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.
its wall, a protoplast adopts a spherical
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
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).
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.
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
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.