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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 22.5The Dynamic Plant Cell Wall

Image plant.jpgThe cell wall surrounding plant cells serves many of the same functions as the extracellular matrix produced by animal cells, even though the two structures are composed of entirely different macromolecules and have a different organization. Like the extracellular matrix, the plant cell wall connects cells into tissues, signals a plant cell to grow and divide, and controls the shape of plant organs. In the past, the plant cell wall was viewed an inanimate rigid box, but it is now recognized as a dynamic structure that plays important roles in controlling the differentiation of plant cells during embryogenesis and growth.

Because the major function of a plant cell wall is to withstand the osmotic turgor pressure of the cell, the cell wall is built for lateral strength. Arranged into layers of cellulose microfibers embedded in a matrix of pectin and hemicellulose, the cell wall is 0.2 μm thick and completely coats the outside of the plant plasma membrane (Figure 22-29). The combination of pressure and strength contributes to the rigidity of a plant. Because the cell wall prevents a cell from expanding, some proteins in the matrix are responsible for loosening the structure of the wall when a cell grows. In addition, the porosity of the matrix permits soluble factors to diffuse across the cell wall and interact with receptors on the plant plasma membrane. However, the cell wall is a selective filter that is more impermeable than the matrices surrounding animal cells. Whereas water and ions diffuse freely in cell walls, diffusion of particles with a diameter greater than ≈4 nm, including proteins with a molecular weight less than 20,000, is reduced. This is one of the reasons that plant hormones are small, water-soluble molecules.

Figure 22-29. Schematic representation of the cell wall of an onion.

Figure 22-29

Schematic representation of the cell wall of an onion. Cellulose and hemicellulose are arranged into at least three layers in a matrix of pectin polymers. The size of the polymers and their separations are drawn to scale. To simplify the diagram, most (more...)

The cell wall undergoes its greatest changes at the meri-stem of a root or shoot tip. These are sites where cells divide and expand. Young cells are connected by thin primary cell walls (Figure 22-30), which can be loosened and stretched to allow subsequent cell elongation. After cell elongation ceases, the cell wall generally is thickened, either by secretion of additional macromolecules into the primary wall or, more usually, by formation of a secondary cell wall composed of several layers. In mature tissues such as the xylem — the tubes that conduct salts and water from the roots through the stems to the leaves (see Figure 16-53) — the cell body degenerates, leaving only the cell wall. The unique properties of wood and of plant fibers such as cotton are due to the molecular properties of the cell walls in the tissues of origin. We begin our discussion with the structure of the cell wall.

Figure 22-30. Light micrograph of young root tip cells of an onion.

Figure 22-30

Light micrograph of young root tip cells of an onion. A thin primary cell wall separates two recently separated cells. [Courtesy of Jim Solliday and Biological Photo Service.]

The Cell Wall Is a Laminate of Cellulose Fibrils in a Pectin and Hemicellulose Matrix

The strength of the cell wall is derived from layers of cellulose microfibrils that are extensively cross-linked by hemicellulose polysaccharide chains. Each microfibril consists of a bundle of linear polymers of glucose residues linked together by β(1→4) glycosidic bonds into a straight glucan chain. In this bonding arrangement, each glucose residue is rotated by 180° around its (1→4) axis relative to an adjacent residue; thus a pair of residues, cellobiose, constitute a subunit (Figure 22-31). Microfibrils are 5 – 15 nm in diameter and can be many micrometers in length. Extensive hydrogen bonding within glucan chains and between adjacent chains makes the microfibril an almost crystalline aggregate. The layers of microfibrils prevents the cell wall from stretching laterally.

Figure 22-31. The structure of cellulose in the plant cell wall.

Figure 22-31

The structure of cellulose in the plant cell wall. (a) Cellulose is a linear polymer consisting of 2000 – 20,000 glucose residues linked together by β(1→4) glycosidic bonds. Because the β(1→4) linkages (more...)

