1
Cell Walls and
Plant Anatomy
Examine a thin section of any higher plant organ with the light microscope,
and you will immediately notice two features (Figure 1.1A). First, the plant
is built from more than one type of cell, and each type can be identified by
its size, its location, and the thickness, organization, and structure of the
wall that surrounds it. Second, the different cell types are neatly cemented
to their neighbors by their walls (Figure 1.1B) in beautiful and reproducible patterns. The structure and function of cells and the developmental
patterns they form are the subject matter of plant anatomy. In this chapter we introduce the role of the cell wall in the development of the various
cell types that are used to build higher plants. We discuss how and where
new cells and therefore new walls arise, emphasizing the involvement of
intracellular structures. We then examine the way in which new walls allow
cells to expand and how walls are modified when cells differentiate. Last, we
discuss the basic cell types of the plant body and how the structural adaptations of their walls suit their particular function.
A. The Derivation of Cells and Their Walls
1. Cells arise in specialized regions of the plant called
meristems.ref1
Plants, like ourselves, start off with the fusion of an egg and a sperm to form
a zygote. However, the plant zygote’s subsequent development differs in
at least two major respects from our own. First, the cell movements and
migrations that characterize animal embryo development are not possible
within a plant, whose cells are stuck to each other by their surrounding cell
walls. The second is that plant development, that is, the elaboration of the
adult plant from the fertilized egg, is not a continuous process. Instead, a
programmed series of cell divisions gives rise to an embryo that contains
epidermis
B
cortex
A
wall of
cell A
(A)
(B)
middle
lamina
wall of
cell B
Figure 1.1 Cell walls are cemented to
each other. (A) Section of Arabidopsis
stem stained with calcofluor, a dye that
binds to the cellulose in walls, making
them fluorescent. (B) It is important to
realize that in sections, what we generally
refer to as the cell wall can also be viewed
as a composite structure formed from two
walls, one contributed by each adjoining
cell, firmly stuck together at a region called
the middle lamella.
2
Chapter 1
Cell Walls and Plant Anatomy
shoot
pole
(A)
shoot apical meristem
100 mm
root
pole
embryo
(C)
(B)
100 mm
100 mm
Figure 1.2 Apical meristems derive from the root and shoot
poles of the embryo. (A) A scanning electron micrograph shows
the shoot apex with two sequentially emerging leaf primordia, seen
here as lateral swellings on either side of the domed apical meristem.
(B) A thin section of a similar apex shows that the youngest leaf
primordium arises from a small group of cells (about 100) in the
outer four or five layers of cells. (C) The root meristem and root cap
of a corn root, showing the orderly files of cells produced. (A and B,
from R.S. Poethig and I.M. Sussex, Planta 165:158–169, 1985.
C, from P.H. Raven, R.F. Evert, and S.E. Eichhorn, Biology of Plants,
4th ed. New York: Worth, 1986.)
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a shoot pole, a root pole, and either one or more seed leaves; and at this
point the embryo’s development is commonly arrested in a seed. Within the
maternal ovule tissue of the seed, the embryo will remain dormant until
seed dispersal and suitable environmental conditions permit it to germinate and grow.
The young plant seedling will grow by a coordinated combination of cell
enlargement and the production of new cells. However, cell proliferation
does not occur evenly throughout the plant body. Following germination,
two specialized groups of cells at opposite ends of the seedling axis, the
shoot and root poles, begin to proliferate actively. Although cell divisions
can occur elsewhere in the plant, these two groups of cells, now called the
shoot apical meristem and the root meristem, respectively, will become a
major source of new cells in the plant. They both perpetuate themselves
and also give rise to new meristems in an iterative process that produces
the characteristic structure of the mature plant, including structures such
as leaves and flowers (Figure 1.2). Other groups of dividing cells can arise in
established tissues, where they are called lateral meristems. These give rise
to lateral roots and, through the cambium and cork cambium contribute to
growth in thickness of the plant. A simple overview of higher plant anatomy
is discussed later (see Panel 1.1).
Meristems themselves have a relatively defined, species-specific structure,
but all are composed of small, densely cytoplasmic cells, generally less than
10 mm in diameter and having thin cell walls (Figure 1.3). Both shoot apical and root meristems contain two distinct populations of cells. A small
subset of cells, called initials, or stem cells, divide surely but slowly. When
The Derivation of Cells and Their Walls
nucleus
vacuole
3
Figure 1.3 Meristematic cells. Typical
meristematic plant cells are small, with
very small vacuoles and thin cell walls. This
example is from a shoot apical meristem.
