Cell wall functions in growth and development —a physical and chemical point of view

The plant cell changes its cell wall architecture during growth and development through synthesis and degradation of wall polysaccharides. Changes of chemical components in the cell wall include not only the synthesis and degradation but also the shift of molecular-weight distribution of certain species of the component polysaccharides. The changes in chemical structure, in turn lead to alteration of physical properties of the cell wall. Changes of physical parameters of cell walls obtained by a physical method accord with the biochemical degradation of polysaccharides. The changes in chemical structures of the cell wall are regulated by plant hormones, stress signals and gene expression. The physical and chemical studies of the cell wall have disclosed that degradation and/or depolymerization of wall polysaccahrides causes decrease in viscosity of the cell wall, leading further extension of the cell wall even under the unchanged osmotic relation. Furthermore, cell walls of outer and inner tissues play different regulatory roles in tissue growth and stem strength was governed by the number of cellulose molecules in the cell wall. Recipient of the Botanical Society Award for Young Scientists, 1990.     

The role of pectin in plant morphogenesis

The presence of a polysaccharidic cell wall distinguishes plant cells from animal cells and is responsible for fundamental mechanistic differences in organ development between the two kingdoms. Due to the presence of this wall, plant cells are unable to crawl and contract. On the other hand, plant cell size can increase by several orders of magnitude and cell shape can change from a simple polyhedron or cube to extremely intricate. This expansive cellular growth is regulated by the interaction between the cell wall and the intracellular turgor pressure. One of the principal cell wall components involved in temporal and spatial regulation of the growth process is pectin. Through biochemical changes to pectin composition and biochemical configuration, the properties of this material can be altered to trigger specific developmental processes. Here, the roles of pectin in three systems displaying rapid growth - the elongation zone of the root, the tip region of the pollen tube, and organ primordia formation at the shoot apical meristem - are reviewed.     

Properties of group I allergens from grass pollen and their relation to cathepsin B, a member of the C1 family of cysteine proteinases.

Expansins are a family of proteins that catalyze pH-dependent long-term extension of isolated plant cell walls. They are divided into two groups, alpha and beta, the latter consisting of the grass group I pollen allergens and their vegetative homologs. Expansins are suggested to mediate plant cell growth by interfering with either structural proteins or the polysaccharide network in the cell wall. Our group reported papain-like properties of beta-expansin of Timothy grass (Phleum pratense) pollen, Phl p 1, and suggested that cleavage of cell wall structural proteins may be the underlying mechanism of expansin-mediated wall extension. Here, we report additional data showing that beta-expansins resemble ancient and modern cathepsin B, which is a member of the papain (C1) family of cysteine proteinases. Using the Pichia pastoris expression system, we show that cleavage of inhibitory prosequences from the recombinant allergen is facilitated by its N-glycosylation and that the truncated, activated allergen shows proteolytic activity, resulting in very low stability of the protein. We also show that deglycosylated, full-length allergen is not activated efficiently and therefore is relatively stable. Motif and homology search tools detected significant similarity between beta-expansins and cathepsins of modern animals as well as the archezoa Giardia lamblia, confirming the presence of inhibitory prosequences, active site and other functional amino-acid residues, as well as a conserved location of these features within these molecules. Lastly, we demonstrate by site-directed mutagenesis that the conserved His104 residue is involved in the catalytic activity of beta-expansins. These results indicate a common origin of cathepsin B and beta-expansins, especially if taken together with their previously known biochemical properties.     

Construction and restructuring of the cellulose-xyloglucan framework in the apoplast as mediated by the xyloglucan-related protein family—A hypothetical scheme

The cellulose-xyloglucan framework functions as the load-bearing structure of the cell wall and constrains cell shape in plants. Xyloglucan cross-links which underpin the framework structure can be modified by transferases and hydrolases encoded by xyloglucan-related protein (XRP) family genes. These enzymes are considered to play critical roles in the construction and restructuring of the three-dimensional structure of the plant cell wall. Although analyses of their protein structures and gene-expression profiles for individual members of XRPs have disclosed their potentially divergent roles in plants, the biochemical reactions catalyzed by individual XRPs and their biochemical implications remain to be clarified. This review focuses on the XRP-catalyzed chemical processes occurring in the apoplast and considers the biochemical steps involved in the construction and restructuring of the cellulose-xyloglucan framework, an ensemble of chemical reactions that are more complicated than commonly supposed.     

Plant glycoside hydrolases involved in cell wall polysaccharide degradation.

