If walls could talk: Commentary

The plant cell wall is very complex, both in structure and function. The wall components and the mechanical properties of the wall have been implicated in conveying information that is important for morphogenesis. Proteoglycans, fragments of polysaccharides and the structural integrity of the wall may relay signals that influence cellular differentiation and growth control. Furthering our knowledge of cell wall structure and function is likely to have a profound impact on our understanding of how plant cells communicate with the extracellular environment.     

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.     

The plant cell wall: Biosynthesis,construction, and functions

The plant cell wall is composed of multiple biopolymers, representing one of the most complex structural networks in nature. Hundreds of genes are involved in building such a natural masterpiece. However, the plant cell wall is the least understood cellular structure in plants. Due to great progress in plant functional genomics,manyachievementshavebeenmadein uncovering cell wall biosynthesis, assembly, and architecture, as well as cell wall regulation and signaling. Such information has significantly advanced our understanding of the roles of the cell wall in many biological and physiological processes and has enhanced our utilization of cell wall materials. The use of cutting-edge technologies such as single-molecule imaging,nuclear magnetic resonance spectroscopy, and atomic force microscopy has provided much insight into the plant cell wall as an intricate nanoscale network, opening up unprecedented possibilities for cell wall research. In this review,we summarize the major advances made in understanding the cell wall in this era of functional genomics, including the latest findings on the biosynthesis, construction, and functions of the cell wall.     

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.     

More than a leak sealant. The mechanical properties of callose in pollen tubes

While callose is a well-known permeability barrier and leak sealant in plant cells, it is largely unknown whether this cell wall polymer can also serve as a load-bearing structure. Since callose occurs in exceptionally large amounts in pollen, we assessed its role for resisting tension and compression stress in this cell. The effect of callose digestion in Solanum chacoense and Lilium orientalis pollen grains demonstrated that, depending on the species, this cell wall polymer represents a major stress-bearing structure at the aperture area of germinating grains. In the pollen tube, it is involved in cell wall resistance to circumferential tension stress, and despite its absence at the growing apex, callose is indirectly involved in the establishment of tension stress resistance in this area. To investigate whether or not callose is able to provide mechanical resistance against compression stress, we subjected pollen tubes to local deformation by microindentation. The data revealed that lowering the amount of callose resulted in reduced cellular stiffness and increased viscoelasticity, thus indicating clearly that callose is able to resist compression stress. Whether this function is relevant for pollen tube mechanics, however, is unclear, as stiffened growth medium caused a decrease in callose deposition. Together, our data provide clear evidence for the capacity of cell wall callose to resist tension and compression stress, thus demonstrating that this amorphous cell wall substance can have a mechanical role in growing plant cells.     

Cell wall traits that influence plant development,immunity, and bioconversion

The architecture of the plant cell wall is highly dynamic, being substantially re‐modeled during growth and development. Cell walls determine the size and shape of cells and contribute to the functional specialization of tissues and organs. Beyond the physiological dynamics, the wall structure undergoes changes upon biotic or abiotic stresses. In this review several cell wall traits, mainly related to pectin, one of the major matrix components, will be discussed in relation to plant development, immunity and industrial bioconversion of biomass, especially for energy production. Plant cell walls are a source of oligosaccharide fragments with a signaling function for both development and immunity. Sensing cell wall damage, sometimes through the perception of released damage‐associated molecular patterns (DAMPs), is crucial for some developmental and immunity responses. Methodological advances that are expected to deepen our knowledge of cell wall (CW) biology will also be presented.     

Pectins: structure, biosynthesis, and oligogalacturonide-related signaling.

Pectin is a family of complex polysaccharides present in all plant primary cell walls. The complicated structure of the pectic polysaccharides, and the retention by plants of the large number of genes required to synthesize pectin, suggests that pectins have multiple functions in plant growth and development. In this review we summarize the current level of understanding of pectin primary and tertiary structure, and describe new methods that may be useful to study localized pectin structure in the plant cell wall. We also discuss progress in our understanding of how pectin is biosynthesized and review the biological activities and possible modes of action of pectic oligosaccharides referred to as oligogalacturonides. We present our view of critical questions regarding pectin structure, biosynthesis, and function that need to be addressed in the coming decade. As the plant community works towards understanding the functions of the tens of thousands of genes expressed by plants, a large number of those genes are likely to be involved in the synthesis, turnover, biological activity, and restructuring of pectin. A combination of genetic, molecular, biochemical and chemical approaches will be necessary to fully understand the function and biosynthesis of pectin.     

