Primary Plant Cell Wall Research Articles

The effect of hemicelluloses on biosynthesis, structure and mechanical performance of bacterial cellulose-hemicellulose hydrogels

The primary plant cell wall (PCW) is a specialized structure composed predominantly of cellulose, hemicelluloses and pectin. While the role of cellulose and hemicelluloses in the formation of the PCW scaffold is undeniable, the mechanisms of how hemicelluloses determine the mechanical properties of PCW remain debatable. Thus, we produced bacterial cellulose–hemicellulose hydrogels as PCW analogues, incorporated with hemicelluloses. Next, we treated samples with hemicellulose degrading enzymes, and explored its structural and mechanical properties. As suggested, difference of hemicelluloses in structure and chemical composition resulted in a variety of the properties studied. By analyzing all the direct and indirect evidences we have found that glucomannan, xyloglucan and arabinoxylan increased the width of cellulose fibers both by hemicellulose surface deposition and fiber entrapment. Arabinoxylan increased stresses and moduli of the hydrogel by its reinforcing effect, while for xylan, increase in mechanical properties was determined by establishment of stiff cellulose–cellulose junctions. In contrast, increasing content of xyloglucan decreased stresses and moduli of hydrogel by its weak interactions with cellulose, while glucomannan altered cellulose network formation via surface deposition, decreasing its strength. The current results provide evidence for structure–dependent mechanisms of cellulose–hemicellulose interactions, suggesting the specific structural role of the latter.

Putative rhamnogalacturonan-II glycosyltransferase identified through callus gene editing which bypasses embryo lethality.

Rhamnogalacturonan II (RG-II) is a structurally complex and conserved domain of the pectin present in the primary cell walls of vascular plants. Borate cross-linking of RG-II is required for plants to grow and develop normally. Mutations that alter RG-II structure also affect cross-linking and are lethal or severely impair growth. Thus, few genes involved in RG-II synthesis have been identified. Here, we developed a method to generate viable loss-of-function Arabidopsis (Arabidopsis thaliana) mutants in callus tissue via CRISPR/Cas9-mediated gene editing. We combined this with a candidate gene approach to characterize the male gametophyte defective 2 (MGP2) gene that encodes a putative family GT29 glycosyltransferase. Plants homozygous for this mutation do not survive. We showed that in the callus mutant cell walls, RG-II does not cross-link normally because it lacks 3-deoxy-D-manno-octulosonic acid (Kdo) and thus cannot form the α-L-Rhap-(1→5)-α-D-kdop-(1→sidechain). We suggest that MGP2 encodes an inverting RG-II CMP-β-Kdo transferase (RCKT1). Our discovery provides further insight into the role of sidechains in RG-II dimerization. Our method also provides a viable strategy for further identifying proteins involved in the biosynthesis of RG-II.

Lead phytochemicals and marine compounds against ceruloplasmin in cancer targeting

In silico docking studies serve as a swift and efficient means to sift through a vast array of natural and synthetic small molecules, aiding in the identification of potential inhibitors for cancer biomarkers. One such biomarker, ceruloplasmin (CP), has been implicated in various tumor types due to its overexpression, earning it recognition as a marker of aggressive tumors. This study focused on pinpointing inhibitors for the CP –Myeloperoxidase (MPO) interaction site, a complex formation known to impede HOCl production, a crucial process for inducing apoptotic cell death in tumor cells. The initial phase of our investigation involved in silico docking studies, which screened a diverse library of phytochemicals and marine compounds. Through this process, we identified several promising drug candidates based on their binding affinities. Subsequently, these candidates underwent rigorous filtration based on Absorption, Distribution, Metabolism, Excretion, and Toxicity (ADMET) properties. Finally, we subjected the selected compounds to molecular dynamics (MDs) simulation to further assess their viability. Lycoperoside F, a steroidal alkaloid glycoside derived from tomatoes (Lycopersicon esculentum), stood out with notable interactions at the binding site. Another noteworthy compound was Xyloglucan (XG) oligosaccharides, predominantly found in the primary cell walls of higher plants. During the subsequent MDs simulations, these interactions were accompanied by highly stable root mean square deviation (RMSD) plots, signifying the consistency and robustness of the observed MDs behavior. XG oligosaccharides demonstrated the highest binding affinity with CP, reaffirming their potential as strong candidates. Additionally, Ardimerin digallate, known as a retroviral ribonuclease H inhibitor for HIV-1 and HIV-2, displayed favorable interactions at the MPO interaction site. Given that promising drug candidates must meet stringent criteria, including non-toxicity, effectiveness, specificity, stability and potency, these phytochemicals have the potential to progress to in vitro studies as CP inhibitors. Ultimately, this could contribute to the suppression of tumor growth, marking a significant step in cancer treatment research. Communicated by Ramaswamy H. Sarma

