This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of ...the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.
•Cellulose interactions with the matrix depend on microfibril shape.•Physical data point to a microfibril made of 18–24 chains, not 36 chains.•Recent structures for cellulose synthases yield insights ...into glucan polymerization.•Revised roles for xyloglucan and pectin in wall structure are emerging.•Direct contact between cellulose microfibrils may control wall extensibility.
The cellulose microfibril has more subtlety than is commonly recognized. Details of its structure may influence how matrix polysaccharides interact with its distinctive hydrophobic and hydrophilic surfaces to form a strong yet extensible structure. Recent advances in this field include the first structures of bacterial and plant cellulose synthases and revised estimates of microfibril structure, reduced from 36 to 18 chains. New results also indicate that cellulose interactions with xyloglucan are more limited than commonly believed, whereas pectin–cellulose interactions are more prevalent. Computational results indicate that xyloglucan binds tightest to the hydrophobic surface of cellulose microfibrils. Wall extensibility may be controlled at limited regions (‘biomechanical hotspots’) where cellulose–cellulose contacts are made, potentially mediated by trace amounts of xyloglucan.
The advent of user-friendly instruments for measuring force/deflection curves of plant surfaces at high spatial resolution has resulted in a recent outpouring of reports of the 'Young's modulus' of ...plant cell walls. The stimulus for these mechanical measurements comes from biomechanical models of morphogenesis of meristems and other tissues, as well as single cells, in which cell wall stress feeds back to regulate microtubule organization, auxin transport, cellulose deposition, and future growth directionality. In this article I review the differences between elastic modulus and wall extensibility in the context of cell growth. Some of the inherent complexities, assumptions, and potential pitfalls in the interpretation of indentation force/deflection curves are discussed. Reported values of elastic moduli from surface indentation measurements appear to be 10- to >1000-fold smaller than realistic tensile elastic moduli in the plane of plant cell walls. Potential reasons for this disparity are discussed, but further work is needed to make sense of the huge range in reported values. The significance of wall stress relaxation for growth is reviewed and connected to recent advances and remaining enigmas in our concepts of how cellulose, hemicellulose, and pectins are assembled to make an extensible cell wall. A comparison of the loosening action of α-expansin and Cel12A endoglucanase is used to illustrate two different ways in which cell walls may be made more extensible and the divergent effects on wall mechanics.
•The distinction between expansins and expansin-like proteins is clarified.•Site-directed mutagenesis reveals expansin's complex binding to plant cell walls.•Expansin action stems from its binding to ...selective sites on cellulose microfibrils.•Ectopic expression of expansin enhances tolerance to water stress.
Expansins were discovered two decades ago as cell wall proteins that mediate acid-induced growth by catalyzing loosening of plant cell walls without lysis of wall polymers. In the interim our understanding of expansins has gotten more complex through bioinformatic analysis of expansin distribution and evolution, as well as through expression analysis, dissection of the upstream transcription factors regulating expression, and identification of additional classes of expansin by sequence and structural similarities. Molecular analyses of expansins from bacteria have identified residues essential for wall loosening activity and clarified the bifunctional nature of expansin binding to complex cell walls. Transgenic modulation of expansin expression modifies growth and stress physiology of plants, but not always in predictable or even understandable ways.
•Cellulose microfibrils have hydrophobic and hydrophilic faces.•The different cellulose faces may facilitate formation of anisotropic lamellae.•Microfibrils move differently during wall stretch ...versus creep that mimics growth.•Side-by-siding sliding of microfibrils may be key to wall enlargement.•Wall indentation stiffness varies with turgor pressure, cell shape and tissue context.
This article briefly reviews recent advances in nano-scale and micro-scale assessments of primary cell wall structure, mechanical behaviors and expansive growth. Cellulose microfibrils have hydrophobic and hydrophilic faces which may selectively bind different matrix polysaccharides and adjacent microfibrils. These distinctive binding interactions may guide partially aligned cellulose microfibrils in primary cell walls to form a planar, load-bearing network within each lamella of polylamellate walls. Consideration of expansive growth of cross-lamellate walls leads to a surprising inference: side-by-side sliding of microfibrils may be a key rate-limiting physical step, potentially targeted by specific wall loosening agents. Atomic force microscopy shows different patterns of microfibril movement during force-driven extension versus enzymatic loosening. Consequently, simulations of cell growth as elastic deformation of isotropic cell walls may need to be augmented to incorporate the distinctive behavior of growing cell walls.