(Courtesy of Ichirou Karahara.)
cell wall
5 mm
they divide, they produce one daughter cell that remains a stem cell and
another daughter cell that enters the cell population of the rest of the meristem. These cells may divide further, but eventually they leave the meristem.
It is largely within these populations of dividing cells that new primary walls
are produced.
2. Walls originate in dividing
cells.ref2
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If we consider a single cell within an apical meristem, it is tempting to think
that its surrounding wall arose in a single process of wall deposition around
the living protoplast. But that would be to ignore the course of development.
In fact, every facet of its polyhedral wall was laid down at a different time,
namely, when that cell was cut off from one of its immediate neighbors by
a cross wall during cell division. In some cases this might be a recent event,
but in others the wall might derive, by growth, from one laid down during
a cell division much earlier, for example, in the embryo. Each wall is thus a
patchwork coat, with each patch reflecting events that happened at different times in the cell’s development. Each cell, consequently, carries within
its wall a historical relationship with all of its neighbors (Figure 1.4).
1
2
1
3
2
2
3
4
4
Figure 1.4 Sequential cell divisions create “walls” with different histories. Schematic diagram, showing how a sequence of
cell divisions creates a series of intersecting cell walls of different ages (shaded gray). The resultant final cell pattern is shown on the
right with the wall of the central cell shown in black. This is usually interpreted as a single cell wall, but developmentally it has been
made by the consecutive addition of different cell plates that eventually intersect with each other. A particular wall (for example,
wall 1) has a history in common with the walls of adjacent cells with which it shares ancestry, rather than necessarily with wall 3, for
example, which has only recently been cut and pasted into it. This example emphasizes the problem of whether a “wall” stops at the
middle lamella or is a single partition that includes the contribution from two neighboring cells.
4
Chapter 1
Cell Walls and Plant Anatomy
25 mm
Figure 1.5 The origin of a new cell wall.
Sequential light micrographs of a dividing
stamen hair cell. The elapsed time in
minutes is shown at the bottom left corner
of each photograph. By 42 minutes the
nucleus has undergone mitosis, cytokinesis
is under way, and the early cell plate
can be seen between the two daughter
nuclei. This extends rapidly outward until
it reaches and fuses with the mother cell
wall. (Courtesy of Peter Hepler.)
With very few exceptions, new cross-walls arise only when cells divide and,
following nuclear division or mitosis, they partition the protoplast in a
process called cytokinesis. Plant cytokinesis is an inside-outward event that
starts with the formation of a small, disklike wall, or cell plate, inside the
cell, 1A2.2/1.05
between the two daughter nuclei. This cell plate extends radially outPCW
ward until it finally reaches and fuses with the mother cell wall, thus cutting
the cell in two (Figure 1.5). The structure that assembles this cell plate is
called the phragmoplast; it originates most commonly from the microtubules of the two half spindles of the mitotic apparatus and has a disklike
structure, with the plus ends of the microtubules on either side ending in
a plane at right angles to the spindle axis. Associated with this structure
are actin filaments, closely aligned with the microtubules. The phragmoplast is surrounded by numerous Golgi stacks, which produce vesicles that
are transported along the microtubules to the center of the phragmoplast.
Here, the vesicles fuse to form a membrane-enclosed disk, the cell plate
(Figure 1.6). The vesicles carry pectic polysaccharides, xyloglucan, and proteins to the cell plate; this subject is discussed in greater detail in Chapter 4.
The polysaccharide callose, a b1-3 glucan, can be detected in the plate at an
early stage, but the cellulose, characteristic of the final wall, is not deposited
until the latest stages of cell plate formation. Intact phragmoplasts that can
continue to make wall material for the cell plate can be isolated from synchronized cells in suspension culture (Figure 1.7).
For complete partition, even in a small meristematic cell, the cell plate must
be extended at its edge until it finally makes contact with the mother cell
wall to produce a cross-wall. The phragmoplast achieves this by continuously disassembling the microtubules at its center and reassembling them
at the edge of the growing cell plate, where they function in guiding new
vesicles to fuse with and extend its margin (Figure 1.8).