The role of lipid-modified proteins in cell wall synthesis and signaling.

The plant cell wall is a complex and dynamic extracellular matrix. Plant primary cell walls are the first line of defense against pathogens and regulate cell expansion. Specialized cells deposit a secondary cell wall that provides support and permits water transport. The composition and organization of the cell wall varies between cell types and species, contributing to the extensibility, stiffness, and hydrophobicity required for its proper function. Recently, many of the proteins involved in the biosynthesis, maintenance, and remodeling of the cell wall have been identified as being post-translationally modified with lipids. These modifications exhibit diverse structures and attach to proteins at different sites, which defines the specific role played by each lipid modification. The introduction of relatively hydrophobic lipid moieties promotes the interaction of proteins with membranes and can act as sorting signals, allowing targeted delivery to the plasma membrane regions and secretion into the apoplast. Disruption of lipid modification results in aberrant deposition of cell wall components and defective cell wall remodeling in response to stresses, demonstrating the essential nature of these modifications. Although much is known about which proteins bear lipid modifications, many questions remain regarding the contribution of lipid-driven membrane domain localization and lipid heterogeneity to protein function in cell wall metabolism. In this update, we highlight the contribution of lipid modifications to proteins involved in the formation and maintenance of plant cell walls, with a focus on the addition of glycosylphosphatidylinositol anchors, N-myristoylation, prenylation, and S-acylation.

Immunochemical Identification of the Main Cell Wall Polysaccharides of the Early Land Plant Marchantia polymorpha.

Plant primary cell walls are composite structures surrounding the protoplast and containing pectins, hemicelluloses, and cellulose polysaccharides, as well as proteins. Their composition changed during the evolution of the green lineage from algae to terrestrial plants, i.e., from an aquatic to a terrestrial environment. The constraints of life in terrestrial environments have generated new requirements for the organisms, necessitating adaptations, such as cell wall modifications. We have studied the cell wall polysaccharide composition of thalli of Marchantia polymorpha, a bryophyte belonging to one of the first land plant genera. Using a collection of specific antibodies raised against different cell wall polysaccharide epitopes, we were able to identify in polysaccharide-enriched fractions: pectins, including low-methylesterified homogalacturonans; rhamnogalacturonan I with arabinan side-chains; and hemicelluloses, such as xyloglucans with XXLG and XXXG modules, mannans, including galactomannans, and xylans. We could also show the even distribution of XXLG xyloglucans and galactomannans in the cell walls of thalli by immunocytochemistry. These results are discussed with regard to the cell wall proteome composition and in the context of the evolution of the green lineage. The cell wall polysaccharides of M. polymorpha illustrate the transition from the charophyte ancestors of terrestrial plants containing xyloglucans, xylans and mannans as hemicelluloses, and embryophytes which do not exhibit mannans as major primary cell wall polysaccharides.

A Pectate Lyase Gene Plays a Critical Role in Xylem Vascular Development in Arabidopsis.

As a major component of the plant primary cell wall, structure changes in pectin may affect the formation of the secondary cell wall and lead to serious consequences on plant growth and development. Pectin-modifying enzymes including pectate lyase-like proteins (PLLs) participate in the remodeling of pectin during organogenesis, especially during fruit ripening. In this study, we used Arabidopsis as a model system to identify critical PLL genes that are of particular importance for vascular development. Four PLL genes, named AtPLL15, AtPLL16, AtPLL19, and AtPLL26, were identified for xylem-specific expression. A knock-out T-DNA mutant of AtPLL16 displayed an increased amount of pectin, soluble sugar, and acid-soluble lignin (ASL). Interestingly, the atpll16 mutant exhibited an irregular xylem phenotype, accompanied by disordered xylem ray cells and an absence of interfascicular phloem fibers. The xylem fiber cell walls in the atpll16 mutant were thicker than those of the wild type. On the contrary, AtPLL16 overexpression resulted in expansion of the phloem and a dramatic change in the xylem-to-phloem ratios. Altogether, our data suggest that AtPLL16 as a pectate lyase plays an important role during vascular development in Arabidopsis.

