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.     

A genome-based approach to study the mechanisms by which cell-wall type is defined and constructed by the collaborative actions of cell-wall-related enzymes

The cellulose/xyloglucan framework underpins the cell wall of most flowering plants, and the processes of construction and restructuring of this framework are considered to be mediated by several different classes of enzymes such as cellulose synthetases, β-1,4 glucanases, xyloglucan endotransglucosylases/hydrolases (XTH) and expansins. The Arabidopsis sequencing project has revealed that these enzymes are encoded, without exception, by large multi-gene families. Comprehensive expression-analyses of the XTH gene family, as assisted by real-time RT-PCR procedure, have revealed that each member of the gene family exhibits an expression profile distinct from the other members. The results obtained thus far support the idea that each member of the XTH gene family is regulated specifically by different sets of plant hormones and is committed to a certain specific process in a specific tissue, at specific stages of development. Based on these considerations, we advance a hypothesis that the cell wall in a certain cell-type is constructed, maintained and restructured by a series of collaborative actions of a set of enzymes that are characteristic of the cell-wall type. This hypothesis assumes that a master gene, specific for each cell type, conducts a set of enzymes required for certain types of cell-wall structure and, thereby, defines the cell-wall type and, hence, cell type, during the process of plant development. Electronic Publication     

Comprehensive approach to genes involved in cell wall modifications in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

The plant cell wall is of supermolecular architecture, and is composed of various types of heterogeneous polymers. A few thousand enzymes and structural proteins are directly involved in the construction processes, and in the functional aspects of the dynamic architecture in Arabidopsis thaliana. Most of these proteins are encoded by multigene families, and most members within each family share significant similarities in structural features, but often exhibit differing expression profiles and physiological functions. Thus, for the molecular dissection of cell wall dynamics, it is necessary to distinguish individual members within a family of proteins. As a first step towards characterizing the processes involved in cell wall dynamics, we have manufactured a gene-specific 70-mer oligo microarray that consists of 765 genes classified into 30 putative families of proteins that are implicated in the cell wall dynamics of Arabidopsis. By using this array system, we identified several sets of genes that exhibit organ preferential expression profiles. We also identified gene sets that are expressed differentially at certain specific growth stages of the Arabidopsis inflorescence stem. Our results indicate that there is a division of roles among family members within each of the putative cell wall-related gene families.     

Major changes in the cell wall during silique development in Arabidopsis thaliana

Fruit development is a highly complex process, which involves major changes in plant metabolism leading to cell growth and differentiation. Changes in cell wall composition and structure play a major role in modulating cell growth. We investigated the changes in cell wall composition and the activities of associated enzymes during the dry fruit development of the model plant Arabidopsis thaliana. Silique development is characterized by several specific phases leading to fruit dehiscence and seed dispersal. We showed that early phases of silique growth were characterized by specific changes in non-cellulosic sugar content (rhamnose, arabinose, xylose, galactose and galacturonic acid). Xyloglucan oligosaccharide mass profiling further showed a strong increase in O-acetylated xyloglucans over the course of silique development, which could suggest a decreased capacity of xyloglucans to be associated with each other or to cellulose. The degree of methylesterification, mediated by the activity of pectin methylesterases (PMEs), decreased over the course of silique growth and dehiscence. The major changes in cell wall composition revealed by our analysis suggest that it could be major determinants in modulating cell wall rheology leading to growth or growth arrest.     

Gravity resistance, another graviresponse in plants--function of anti-gravitational polysaccharides]

The involvement of anti-gravitational polysaccharides in gravity resistance, one of two major gravity responses in plants, was discussed. In dicotyledons, xyloglucans are the only cell wall polysaccharides, whose level, molecular size, and metabolic turnover were modified under both hypergravity and microgravity conditions, suggesting that xyloglucans act as anti-gravitational polysaccharides. In monocotyledonous Poaceae, (1-->3),(1-->4)-beta glucans, instead of xyloglucans, were shown to play a role as anti-gravitational polysaccharides. These polysaccharides are also involved in plant responses to other environmental factors, such as light and temperature, and to some phytohormones, such as auxin and ethylene. Thus, the type of anti-gravitational polysaccharides is different between dicotyledons and Poaceae, but such polysaccharides are universally involved in plant responses to environmental and hormonal signals. In gravity resistance, the gravity signal may be received by the plasma membrane mechanoreceptors, transformed and transduced within each cell, and then may modify the processes of synthesis and secretion of the anti-gravitational polysaccharides and the cell wall enzymes responsible for their degradation, as well as the apoplastic pH, leading to the cell wall reinforcement. A series of events inducing gravity resistance are quite independent of those leading to gravitropism.     

