension. We conclude that post-translational modifications are vital for the proper functioning of wall proteins. N-glycosylation is crucial for their proper folding and stability. Extracellular compartments could also serve as a sink for improperly folded proteins during the unfolded protein response.

Specific targeting of a plasmodesmal protein affecting cell-to-cell communication

Plasmodesmata provide the cytoplasmic conduits for cell-to-cell communication throughout plant tissues and participate in a diverse set of non–cell-autonomous functions. Despite their central role in growth and development and defence, resolving their modus operandi remains a major challenge in plant biology. Features of protein sequences and/or structure that determine protein targeting to plasmodesmata were previously unknown. We identify here a novel family of plasmodesmata-located proteins (called PDLP1) whose members have the features of type I membrane receptor-like proteins. We focus our studies on the first identified type member (namely At5g43980, or PDLP1a) and show that, following its altered expression, it is effective in modulating cell-to-cell trafficking. PDLP1a is targeted to plasmodesmata via the secretory pathway in a Brefeldin A–sensitive and COPII-dependent manner, and resides at plasmodesmata with its C-terminus in the cytoplasmic domain and its N-terminus in the apoplast. Using a deletion analysis, we show that the single transmembrane domain (TMD) of PDLP1a contains all the information necessary for intracellular targeting of this type I membrane protein to plasmodesmata, such that the TMD can be used to target heterologous proteins to this location. These studies identify a new family of plasmodesmal proteins that affect cell-to-cell communication. They exhibit a mode of intracellular trafficking and targeting novel for plant biology and provide technological opportunities for targeting different proteins to plasmodesmata to aid in plasmodesmal characterisation.

In plants, cylindrical, microscopic channels called plasmodesmata provide intracellular connections between cells for communication and material transport, and are important for many aspects of plant growth and defence. We identify a novel family of plasmodesmata-located proteins (called PDLP1) with features of type I membrane receptor-like proteins. In line with the potential for this protein to regulate molecular movement from cell to cell, we show that altered expression of the protein changes the efficiency of protein diffusion from plasmodesmata. We have also analysed the manner in which PDLP1 is transported to plasmodesmata. We show that the single transmembrane domain (TMD) of the protein contains all the information necessary for targeting to plasmodesmata and that proper targeting depends upon specific interactions with other factors within the membrane. Notably, a single amino acid close to the C-terminus of the TMD is critical for determining the intracellular destination. Further, by fusing the TMD to yellow fluorescent protein, we establish that the TMD can be used to target heterologous proteins to plasmodesmata.

Little is know about the structure and function of plant cell-to-cell connections, called plasmodesmata. This paper describes a new family of plasmodesmal proteins and the processes controlling their subcellular trafficking.

A plasmodesmata-associated beta-1,3-glucanase in Arabidopsis

Plasmodesmal conductivity is regulated in part by callose turnover, which is hypothesized to be determined by beta-1,3-glucan synthase versus glucanase activities. A proteomic analysis of an Arabidopsis thaliana plasmodesmata (Pd)-rich fraction identified a beta-1,3-glucanase as present in this fraction. The protein encoded by the putative plasmodesmal associated protein (ppap) gene, termed AtBG_ppap, had previously been found to be a post-translationally modified glycosylphosphatidylinositol (GPI) lipid-anchored protein. When fused to green fluorescent protein (GFP) and expressed in tobacco (Nicotiana tabacum) or Nicotiana benthamiana epidermal cells, this protein displays fluorescence patterns in the endoplasmic reticulum (ER) membrane system, along the cell periphery and in a punctate pattern that co-localizes with aniline blue-stained callose present around the Pd. Plasma membrane localization was verified by co-localization of AtBG_ppap:GFP together with a plasma membrane marker N-[3-triethylammoniumpropyl]-4-[p-diethylaminophenylhexatrienyl] pyridinium dibromide (FM4-64) in plasmolysed cells. In Arabidopsis T-DNA insertion mutants that do not transcribe AtBG_ppap, functional studies showed that GFP cell-to-cell movement between epidermal cells is reduced, and the conductivity coefficient of Pd is lower. Measurements of callose levels around Pd after wounding revealed that callose accumulation in the mutant plants was higher. Taken together, we suggest that AtBG_ppap is a Pd-associated membrane protein involved in plasmodesmal callose degradation, and functions in the gating of Pd.

Plasmolysis and recovery of different cell types in cryoprotected shoot tips of Mentha X piperita

Successful cryopreservation of plant shoot tips is dependent upon effective desiccation through osmotic or physical processes. Microscopy techniques were used to determine the extent of cellular damage and plasmolysis that occurs in peppermint (Mentha x piperita) shoot tips during the process of cryopreservation, using the cryoprotectant plant vitrification solution 2 (PVS2) (30% glycerol, 15% dimethyl sulfoxide, 15% ethylene glycol, 0.4 M sucrose) prior to liquid-nitrogen exposure. The meristem cells were the smallest and least plasmolyzed cell type of the shoot tips, while the large, older leaf and lower cortex cells were the most damaged. When treated with cryoprotectant solutions, meristem cells exhibited concave plasmolysis, suggesting that this cell type has a highly viscous protoplasm, and protoplasts have many cell wall attachment sites. Shoot tip cells were most severely plasmolyzed after PVS2 treatment, liquid-nitrogen exposure, and warming in 1.2 M sucrose. Successful recovery may be dependent upon surviving the plasmolytic conditions induced by warming and diluting treated shoot tips in 1.2 M sucrose solutions. In peppermint shoot tips, clumps of young meristem or young leaf cells survive the cryopreservation process and regenerate plants containing many shoots. Cryoprotective treatments that favor survival of small, meristematic cells and young leaf cells are most likely to produce high survival rates after liquid-nitrogen exposure.