Grazing-incidence diffraction reveals cellulose and pectin organization in hydrated plant primary cell wall

The primary cell wall is highly hydrated in its native state, yet many structural studies have been conducted on dried samples. Here, we use grazing-incidence wide-angle X-ray scattering (GIWAXS) with a humidity chamber, which enhances scattering and the signal-to-noise ratio while keeping outer onion epidermal peels hydrated, to examine cell wall properties. GIWAXS of hydrated and dried onion reveals that the cellulose (110/1overline{1}0) lattice spacing decreases slightly upon drying, while the (200) lattice parameters are unchanged. Additionally, the (110/1overline{1}0) diffraction intensity increases relative to (200). Density functional theory models of hydrated and dry cellulose microfibrils corroborate changes in crystalline properties upon drying. GIWAXS also reveals a peak that we attribute to pectin chain aggregation. We speculate that dehydration perturbs the hydrogen bonding network within cellulose crystals and collapses the pectin network without affecting the lateral distribution of pectin chain aggregates.

In silico studies of plant primary cell walls - structure and mechanics.

Primary plant cell wall (PCW) is a highly organized network, its performance is dependent on cellulose, hemicellulose and pectic polysaccharides, their properties, interactions and assemblies. Their mutual relationships and functions in the cell wall can be better understood by means of conceptual models of their higher-order structures. Knowledge unified in the form of a conceptual model allows predictions to be made about the properties and behaviour of the system under study. Ongoing research in this field has resulted in a number of conceptual models of the cell wall. However, due to the currently limited research methods, the community of cell wall researchers have not reached a consensus favouring one model over another. Herein we present yet another research technique - numerical modelling - which is capable of resolving this issue. Even at the current stage of development of numerical techniques, due to their complexity, the in silico reconstruction of PCW remains a challenge for computational simulations. However, some difficulties have been overcome, thereby making it possible to produce advanced approximations of PCW structure and mechanics. This review summarizes the results concerning the simulation of polysaccharide interactions in PCW with regard to network fine structure, supramolecular properties and polysaccharide binding affinity. The in silico mechanical models presented herein incorporate certain physical and biomechanical aspects of cell wall architecture for the purposes of undertaking critical testing to bring about advances in our understanding of the mechanisms controlling cells and limiting cell wall expansion.

Pectin, Lignin and Disease Resistance in Brassica napus L.: An Update

The plant cell wall is dynamically modified during host–pathogen interactions and acts as a crucial factor controlling plant immunity. In the context of recently revised models of plant primary cell walls (PCWs), pectin is considered to be important in determining the mechanical properties of PCWs. A secondary cell wall is present in some cell types and lignin is normally present and acts to strengthen wall rigidity. In this review, we summarize the recent advances in understanding cell-wall-mediated defense responses against pathogens in Brassica napus L. (B. napus). A major part of this response involves pectin and lignin, and these two major cell wall components contribute greatly to immune responses in B. napus. Crosstalk between pectin and lignin metabolism has been detected in B. napus upon pathogen infection, suggesting a synergistic action of pectin and lignin metabolism in regulating cell wall integrity as well as wall-mediated immunity. The transcriptional regulation of cell-wall-mediated immunity in B. napus along with that in Arabidopsis is discussed, and directions for future work are proposed for a better understanding of wall-mediated plant immunity in B. napus.