Cellular localization of Arabidopsis xyloglucan endotransglycosylase-related proteins during development and after wind stimulation.

A gene family encoding xyloglucan endotransglycosylase (XET)-related proteins exists in Arabidopsis. TCH4, a member of this family, is strongly up-regulated by environmental stimuli and encodes an XET capable of modifying cell wall xyloglucans. To investigate XET localization we generated antibodies against the TCH4 carboxyl terminus. The antibodies recognized TCH4 and possibly other XET-related proteins. These data indicate that XETs accumulate in expanding cell, at the sites of intercellular airspace formation, and at the bases of leaves, cotyledons, and hypocotyls. XETs also accumulated in vascular tissue, where cell wall modifications lead to the formation of tracheary elements and sieve tubes. Thus, XETs may function in modifying cell walls to allow growth, airspace formation, the development of vasculature, and reinforcement of regions under mechanical strain. Following wind stimulation, overall XET levels appeared to decrease in the leaves of wind-stimulated plants. However, consistent with an increase in TCH4 mRNA levels following wind, there were regions that showed increased immunoreaction, including sites around cells of the pith parenchyma, between the vascular elements, and within the epidermis. These results indicate that TCH4 may contribute to the adaptive changes in morphogenesis that occur in Arabidopsis following exposure to mechanical stimuli.     

XTH acts at the microfibril-matrix interface during cell elongation

Sulphorhodamine-labelled oligosaccharides of xyloglucan are incorporated into the cell wall of Arabidopsis and tobacco roots, and of cultured Nicotiana tabacum cells by the transglucosylase (XET) action of XTHs. In the cell wall of diffusely growing cells, the subcellular pattern of XET action revealed a 'fibrillar' pattern, different from the xyloglucan localization. The fibrillar fluorescence pattern had no net orientation in spherical cultured cells. It changed to transverse to the long axis when the cells started to elongate, a feature mirroring the rearrangements of cortical microtubules and the accompanying cellulose deposition. Interference with the polymerization of microtubules and with cellulose deposition inhibited this strong and 'fibrillar'-organized XET-action, whereas interference with actin-polymerization only decreased the intensity of enzyme action. Epidermal cells of a mutant with reduced cellulose synthesis also had low XET action. Root hairs (tip-growing cells) exhibited high XET-action over all their length, but lacked the specific parallel pattern. In both diffuse- and tip-growing cell types extraction of the incorporated fluorescent xyloglucans by a xyloglucan-specific endoglucanase reduced the fluorescence, but the 'fibrillar' appearance in diffuse growing cells was not eliminated. These results show that XTHs act on the xyloglucans attached to cellulose microfibrils. After incorporation of the fluorescent oligosaccharides, the xyloglucans decorate the cellulose microfibrils and become inaccessible to hydrolytic enzymes.     

Effects of xylan and starch on secretome of the basidiomycete Phanerochaete chrysosporium grown on cellulose

Lignocellulosic biomass contains cellulose and xylan as major structural components, and starch as a storage polysaccharide. In the present study, we have used comparative secretomic analysis to examine the effects of xylan and starch on the expression level of proteins secreted by the basidiomycete Phanerochaete chrysosporium grown on cellulose,. Forty-seven spots of extracellular proteins expressed by P. chrysosporium separated by two-dimensional electrophoresis were identified by liquid chromatography-tandem mass spectrometry analysis. Addition of starch to the cellulolytic culture did not affect fungal growth significantly, but did decrease the production of total extracellular enzymes, including cellulases and xylanases. In contrast, addition of xylan increased mycelial volume and the production of extracellular proteins. Xylan increased synthesis of several glycoside hydrolase (GH) family 10 putative endoxylanases and a putative glucuronoyl esterase belonging to carbohydrate esterase family 15, for which plant cell wall xylan may be a substrate. Moreover, cellobiose dehydrogenase and GH family 61 proteins, which are known to promote cellulose degradation, were also increased in the presence of xylan. These enzymes may contribute to degradation by the fungus of not only cellulose but also complex carbohydrate components of the plant cell wall.     