As a cell matures, it lays down an inner secondary wall (Figure 22-32). This inner wall may have several layers; within each layer the cellulose fibrils are parallel to one another, but the orientation differs in adjacent layers. Such a plywood-like construction adds considerable strength to the wall. Befitting its central structural role, cellulose makes up 20 – 30 percent of the wall’s dry weight and is the most abundant molecule in the cell wall.

Figure 22-32. The structure of the secondary cell wall, built up of a series of layers of cellulose.

Figure 22-32

The structure of the secondary cell wall, built up of a series of layers of cellulose. In each layer, the cellulose fibrils run more or less in the same direction, but the direction varies in different layers. As plant cells grow, they deposit new layers (more...)

Two other polysaccharide molecules, hemicellulose and pectins, are major constituents of the cell wall. Cellulose microfibrils are cross-linked by hemicelluloses, highly branched polysaccharides with a backbone of about 50 β(1→4)-linked sugars of a single type. Hemicelluloses are linked by hydrogen bonds to the surface of cellulose microfibrils. The hemicellulose branches help bind the microfibrils to one another and to other matrix components, particularly the pectins. This interlinked network of pectin and hemicellulose helps bind adjacent cells to each other and cushion them. The gel-like property of the cell wall is derived in part from pectins. Like hyaluronan, pectin contains multiple negatively charged saccharides that bind cations such as Ca2+ and become highly hydrated. When purified, pectin binds water and forms a gel — hence the use of pectins in many processed foods. Pectins are particularly abundant in the middle lamella, the layer between the cell walls of adjacent cells. Treatment of tissues with pectinase or other enzymes that degrade pectin frequently causes cells with their walls to separate from one another.

Cell Walls Contain Lignin and an Extended Hydroxyproline-Rich Glycoprotein

As much as 15 percent of the primary cell wall may be composed of extensin, a glycoprotein made up of roughly 300 amino acids. Extensin, like collagen, contains abundant hydroxyproline (Hyp) and about half its length represents variations of the four-residue sequence Ser-Hyp-Hyp-Hyp. Most of the hydroxyprolines are glycosylated with chains of three or four arabinose residues, and the serines are linked to galactose. Thus extensin is about 65 percent carbohydrate, and its protein backbone forms an extended rodlike helix with carbohydrates protruding outward. Extensins, like other cell-wall proteins, are incorporated into the insoluble polysaccharide network and are believed to have a structural role, forming the scaffolding upon which the cell-wall architecture is formed.

Lignin — a complex, insoluble polymer of phenolic residues  — associates with cellulose and is a strengthening material in all cell walls. It is particularly abundant in wood, where it accumulates in primary cell walls and in the secondary walls of the xylem. Like cartilage proteoglycans, lignin resists compression forces on the matrix. Particularly for soil-grown plants, lignin is essential for strengthening the xylem tubes to enable them to conduct water and salts over long distances. Lignin also protects the plant against invasion by pathogens and against predation by insects or other animals.

A Plant Hormone, Auxin, Signals Cell Expansion

Cell growth in higher plants frequently occurs without an increase in the volume of the cytosol. Because of the low ionic strength of the cell wall, water tends to leave it and enter the cytosol and vacuole, causing the cell to expand. A localized loosening of the primary cell wall, induced by auxin, allows the cell to expand in a particular direction; the size and shape of a plant are determined primarily by the amount and direction of this enlargement (Figure 22-33a). Individual plant cells can increase in size very rapidly by loosening the wall and pushing the cytosol and plasma membrane outward against it. The increase in cell volume is due only to the expansion of the intracellular vacuole by uptake of water. We can appreciate the magnitude of this phenomenon by considering that if all cells in a redwood tree were reduced to the size of a typical liver cell (≈20 mm in diameter), the tree would have a maximum height of only 1 meter.

Figure 22-33. Elongation of plant cells.

Figure 22-33

Elongation of plant cells. (a) Change in structure of a plant cell during elongation. Uptake of water causes an internal pressure (turgor); in the presence of auxin, the cell wall is loosened, and the turgor pressure against the loosened wall leads to (more...)