The cell wall plays a key role in controlling the size and shape of the plant cell during plant development and in the interactions of the plant with its environment. The cell wall structure is complex and contains various components such as polysaccharides, lignin and proteins whose composition and concentration change during plant development and growth. Many studies have revealed changes in cell walls which occur during cell division, expansion, and differentiation and in response to environmental stresses; i.e. pathogens or mechanical stress. Although many proteins and enzymes are necessary for the control of cell wall organization, little information is available concerning them. An important advance was made recently concerning cell wall organization as plant enzymes that belong to the superfamily of glycoside hydrolases and transglycosidases were identified and characterized; these enzymes are involved in the degradation of cell wall polysaccharides. Glycoside hydrolases have been characterized using molecular, genetic and biochemical approaches. Many genes encoding these enzymes have been identified and functional analysis of some of them has been performed. This review summarizes our current knowledge about plant glycoside hydrolases that participate in the degradation and reorganisation of cell wall polysaccharides in plants focussing particularly on those from Arabidopsis thaliana.     

The structure, function, and biosynthesis of plant cell wall pectic polysaccharides

Plant cell walls consist of carbohydrate, protein, and aromatic compounds and are essential to the proper growth and development of plants. The carbohydrate components make up ∼90% of the primary wall, and are critical to wall function. There is a diversity of polysaccharides that make up the wall and that are classified as one of three types: cellulose, hemicellulose, or pectin. The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, are a class of molecules defined by the presence of galacturonic acid. The pectic polysaccharides include the galacturonans (homogalacturonan, substituted galacturonans, and RG-II) and rhamnogalacturonan-I. Galacturonans have a backbone that consists of α-1,4-linked galacturonic acid. The identification of glycosyltransferases involved in pectin synthesis is essential to the study of cell wall function in plant growth and development and for maximizing the value and use of plant polysaccharides in industry and human health. A detailed synopsis of the existing literature on pectin structure, function, and biosynthesis is presented.     

WAKs; cell wall associated kinases.

Students of metazoan biology have traditionally viewed the extracellular matrix (ECM) as a substrate with which cells interact to participate in developmental pattern formation and define a specific location. In contrast, the plant cell wall has been viewed as a cage that limits and thus directs plant cell morphology, and perhaps for this reason many have shied away from calling the plant cell wall the ECM. The recent discovery of a variety of receptor molecules and their ligands on the surface of plant cells and the intimate role cell walls play in development should direct our thinking toward a more dynamic view of the plant cell wall. A recent example, is the discovery of wall associated kinases (WAKs), which may well signal between the ECM and the cell and are required for cell expansion.     

Glycosyltransferases and cell wall biosynthesis: novel players and insights

Plants need an enormous biosynthetic machinery to synthesize the complex polysaccharides that are present in the plant cell wall. The isolation, characterization and mapping of wall mutants, together with biochemical approaches, have led to significant advances in our understanding of both wall polysaccharide synthesis at a molecular level and the function of polysaccharides in plant growth and development. Moreover, potential regulation mechanisms and associated protein factors are emerging from recent data.     

Root hair sweet growth

Root hairs are single cells specialized in the absorption of water and nutrients from the soil. Growing root hairs require intensive cell-wall changes to accommodate cell expansion at the apical end by a process known as tip or polarized growth. We have recently shown that cell wall glycoproteins such as extensins (EXTs) are essential components of the cell wall during polarized growth. Proline hydroxylation, an early posttranslational modification of cell wall EXTs that is catalyzed by prolyl 4-hydroxylases (P4Hs), defines the subsequent O-glycosylation sites in EXTs. Biochemical inhibition or genetic disruption of specific P4Hs resulted in the blockage of polarized growth in root hairs. Our results demonstrate that correct hydroxylation and also further O-glycosylation on EXTs are essential for cell-wall self-assembly and, hence, root hair elongation. The changes that O-glycosylated cell-wall proteins like EXTs undergo during cell growth represent a starting point to unravel the entire biochemical pathway involved in plant development.Key words: cell wall, O-glycoproteins, extensins, proline hydroxylation, polarized growth, root hairs, P4H     

2-Fluoro-L-Fucose Is a Metabolically Incorporated Inhibitor of Plant Cell Wall Polysaccharide Fucosylation