Ectomycorrhizas of two species of <Emphasis Type="Italic">Tuber</Emphasis> (clade Puberulum) in the Mexican subtropical cloud forest

Expansins are non-enzymatic cell wall proteins that mediate plant growth by catalyzing loosening of cell walls without lysing the wall polymers. Advances in the field of bioinformatics have facilitated the prediction of the members of expansin gene family across several model plants. Expansins constitutes into four sub-families; α-expansin, β-expansin, expansin-like A and expansin-like B. Biological functions of expansin gene family include diverse aspects of plant growth and development, shoot and root elongation, leaf morphogenesis, flower and fruit development, embryogenesis, pollen tube growth, stress tolerance, etc. Recent studies have demonstrated the role of expansins in plant-symbiotic interactions. The present review reveals the factors that govern plant-arbuscular mycorrhizal fungi (AMF) and legume-rhizobia symbioses; and the genes that participate in these diverse symbiont interactions. Further, we focus on the expression profiles and the functions of expansins during plant-AMF and legume-rhizobia interactions. The key roles of expansin proteins during AMF invasion, arbuscule formation, rhizobial infection and nodule organogenesis were uncovered during symbioses. This review summarizes discoveries that support the key and versatile roles of various expansin members in the plant-mycorrhizal and legume-rhizobial symbioses.     

Ethylene regulation of fruit softening and cell wall disassembly in Charentais melon

Cell wall disassembly in ripening fruit is highly complex, involving the dismantling of multiple polysaccharide networks by diverse families of wall-modifying proteins. While it has been reported in several species that multiple members of each such family are expressed in the same fruit tissue, it is not clear whether this reflects functional redundancy, with protein isozymes from a single enzyme class performing similar roles and contributing equally to wall degradation, or whether they have discrete functions, with some isoforms playing a predominant role. Experiments reported here sought to distinguish between cell wall-related processes in ripening melon that were softening-associated and softening-independent. Cell wall polysaccharide depolymerization and the expression of wall metabolism-related genes were examined in transgenic melon (Cucumis melo var. cantalupensis Naud.) fruit with suppressed expression of the 1-aminocyclopropane-1-carboxylate oxidase (ACO) gene and fruits treated with ethylene and 1-methylcyclopropene (1-MCP). Softening was completely inhibited in the transgenic fruit but was restored by treatment with exogenous ethylene. Moreover, post-harvest application of 1-MCP after the onset of ripening completely halted subsequent softening, suggesting that melon fruit softening is ethylene-dependent. Size exclusion chromatography of cell wall polysaccharides, from the transgenic fruits, with or without exogenous ethylene, indicated that the depolymerization of both pectins and xyloglucans was also ethylene dependent. However, northern analyses of a diverse range of cell wall-related genes, including those for polygalacturonases, xyloglucan endotransglucosylase/hydrolases, expansin, and beta-galactosidases, identified specific genes within single families that could be categorized as ethylene-dependent, ethylene-independent, or partially ethylene-dependent. These results support the hypothesis that while individual cell wall-modifying proteins from each family contribute to cell wall disassembly that accompanies fruit softening, other closely related family members are regulated in an ethylene-independent manner and apparently do not directly participate in fruit softening.     

The COBRA family of putative GPI-anchored proteins in Arabidopsis. A new fellowship in expansion

Identification of regulatory molecules that determine the extent and direction of expansion is necessary to understand how cell morphogenesis is controlled in plants. We recently identified COB (COBRA) as a key regulator of the orientation of cell expansion in the root. Analysis of the Arabidopsis genome sequence indicated that COB belongs to a multigene family consisting of 12 members, all predicted to encode glycosylphosphatidylinositol-anchored proteins. All but two of the COBL (COB-like) genes are expressed in most organs examined, suggesting possible redundancy. Sequence comparisons, phylogenetic analyses, and exon-intron positions revealed that the COB family is composed of two main subgroups sharing a common architecture, one subgroup being characterized by an additional N-terminal domain. Identification of expressed sequence tags corresponding to potential orthologs in other plant species suggested that COB-related functions are required in all vascular plants. Together, these results indicate that COB family members are likely to be important new players at the plasma membrane-cell wall interface.     

Proteinases in fungal morphogenesis

Fungal morphogenesis is a regulated series of events, leading to changes from one state to another, in which proteolysis could be regarded as one of the controlling functions. Proteinases are essential for the supply of amino acids, selective inactivation of specific growth phase proteins not required during development and for the activation and modification of the enzymes involved in cell wall synthesis. A critical evaluation of the role of proteinases as a biochemical correlate in fungal morphogenesis is discussed.     