At about the time the cell plate finally reaches the mother cell wall and separates the two daughter cells, callose is removed and cellulose deposition
begins on each face of the plate, allowing us to finally distinguish two separate “walls” within the partition, one being made by and belonging to one
daughter cell and one being made by and belonging to the other.
3. Plant organ development depends on precise control of
the plane of cell division and of cell expansion.ref3
A single isolated plant cell grown in culture can successfully divide and
expand or elongate. If its cell wall is removed, however, the remaining protoplast, although living, can neither divide nor elongate unless it regenerates
its wall. Correspondingly, the walled cell cannot divide or elongate unless
its cytoskeleton is both intact and functional. The proliferation and growth
of plant cells depends on a functional interaction between elements inside
the plasma membrane—in particular the cytoskeleton—and the cell wall
outside the plasma membrane. However, during tissue development, these
The Derivation of Cells and Their Walls
cell A in telophase
5
cell B in cytokinesis
new cell plate
5 mm
Figure 1.6 The cell plate and dividing cells. This electron micrograph is of a thin longitudinal section through a maize
root meristem. The long axis of the root runs from left to right. The large central cell has already divided once to form two
new cells, A and B, with a thin young primary wall separating them. The two daughter cells are each now dividing again.
Cell A is in late telophase, and the two separated sets of chromosomes are visible. Cell B has reached the next stage of
cell division, and between the two re-forming nuclei an early stage of vesicle accumulation and cell plate formation can be
seen. (Courtesy of Adrian Turner.)
cell-autonomous activities are tightly coupled with those of neighboring
cells to generate the precise and reproducibly structured organs we find in
the mature plant.
There are additional constraints on the generation of plant form. We have
already mentioned that the cells of plants are immobile and that, with
very few exceptions, adjacent cells are firmly stuck together and are thus
prevented from expanding at different rates and “slipping” relative to one
another. Thus, local cell-cell interactions are central to
coordination
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1A2.3/1.06
and control of plant growth.
We are now in a position to appreciate that, in many cases, it is the accurate
placing of new cross-walls during cell division, followed by the subsequent
controlled expansion and/or elongation of the resultant cells, that has
(A)
(B)
10 mm
Figure 1.7 Polysaccharide synthesis in
isolated phragmoplasts. Radioactive
precursors are incorporated into wall
polysaccharides by phragmoplasts isolated
from tobacco cells in culture. These retain
all the cytoplasmic organelles usually
associated with the phragmoplast,
including Golgi stacks. (A) When the
phragmoplasts are fed with radioactive
UDP-xylose, this marker is incorporated
into matrix polysaccharides in the Golgi
stacks surrounding the cell plate (arrows)
and can be revealed by autoradiography
as black silver grains. (Golgi stacks cannot
transport the product to the cell plate
within the isolated phragmoplast.)
(B) When phragmoplasts are fed with
labeled UDP-glucose, the label is
incorporated into callose by enzymes
located in the membrane of the cell plate.
Here the silver grains are located directly
over the cell plate (arrows). (From
T. Kakimoto and H. Shibaoka, Plant Cell
Physiol. 33:353–361, 1992.)
6
Chapter 1
Cell Walls and Plant Anatomy
(A)
Figure 1.8 The phragmoplast and cell plate. (A) Sequence of
events in phragmoplast formation. (B) A light micrograph of a plant
cell entering cytokinesis. The cell has been stained to show both the
microtubules and the two sets of daughter chromosomes. The clear
region where the new cell plate is being assembled is indicated by
the arrowheads. (B, courtesy of Andrew Bajer.)
(B)
50 mm
a major influence on plant organ development. For example, in the first
early divisions of the zygote of the model plant Arabidopsis (Figure 1.9),
the precise sequential placing of the first cross-walls that produces the precise pattern of cells in the embryo appears to be essential for subsequent
development. Mutations that affect these early embryonic division planes
produce severely deformed or nonviable embryos. Later in development,
common patterns of division and expansion often emerge. Thus, meristems
often contain cells that reiterate transverse divisions to produce extended
files of cells (Figure 1.10).