The specificity of pectate lyase VdPelB from Verticilium dahliae is highlighted by structural, dynamical and biochemical characterizations

Pectins, complex polysaccharides and major components of the plant primary cell wall, can be degraded by pectate lyases (PLs). PLs cleave glycosidic bonds of homogalacturonans (HG), the main pectic domain, by β-elimination, releasing unsaturated oligogalacturonides (OGs). To understand the catalytic mechanism and structure/function of these enzymes, we characterized VdPelB from Verticillium dahliae. We first solved the crystal structure of VdPelB at 1.2 Å resolution showing that it is a right-handed parallel β-helix structure. Molecular dynamics (MD) simulations further highlighted the dynamics of the enzyme in complex with substrates that vary in their degree of methylesterification, identifying amino acids involved in substrate binding and cleavage of non-methylesterified pectins. We then biochemically characterized wild type and mutated forms of VdPelB. Pectate lyase VdPelB was most active on non-methylesterified pectins, at pH 8.0 in presence of Ca2+ ions. The VdPelB-G125R mutant was most active at pH 9.0 and showed higher relative activity compared to native enzyme. The OGs released by VdPelB differed to that of previously characterized PLs, showing its peculiar specificity in relation to its structure. OGs released from Verticillium-partially tolerant and sensitive flax cultivars differed which could facilitate the identification VdPelB-mediated elicitors of defence responses.

Cloning, expression, and characterization of two pectate lyases isolated from the sheep rumen microbiome.

Pectate lyases (Pels) have a vital function in degradation of the primary plant cell wall and the middle lamella and have been widely used in the industry. In this study, two pectate lyase genes, IDSPel16 and IDSPel17, were cloned from a sheep rumen microbiome. The recombinant enzymes were expressed in Escherichia coli and functionally characterized. Both IDSPel16 and IDSPel17 proteins had an optimal temperature of 60 ℃, and an optimal pH of 10.0. IDSPel16 was relatively stable below 60°C, maintaining 77.51% residual activity after preincubation at 60°C for 1h, whereas IDSPel17 denatured rapidly at 60°C. IDSPel16 was relatively stable between pH 6.0 and 12.0, after pretreatment for 1h, retaining over 60% residual activity. IDSPel16 had high activity towards polygalacturonic acid, with a Vmax of 942.90 ± 68.11, whereas IDSPel17 had a Vmax of only 28.19 ± 2.23μmol/min/mg. Reaction product analyses revealed that IDSPel17 liberated unsaturated digalacturonate (uG2) and unsaturated trigalacturonate (uG3) from the substrate, indicating a typical endo-acting pectate lyase (EC 4.2.2.2). In contrast, IDSPel16 initially generated unsaturated oligogalacturonic acids, then converted these intermediates into uG2 and unsaturated galacturonic acid (uG1) as end products, a unique depolymerization profile among Pels. To the best of our knowledge, the IDSPel16 discovered with both endo-Pel (EC 4.2.2.2) and exo-Pel (EC 4.2.2.9) activities. These two pectate lyases, particularly the relatively thermo- and pH-stable IDSPel16, will be of interest for potential application in the textile, food, and feed industries. KEY POINTS: • Two novel pectate lyase genes, IDSPel16 and IDSPel17, were isolated and characterized from the sheep rumen microbiome. • Both IDSPel16 and IDSPel17 are alkaline pectate lyases, releasing unsaturated digalacturonate and unsaturated trigalacturonate from polygalacturonic acid. • IDSPel16, a bifunctional pectate lyase with endo-Pel (EC 4.2.2.2) and exo-Pel (EC 4.2.2.9) activities, could be a potential candidate for industrial application.

Cellulose in Secondary Xylem of Cactaceae: Crystalline Composition and Anatomical Distribution.

Cellulose is the main polymer that gives strength to the cell wall and is located in the primary and secondary cell walls of plants. In Cactaceae, there are no studies on the composition of cellulose. The objective of this work was to analyze the crystallinity composition and anatomical distribution of cellulose in Cactaceae vascular tissue. Twenty-five species of Cactaceae were collected, dried, and milled. Cellulose was purified and analyzed with Fourier transform infrared spectroscopy, the crystallinity indexes were calculated, and statistical analyzes were performed. Stem sections were fixed, cut, and stained with safranin O/fast green, for observation with epifluorescence microscopy. The crystalline cellulose ratios had statistical differences between Echinocereus pectinatus and Coryphantha pallida. All cacti species presented a higher proportion of crystalline cellulose. The fluorescence emission of the cellulose was red in color and distributed in the primary wall of non-fibrous species; while in the fibrous species, the distribution was in the pits. The high percentages of crystalline cellulose may be related to its distribution in the non-lignified parenchyma and primary walls of tracheary elements with helical or annular thickenings of non-fibrous species, possibly offering structural rigidity and forming part of the defense system against pathogens.

