Plant cells encase themselves within a complex polysaccharide wall, which constitutes the raw material that is used to manufacture textiles, paper, lumber, films, thickeners and other products. The plant cell wall is also the primary source of cellulose, the most abundant and useful biopolymer on the Earth. The cell wall not only strengthens the plant body, but also has key roles in plant growth, cell differentiation, intercellular communication, water movement and defence. Recent discoveries have uncovered how plant cells synthesize wall polysaccharides, assemble them into a strong fibrous network and regulate wall expansion during cell growth.

/pdf/growth-of-the-plant-cell-wall-4k81okl2ih.pdf

Growth of the plant cell wall

One of the defining features of plants is a body plan based on the physical properties of cell walls. Structural analyses of the polysaccharide components, combined with high-resolution imaging, have provided the basis for much of the current understanding of cell walls. The application of genetic methods has begun to provide new insights into how walls are made, how they are controlled, and how they function. However, progress in integrating biophysical, developmental, and genetic information into a useful model will require a system-based approach.

/pdf/toward-a-systems-approach-to-understanding-plant-cell-walls-375h7arxvl.pdf

Toward a Systems Approach to Understanding Plant Cell Walls

https://www.cell.com/cell/pdf/S0092-8674(02)00721-3.pdf

BES1 Accumulates in the Nucleus in Response to Brassinosteroids to Regulate Gene Expression and Promote Stem Elongation

Cellulose microfibrils play essential roles in the organization of plant cell walls, thereby allowing a growth habit based on turgor. The fibrils are made by 30 nm diameter plasma membrane complexes composed of approximately 36 subunits representing at least three types of related CESA proteins. The complexes assemble in the Golgi, where they are inactive, and move to the plasma membrane, where they become activated. The complexes move through the plasma membrane during cellulose synthesis in directions that coincide with the orientation of microtubules. Recent, simultaneous, live-cell imaging of cellulose synthase and microtubules indicates that the microtubules exert a direct influence on the orientation of cellulose deposition. Genetic studies in Arabidopsis have identified a number of genes that contribute to the overall process of cellulose synthesis, but the role of these proteins is not yet known.

Cellulose Synthesis in Higher Plants

Proteome analysis implies the ability to separate proteins as a first step prior to characterization. Thus, the overall performance of the analysis strongly depends on the performance of the separation tool, usually two-dimensional electrophoresis. This review shows how two-dimensional electrophoresis performs with membrane proteins from bacteria or animal or vegetable cells and tissues, the recent progress in this field, and it examines future prospects in this area.

Membrane proteins and proteomics: un amour impossible?

Endo‐1,4‐β‐d‐glucanases (EGases) form a large family of hydrolytic enzymes in prokaryotes and eukaryotes. In higher plants, potential substrates in vivo are xyloglucan and non‐crystalline cellulose in the cell wall. Gene expression patterns suggest a role for EGases in various developmental processes such as leaf abscission, fruit ripening and cell expansion. Using Arabidopsis thaliana genetics, we demonstrate the requirement of a specialized member of the EGase family for the correct assembly of the walls of elongating cells. KORRIGAN ( KOR ) is identified by an extreme dwarf mutant with pronounced architectural alterations in the primary cell wall. The KOR gene was isolated and encodes a membrane‐anchored member of the EGase family, which is highly conserved between mono‐ and dicotyledonous plants. KOR is located primarily in the plasma membrane and presumably acts at the plasma membrane–cell wall interface. KOR mRNA was found in all organs examined, and in the developing dark‐grown hypocotyl, mRNA levels were correlated with rapid cell elongation. Among plant growth factors involved in the control of hypocotyl elongation (auxin, gibberellins and ethylene) none significantly influenced KOR ‐mRNA levels. However, reduced KOR ‐mRNA levels were observed in det2 , a mutant deficient for brassinosteroids. Although the in vivo substrate remains to be determined, the mutant phenotype is consistent with a central role for KOR in the assembly of the cellulose–hemicellulose network in the expanding cell wall.

/pdf/a-plasma-membrane-bound-putative-endo-1-4-b-d-glucanase-is-2b75pmzu4q.pdf

A plasma membrane‐bound putative endo‐1,4‐β‐D‐glucanase is required for normal wall assembly and cell elongation in Arabidopsis

Korrigan (kor) is a dwarf mutant of Arabidopsis thaliana (L.) Heynh. that is deficient in a membrane-bound endo-1,4-β-glucanase. The effect of the mutation on the pectin network has been studied in kor by microscopical techniques associated with various probes specific for different classes of pectic polysaccharides. The localisation of native crystalline cellulose was also examined using the cellobiohydrolase I-gold probe. The investigations were focused on the external cell walls of the epidermis, a cell layer that, in a number of plant species, has been shown to be growth limiting. Anionic sites associated with pectic polymers were quantified using the cationic gold probe. Homogalacturonans were quantified using polyclonal anti-polygalacturonic acid/rhamnogalacturonan I antibodies recognising polygalacturonic acid, and monoclonal JIM7 and JIM5 antibodies recognising homogalacturonans with a high or low degree of methyl-esterification, respectively. Rhamnogalacturonans were quantified with two monoclonal antibodies, LM5, recognising β-1,4 galactan side chains of rhamnogalacturonan I, and CCRCM2. Our results show a marked increase in homogalacturonan epitopes and a decrease in rhamnogalacturonan epitopes in kor compared to the wild type. A substantial decrease in cellobiohydrolase I-gold labelling was also observed in the mutant cell walls. These findings demonstrate that a deficiency in an endo-1,4-β-glucanase, which is in principle not directly implicated in pectin metabolism, can induce important changes in pectin composition in the primary cell wall. The changes indicate the existence of feedback mechanisms controlling the synthesis and/or deposition of pectic polysaccharides in primary cell walls.