The monosaccharide L-fucose (L-Fuc) is a common component of plant cell wall polysaccharides and other plant glycans, including the hemicellulose xyloglucan, pectic rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II), arabinogalactan proteins, and N-linked glycans. Mutations compromising the biosynthesis of many plant cell wall polysaccharides are lethal, and as a result, small molecule inhibitors of plant cell wall polysaccharide biosynthesis have been developed because these molecules can be applied at defined concentrations and developmental stages. In this study, we characterize novel small molecule inhibitors of plant fucosylation. 2-fluoro-L-fucose (2F-Fuc) analogs caused severe growth phenotypes when applied to Arabidopsis seedlings, including reduced root growth and altered root morphology. These phenotypic defects were dependent upon the L-Fuc salvage pathway enzyme L-Fucose Kinase/ GDP-L-Fucose Pyrophosphorylase (FKGP), suggesting that 2F-Fuc is metabolically converted to the sugar nucleotide GDP-2F-Fuc, which serves as the active inhibitory molecule. The L-Fuc content of cell wall matrix polysaccharides was reduced in plants treated with 2F-Fuc, suggesting that this molecule inhibits the incorporation of L-Fuc into these polysaccharides. Additionally, phenotypic defects induced by 2F-Fuc treatment could be partially relieved by the exogenous application of boric acid, suggesting that 2F-Fuc inhibits RG-II biosynthesis. Overall, the results presented here suggest that 2F-Fuc is a metabolically incorporated inhibitor of plant cellular fucosylation events, and potentially suggest that other 2-fluorinated monosaccharides could serve as useful chemical probes for the inhibition of cell wall polysaccharide biosynthesis.     

Evidence that hydroxyl radicals mediate auxin-induced extension growth

Reactive oxygen intermediates, i.e. the superoxide radical (O*-)(2), hydrogen peroxide (H2O2) and the hydroxyl radical (*OH), are generally regarded as harmful products of oxygenic metabolism causing cell damage in plants, animals and microorganisms. However, oxygen radical chemistry may also play a useful role in polymer breakdown leading to wall loosening during extension growth of plant cells controlled by the phytohormone auxin. Backbone cleavage of cell wall polysaccharides can be accomplished in vitro by (*OH) produced from H2O2 in a Fenton reaction or in a reaction catalyzed by peroxidase supplied with O2 and NADH. Here, we show that coleoptile growth of maize seedlings is accompanied by the release of reactive oxygen intermediates in the cell wall. Auxin promotes release of (O*-)(2) and subsequent generation of (*OH)when inducing elongation growth. Experimental generation of (*OH) in the wall causes an increase in wall extensibility in vitro and replaces auxin in inducing growth. Auxin-induced growth can be inhibited by scavengers of (O*-)(2), H2O2 or (*OH), or inhibitors interfering with the formation of these molecules in the cell wall. These results provide the experimental background for a novel hypothesis on the mechanism of plant cell growth in which (*OH), produced from (O*-)(2) and H2O2 by cell wall peroxidase, acts as a wall-loosening agent.     

The biophysics of differential growth

The intrinsic control of uniform and differential growth of plant cells can be traced to a small number of physical parameters. These are cell wall rheology, membrane and tissue hydraulic conductivity, and membrane and tissue solute transport. Water and solute effects are manifested as alterations in turgor pressure. Environmental and biochemical processes always channel their effects through one or more of these parameters. Technical developments such as the pressure probe and Instron tensiometer, together with a reappraisal of older techniques, are beginning to allow assessment of the relative roles of these factors. Although the importance of cell wall rheology is becoming increasingly apparent, there is still insufficient information to allow generalized conclusions regarding the role of turgor pressure in differential growth. This review considers attempts to correlate these parameters with observed anatomical growth patterns.     

Effect of temperature on plant elongation and cell wall extensibility

Lockhart equation was derived for explaining plant cell expansion where both cell wall extension and water uptake must occur concomitantly. Its fundamental contribution was to express turgor pressure explicitly in terms of osmosis and wall mechanics. Here we present a new equation in which pressure is determined by temperature. It also accounts for the role of osmosis and consequently the role of water uptake in growing cell. By adopting literature data, we also attempt to report theoretically the close relation between plant elongation and cell wall extensibility. This is accomplished by the modified equation of growth solved for various temperatures in case of two different species. The results enable to interpret empirical data in terms of our model and fully confirm its applicability to the investigation of the problem of plant cell extensibility in function of environmental temperature. Moreover, by separating elastic effects from growth process we specified the characteristic temperature common for both processes which corresponds to the resonance energy of biochemical reactions as well as to the rapid softening of the elastic modes toward the high temperature end where we encountered viscoelastic and/or plastic behavior as dominating. By introducing analytical formulae connected with growth and elastic properties of the cell wall, we conclude with the statement how these both processes contribute quantitatively to the resonance-like shape of the elongation curve. In addition, the tension versus temperature "phase diagram" for a living plant cell is presented.     