Comparison of Trichoderma longibrachiatum Xyloglucanase Production Using Tamarind (Tamarindus indica) and Jatoba (Hymenaea courbaril) Seeds: Factorial Design and Immobilization on Ionic Supports

Xyloglucan (XG) is the predominant hemicellulose in the primary cell wall of superior plants. It has a fundamental role in controlling the stretching and expansion of the plant cell wall. There are five types of enzymes known to cleave the linear chain of xyloglucan, and the most well-known is xyloglucanase (XEG). The immobilization process can be used to solve problems related to stability, besides the economic benefits brought by the possibility of its repeated use and recovery. Therefore, this study aims at the optimization of the xyloglucanase production of Trichoderma longibrachiatum using a central composite rotatable design (CCRD) with tamarind and jatoba seeds as carbon sources, as well as XEG immobilization on ionic supports, such as MANAE (monoamine-N-aminoethyl), DEAE (diethylaminoethyl)-cellulose, CM (carboxymethyl)-cellulose, and PEI (polyethyleneimine). High concentrations of carbon sources (1.705%), at a temperature of 30 °C and under agitation for 72 h, were the most favorable conditions for the XEG activity from T. longibrachiatum with respect to both carbon sources. However, the tamarind seeds showed 23.5% higher activity compared to the jatoba seeds. Therefore, this carbon source was chosen to continue the experiments. The scaling up from Erlenmeyer flasks to the bioreactor increased the XEG activity 1.27-fold (1.040 ± 0.088 U/mL). Regarding the biochemical characterization of the crude extract, the optimal temperature range was 50–55 °C, and the optimal pH was 5.0. Regarding the stabilities with respect to pH and temperature, XEG was not stable for prolonged periods, which was crucial to immobilizing it on ionic resins. XEG showed the best immobilization efficiency on CM-cellulose and DEAE-cellulose, with activities of 1.16 and 0.89 U/g of the derivative (enzyme plus support), respectively. This study describes, for the first time in the literature, the immobilization of a fungal xyloglucanase using these supports.

An Arabidopsis thaliana arabinogalactan-protein (AGP31) and several cationic AGP fragments catalyse the boron bridging of rhamnogalacturonan-II

Rhamnogalacturonan-II (RG-II) is a complex pectic domain in plant primary cell walls. In vivo, most RG-II domains are covalently dimerised via borate diester bridges, essential for correct cell-wall assembly, but the dimerisation of pure RG-II monomers by boric acid in vitro is extremely slow. Cationic ‘chaperones’ can promote dimerisation, probably by overcoming the mutual repulsion between neighbouring anionic RG-II molecules. Highly effective artificial chaperones include Pb2+ and polyhistidine, but the proposed natural chaperones remained elusive. We have now tested cationic peptide fragments of several Arabidopsis thaliana arabinogalactan-proteins (AGPs) as candidates. Fragments of AGP17, 18, 19 and 31 were effective, typically at ∼25 µg/ml (9–19 µM), promoting the boron bridging of 16–20 µM monomeric RG-II at pH 4.8 in vitro. Native AGP31 glycoprotein was also effective, and hexahistidine was moderately so. All chaperones tested interacted reversibly with RG-II and were not consumed during the reaction; thus they acted catalytically, and may constitute the first reported boron-acting enzyme activity, an RG-II borate diesterase. Many of the peptide chaperones became less effective catalysts at higher concentration, which we interpret as due to the formation of RG-II–peptide complexes with a net positive charge, as mutually repulsive as negatively charged pure RG-II molecules. The four unique AGPs studied here may serve an enzymic role in the living plant cell, acting on RG-II within Golgi cisternae and/or in the apoplast after secretion. In this way, RG-II and specific AGPs may contribute to cell-wall assembly and hence plant cell expansion and development.

