Functions Of Plant Cell Walls In Planta

Cell walls have long been considered as dead extracellular material possessing solely static structural roles, namely to counteract the turgor pressure and thus give strength and form to plant cells, tissues, organs, and organisms. This view has changed dramatically with the insight modern analytical techniques and molecular probes have allowed into the fine structure and apparent highly complex spatial and temporal regulation of cell wall components and architecture (1). It was concluded that the cell wall must have important functional roles, e.g., in cell-cell communication, transport of metabolites, differentiation, and development. To indicate this new appreciation of the cell wall as a dynamic and regulatory extracellular organelle, some authors prefer to call it an extracellular matrix. The growing realization that this matrix forms an integral and essential part of the plant cell may eventually even lead us to abandon the term extracellular in favor of, e.g., pericellular.

In spite of these considerations, plant cell walls do play important static structural roles. First, they have to bear the turgor pressure, which results in enormous tangentially oriented stress forces in the cell wall plane (2). This stress is taken up by the fibrillar component in the cell wall, namely the cellulose microfibrils, which are interconnected and held in place by hemi-celluloses. Second, the cell wall has to take up the pressure exerted on a plant cell by the surrounding tissue. This pressure is taken up by the matrix in which the fibrils are embedded, made up of polysaccharides containing uronic acids, namely pectins and glucuronoarabinoxylans, or—more precisely—by the water molecules held in the cell walls by these negatively

Figure 1 In vivo roles and ex vivo uses of plant cell walls and their components. Cell walls fulfill many roles—both structural and functional—in the life of a plant, and different components are responsible for the different roles. Biotechnology may aim at improving these in vivo roles according to human interest, e.g., increasing resistance of crop plants against pathogens (HR, hypersensitive reaction). Ex vivo, different cell wall components can be used according to their physical properties and, again, biotechnology may aim at improving these properties.

Figure 1 In vivo roles and ex vivo uses of plant cell walls and their components. Cell walls fulfill many roles—both structural and functional—in the life of a plant, and different components are responsible for the different roles. Biotechnology may aim at improving these in vivo roles according to human interest, e.g., increasing resistance of crop plants against pathogens (HR, hypersensitive reaction). Ex vivo, different cell wall components can be used according to their physical properties and, again, biotechnology may aim at improving these properties.

charged polymers. In extreme cases, this water is replaced by the incorporation of lignin, a heavily cross-linked three-dimensional polyphenolic network able to withstand very high pressures.

As the hydrophobic lignin replaces the water in the cell wall, lignification also renders cell walls impermeable to water. Other ways to achieve the same goal—impermeability to water—are the incrustation with suberin or the deposition of cutin along with cuticular and epicuticular waxes. These layers form essential barriers to the free transport of water and solutes, such as nutrients and assimilates. Suberin in the Casparian strand of the endo-dermis effectively seals the central cylinder of the root from the surrounding cortex tissue, allowing control over the uptake of nutrients and water from the soil. Lignin in the tracheary elements of the xylem allows water and solute transport from the roots to the aboveground plant parts by stabilizing the vessel cell walls against the strong internal negative pressure of the transpiration stream and by preventing losses to the surrounding tissue. Cutin and the waxes protect the aboveground plant organs from desiccation by restricting the transpiration to the stomata with their regulated diffusion resistance.

Cell walls restrict and dictate cell size and form (3). Consequently, growth is possible only when the cell walls partially yield to turgor pressure. In effect, plant growth is growth of the plant cell walls. Similarly, the shape of a plant is formed by the shape of the plant cell walls. The processes taking place in the cell wall during growth have been the subject of intense research over many decades (2). Although we are still far from understanding this highly complex and tightly regulated process, it appears today that the hemicelluloses interconnecting cellulose fibrils play a crucial role in this process. Enzymes loosening and retightening these connections, such as ex-pansins and xyloglucanendotransglycosylases, are critically involved in cell wall growth (4-6). Shape of a plant cell, on the other hand, appears to be dictated by the orientation of the cellulose fibrils that is most likely brought about by the movement of the cellulose synthase complex in the plasma membrane, which may or may not be guided and driven by the cytoskele-ton (7,8).

In contrast to the preceding morphological roles of plant cell walls, our understanding of the functional roles of cell walls in the physiology of a plant still remains rather patchy. It is clear that cell walls both separate and connect neighboring cells in a tissue, and it can be anticipated that information and metabolites travel from cell to cell both symplastically, i.e., via plasmodesmata, and apoplastically, i.e., across the cell wall. One example of apoplastic transport of metabolites is sucrose on its way from source mesophyll cells to the phloem and from the phloem to sink cells in the root or fruit tissues (9). Sucrose in the cell wall may be hydrolyzed into its constituent hexoses by apoplastic invertase (10), and the glucose produced can act as a local signal in sugar sensing influencing the metabolic state of the plant cells (11,12). Another example of information-bearing molecules traveling in the apoplast is auxin, which is thought to be moved through the plant by polar secretion into the cell wall and uptake from the cell wall by adjacent cells (13).

Although the water present in the cell wall provides the space for intercellular travel of molecules, transport in the cell wall is not unrestricted. We have already seen that special elaborations of the cell wall, such as suberin in the Casparian band, can completely stop flow of water and solutes. In addition, the pore size in the cell wall determines the diffusion rate of larger molecules, such as proteins. The size exclusion limit of plant cell walls has been estimated for globular proteins at between 10 and 30 (and sometimes up to 100) kDa (14). Different types of cross-links between pectins appear to be responsible for the porosity of the plant cell walls (15). Phenolic cross-links possibly involving tyrosine residues of cell wall (glyco)proteins have been postulated to restrict porosity further (16). It remains to be seen whether plant cells are able to change the pore size of their surrounding cell walls actively in reaction to internal or external stimuli such as pathogen attack (17).