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.     

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.     

Whole plant cell wall characterization using solution-state 2D NMR

Recent advances in nuclear magnetic resonance (NMR) technology have made it possible to rapidly screen plant material and discern whole cell wall information without the need to deconstruct and fractionate the plant cell wall. This approach can be used to improve our understanding of the biology of cell wall structure and biosynthesis, and as a tool to select plant material for the most appropriate industrial applications. This is particularly true in an era when renewable materials are vital to the emerging bio-based economies. This protocol describes procedures for (i) the preparation and extraction of a biological plant tissue, (ii) solubilization strategies for plant material of varying composition and (iii) 2D NMR acquisition (for typically 15 min-5 h) and integration methods used to elucidate lignin subunit composition and lignin interunit linkage distribution, as well as cell wall polysaccharide profiling. Furthermore, we present data that demonstrate the utility of this new NMR whole cell wall characterization procedure with a variety of degradative methods traditionally used for cell wall compositional analysis.     

Cell wall integrity maintenance in plants: Lessons to be learned from yeast?

The plant cell wall is involved in different biological processes like cell morphogenesis and response to biotic/abiotic stress. Functional integrity of the wall is apparently being maintained during these processes by changing structure/composition and coordinating cell wall with cellular metabolism. In S.cerevisiae a well-characterized mechanism exists that is maintaining functional integrity of yeast the cell wall during similar processes. During the last years it has become obvious that plants have evolved a mechanism to monitor and maintain functional integrity of their cell walls. However, our understanding of the mechanism is rather limited. The available evidence suggests that similar signaling cascades may be involved and particular protein activities may be conserved between plants and yeast. Here we review the available evidence briefly and highlight similarities between yeast and plants that could help us to understand the mode of action of the signaling cascades maintaining plant cell wall integrity.     

植物果胶的生物合成与功能

果胶作为植物细胞壁多糖之一,其结构和功能非常复杂。果胶主要由同型半乳糖醛酸聚糖(HG)、鼠李半乳糖醛酸聚糖I (RGI)和鼠李半乳糖醛酸聚糖II (RGII)组成。果胶类成分在维持细胞壁结构的完整性以及细胞间黏附和信号转导等方面发挥重要作用。研究果胶类成分的结构、分布和功能,对理解细胞壁高级结构的构建和功能具有重要意义...     

植物细胞壁的研究进展

随着实验技术的发展尤其是分子生物学技术的应用,植物细胞壁的研究已取得丰硕的成果,现在已基本清楚了细胞壁的解剖结构及化学组成,但细胞壁的发生发育、细胞壁的功能和细胞壁的各种组成成份--木质素、纤维素等的研究及利用尚需进一步研究探讨。本文在总结现有研究结果的基础上,对细胞壁的研究重点提出了初步的构想以共商榷。     

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.     

Cellulose synthesis in two secondary cell wall processes in a single cell type

Plant cells have a rigid cell wall that constrains internal turgor pressure yet extends in a regulated and organized manner to allow the cell to acquire shape. The primary load-bearing macromolecule of a plant cell wall is cellulose, which forms crystalline microfibrils that are organized with respect to a cell''s function and shape requirements. A primary cell wall is deposited during expansion whereas secondary cell wall is synthesized post expansion during differentiation. A complex form of asymmetrical cellular differentiation occurs in Arabidopsis seed coat epidermal cells, where we have recently shown that two secondary cell wall processes occur that utilize different cellulose synthase (CESA) proteins. One process is to produce pectinaceous mucilage that expands upon hydration and the other is a radial wall thickening that reinforced the epidermal cell structure. Our data illustrate polarized specialization of CESA5 in facilitating mucilage attachment to the parent seed and CESA2, CESA5 and CESA9 in radial cell wall thickening and formation of the columella. Herein, we present a model for the complexity of cellulose biosynthesis in this highly differentiated cell type with further evidence supporting each cellulosic secondary cell wall process.     