The continuous cell proliferation activity of meristems creates a population
pcw 1A2.5/1.08
of cells that will divide less frequently, will expand, and will eventually differentiate into mature cell types. Both the spatial and temporal controls over
the plant cell cycle and the spatial and temporal controls over cell expansion have to be tight and, more important, coordinated with neighboring
cells. Although such controls are likely to include mechanical constraints,
environmental and physiological cues, and the action of conventional plant
growth factors, the detailed molecular machinery through which they work
has not been fully elucidated.
Figure 1.9 Early sequence of divisions
in the Arabidopsis zygote. The fertilized
egg (A) divides transversely to form an
upper cell (gray) that will form the embryo
proper, and a lower cell (white) that will
form the suspensor and part of the root
meristem (B). The embryo proper goes
through a series of stereotyped divisions
(C–F) that include, for example, some
periclinal divisions (E) that distinguish an
outer layer of cells that will form the entire
shoot epidermis (dark gray). In some other
species, early embryonic divisions are much
less organized.
Despite our emphasis so far on the pattern of sequential divisions, it should
be noted that plant structure is characterized by flexibility and plasticity,
and it is the interaction between the separate processes of cell proliferation
and cell expansion that generates plant form. In some cases, properties of
the whole organ can override or entrain the contributions of division and
expansion at the cellular level, as in the maize mutant tangled. In a normal
maize plant, the leaf epidermis has a highly ordered pattern of longitudinal
and transverse divisions that contribute to the width and length, respectively, of the leaf. By contrast, in the tangled mutant, the leaf epidermis has
large numbers of cells with aberrant division planes. Despite the rather
(A)
(B)
(C)
(D)
(E)
(F)
The Derivation of Cells and Their Walls
epidermis
cortex
endodermis
Figure 1.10 Root meristems produce
organized files of cells. (A) A scanning
electron micrograph of an Arabidopsis root
tip showing the longitudinal files of cells
produced by transverse divisions of the
epidermal cells. The diameter of this root
is exactly comparable to a human hair.
(B) A longitudinal section of a similar root
shows that the files of cells can be traced
back to their origins within the meristem.
The epidermis in this section is largely
covered by cells of the root cap.
(A, courtesy of Paul Linstead.)
root cap
(B)
(A)
50 mm
muddled pattern of resulting cell walls, the overall shape of the mutant leaf is
remarkably similar to that of the wild type (Figure 1.11). The TANGLED gene
is required for the proper positioning of the cytoskeletal arrays involved in
the formation and placement of new cell plates, which we discuss next. The
way in which larger-scale structures and their growth patterns can affect
cell division and expansion patterns remains obscure.
(A)
7
(B)
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Figure 1.11 Division plane control.
Scanning electron micrographs of the
surface of maize leaf primordia. (A) Wild
type and (B) the mutant tangled. Despite
the disorganized division planes, the
mature leaf that finally develops looks
remarkably normal in shape. (Courtesy of
Laurie Smith.)
8
Chapter 1
Cell Walls and Plant Anatomy
forming
mitotic
spindle
nucleus
preprophase
band of
microtubules
(A)
(B)
Figure 1.12 The preprophase band. In
these living tobacco cells in culture, the
microtubules are stained because the cell
is expressing a green fluorescent protein
fused to a microtubule-binding protein.
The microtubules in the dividing cell
gradually accumulate in the preprophase
band (A–C) and also begin to form the
mitotic spindle (C). Cortical microtubules
can be seen in the nondividing cells.
(D) The preprophase band encircles the
cell, defining the future division plane.
(A–C, courtesy of Henrik Buschmann.)
(C)
(D)
4. Cytoskeletal elements predict the position of the new
cross-wall before the cell divides.ref4
Thin sections of healthy plant tissue show clear, reproducible, and speciesspecific patterns of cells. This consistent anatomy strongly indicates that,
during the cell divisions that give rise to the tissue, the positioning of new
cross-walls is an extremely accurate process. What mechanisms exist within
plant tissues to ensure such precision in cell partitioning? In almost every
PCW 1A4.1/1.12
case, shortly after DNA replication has finished, cytoskeletal elements rearrange themselves within the cell into a structure just inside the plasma
membrane that predicts the site where the new cell plate will later meet the
mother cell wall. The most obvious feature of this structure is a tight bundle of
microtubules lying just beneath the plasma membrane that has been called
the preprophase band (Figure 1.12). The ability of this band to prefigure the
exact future division plane is true for both symmetric and asymmetric cell
divisions (Figure 1.13). The nucleus at the early prophase stage of mitosis,
with its condensing chromosomes, typically lies at the center of the plane
defined by the preprophase band and appears to be held there by cytoplasmic strands that also contain both microtubules and actin filaments. One
might think of the whole structure as rather like a bicycle wheel in which the
rim represents the preprophase band, the hub represents the nucleus, and
the spokes represent the microtubules and actin filaments radiating from
the nucleus to the preprophase band (Figure 1.14 and Figure 1.15).