Magnaporthe oryzae Chloroplast Targeting Endo-β-1,4-Xylanase I MoXYL1A Regulates Conidiation, Appressorium Maturation and Virulence of the Rice Blast Fungus

Endo-β-1,4-Xylanases are a group of extracellular enzymes that catalyze the hydrolysis of xylan, a principal constituent of the plant primary cell wall. The contribution of Endo-β-1,4-Xylanase I to both physiology and pathogenesis of the rice blast fungus M. oryzae is unknown. Here, we characterized the biological function of two endoxylanase I (MoXYL1A and MoXYL1B) genes in the development of M. oryzae using targeted gene deletion, biochemical analysis, and fluorescence microscopy. Phenotypic analysis of ∆Moxyl1A strains showed that MoXYL1A is required for the full virulence of M. oryzae but is dispensable for the vegetative growth of the rice blast fungus. MoXYL1B, in contrast, did not have a clear role in the infectious cycle but has a critical function in asexual reproduction of the fungus. The double deletion mutant was severely impaired in pathogenicity and virulence as well as asexual development. We found that MoXYL1A deletion compromised appressorium morphogenesis and function, leading to failure to penetrate host cells. Fluorescently tagged MoXYL1A and MoXYL1B displayed cytoplasmic localization in M. oryzae, while analysis of MoXYL1A-GFP and MoXYL1B-GFP in-planta revealed translocation and accumulation of these effector proteins into host cells. Meanwhile, sequence feature analysis showed that MoXYL1A possesses a transient chloroplast targeting signal peptide, and results from an Agrobacterium infiltration assay confirmed co-localization of MoXYL1A-GFP with ChCPN10C-RFP in the chloroplasts of host cells. MoXYL1B, accumulated to the cytoplasm of the host. Taken together, we conclude that MoXYL1A is a secreted effector protein that likely promotes the virulence of M. oryzae by interfering in the proper functioning of the host chloroplast, while the related xylanase MoXYL1B does not have a major role in virulence of M. oryzae.

Eudicot primary cell wall glucomannan is related in synthesis, structure, and function to xyloglucan.

Hemicellulose polysaccharides influence assembly and properties of the plant primary cell wall (PCW), perhaps by interacting with cellulose to affect the deposition and bundling of cellulose fibrils. However, the functional differences between plant cell wall hemicelluloses such as glucomannan, xylan, and xyloglucan (XyG) remain unclear. As the most abundant hemicellulose, XyG is considered important in eudicot PCWs, but plants devoid of XyG show relatively mild phenotypes. We report here that a patterned β-galactoglucomannan (β-GGM) is widespread in eudicot PCWs and shows remarkable similarities to XyG. The sugar linkages forming the backbone and side chains of β-GGM are analogous to those that make up XyG, and moreover, these linkages are formed by glycosyltransferases from the same CAZy families. Solid-state nuclear magnetic resonance indicated that β-GGM shows low mobility in the cell wall, consistent with interaction with cellulose. Although Arabidopsis β-GGM synthesis mutants show no obvious growth defects, genetic crosses between β-GGM and XyG mutants produce exacerbated phenotypes compared with XyG mutants. These findings demonstrate a related role of these two similar but distinct classes of hemicelluloses in PCWs. This work opens avenues to study the roles of β-GGM and XyG in PCWs.

Dynamics of cell wall polysaccharides during the elongation growth of rye primary roots.

In cells of growing rye roots, xyloglucans and homogalacturonans demonstrate developmental stage specificity, while different xylans have tissue specificity. Mannans, arabinans and galactans are also detected within the protoplast. Mannans form films on sections of fresh material. The primary cell walls of plants represent supramolecular exocellular structures that are mainly composed of polysaccharides. Cell wall properties and architecture differ between species and across tissues within a species. We revised the distribution of cell wall polysaccharides and their dynamics during elongation growth and histogenesis in rye roots using nonfixed material and the spectrum of antibodies. Rye is a member of the Poaceae family and thus has so-called type II primary cell walls, which are supposed to be low in pectins and xyloglucans and instead have arabinoxylans and mixed-linkage glucans. However, rye cell walls at the earliest stages of cell development were enriched with the epitopes of xyloglucans and homogalacturonans. Mixed-linkage glucan, which is often considered an elongation growth-specific polysaccharide in plants with type II cell walls, did not display such dynamics in rye roots. The cessation of elongation growth and even the emergence of root hairs were not accompanied by the disappearance of mixed-linkage glucans from cell walls. The diversity of xylan motifs recognized by different antibodies was minimal in the meristem zone of rye roots, but this diversity increased and showed tissue specificity during root growth. Antibodies specific for xyloglucans, galactans, arabinans and mannans bound the cell content. When rye root cells were cut, the epitopes of xyloglucans, galactans and arabinans remained within the cell content, while mannans developed net-like or film-like structures on the surface of sections.