Plasmodesmata - membrane tunnels with attitude

Plasmodesmata are doors in the rigid cell wall. In multicellular tissues, they allow the passage of molecules needed to create physiological gradients and, by closure, symplastic boundaries, which are necessary for the fundamental processes of plant growth, development and defence. Despite this central role in plant growth our knowledge of their contribution has been hindered by difficulties in biochemical and molecular characterisation. Recent advances in proteomic, biochemical, cell biological and genetic analysis of their structure and function is showing that plasmodesmata are plastic yet highly regulated structures. They require the perception of small molecule signals (such as reactive oxygen species) to activate local changes in the cell wall that place physical constraints on the channel. This article reviews recent evidence that highlights the roles of the membrane subcomponents both as structural elements and as environments for resident signalling molecules.     

Formation of plant cell wall supramolecular structure

Plant cell wall is an example of a widespread natural supramolecular structure: its components are considered to be the most abundant organic compounds renewable by living organisms. Plant cell wall includes numerous components, mainly polysaccharidic; its formation is largely based on carbohydrate-carbohydrate interactions. In contrast to the extracellular matrix of most other organisms, the plant cell compartment located outside the plasma membrane is so structured that has been named “wall”. The present review summarizes data on the mechanisms of formation of this supramolecular structure and considers major difficulties and results of research. Existing approaches to the study of interactions between polysaccharides during plant cell wall formation have been analyzed, including: (i) characterization of the structure of natural polysaccharide complexes obtained during cell wall fractionation; (ii) analysis of the interactions between polysaccharides “at mixing in a tube”; (iii) study of the interactions between isolated individual plant cell wall matrix polysaccharides and microfibrils formed by cellulose-synthesizing microorganisms; and (iv) investigation of cell wall formation and modification directly in plant objects. The key stages in formation of plant cell wall supramolecular structure are defined and characterized as follows: (i) formation of cellulose microfibrils; (ii) interactions between matrix polysaccharides within Golgi apparatus substructures; (iii) interaction between matrix polysaccharides, newly secreted outside the plasma membrane, and cellulose microfibrils during formation of the latter; (iv) packaging of the formed complexes and individual polysaccharides in cell wall layers; and (v) modification of deposited cell wall layers.     

Determining the polysaccharide composition of plant cell walls

The plant cell wall is a chemically complex structure composed mostly of polysaccharides. Detailed analyses of these cell wall polysaccharides are essential for our understanding of plant development and for our use of plant biomass (largely wall material) in the food, agriculture, fabric, timber, biofuel and biocomposite industries. We present analytical techniques not only to define the fine chemical structures of individual cell wall polysaccharides but also to estimate the overall polysaccharide composition of cell wall preparations. The procedure covers the preparation of cell walls, together with gas chromatography-mass spectrometry (GC-MS)-based methods, for both the analysis of monosaccharides as their volatile alditol acetate derivatives and for methylation analysis to determine linkage positions between monosaccharide residues as their volatile partially methylated alditol acetate derivatives. Analysis time will vary depending on both the method used and the tissue type, and ranges from 2 d for a simple neutral sugar composition to 2 weeks for a carboxyl reduction/methylation linkage analysis.     

果胶甲酯酶抑制蛋白(PMEIs)的功能性研究进展

细胞壁是一种复杂的动态网络结构,在植物生长发育、胁迫应答和免疫抗性过程中起着重要的调控和防御作用。果胶(pectin)是细胞初生壁结构中多糖的主要成分之一;其中,同型半乳糖醛酸聚糖(HG)是果胶多糖组分中含量最丰富的线性聚合物。HG的甲基酯化程度变化会导致其酶解形成凝胶,从而影响果胶结构的稳定性。果胶甲酯酶抑制蛋白(PMEIs)通过翻译后机制调控果胶甲酯酶(PMEs)活性,微调果胶多糖甲酯化修饰平衡后,维持细胞壁的完整性和生物力学特性。研究发现,PMEI-PME互作调控果胶甲酯化修饰的稳态是决定细胞黏附、细胞壁硬度和弹性以及器官形态发生的关键因素,同时也是细胞壁应对逆境、释放抗性信号和免疫防御的分子模式。主要对PMEIs在调节植物器官发育过程和应对不同胁迫因子发挥的抗逆功能及调控机制等最新研究进展作出综述。鉴于PMEIs在木本植物中的体内生理活性和调控机制仍有待探索,可为后续填补该领域的研究空白提供理论依据和策略参考。