Immunological approaches have shown that the wall surrounding a plant cell is far from being a uniform structure. Owing to the availability of a set of two complementary antibodies, the distribution of methyl esterified versus non-methyl esterified pectins has been analyzed in many plant tissues, and distinct distributions both over the thickness and in the plane of plant cell walls have been found (18). Even more spectacular are the distributions of certain proteins that form minor constituents of the cell walls. These epitopes are sometimes restricted to a very small number of cells and their presence may indicate or dictate determination of cell fate, e.g., to become metaxylem cells (19,20). Also, cell polarity may be determined by markers immobilized in the cell wall, as evidenced by a unique localized cell wall domain stabilizing the spatial orientation of the fucus zygote (21). The role of cell walls in the development of plant cells, tissues, organs, and organisms is likely to be a most deserving field for future research.

Cell walls are also of paramount importance in the defense of plants against pests and pathogens and in their tolerance to stress (22). Forming the outer shell of plant cells and, consequently, of total plants, cell walls— and most notably outer epidermal cell walls—are the first barrier put up by the plant against external biotic and abiotic threats. Cell walls are the single most important protection of plants against the myriads of microorganisms that otherwise would be potential plant pathogens. Whereas successful pathogens must have evolved means to degrade and/or penetrate standard plant cell walls, plants have counteracted this attack by a multitude of pre- or postinfection cell wall modifications, such as callóse deposition to form a periplasmic papilla, production of active oxygen species in an odixative burst for the peroxidative cross-linking of phenolic acids and proteins, or deposition of suberin or lignin, which as a true polymer is extremely difficult to degrade enzymatically. Active oxygen species produced in the cell wall have also been supposed to act as second messengers inducing the hypersensitive reaction (23), a form of programmed cell death involved in plant disease defense that in some cases may be brought about by the intracellular performance of a process usually confined to the cell wall, namely radical coupling of monolignols to the lignin polymer (24).

The cuticle including the epicuticular wax layers does not only protect plants from desiccation. The microroughness of the plant surface brought about by the wax crystals creates the Lotus effect ensuring that dust particles and fungal spores are easily washed away by runoff raindrops (25). The extreme hydrophobicity of the epicuticular wax prevents the formation of a continuous water layer so that motile bacteria cannot easily reach stomates (26). Wax protrusions may protectively cover stomatal openings, effectively reducing stomatal transpiration and preventing recognition of stomates by fungal pathogens searching for easy ingress points (27). Cell walls form the arena where potential microbial pathogens deploy their pathogenicity factors, often a plethora of plant cell wall hydrolyzing enzymes, and where host plant cells mount their defenses, e.g., in the form of pathogenesis-related (pr-) proteins such as inhibitors of the microbial hydrolases or hydrolytic enzymes attacking the microbial cell walls (28).

Not all of the microbes interacting with a plant are potential foes. Both the rhizosphere and the phylloplane are complex ecosystems colonized by a multitude of mutualistic or commensal microbes. The interactions of these microorganisms with the plant are surface interactions and, as such, interactions of the cell walls of both partners, at least initially. However, little is known about the molecular details of these interactions, except for arbuscular mycorrhiza and nitrogen-fixing rhizobia interacting with plant root cells. The fungi and bacteria involved in these interactions form symbiosome structures within the host cells, similar to the haustorial complexes of bio-trophic pathogenic fungi such as the rusts and mildews (29). Cell wall penetration must be accomplished by these symbiotic microorganisms in a manner preventing the elicitation of the active resistance mechanisms just described for the defense against pathogenic microorganisms. Mutualists may achieve this by actively suppressing plant defense responses such as callóse deposition (30) or by inducing cell wall autolytic processes within the plant cell (31) reminiscent of phragmosome formation during cell division (32). Similarly, successful biotrophic pathogens must continuously sup press resistance reactions by the penetrated host cells, and they may achieve this by generating plant cell wall fragments acting as endogenous suppressors of disease resistance elicitation (33).

Cell walls do not only form the stage of interaction between plant cells and microbial symbionts and pathogens. Cell walls of the male pollen tube and the female pistil cells are also the site where self-recognition in the self-incompatibility system prevents self-pollination (29). This system has been elucidated on the molecular level in Brassica, where a receptor kinase in the plasma membrane of the stigma epidermal cells (34) cooperates with a stigma cell wall glycoprotein (35) in the recognition of a cysteine-rich pollen cell wall protein (36). The outcome of this molecular interaction is most likely a multiple response certainly including cell wall modifications that eventually arrest further pollen tube growth (37).

The examples described here are recent corroborations of the much older oligosaccharin concept postulating oligosaccharide fragments from plant cell walls as hormone-like signal molecules influencing plant metabolism and development (38). Intermediate-sized products of fungal endo-polygalacturonase digestion of plant pectins act as endogenous elicitors of active defense reactions in plants. Small products of pectin digestion act as endogenous suppressors of elicitor-induced plant resistance reactions. The biological activity of the endogenous suppressors that are beneficial to the fungal plant pathogens but not to the host plant suggests that these may mimic as yet unknown endogenous plant signals involved in determining differentiation events in plant cells. Indeed, oligomeric fragments of pectin and xyloglucan have been implicated as tissue hormones regulating plant growth and development. Similarly, the nodulation factors of rhizobia initiating meristem formation may mimic endogenous plant signal molecules. It has been speculated that the diverse family of plant arabinogalactan proteins may form a cell wall-located reservoir of such signal molecules involved in local differentiation and development.

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