subsidiary
cell
Figure 1.13 A preprophase band
predicts a future asymmetric cell
division. In the epidermis of a grass leaf
(rye) a series of divisions gives rise to a
mature stoma flanked by two guard cells
and two subsidiary cells. (A) The subsidiary
cells arise by an asymmetric division in
the subsidiary cell mother cell (B) that is
predicted by a curved preprophase band.
(C) The immunofluorescence image shows
the microtubules of the preprophase band
revealed by an antibody to tubulin.
(C, from S.-O. Cho and S.M. Wick,
J. Cell Sci. 92:581–594, 1989.)
20 mm
(A)
stomatal
guard cell
preprophase band in
subsidiary cell mother cell
(B)
(C)
10 mm
The Derivation of Cells and Their Walls
9
Figure 1.14 Microtubules in the
preprophase band. These electron
micrographs of a thin section of a leaf
epidermal cell from sugar cane show the
preprophase band of microtubules just
beneath the plasma membrane. The cell
in this case is part of the stomatal complex
and is dividing asymmetrically to produce
a smaller daughter cell that will form the
stomatal subsidiary cell. (Courtesy of
C.H. Busby.)
It may seem paradoxical that the microtubules of the preprophase band,
which appear to predict so accurately the position of the future cross-wall,
actually disappear as the cell enters mitosis. The nature of the “molecular
memory” that remains at thePCW
cell1A4.3/1.14
surface appears to be produced by remodeling of the plasma membrane in the region of the preprophase band by
endocytic vesicles that remove selected membrane proteins. The significance of this is explored in the next section.
The signals or cues that are used by a cell to position the preprophase band
are largely unknown. However, one signal used by many cells to determine
the division plane is mechanical stress. Artificially applied pressure on living
plant tissue commonly triggers new cell divisions in which the new crosswalls are oriented at right angles to the applied pressure. To what extent
such cues operate to determine the position of preprophase bands during
normal development is not known.
5. Actin filaments help to position new cross-walls.ref5
We have seen how placement of the new cross-wall during cell division is
faithfully predicted by the position of the preprophase band of microtubules.
Both environmental cues—for example, physical pressure—and local factors can influence cross-wall positioning. In particular, the shape of the cell
and the local cellular geometry of neighboring cells influence the decision
about where the preprophase band, and hence the future cross-wall, will
be located. For example, long cells tend to divide with the new cross-wall at
right angles to the long axis of the cell. A cross-wall in one cell also tends to
inhibit the placing of a new cross-wall directly opposite in the neighboring
cell. In other words, three-way junctions (places at which three cells contact
each other) are strongly favored over four-way junctions (Figure 1.16). In
groups of cells in which the final pattern of divisions is highly repeatable,
such as the stomatal divisions in Figure 1.13, the extending cell plate meets
the mother cell wall at the predetermined site with an accuracy of less than
1 mm, and we must consider now how this relates to the earlier positioning
of the preprophase band.
Following the disappearance of the microtubules from the preprophase
band, the actin filaments that remain appear to have several major roles
in subsequent events. First, an actin-free zone persists in place of the
preprophase band, probably originating by local endocytosis removing
membrane-associated actin-binding proteins. This in some way marks that
cortical region of the cell with which the new cross-wall will fuse. Groups
of actin filaments, extending from the surface of the nucleus to a region at
each end of the cell, create a force on the developing spindle that helps align
it perpendicular to the plane of the preprophase band (Figure 1.17A–C). This
ensures that, following chromosome separation, the developing phragmo-
20 mm
Figure 1.15 A preprophase band in a
large dividing vacuolate epidermal cell.
