Components And Architecture Of Plant Cell Walls

In reviewing the structural and functional roles of plant cell walls, we have already named most of their constituents, and we have indicated which of the different components contribute to the specific functions. In this section, we will briefly summarize our current knowledge of the structure of these components and our understanding of how they interact to assemble into the complex and dynamic three-dimensional pericellular matrix. Over the past decades, a number of ever more refined models of plant cell wall architecture have been proposed, mostly based on detailed investigations of the primary cell walls of suspension-cultured cells (2,39-42). This prototype cell wall of dicot plants is opposed by the somewhat different cell wall of some— the commelinoid—monocot plants, most notably the grasses and cereals (43). The general architectural plan of both types of cell walls, however, is identical: stress-bearing fibrils are embedded in a pressure-bearing matrix (Fig. 2).

In both types of plant cell walls, the fibrillar component is represented by cellulose microfibrils, accounting for about one third of the cell wall dry pectin Ca++

pectin Ca++

Figure 2 Simplified model for the primary cell wall of dicot plants. Plant cell walls are composite materials consisting of stress-bearing fibers made of cellulose embedded in a pressure-bearing matrix consisting of hemicelluloses and pectins. Parallel linear cellulose molecules form the crystalline core of the microfibrils. In a typical primary cell wall of dicot plants, these are thought to be covered by hemicellulosic xyloglucan molecules. Individual xyloglucan molecules may span the interfibrillar space, their ends hydrogen bonded to the surface of different cellulose microfibrils, thus forming a cellulose-xylo-glucan net. Hydrophilic, galacturonic acid-containing pectin molecules consisting of linear "smooth" regions and branched "hairy" regions fill in the meshes of this net. The smooth homogalacturonan regions can cross-link via Ca2+ bridges, thus forming a second, independent pectin-Ca2+ net. A third, independent net of cross-linked glycoproteins may exist in the primary cell wall, possibly involved in defining the precise spacing of the polysaccharide components.

Figure 2 Simplified model for the primary cell wall of dicot plants. Plant cell walls are composite materials consisting of stress-bearing fibers made of cellulose embedded in a pressure-bearing matrix consisting of hemicelluloses and pectins. Parallel linear cellulose molecules form the crystalline core of the microfibrils. In a typical primary cell wall of dicot plants, these are thought to be covered by hemicellulosic xyloglucan molecules. Individual xyloglucan molecules may span the interfibrillar space, their ends hydrogen bonded to the surface of different cellulose microfibrils, thus forming a cellulose-xylo-glucan net. Hydrophilic, galacturonic acid-containing pectin molecules consisting of linear "smooth" regions and branched "hairy" regions fill in the meshes of this net. The smooth homogalacturonan regions can cross-link via Ca2+ bridges, thus forming a second, independent pectin-Ca2+ net. A third, independent net of cross-linked glycoproteins may exist in the primary cell wall, possibly involved in defining the precise spacing of the polysaccharide components.

weight. In the dicot cell wall, each cellulose microfibril is thought to be completely wrapped in an envelope of xyloglucan chains mediating the surface interaction between the fibrillar component and the surrounding matrix (44). Xyloglucan molecules are thought to span the interfibrillar space, their ends hydrogen bonding to different cellulose microfibrils. The cellulose and xyloglucan molecules thus form a three-dimensional scaffolding network in the dicot cell wall. In commelinoid monocot cell walls, xyloglucan is mostly replaced by glucuronoarabinoxylan chains, which are similarly hydrogen bonded to several cellulose microfibrils, thus leading to a cellulose-xylan network (45).

The cellulose-xyloglucan network of the dicot cell wall is embedded in the pectic matrix, which makes up about one third of the dry weight of the cell wall (46). The pectins are believed to form a second, independent network in the cell wall that may not be covalently linked to the cellulose-xyloglucan network. The intermolecular cross-links knitting pectic polysaccharides together are most likely Ca2+ bridges between nonesterified stretches of homogalacturonan (47), hydrophobic interactions between methyl esterified stretches of homogalacturonan, and, possibly, boron dies-ters between rhamnogalacturonan II stretches (48). Commelinoid monocot cell walls contain the same pectic polymers as dicot walls but at much lower contents (below 10%). Their role as a water-binding matrix component is substituted by the major hemicellulose, glucuronoarabinoxylan, which alone can account for about half of the dry weight of the commelinoid monocot wall (49). It can be expected that the same types of interpectic cross-links exist in the commelinoid monocot cell wall as discussed for the dicot wall. Many of the arabinose residues of the glucuronoarabinoxylan carry ferulic acid or p-coumaric acid esters, and these may dimerize oxidatively to form diphenolic acid cross-links possibly substituting for missing interpectic links (50). Thus, a pectin and/or a glucuronoarabinoxylan-phenolic acid network may exist in commelinoid monocot cell walls.

Both types of cell wall contain structural glycoproteins—roughly 1020% of the dry weight of the dicot wall compared with about 2-10% in the commelinoid monocot wall—which may form a third independent network in the primary plant cell wall (51). The major protein component of dicot walls appears to be the hydroxyproline-rich glycoprotein (HRGP) extensin, but proline-rich proteins (PRPs) and glycine-rich proteins (GRPs) may be involved as well. The only cross-links proposed so far to hold the protein network together are intermolecular isodityrosine bridges, but their existence still awaits experimental support.

