The 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.
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 of the hemicellulose cross-links are not shown. [Adapted from M. McCann and K. R. Roberts, 1991, in C. Lloyd, ed., The Cytoskeletal Basis of Plant Growth and Form, p. 126.]
). 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.
A thin primary cell wall separates two recently separated cells. [Courtesy of Jim Solliday and Biological Photo Service.]
), 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.
(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 cause alternating glucose residues to be rotated by 180°, a pair of residues constitute a repeating unit, the cellobiose monomer; these monomers polymerize into straight glucan chains. The chains pack together to form rodlike microfibrils, which are stabilized by hydrogen bonds between the chains. Each glucan chain is polar because its two ends are distinct, and all the chains in a microfibril have the same polarity. (b) A rotary shadowed platinum replica of a rapidly frozen, deep-etched onion cell wall shows the arrangement of cellulose fibers and thinner cross-links presumably composed of hemicellulose or pectin. The scale bar is 200 nm. [Part (b) from: M. C. McCann, B. Wells, and K. Roberts, 1990, Journal of Cell Science 96:327; courtesy of Keith Roberts.]
). 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.
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 of cellulose adjacent to the plasma membrane. Thus the oldest layers are in the primary wall (the outer wall) and in the middle lamella (the pectin-rich part of the cell wall laid down between two daughter cells as they cleave during cell division). Younger regions of the wall — collectively the secondary cell wall — are laid down as successive layers, adjacent to the plasma membrane. The cytoplasms of adjacent cells are usually connected by plasmodesmata that run through the layers of the cell walls.
). 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.
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.
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) 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 elongation. (b) Proposed mechanism of cell-wall loosening in plant cells. [Part (b) adapted from L. Taiz, 1994, Proc. Nat’l. Acad. Sci. USA 91:7387.]
). 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.
). 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.
(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 or hydrogen bonds between the cellulose fibers will cause the paper strip to elongate by x amount. The movement of the clamp is recorded. (b) Treatment of a paper strip with expansin at pH 4.5 (red) results in the reversible weakening of the cellulose molecule. In contrast, cellulase irreversibly weakens paper by breaking covalent bonds in the polymer. Control experiments show that the weakening is not caused by the pH 4.5 solution and is dependent on active protein.
). 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.
). 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.
(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, a large integral membrane protein arranged into a rosette of subunits, on the outer face of the plasma membrane and spontaneously assemble into cellulose microfibrils. As the long insoluble cellulose fibrils form, the synthase moves in the plasma membrane (red arrows) parallel to the underlying microtubule network. Thus in a growing cell, the new fibrils are arranged in circumferential rings perpendicular to the direction of elongation. (b) A fluorescence micrograph of GFP fused to the microtubule binding domain of MAP-4. The domain highlights the cortical belt of microtubules and shows their parallel orientation beneath the cell wall. [Part (b) adapted from R. Cyr, 1998, Plant Cell 10:1927; courtesy of R. Cyr].
).
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.
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 of adjacent cells. Not shown is a gating complex that fills the channel and controls transport of materials through plasmodesmata.
).
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+.
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.]
; 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.
Cellulose is a large, linear glucose polymer that assembles spontaneously into microfibrils stabilized by hydrogen bonding.
).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.
).Cell-cell communication takes place through plasmodesmata, which allow small molecules such as sucrose to pass between the cells.