The ability of auxin (indole-3-acetic acid) to rapidly induce cell elongation was first demonstrated in classical experiments on coleoptiles from grasses and oats. According to the acid-growth hypothesis, auxin stimulates proton secretion at the “growing” end of the cell by activating (directly or indirectly) a proton pump bound to the plasma membrane (Figure 22-33b). As a result, the pH of the cell wall near this region of the plasma membrane falls from the normal 7.0 to as low as 4.5. The low pH activates a class of wall proteins, termed expansins, that disrupt the hydrogen bonding between cellulose microfibrils, causing the laminate structure of the cell wall to loosen. With the rigidity of the wall reduced, the cell can elongate.

Expansins were discovered and purified using a novel biochemical assay on pure cellulose paper, since paper, like the plant cell walls from which it is made, derives its mechanical strength from hydrogen bonding between cellulose microfibrils. Extracts of plant cell walls were tested for their ability to mechanically weaken paper at pH values between 3.0 and 5.0, but not at pH 7 (Figure 22-34). The expansin-triggered loosening of the wall is reversed when the pH is raised back to 7.0, showing that expansin does not break covalent bonds in cellulose. Additional evidence for the acid-growth hypothesis stems from studies of the fungal compound fusicoccin. Like auxin, fusicoccin induces rapid cell elongation and triggers proton pumping out of sensitive cells, with accompanying localized wall loosening. The action of fusicoccin or auxin can be blocked by permeating the cell wall with buffers that prevent the extracellular pH from being lowered.

Figure 22-34. Experimental demonstration that expansin loosens hydrogen bonds.

Figure 22-34

Experimental demonstration that expansin loosens hydrogen bonds. (a) In an elastometer, a paper strip is clamped at both ends and immersed in a solution. One end is attached to a weight, while the other end is held fixed. Agents that break the covalent (more...)

Cellulose Fibrils Are Synthesized and Oriented at the Plant Cortex

Cellulose microfibrils are synthesized on the exoplasmic face of the plasma membrane from UDP-glucose and ADP-glucose formed in the cytosol. The polymerizing enzyme, called cellulose synthase, is thought to be a large complex of many identical subunits, each of which “spins out” glucan chains that spontaneously form microfibrils (see Figure 22-31). The long microfibrils are insoluble, which probably explains why they are not formed within the cell. In contrast, soluble hemicellulose and pectin molecules are synthesized in the Golgi complex and secreted at the cell surface, where they cross-link the cellulose microfibrils into the matrix of the cell wall. In the primary cell wall of elongating cells, newly made cellulose microfibrils encircle the cell like a belt perpendicular to the axis.

Experiments with elongating root tip cells suggest that in the primary wall, at least, microtubules influence the direction of cellulose deposition. These cells have oriented bands or rings of microtubules located just under the plasma membrane; these microtubules are transverse to the direction of elongation but parallel to many of the cellulose microfibrils in the primary cell wall of the elongating cell (Figure 22-35). Moreover, disruption of the microtubular network by drugs eventually disrupts the pattern of cellulose disposition. Thus, many investigators believe that cellulose synthase complexes move within the plane of the plasma membrane, as cellulose is formed, in directions determined by the underlying microtubule cytoskeleton. Any linkage, however, between the microtubules and cellulose synthase remains to be determined. Interestingly, in gliding bacteria, the synthase is immobile in the membrane. Consequently, a bacterial cell is thought to use the flow of cellulose molecules to affect motility.

Figure 22-35. Microtubules and cellulose synthesis in an elongating root tip cell.

Figure 22-35

Microtubules and cellulose synthesis in an elongating root tip cell. (a) Circumferential rings of microtubules lie just inside the plasma membrane, perpendicular to the direction of cell elongation. Glucan chains are synthesized by cellulose synthase, (more...)