The Arabidopsis XET-related gene family: environmental and hormonal regulation of expression

Enzymes that modify cell wall components most likely play critical roles in altering size, shape, and physical properties of plant cells. Regulation of such modifying activity is expected to be important during morphogenesis and in eliciting developmental and physiological alterations that arise in response to environmental conditions. Previous work has shown that the Arabidopsis TCH4 gene encodes a xyloglucan endotransglycosylase (XET) which acts on the major hemicellulose of the plant cell wall. The expression of TCH4 is dramatically upregulated in response to several environmental stimuli (including touch, wind, darkness, heat shock, and cold shock) as well as the growth-enhancing hormones, auxin and brassinosteroids. This paper reports the presence of an extensive X ET ,related (XTR) gene family in Arabidopsis. In addition to TCH4, this family includes two previously identified genes, EXT and Meri-5, and at least five additional genes. The cDNAs of the XTR family share between 46 and 79% sequence identity and the predicted XTR proteins share from 37 to 84% identity. All eight proteins include potential N-terminal signal sequences and most have a conserved motif (DEIDFEFLG) that is also found in Bacillusβ-glucanase and may be important for enzyme activity. The members of the XTR gene family are differentially sensitive to environmental and hormonal stimuli. Magnitude and kinetics of regulation are distinct for the different genes. Differential regulation of expression of this complex gene family suggests a recruitment of related, yet distinct, cell wall-modifying enzymes that may control the properties of cell walls and tissues during development and in response to environmental cues.     

LytM factors affect the recruitment of autolysins to the cell division site in Caulobacter crescentus

Most bacteria possess a peptidoglycan cell wall that determines their morphology and provides mechanical robustness during osmotic challenges. The biosynthesis of this structure is achieved by a large set of synthetic and lytic enzymes with varying substrate specificities. Although the biochemical functions of these proteins are conserved and well‐investigated, the precise roles of individual factors and the regulatory mechanisms coordinating their activities in time and space remain incompletely understood. Here, we comprehensively analyze the autolytic machinery of the alphaproteobacterial model organism Caulobacter crescentus, with a specific focus on LytM‐like endopeptidases, soluble lytic transglycosylases and amidases. Our data reveal a high degree of redundancy within each protein family but also specialized functions for individual family members under stress conditions. In addition, we identify two lytic transglycosylases and an amidase as new divisome components that are recruited to midcell at distinct stages of the cell cycle. The midcell localization of these proteins is affected by two LytM factors with degenerate catalytic domains, DipM and LdpF, which may serve as regulatory hubs coordinating the activities of multiple autolytic enzymes during cell constriction and fission respectively. These findings set the stage for in‐depth studies of the molecular mechanisms that control peptidoglycan remodeling in C. crescentus.     

Glycosylphosphatidylinositol-anchored proteins are required for cell wall synthesis and morphogenesis in Arabidopsis

Mutations at five loci named PEANUT1-5 (PNT) were identified in a genetic screen for radially swollen embryo mutants. pnt1 cell walls showed decreased crystalline cellulose, increased pectins, and irregular and ectopic deposition of pectins, xyloglucans, and callose. Furthermore, pnt1 pollen is less viable than the wild type, and pnt1 embryos were delayed in morphogenesis and showed defects in shoot and root meristems. The PNT1 gene encodes the Arabidopsis thaliana homolog of mammalian PIG-M, an endoplasmic reticulum-localized mannosyltransferase that is required for synthesis of the glycosylphosphatidylinositol (GPI) anchor. All five pnt mutants showed strongly reduced accumulation of GPI-anchored proteins, suggesting that they all have defects in GPI anchor synthesis. Although the mutants are seedling lethal, pnt1 cells are able to proliferate for a limited time as undifferentiated callus and do not show the massive deposition of ectopic cell wall material seen in pnt1 embryos. The different phenotype of pnt1 cells in embryos and callus suggest a differential requirement for GPI-anchored proteins in cell wall synthesis in these two tissues and points to the importance of GPI anchoring in coordinated multicellular growth.     

