Nicholas Carpita
Maureen McCann

The protoplast is the cell’s way of making more wall. —Joe Varner

The shape of a plant cell is dictated largely by its cell wall. When a living plant cell is treated with cell-wall–degrading enzymes to remove the wall, the resulting membrane-bound protoplast is invariably spherical (Fig. 2.1). In living cells, the cell wall constrains the rate and direction of cell growth, exerting a profound influence on plant development and morphology. Cell walls contribute to the functional specialization of cell types. Within Zinnia leaves, for example, the shape of spongy parenchyma cells maximizes both the volume of the intercellular spaces and the surface area available for gas exchange (Fig. 2.2A), whereas the branched structure of trichomes (Fig. 2.2D) may be adapted for sending mechanical stimuli. In contrast, the papillae-shaped epidermal cells of a snapdragon petal reflect light, attracting the attention of pollinators (Fig. 2.2B). In some cells, including tracheary elements (Fig. 2.2C), the protoplast disintegrates during development, and the mature cell consists entirely of cell wall.

Figure 2.1 Without its wall, a protoplast adopts a spherical form.

(A)	(B)
(C)	(D)

Figure 2.2
A developing cell can change its wall architecture to provide myriad forms. (A) The spongy parenchyma of a Zinnia leaf minimizes cell contact and maximizes cell surface for gas exchange. (B) The specialized shapes of these epidermal cells reflect light to enrich the colors of a snapdragon petal. (C) The secondary thickenings of a tracheid prevent collapse of the wall from the tension created by the transpirational pull. (D) An Arabidopsis trichome is an exquisitely branched modified epidermal cell.

The plant cell wall is a dynamic compartment that changes throughout the life of the cell. The new primary cell wall (Fig. 2.3) is born in the cell plate during cell division (see Chapter 5) and rapidly increases in surface area during cell expansion, in some cases by more than a hundred-fold. The middle lamella forms the interface between the primary walls of neighboring cells. Finally, at differentiation, many cells elaborate within the primary wall a secondary cell wall (Fig. 2.4), building complex structures uniquely suited to the cell’s function. The plant cell wall is a highly organized composite of many different polysaccharides, proteins, and aromatic substances. Some structural molecules act as fibers, others as a cross-linked matrix, analogous to the glass-fibers- and-plastic matrix in fiberglass. The molecular composition and arrangements of the wall polymers differ among species, among tissues of a single species, among individual cells, and even among regions of the wall around a single protoplast (Fig. 2.5). Not all specialized functions of cell walls are structural. Some cells walls contain molecules that affect patterns of development and mark a cell’s position within the plant. Walls contain signaling molecules that participate in cell–cell and wall–nucleus communication. Fragments of cell wall polysaccharides may elicit the secretion of defense molecules, and the wall may become impregnated with protein and lignin to armor it against invading fungal and bacterial pathogens (Fig. 2.6; see also Chapters 21 and 24). In other instances, the walls participate in early recognition of symbiotic nitrogen-fixing bacteria (see Chapter 16). Surface molecules on cell walls also allow plants to distinguish their own cells from foreign cells in pollen-style interactions (Fig. 2.7; see also Chapter 19).

Figure 2.3 The primary walls of cells are capable of expansion. The middle lamella is formed during cell division and grows coordinately during cell expansion. Contact between certain cells is maintained by the middle lamella, and the cell corners are often filled with pectin-rich polysaccharides. In older cells (not shown), the material in the cell corners is sometimes degraded and an air space forms.

(A)

(B)

Figure 2.4 When they have achieved their final size and shape, some cells elaborate a multilayered secondary wall within the primary wall. In the diagram (A), the lumen of the cell is sometimes surrounded by several distinct kinds of secondary walls (here, 1–S3), with the original primary wall (CW1) and the middle lamella (ML) constituting the outermost layers. As shown in this example of a fiber cell from a young stem of a locust tree, fibers may also contain “warts” (W), which are a last stage of wall thickening before the protoplast disintegrates (B).

(A)	(B)	(C)
Figure 2.5 The production of complex, chemically diverse molecules within cell walls is developmentally regulated. (A, B) Staining walls of the oat root with two antibodies against arabinogalactan proteins demonstrates that different members of this class of molecules occur in specific tissues. One epitope is present only in cortical cell walls (A), whereas a second epitope is present only in walls of the vascular cylinder (arrows) and epidermis (B). (C) Antibodies against methyl-esterified pectin and unesterified pectins show that these epitopes are enriched not only in different cells but also in different regions of the walls of a single cell. In this cross-section of an Arabidopsis stem, domains enriched in methyl-esterified pectin are labeled in green, whereas deesterified pectins, shown in yellow, are found in specialized cells, such as the outer epidermal wall (oe), fiber cells of phloem (pf), some cell corners and cross-walls, and tracheary elements of the xylem (te). The autofluorescence of lignin is shown in red.