Altered pectin composition in primary cell walls of korrigan, a dwarf mutant of Arabidopsis deficient in a membrane-bound endo-1,4-β-glucanase

Flax fibers have been the subject of many biochemical studies, which revealed that cellulose and pectins are the major constituents of their walls. In contrast, little is known about the location of these polymers within the walls of mature fibers by microscopic methods. This has been technically hampered by the very thick secondary wall of fibers, resulting in inadequate tissue preservation unsuitable for immunogold microscopy. In this study, we adapted the basic chemical fixation, dehydration and infiltration methods to achieve a good preservation of the cell structures of mature fibers and reduced damage to antigens. We were able to apply postembedding immunocytochemical techniques to map the location of various pectic epitopes within the walls of mature fibers. Our immunolabeling data show that homogalacturonans were exclusively found in the middle lamellae and the cell junctions but were not detectable in the secondary wall. In contrast, rhamnogalacturonan I (RG I)-associated epitopes, as well as galactan and arabinan epitopes, were abundantly distributed over the secondary wall of mature fibers.

https://journals.sagepub.com/doi/pdf/10.1177/002215540104901206

Microscopic studies on mature Flax fibers embedded in LR white: Immunogold localization of cell wall matrix polysaccharides

The deposition and formation of a thick secondary wall is a major event in the differentiation of flax (Linum usitatissimum) fibers. This wall is cellulose-rich; but it also contains significant amounts of other matrix polymers which are noncellulosic such as pectins. We have used immunocytochemical techniques with antibodies specific for various epitopes associated with either pectins or arabinogalactan proteins (AGPs) to investigate the distribution of these polymers within the walls of differentiating young fibers of 1- and 2-week-old plants. Our results show that different epitopes exhibit distinct distribution patterns within fiber walls. Unesterified pectins recognized by polygalacturonic acid-rhamnogalacturonan I (PGA/RG-I) antibodies and rhamnogalacturonan II recognized by anti-RG-II-borate complex antibodies are localized all over the secondary wall of fibers. PGA/RG-I epitopes, but not RG-II epitopes, are also present in the middle lamellae and cell junctions. In marked contrast, β-(1→4) galactans recognized by the LM5 monoclonal antibody and AGP epitopes recognized by anti-β-(1→6) galactan and LM2 antibodies are primarily located in the half of the secondary wall nearest the plasma membrane. LM2 epitopes, present in 1-week-old fibers, are undetectable later in development, suggesting a regulation of the expression of certain AGP epitopes. In addition, localization of cellulose with the cellobiohydrolase I-gold probe reveals distinct subdomains within the secondary walls of young fibers. These findings indicate that, in addition to cellulose, early-developing flax fibers synthesize and secrete different pectin and AGP molecules.

Immunocytochemical characterization of early-developing flax fiber cell walls

Four proteins were isolated from depectinised elementary fibres of flax (Linum usitatissimum L.), using either alkali or cellulase digestion treatments. All the four proteins were characterized by a deficiency or low contents of hydroxyproline and by high levels of glutamic acid/glutamine and/or aspartic acid/asparagine. The two proteoglycans solubilized with cellulase strongly reacted with β-glucosyl Yariv reagent but not with α-glucosyl Yariv reagent and contained appreciable amounts of alanine, glycine, serine and threonine, suggesting a relationship with cell wall hydroxyproline-deficient arabinogalactan-proteins. The two alkali-extracted proteins did not show any reaction with β-glucosyl Yariv dye. Due to the harsh treatment, they might only partially represent the original proteins. Due to its high level of glycine (41%), one of these proteins might be classified as a glycine-rich protein. The latter polypeptide, of low molecular molar mass, contained 14.6% leucine and might consist of a domain related to leucine-rich proteins. The data show that these proteins and arabinogalactan-protein-like proteoglycans were strongly associated with the secondary walls of flax fibres. Their presence in small amounts (0.1–0.4%), raises the problem of their putative structural role.

Isabelle His

Papers

A plasma membrane‐bound putative endo‐1,4‐β‐D‐glucanase is required for normal wall assembly and cell elongation in Arabidopsis

Altered pectin composition in primary cell walls of korrigan, a dwarf mutant of Arabidopsis deficient in a membrane-bound endo-1,4-β-glucanase

Microscopic studies on mature Flax fibers embedded in LR white: Immunogold localization of cell wall matrix polysaccharides

Immunocytochemical characterization of early-developing flax fiber cell walls

Identification and partial characterization of proteins and proteoglycans encrusting the secondary cell walls of flax fibres.