Microtubules and the tax payer

Plant microtubules have evolved into a versatile tool to link environmental signals into flexible morphogenesis. Cortical microtubules define the axiality of cell expansion by control of cellulose orientation. Plant-specific microtubule structures such as preprophase band and phragmoplast determine symmetry and axiality of cell divisions. In addition, microtubules act as sensors and integrators for stimuli such as mechanic load, gravity, but also osmotic stress, cold and pathogen attack. Many of these functions are specific for plants and involve specific proteins or the recruitment of proteins to new functions. The review aims to ventilate the potential of microtubule-based strategies for biotechnological application by highlighting representative case studies. These include reorientation of cortical microtubules to increase lodging resistance, control of microtubule dynamics to alter the gravity-dependent orientation of leaves, the use of microtubules as sensitive thermometers to improve adaptive cold tolerance of chilling and freezing sensitive plants, the reduction of microtubule treadmilling to inhibit cell-to-cell transport of plant viruses, or the modulation of plant defence genes by pharmacological manipulation of microtubules. The specificity of these responses is controlled by a great variety of specific associated proteins opening a wide field for biotechnological manipulation of plant architecture and stress tolerance.     

Microtubules and the tax payer

Plant microtubules have evolved into a versatile tool to link environmental signals into flexible morphogenesis. Cortical microtubules define the axiality of cell expansion by control of cellulose orientation. Plant-specific microtubule structures such as preprophase band and phragmoplast determine symmetry and axiality of cell divisions. In addition, microtubules act as sensors and integrators for stimuli such as mechanic load, gravity, but also osmotic stress, cold and pathogen attack. Many of these functions are specific for plants and involve specific proteins or the recruitment of proteins to new functions. The review aims to ventilate the potential of microtubule-based strategies for biotechnological application by highlighting representative case studies. These include reorientation of cortical microtubules to increase lodging resistance, control of microtubule dynamics to alter the gravity-dependent orientation of leaves, the use of microtubules as sensitive thermometers to improve adaptive cold tolerance of chilling and freezing sensitive plants, the reduction of microtubule treadmilling to inhibit cell-to-cell transport of plant viruses, or the modulation of plant defence genes by pharmacological manipulation of microtubules. The specificity of these responses is controlled by a great variety of specific associated proteins opening a wide field for biotechnological manipulation of plant architecture and stress tolerance.     

Differentiated roles for MreB‐actin isologues and autolytic enzymes in Bacillus subtilis morphogenesis

Cell morphogenesis in most bacteria is governed by spatiotemporal growth regulation of the peptidoglycan cell wall layer. Much is known about peptidoglycan synthesis but regulation of its turnover by hydrolytic enzymes is much less well understood. Bacillus subtilis has a multitude of such enzymes. Two of the best characterized are CwlO and LytE: cells lacking both enzymes have a lethal block in cell elongation. Here we show that activity of CwlO is regulated by an ABC transporter, FtsEX, which is required for cell elongation, unlike cell division as in Escherichia coli. Actin‐like MreB proteins are thought to play a key role in orchestrating cell wall morphogenesis. B. subtilis has three MreB isologues with partially differentiated functions. We now show that the three MreB isologues have differential roles in regulation of the CwlO and LytE systems and that autolysins control different aspects of cell morphogenesis. The results add major autolytic activities to the growing list of functions controlled by MreB isologues in bacteria and provide new insights into the different specialized functions of essential cell wall autolysins.     

OsCD1 encodes a putative member of the cellulose synthase-like D sub-family and is essential for rice plant architecture and growth

The cell wall plays important roles in plant architecture and morphogenesis. The cellulose synthase-like super-families were reported to contain glycosyltransferases motif and are required for the biosynthesis of cell wall polysaccharides. Here, we describe a curled leaf and dwarf mutant, cd1, in rice, which exhibits multiple phenotypic traits such as the reduction of plant height and leaf width, curled leaf morphology and a decrease in the number of grains and in the panicle length. Map-based cloning indicates that a member of the cellulose synthase-like D (CSLD) group is a candidate for OsCD1. RNAi transgenic plants with the candidate CSLD gene display a similar phenotype to the cd1 mutant, suggesting that OsCD1 is a member of the CSLD sub-family. Furthermore, sequence analysis indicates that OsCD1 contains the common D,D,D,QXXRW motif, which is a feature of the cellulose synthase-like super-family. Analysis of OsCD1 promoter with GUS fusion expression shows that OsCD1 exhibits higher expression in young meristem tissues such as fresh roots, young panicle and stem apical meristem. Cell wall composition analysis reveals that cellulose content and the level of xylose are significantly reduced in mature culm owing to loss of OsCD1 function. Take together, the work presented here is useful for expanding the understanding of cell wall biosynthesis.