Cell wall-localized BETA-XYLOSIDASE4 contributes to immunity of Arabidopsis against Botrytis cinerea.

Plant cell walls constitute physical barriers that restrict access of microbial pathogens to the contents of plant cells. The primary cell wall of multicellular plants predominantly consists of cellulose, hemicellulose, and pectin, and its composition can change upon stress. BETA-XYLOSIDASE4 (BXL4) belongs to a seven-member gene family in Arabidopsis (Arabidopsis thaliana), one of which encodes a protein (BXL1) involved in cell wall remodeling. We assayed the influence of BXL4 on plant immunity and investigated the subcellular localization and enzymatic activity of BXL4, making use of mutant and overexpression lines. BXL4 localized to the apoplast and was induced upon infection with the necrotrophic fungal pathogen Botrytis cinerea in a jasmonoyl isoleucine-dependent manner. The bxl4 mutants showed a reduced resistance to B. cinerea, while resistance was increased in conditional overexpression lines. Ectopic expression of BXL4 in Arabidopsis seed coat epidermal cells rescued a bxl1 mutant phenotype, suggesting that, like BXL1, BXL4 has both xylosidase and arabinosidase activity. We conclude that BXL4 is a xylosidase/arabinosidase that is secreted to the apoplast and its expression is upregulated under pathogen attack, contributing to immunity against B. cinerea, possibly by removal of arabinose and xylose side-chains of polysaccharides in the primary cell wall.

The Cell Wall Proteome of Marchantia polymorpha Reveals Specificities Compared to Those of Flowering Plants.

Primary plant cell walls are composite extracellular structures composed of three major classes of polysaccharides (pectins, hemicelluloses, and cellulose) and of proteins. The cell wall proteins (CWPs) play multiple roles during plant development and in response to environmental stresses by remodeling the polysaccharide and protein networks and acting in signaling processes. To date, the cell wall proteome has been mostly described in flowering plants and has revealed the diversity of the CWP families. In this article, we describe the cell wall proteome of an early divergent plant, Marchantia polymorpha, a Bryophyte which belong to one of the first plant species colonizing lands. It has been possible to identify 410 different CWPs from three development stages of the haploid gametophyte and they could be classified in the same functional classes as the CWPs of flowering plants. This result underlied the ability of M. polymorpha to sustain cell wall dynamics. However, some specificities of the M. polymorpha cell wall proteome could be highlighted, in particular the importance of oxido-reductases such as class III peroxidases and polyphenol oxidases, D-mannose binding lectins, and dirigent-like proteins. These proteins families could be related to the presence of specific compounds in the M. polymorpha cell walls, like mannans or phenolics. This work paves the way for functional studies to unravel the role of CWPs during M. polymorpha development and in response to environmental cues.

Modeling the molecular structures and dynamics responsible for the remarkable mechanical properties of a plant cell wall.

The primary plant cell wall is a hydrated meshwork of polysaccharides that is strong enough to withstand large mechanical stresses imposed by turgor while remaining pliant in ways that permit growth. To understand how its macromolecular architecture produces its complex mechanical properties, Zhang et al.1 computationally assembled a realistic network of cellulose microfibrils, hemicellulose, and pectin. The simulated wall responded to computationally applied stress like the real wall on which it was based. The model showed the location and chemical identity of stress-bearing components. It showed that cellulose microfibril interactions and movements dominated the wall's mechanical behavior, while hemicellulose and pectin had surprisingly minor effects.