Plant cells build nanofibrillar walls that are central to plant growth, morphogenesis and mechanics. Starting from simple sugars, three groups of polysaccharides, namely, cellulose, hemicelluloses ...and pectins, with very different physical properties are assembled by the cell to make a strong yet extensible wall. This Review describes the physics of wall growth and its regulation by cellular processes such as cellulose production by cellulose synthase, modulation of wall pH by plasma membrane H
-ATPase, wall loosening by expansin and signalling by plant hormones such as auxin and brassinosteroid. In addition, this Review discusses the nuanced roles, properties and interactions of cellulose, matrix polysaccharides and cell wall proteins and describes how wall stress and wall loosening cooperatively result in cell wall growth.
Xyloglucan is widely believed to function as a tether between cellulose microfibrils in the primary cell wall, limiting cell enlargement by restricting the ability of microfibrils to separate ...laterally. To test the biomechanical predictions of this "tethered network" model, we assessed the ability of cucumber (Cucumis sativus) hypocotyl walls to undergo creep (long-term, irreversible extension) in response to three family-12 endo-β-1, 4-glucanases that can specifically hydrolyze xyloglucan, cellulose, or both. Xyloglucan-specific endoglucanase (XEG from Aspergillus aculeatus) failed to induce cell wall creep, whereas an endoglucanase that hydrolyzes both xyloglucan and cellulose (Cell2A from Hypocrea jecorina) induced a high creep rate. A cellulose-specific endoglucanase (CEG from Aspergillus niger) did not cause cell wall creep, either by itself or in combination with XEG. Tests with additional enzymes, including a family-5 endoglucanase, confirmed the conclusion that to cause creep, endoglucanases must cut both xyloglucan and cellulose. Similar results were obtained with measurements of elastic and plastic compliance. Both XEG and Cell2A hydrolyzed xyloglucan in intact walls, but Cell2A could hydrolyze a minor xyloglucan compartment recalcitrant to XEG digestion. Xyloglucan involvement in these enzyme responses was confirmed by experiments with Arabidopsis (Arabidopsis thaliana) hypocotyls, where Cell2A induced creep in wild-type but not in xyloglucan-deficient (xxt1/xxt2) walls. Our results are incompatible with the common depiction of xyloglucan as a load-bearing tether spanning the 20-to 40-nm spacing between cellulose microfibrils, but they do implicate a minor xyloglucan component in wall mechanics. The structurally important xyloglucan may be located in limited regions of tight contact between microfibrils.
Microbial Expansins Cosgrove, Daniel J
Annual review of microbiology,
09/2017, Letnik:
71
Journal Article
Recenzirano
Expansins are small proteins that loosen plant cell walls and cellulosic materials without lytic activity. First discovered in plants, expansin genes are found in the genomes of numerous bacteria and ...fungi that interact with plants in pathogenic and mutualistic patterns, as well as in microbes that feed on plant debris. Horizontal gene transfer from plants to microbes and between microbes accounts for expansins' irregular taxonomic distribution. Expansins facilitate plant colonization by Bacillus, Clavibacter, and Trichoderma species, a list likely to grow as knowledge of microbial expansin function deepens. Studies have documented a synergistic action of expansins for cellulose digestion by cellulases, but only rarely to an extent that is commercially relevant. Expansins' biophysical actions remain enigmatic because of limited understanding of cell wall structure. Deeper understanding of microbial expansins may lead to novel approaches for biomass deconstruction and biocontrol of plant diseases.
The discovery of xyloglucan and its ability to bind tightly to cellulose has dominated our thinking about primary cell wall structure and its connection to the mechanism of cell enlargement for 40 ...years. Gene discovery has advanced our understanding of the synthesis of xyloglucan in the past decade, and at the same time new and unexpected results indicate that xyloglucan's role in wall structure and wall extensibility is more subtle than commonly believed. Genetic deletion of xyloglucan synthesis does not greatly disable cell wall functions. Nuclear magnetic resonance studies indicate that pectins, rather than xyloglucans, make the majority of contacts with cellulose surfaces. Xyloglucan binding may be selective for specific (hydrophobic) surfaces on the cellulose microfibril, whose structure is more complex than is commonly portrayed in cell wall cartoons. Biomechanical assessments of endoglucanase actions challenge the concept of xyloglucan tethering. The mechanically important xyloglucan is restricted to a minor component that appears to be closely intertwined with cellulose at limited sites ('biomechanical hotspots') of direct microfibril contact; these may be the selective sites of cell wall loosening by expansins. These discoveries indicate that wall extensibility is less a matter of bulk viscoelasticity of the matrix polymers and more a matter of selective control of slippage and separation of microfibrils at specific and limited sites in the wall.
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