Plasmodesmata Directly Connect the Cytosol of Adjacent Cells in Higher Plants

Even though plant cells are bounded by a cell wall, they communicate through specialized cell-cell junctions called plasmodesmata, which extend through the adjacent cell walls. Like gap junctions, plasmodesmata are open channels that connect the cytosol of adjacent cells and permit the diffusion of molecules with a molecular weight up to 1000, including a variety of metabolic and signaling compounds. However, during the trafficking of macromolecules, this limit increases to greater than 10,000 MW. The diameter of the cytosol-filled channel is about 60 nm, and plasmodesmata can traverse cell walls up to 90 nm thick. Depending on the plant type, the density of plasmodesmata varies from 1 to 10 per mm2, and even the smallest meristematic cells (the growing cells at the tips of roots or stems) have more than 1000 interconnections with their neighbors. Plasmo-desmata differ from gap junctions in two significant aspects. The plasma membranes of the adjacent cells extend continuously through each plasmodesma, whereas the membranes of cells at a gap junction are not continuous with each other. In addition, an extension of the endoplasmic reticulum called a desmotubule passes through the ring of cytosol, the annulus, connecting the cytosol of adjacent cells (Figure 22-36).

Figure 22-36. The structure of plasmodesmata.

Figure 22-36

The structure of plasmodesmata. A plasmodesma is a plasma membrane – lined channel through the cell wall. Note the desmotubule, an extension of the endoplasmic reticulum, and the annulus, a ring of cytosol that interconnects the cytosol (more...)

Much evidence establishes that plasmodesmata are in fact used in cell-cell communication. For instance, fluorescent water-soluble chemicals microinjected into plant cells spread to the cytoplasm of adjacent cells but not into the cell wall. Many types of molecules spread from cell to cell through plasmodesmata, including proteins, nucleic acids, metabolic products, and plant viruses. Soluble molecules pass through the cytosolic annulus, but membrane-bound molecules may pass from cell to cell via the desmotubule. Transport of such substances is proportional to the number of plasmodesmata and does not occur between cells not connected by such junctions. The permeability of the plasmodesmata to these molecules is regulated in response to developmental, physiological, or environmental changes. As with gap junctions, transport through plasmodesmata is reversibly inhibited by an elevation in cytosolic Ca2+.

As discussed in Chapter 16, phloem vessels transport sucrose and other metabolites throughout a plant from their sites of synthesis in the leaves. In formation of the long, narrow sieve-tube cells composing a phloem vessel, the primary cell wall thickens and the nucleus, vacuole, and other internal organelles are lost, although the plasma membrane is retained. In each end wall, called the sieve plate, the plasmodesmata expand to form large pores that facilitate fluid movement (Figure 22-37; see also Figure 16-53). Numerous plasmodesmata connect sieve-tube cells with companion cells located along the length of a phloem vessel. Substances pass in and out of the sieve-tube cells through these plasmodesmata.

Figure 22-37. Cross section of the phloem from Curcurbita.

Figure 22-37

Cross section of the phloem from Curcurbita. Note the large pores in the sieve plate and companion cells, which lie adjacent to phloem vessels. [Courtesy of J. R. Waaland and Biological Photo Service.]

SUMMARY

  •  Cellulose is a large, linear glucose polymer that assembles spontaneously into microfibrils stabilized by hydrogen bonding.
  •  The plant cell wall is a matrix of cellulose microfibrils cross-linked by hemicellulose, pectin, and extensin (see Figure 22-29).
  •  Plant cells grow by localized loosening of the cell walls and expansion of the vacuole resulting from osmotic influx of water.
  •  The acid-growth hypothesis states that cell-wall expansion is caused by an acid-dependent activation of the cell-wall protein expansin, which loosens hydrogen-bonded cellulose microfibrils.
  •  The plant hormone auxin induces acid secretion at the membrane, leading to activation of expansin.
  •  Cellulose synthase in the plasma membrane synthesizes new cellulose microfibrils on the exoplasmic face. Cortical microtubules underlying the membrane are thought to determine the orientation of the cellulose microfibrils (see Figure 22-35).
  •  Cell-cell communication takes place through plasmodesmata, which allow small molecules such as sucrose to pass between the cells.
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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21709