The classical cell wall models depicting primary plant cell walls as a largely covalently cross-linked system of the constituent polysaccharides and proteins have been extended and improved to yield a composite cell wall model with a more flexible and dynamic architecture (2). According to our current view, primary cell walls of both dicot and monocot plants are built of the three independent interwoven networks already described—the cel-lulose-glycan network, the pectin network, and the proteinaceous network —held together by noncovalent links. Primary cell walls are capable of enormous rates of largely planar, two-dimensional growth, believed to be restricted by the cellulose-glycan network. In dicot cell walls, controlled activity of expansins and xyloglucanendotransglycosylases is supposed to allow controlled yielding of this network, so that the orientation of the cellulose microfibrils determines the direction of cell elongation. In contrast, some of the xyloglucan-poor monocot cell walls may, during phases of elongation growth, produce an additional polysaccharide component—a mixed-linkage ¡3-1,3-/3-1,4-glucan—that may hydrogen bond to cellulose and may transiently replace the xylan, which appears to be turned over rapidly during growth (52,53).

In contrast to the artificial situation of plant cells growing in liquid suspension where all cells are permanently in a similar state of differentiation, preferentially rapid spherical growth, cells of many different types, and in many different states of differentiation, interact with each other to form a highly complex tissue in the intact plant. Cultured plant cells are surrounded only by a primary cell wall. In a plant tissue, all cells still in the process of growing and even many fully differentiated cells, such as leaf mesophyll cells, also contain only a primary and no secondary cell wall. However, many specialized plant cells produce highly sophisticated secondary cell walls, e.g., collenchyma and sclerenchyma cells, tracheids, and pollen cells. Moreover, all the different cells that make up a tissue are "glued" together by the middle lamella, a structure that may not be present in cultured cells, at least if they grow as a very fine suspension of individual cells. Furthermore, it appears that the primary wall of a cell in a tissue is much less uniform than that of a cell grown in suspension culture.

Immunocytological electron microscopic studies using antibodies to different cell wall components revealed microdomains in the seemingly uniform primary cell wall exhibiting striking differences in their molecular composition (54). Antibodies cross-reacting with non-methyl esterified epitopes of pectin stained the middle lamella, especially in cell corners, but also lined the inner surface of the primary cell walls along the plasma membrane. Antibodies cross-reacting with highly methyl esterified pectic components, in contrast, evenly bound across the whole width of the primary cell wall. Cell wall angles where three or more different cells meet are distinct from cell wall stretches in contact with a single neighboring cell, and these again differ from areas that face the intercellular space. The mature primary cell wall of about 80 nm thickness consists of only about three layers of cellulose microfibrils (55). Each layer, thus, resides in a different nanoenvironment, the outer layer facing the pectic middle lamella, the middle layer surrounded by the other two, and the inner layer facing the plasma membrane or, later, the secondary cell wall layers. Virtually nothing is known about the interaction of the primary cell wall with these neighboring layers. These novel micro- and nanoscopic approaches will certainly shed new light on our understanding of the plant cell wall, but they can be expected to pose more questions initially than they will answer—e.g., concerning assembly and regulation of this enormous complexity outside the cytoplasm.

To make things even more complex, many cells that fulfill special functions within a plant tissue exhibit striking and often highly localized modifications of the standard primary plant cell wall described so far. Epidermal cells have to play multiple roles in growth restriction and stress protection, necessitating modifications of the outer primary cell walls. These are thicker than the other epidermal cell walls, and the cutin monomers and cuticular and epicuticular waxes produced in epidermal cells are transported to the outer surface of this outer periclinal wall to reach the plant surface, where they are laid down in layers and where the cutin is polymerized in situ. One of the most prominent examples of secondary cell walls is certainly the intricate elaboration of cell wall thickenings in tracheary elements of the xylem, where additional cellulose fibers and embedding matrix polymers are laid down only on certain parts of the cell walls (56). Eventually, lignin is incorporated into the walls of these cells. The polymerization of the mono-lignols in muro leads to a growing lignin polymer infiltrating the primary cell wall, displacing the water in the process. Eventually, when the polymerization process is complete, the lignin polymer serves to cement in place all the other components of the cell wall. The wall is then no longer permeable to water, and the mature tracheid dies. The ensuing loss of turgor pressure would result in a shrinking of the dead cell and most likely a collapse due to the negative pressure (tension) of the transpiration stream, were it not for the lignin impregnating and stiffening the cell wall of the tracheid.

Clearly, the wall of an individual cell is a highly differentiated and coordinated organelle with domains differing in structure and—most likely —also in function. In addition, we can expect a highly dynamic temporal differentiation to be superimposed on this complex spatial differentiation. Cells are born in meristematic zones where cell division occurs, they undergo elongation and differentiation, they may eventually senesce, and they will finally die. The wall of a cell most likely changes continuously during these processes, but today little is known of what these changes are and what they may entail.

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