The plant cell wall is an elaborate extracellular matrix that encloses each cell in a plant. It was the thick cell walls of cork, visible in a primitive microscope, that in 1663 enabled Robert Hooke to distinguish and name cells for the first time. The walls of neighboring plant cells, cemented together to form the intact plant (Figure 19-68), are generally thicker, stronger, and, most important of all, more rigid than the extracellular matrix produced by animal cells. In evolving relatively rigid walls, which can be up to many micrometers thick, early plant cells forfeited the ability to crawl about and adopted a sedentary life-style that has persisted in all present-day plants.
All cell walls in plants have their origin in dividing cells, as the cell plate forms during cytokinesis to create a new partition wall between the daughter cells (discussed in Chapter 18). The new cells are usually produced in special regions called meristems (discussed in Chapter 21), and they are generally small in comparison with their final size. To accommodate subsequent cell growth, their walls, called primary cell walls, are thin and extensible, although tough. Once growth stops, the wall no longer needs to be extensible: sometimes the primary wall is retained without major modification, but, more commonly, a rigid, secondary cell wall is produced by depositing new layers inside the old ones. These may either have a composition similar to that of the primary wall or be markedly different. The most common additional polymer in secondary walls is lignin, a complex network of phenolic compounds found in the walls of the xylem vessels and fiber cells of woody tissues.The plant cell wall thus has a “skeletal” role in supporting the structure of the plant as a whole, a protective role as an enclosure for each cell individually, and a transport role, helping to form channels for the movement of fluid in the plant. When plant cells become specialized, they generally adopt a specific shape and produce specially adapted types of walls, according to which the different types of cells in a plant can be recognized and classified (Figure 19-69; see also Panel 21-3).
Although the cell walls of higher plants vary in both composition and organization, they are all constructed, like animal extracellular matrices, using a structural principle common to all fiber-composites, including fibreglass and reinforced concrete. One component provides tensile strength, while another, in which the first is embedded, provides resistance to compression. While the principle is the same in plants and animals, the chemistry is different. Unlike the animal extracellular matrix, which is rich in protein and other nitrogen-containing polymers, the plant cell wall is made almost entirely of polymers that contain no nitrogen, including cellulose and lignin. Trees make a huge investment in the cellulose and lignin that comprise the bulk of their biomass. For a sedentary organism that depends on CO2, H2O and sunlight, these two abundant biopolymers represent “cheap,” carbon-based, structural materials, helping to conserve the scarce fixed nitrogen available in the soil that generally limits plant growth.
In the cell walls of higher plants, the tensile fibers are made from the polysaccharide cellulose, the most abundant organic macromolecule on Earth, tightly linked into a network by cross-linking glycans. In primary cell walls, the matrix in which the cellulose network is embedded is composed of pectin, a highly hydrated network of polysaccharides rich in galacturonic acid. Secondary cell walls contain additional components, such as lignin, which is hard and occupies the interstices between the other components, making the walls rigid and permanent. All of these molecules are held together by a combination of covalent and noncovalent bonds to form a highly complex structure, whose composition, thickness and architecture depends on the cell type.
We focus here on the primary cell wall and the molecular architecture that underlies its remarkable combination of strength, resilience, and plasticity, as seen in the growing parts of a plant.
The aqueous extracellular environment of a plant cell consists of the fluid contained in the walls that surround the cell. Although the fluid in the plant cell wall contains more solutes than does the water in the plant's external milieu (for example, soil), it is still hypotonic in comparison with the cell interior. This osmotic imbalance causes the cell to develop a large internal hydrostatic pressure, or turgor pressure, that pushes outward on the cell wall, just as an inner tube pushes outward on a tire. The turgor pressure increases just to the point where the cell is in osmotic equilibrium, with no net influx of water despite the salt imbalance (see Panel 11-1, pp. 628–629). This pressure is vital to plants because it is the main driving force for cell expansion during growth, and it provides much of the mechanical rigidity of living plant tissues. Compare the wilted leaf of a dehydrated plant, for example, with the turgid leaf of a well-watered one. It is the mechanical strength of the cell wall that allows plant cells to sustain this internal pressure.