Wall extensibility: its nature, measurement and relationship to plant cell growth

Expansive growth of plant cells is controlled principally by processes that loosen the wall and enable it to expand irreversibly. The central role of wall relaxation for cell expansion is reviewed. The most common methods for assessing the extension properties of plant cell walls ( wall extensibility') are described, categorized and assessed critically. What emerges are three fundamentally different approaches which test growing cells for their ability (a) to enlarge at different values of turgor, (b) to induce wall relaxation, and (c) to deform elastically or plastically in response to an applied tensile force. Analogous methods with isolated walls are similarly reviewed. The results of these different assays are related to the nature of plant cell growth and pertinent biophysical theory. I argue that the extensibilities' measured by these assays are fundamentally different from one another and that some are more pertinent to growth than others.     

The plot thickens: New perspectives of primary cell wall modification

Recent studies have further confirmed the ubiquity of cell wall restructuring during plant growth and development, and have emphasized the fact that our understanding of the breadth of molecular processes that mediate wall modification is still rudimentary. In the past few years, both enzymatic and non-enzymatic agents that apparently contribute to wall disassembly have been identified, and it is likely that additional mechanisms will continue to be revealed. These discoveries are being propelled by the development of new biochemical and biophysical assays, by database mining in the wake of the explosion of plant sequence information from genome sequencing and expressed sequence tags, and by a variety of strategies used to catalog the cell wall proteome. The daunting question of how these mechanistically diverse and complex processes are coordinated remains unresolved.     

Mechanical modeling and structural analysis of the primary plant cell wall

Plant cell growth is a fundamental process during plant development whose spatial and temporal dynamics are controlled by the cell wall. Modeling mechanical aspects of cell growth therefore requires the integration of structural cell wall details with quantitative biophysical parameters. Recent advances in microscopic techniques and mechanical modeling have made significant contributions to the field of cell wall biomechanics. Live observation of cellulose microfibrils at high z-resolution now enables determining the dynamic orientation of these polymers in the different wall layers of growing cells. Mechanical modeling approaches have been developed to operate at the scale of individual molecules and will thus be able to exploit the availability of the high-resolution structural data. The combination of these techniques has the potential to make a significant and quantitative contribution to our understanding of plant growth and development.     

Cell Wall Modifying Proteins Mediate Plant Acclimatization to Biotic and Abiotic Stresses

Cell wall modification is an important aspect of plant acclimation to environmental stresses. Structural changes of the existing cell wall mediated by various cell wall modifying proteins help a plant adjust to environmental changes by regulating growth and policing the entry of biotic agents. For example, accelerated shoot growth during submergence and shading allows some plants to escape these unfavorable conditions. This is mediated by the regulation of wall modifying proteins that alter cell wall structure and allow it to yield to turgor, thus fueling cellular expansion. Regulation of cell wall protein activity results in growth modulation during drought, where maintenance of root growth through changes in wall extensibility is an important adaptation to water deficit. Freeze-tolerant plants adjust their cell wall properties to prevent freezing-induced dehydration and also use the cell wall as a barrier against ice crystal propagation. Cell wall architecture is an important determinant of plant resistance to biotic stresses. A rigid cell wall can fend off pathogen attack by forming an impenetrable, physical barrier. When breached, products released during wall modification can trigger plant defense signaling. This review documents and discusses studies demonstrating the importance of timely cell wall modification during plant stress responses by focusing on a well-researched subset of wall modifying proteins.     

Expansin的研究进展

随着对植物生长机制的不断深入研究,发现expansin蛋白具明显而广泛的促进生长的作用。简述了expansin蛋白的生化特性及其对细胞壁的松弛机制,同时介绍了expansin在水稻中的组织定位。     

Cell wall carbohydrates as signals in plants

Plant and fungal cells are surrounded by a cell wall rich in diverse polysaccharides and proteins. It has become apparent in recent years that the carbohydrates in the cell wall function not only to maintain cell shape and integrity, but also may serve as signals in plants. This review summarizes the evidence that biologically-active oligosaccharides (oligosaccharins) released from plant or microbial cell walls can serve as signals to regulate plant defense and plant growth and development. The oligosaccharins discussed include the fungal-derived hepta-β-glucoside and the plant cell wall-derived oligogalacturonides and xyloglucans. Possible mechanisms by which oligosaccharins may exert their effects on plant cells are discussed.     

Expansin的研究进展

随着对植物生长机制的不断深入研究,发现expansin蛋白具明显而广泛的促进生长的作用。简述了expansin蛋白的生化特性及其对细胞壁的松弛机制,同时介绍了expansin在水稻中的组织定位。