At-4/1, an interactor of the Tomato spotted wilt virus movement protein, belongs to a new family of plant proteins capable of directed intra- and intercellular trafficking

The Tomato spotted wilt virus (TSWV) encoded NSm movement protein facilitates cell-to-cell spread of the viral genome through structurally modified plasmodesmata. NSm has been utilized as bait in yeast two-hybrid interaction trap screenings. As a result, a protein of unknown function, called At-4/1, was isolated from an Arabidopsis thaliana GAL4 activation domain-tagged cDNA library. Using polyclonal antibodies against bacterially expressed At-4/1, Western blot analysis of protein extracts isolated from different plant species as well as genome database screenings showed that homologues of At-4/1 seemed to be encoded by many vascular plants. For subcellular localization studies, At-4/1 was fused to green fluorescent protein, and corresponding expression vectors were used in particle bombardment and agroinfiltration assays. Confocal laser scannings revealed that At-4/1 assembled in punctate spots at the cell periphery. The protein accumulated intracellularly in a polarized fashion, appearing in only one-half of a bombarded epidermal cell, and, moreover, moved from cell to cell, forming twin-structured bodies seemingly located at both orifices of the plasmodesmatal pore. In coexpression studies, At-4/1 colocalized with a plant virus movement protein TGBp3 known to reside in endoplasmic reticulum-derived membrane structures located in close vicinity to plasmodesmata. Thus, At-4/1 belongs to a new family of plant proteins capable of directed intra- and intercellular trafficking.

Arabidopsis cell wall proteome defined using multidimensional protein identification technology

With the completion of the sequencing of the Arabidopsis genome and the recent advances in proteomic technology, the identification of proteins from highly complex mixtures is now possible. Rather than using gel electrophoresis and peptide mass fingerprinting, we have used multidimensional protein identification technology (MudPIT) to analyse the "tightly-bound" proteome for purified cell walls from Arabidopsis cell suspension cultures. Using bioinformatics for the prediction of signal peptides for targeting to the secretory pathway and for the absence of ER retention signal, 89 proteins were selected as potential extracellular proteins. Only 33% of these were identified in previous proteomic analyses of Arabidopsis cell walls. A functional classification revealed that a large proportion of the proteins were enzymes, notably carbohydrate active enzymes, peroxidases and proteases. Comparison of all the published proteomic analyses for the Arabidopsis cell wall identified 268 non-redundant genes encoding wall proteins. Sixty of these (22%) were derived from our analysis of tightly-bound wall proteins.

In planta secretion of a calreticulin by migratory and sedentary stages of root-knot nematode

Esophageal secretions from endoparasitic sedentary nematodes are thought to play key roles throughout plant parasitism, in particular during the invasion of the root tissue and the initiation and maintenance of the nematode feeding site (NFS) essential for nematode development. The secretion in planta of esophageal cell-wall-degrading enzymes by migratory juveniles has been shown, suggesting a role for these enzymes in the invasion phase. Nevertheless, the secretion of an esophageal gland protein into the NFS by nematode sedentary stages has never been demonstrated. The calreticulin Mi-CRT is a protein synthesized in the esophageal glands of the root-knot nematode Meloidogyne incognita. After three-dimensional modeling of the Mi-CRT protein, a surface peptide was selected to raise specific antibodies. In planta immunolocalization showed that Mi-CRT is secreted by migratory and sedentary stage nematodes, suggesting a role for Mi-CRT throughout parasitism. During the maintenance of the NFS, the secreted Mi-CRT was localized outside the nematode at the tip of the stylet. In addition, Mi-CRT accumulation was observed along the cell wall of the giant cells that compose the feeding site, providing evidence for a nematode esophageal protein secretion into the NFS.

Regulation of short-distance transport of RNA and protein

The intercellular trafficking of proteins and RNAs has emerged as a novel mechanism of cell-cell communication in plant development. Plasmodesmata (PD), intercellular cytoplasmic channels, have a central role in cell-cell trafficking of regulatory proteins and RNAs. Recent studies have demonstrated that plants use either a selective or a non-selective PD trafficking pathway for regulatory proteins. Moreover, plants have developed strategies to regulate both selective and non-selective movement. Recent work has focused especially on integrating the recent understanding of the function and mechanisms of intercellular macromolecule movement through PD.

Digging deeper into the plant cell wall proteome

The proteome of the plant cell wall/apoplast is less well characterized than those of other subcellular compartments. This largely reflects the many technical challenges involved in extracting and identifying extracellular proteins, many of which resist isolation and identification, and in capturing a population that is both comprehensive and relatively uncontaminated with intracellular proteins. However, a range of disruptive techniques, involving tissue homogenization and subsequent sequential extraction and non-disruptive approaches has been developed. These approaches have been complemented more recently by other genome-scale screens, such as secretion traps that reveal the genes encoding proteins with N-terminal signal peptides that are targeted to the secretory pathway, many of which are subsequently localized in the wall. While the size and complexity of the wall proteome is still unresolved, the combination of experimental tools and computational prediction is rapidly expanding the catalog of known wall-localized proteins, suggesting the unexpected extracellular localization of other polypeptides and providing the basis for further exploration of plant wall structure and function.