The cellulose molecules provide tensile strength to the primary cell wall. Each molecule consists of a linear chain of at least 500 glucose residues that are covalently linked to one another to form a ribbonlike structure, which is stabilized by hydrogen bonds within the chain (Figure 19-70). In addition, intermolecular hydrogen bonds between adjacent cellulose molecules cause them to adhere strongly to one another in overlapping parallel arrays, forming a bundle of about 40 cellulose chains, all of which have the same polarity. These highly ordered crystalline aggregates, many micrometers long, are called cellulose microfibrils, and they have a tensile strength comparable to steel. Sets of microfibrils are arranged in layers, or lamellae, with each microfibril about 20–40 nm from its neighbors and connected to them by long cross-linking glycan molecules that are bound by hydrogen bonds to the surface of the microfibrils. The primary cell wall consists of several such lamellae arranged in a plywoodlike network (Figure 19-71).
The cross-linking glycans are a heterogeneous group of branched polysaccharides that bind tightly to the surface of each cellulose microfibril and thereby help to cross-link microfibrils into a complex network. Their function is analogous to that of the fibril-associated collagens discussed earlier (see Figure 19-49). There are many classes of cross-linking glycans, but they all have a long linear backbone composed of one type of sugar (glucose, xylose, or mannose) from which short side chains of other sugars protrude. It is the backbone sugar molecules that form hydrogen bonds with the surface of cellulose microfibrils, cross-linking them in the process. Both the backbone and the side-chain sugars vary according to the plant species and its stage of development.
Coextensive with this network of cellulose microfibrils and cross-linking glycans is another cross-linked polysaccharide network based on pectins (see Figure 19-71). Pectins are a heterogeneous group of branched polysaccharides that contain many negatively charged galacturonic acid units. Because of their negative charge, pectins are highly hydrated and associated with a cloud of cations, resembling the glycosaminoglycans of animal cells in the large amount of space they occupy (see Figure 19-37). When Ca2+ is added to a solution of pectin molecules, it cross-links them to produce a semirigid gel (it is pectin that is added to fruit juice to make jelly). Certain pectins are particularly abundant in the middle lamella, the specialized region that cements together the walls of adjacent cells (see Figure 19-71); here, Ca2+ cross-links are thought to help hold cell-wall components together. Although covalent bonds also play a part in linking the components together, very little is known about their nature. Regulated separation of cells at the middle lamella underlies such processes as the ripening of tomatoes and the abscission (detachment) of leaves in the fall.
In addition to the two polysaccharide-based networks that are present in all plant primary cell walls, proteins can contribute up to about 5% of the wall's dry mass. Many of these proteins are enzymes, responsible for wall turnover and remodelling, particularly during growth. Another class of wall proteins contains high levels of hydroxyproline, as in collagen. These proteins are thought to strengthen the wall, and they are produced in greatly increased amounts as a local response to attack by pathogens. From the genome sequence of Arabidopsis, it has been estimated that more than 700 genes are required to synthesize, assemble, and remodel the plant cell wall. Some of the main polymers found in the primary and secondary cell wall are listed in Table 19-8.
For a plant cell to grow or change its shape, the cell wall has to stretch or deform. Because of their crystalline structure, however, individual cellulose microfibrils are unable to stretch. Thus, stretching or deformation of the cell wall must involve either the sliding of microfibrils past one another, the separation of adjacent microfibrils, or both. As we discuss next, the direction in which the growing cell enlarges depends in part on the orientation of the cellulose microfibrils in the primary wall, which in turn depends on the orientation of microtubules in the underlying cell cortex at the time the wall was deposited.
The final shape of a growing plant cell, and hence the final form of the plant, is determined by controlled cell expansion. Expansion occurs in response to turgor pressure in a direction that depends in part on the arrangement of the cellulose microfibrils in the wall. Cells, therefore, anticipate their future morphology by controlling the orientation of microfibrils that they deposit in the wall. Unlike most other matrix macromolecules, which are made in the endoplasmic reticulum and Golgi apparatus and are secreted, cellulose, like hyaluronan, is spun out from the surface of the cell by a plasma-membrane-bound enzyme complex (cellulose synthase), which uses as its substrate the sugar nucleotide UDP-glucose supplied from the cytosol. As they are being synthesized, the nascent cellulose chains assemble spontaneously into microfibrils that form on the extracellular surface of the plasma membrane—forming a layer, or lamella, in which all the microfibrils have more or less the same alignment (see Figure 19-71). Each new lamella forms internally to the previous one, so that the wall consists of concentrically arranged lamellae, with the oldest on the outside. The most recently deposited microfibrils in elongating cells commonly lie perpendicular to the axis of cell elongation (Figure 19-72). Although the orientation of the microfibrils in the outer lamellae that were laid down earlier may be different, it is the orientation of these inner lamellae that is thought to have a dominant influence on the direction of cell expansion (Figure 19-73).
An important clue to the mechanism that dictates this orientation came from observations of the microtubules in plant cells. These are arranged in the cortical cytoplasm with the same orientation as the cellulose microfibrils that are currently being deposited in the cell wall in that region. These cortical microtubules form a cortical array close to the cytosolic face of the plasma membrane, held there by poorly characterized proteins (Figure 19-74). The congruent orientation of the cortical array of microtubules (lying just inside the plasma membrane) and cellulose microfibrils (lying just outside) is seen in many types and shapes of plant cells and is present during both primary and secondary cell-wall deposition, suggesting a causal relationship.
If the entire system of cortical microtubules is disassembled by treating a plant tissue with a microtubule-depolymerizing drug, the consequences for subsequent cellulose deposition are not as straightforward as might be expected. The drug treatment has no effect on the production of new cellulose microfibrils, and in some cases cells can continue to deposit new microfibrils in the preexisting orientation. Any developmental change in the microfibril pattern that would normally occur between successive lamellae, however, is invariably blocked. It seems that a preexisting orientation of microfibrils can be propagated even in the absence of microtubules, but any change in the deposition of cellulose microfibrils requires that intact microtubules be present to determine the new orientation.
These observations are consistent with the following model. The cellulose-synthesizing complexes embedded in the plasma membrane are thought to spin out long cellulose molecules. As the synthesis of cellulose molecules and their self-assembly into microfibrils proceeds, the distal end of each microfibril presumably forms indirect cross-links to the previous layer of wall material as it becomes integrated into the texture of the wall. At the growing, proximal end of each microfibril, the synthesizing complexes would therefore need to move through the membrane in the direction of synthesis. Since the growing cellulose microfibrils are stiff, each layer of microfibrils would tend to be spun out from the membrane in the same orientation as the previously laid down layer, with the cellulose synthase complex following along the preexisting tracks of oriented microfibrils outside the cell. Oriented microtubules inside the cell, however, can change this predetermined direction in which the synthase complexes move: they can create boundaries in the plasma membrane that act like the banks of a canal to constrain movement of the synthase complexes (Figure 19-75). In this view, cellulose synthesis can occur independently of microtubules but is constrained spatially when cortical microtubules are present to define membrane domains within which the enzyme complex can move.
Plant cells can change their direction of expansion by a sudden change in the orientation of their cortical array of microtubules. Because plant cells cannot move (being constrained by their walls), the entire morphology of a multicellular plant depends on the coordinated, highly patterned control of cortical microtubule orientations during plant development. It is not known how the organization of these microtubules is controlled, although it has been shown that they can reorient rapidly in response to extracellular stimuli, including low-molecular-weight plant growth regulators such as ethylene and gibberellic acid (see Figure 21-113).
Plant cells are surrounded by a tough extracellular matrix in the form of a cell wall, which is responsible for many of the unique features of a plant's life style. The cell wall is composed of a network of cellulose microfibrils and cross-linking glycans embedded in a highly cross-linked matrix of pectin polysaccharides. In secondary cell walls, lignin may be deposited. A cortical array of microtubules can determine the orientation of newly deposited cellulose microfibrils, which in turn determines directional cell expansion and therefore the final shape of the cell and, ultimately, of the plant as a whole.