The cell is fluorescently labeled with an
antibody to tubulin and the original image
is three-dimensional, made by combining
PCW 1A4.4/1.15
a stack of separate optical sections. The
nucleus, in the center of the cell, is clearly
connected to the cortical preprophase
band of microtubules by radial bundles
of microtubules. (From C.W. Lloyd, ed.,
Cytoskeletal Basis of Plant Growth and
Form. London: Academic Press,
pp. 245–257, 1991.)
10
Chapter 1
(A)
(B)
(C)
(D)
(E)
(F)
pcw 1A5.1/1.16b-f
Cell Walls and Plant Anatomy
30 mm
Figure 1.16 Cell division planes and the preference for threeway junctions. (A) A scanning electron micrograph of the surface
of an oilseed rape embryo reveals that junctions are almost invariably
between three cells rather than four or more. (B–F) These preferred
Y-shaped, three-way junctions arise because new cell walls that are
inserted during cell division generally avoid a preexisting junction as
is shown below schematically. (B, C) A new division wall is inserted
transversely in the left-hand cell of two young expanding cells.
(D) As the cells expand it is thought that the new wall lags behind
and the common wall buckles to produce a Y-shaped three-way
junction. When the right-hand cell divides, the preprophase band
avoids this vertex, and when the new division wall is complete (E),
it is staggered with respect to the earlier wall. (F) This junction, too,
will eventually become Y-shaped. Similar, and well understood,
geometries occur in rafts of soap bubbles, but the molecular basis
for selecting the preprophase band site, although influenced in part
by geometry and mechanical forces, is by no means completely
understood. (A, courtesy of Lloyd Peto and Kim Findlay.)
plast, and therefore the developing cell plate, is in the plane defined by the
preprophase band. Second, the groups of radial actin filaments persist and
become attached to the edge of the growing cell plate. These filaments exert
tension and guide the extending plate accurately to its final cortical position
where the actin-free zone was established. The accuracy of this guidance
system is particularly obvious in the highly vacuolate cells of the cambium,
where the growing edge of the plate can be guided over a considerable distance to its final predetermined site (Figure 1.17D–F).
6. The new cross-wall must join and fuse with the mother
cell wall.ref6
When the extending cell plate finally meets its appointed destination on
the mother cell wall, further events must occur before the formation of two
new daughter cells is completed. The membrane that surrounds the cell
plate is topologically equivalent to the plasma membrane of the mother
cell, and when these meet they fuse, initially at local sites, and later completely to effect the separation of two daughter protoplasts. At this stage,
the new cross-wall, as it has now become, has a relatively homogeneous
interior, and at this stage or shortly before, cellulose begins to be deposited.
Since the cellulose is made at the plasma membrane (see Chapters 4 and
5), production of this structural polysaccharide takes place simultaneously
at both faces of the new cross-wall. Cellulose fibers from one face do not
intermingle with those from the other, and therefore, an “exclusion zone,”
rich in pectic polysaccharides, is created in the center of the wall. This is
called the middle lamella. At the junction between the mother cell and the
new cross-wall, further events take place. In this region, local dissolution of
the mother cell wall takes place, allowing the newly created middle lamella
to extend and fuse with the middle lamella of the surrounding mother cell
wall. It is as though the old wall were cut and the new walls were pasted in
(Figure 1.18). The two new cross-walls, as they now are, become successfully
integrated into the structure of the mother cell wall and finally create two
daughter cells, each with its own new wall. The new three-way junctions of
middle lamella that are created are important sites for future events (such
as air space formation), which are discussed in Chapter 6 (Concept 6C5).
7. A plant is constructed from two compartments:
the apoplast and the symplast.ref7
During the formation of new cross-walls, tubular elements of endoplasmic
reticulum become trapped in the cell plate at the stage of vesicle fusion.
The resulting channels are structurally elaborated into more permanent
The Derivation of Cells and Their Walls
anaphase
chromosomes
nucleus
preprophase
band of
microtubules
cell plate
11
preprophase
band
actinfree
zone
(A)
actin
(B)
(C)
actin
(D)
(E)
(F)
Figure 1.17 The role of the cytoskeleton in division plane alignment. (A) As a vacuolated dividing cell enters
prophase, the preprophase band of microtubules at the cell cortex marks the site of the future division plane.
Microtubules and actin also extend through the cytoplasm and connect with the nucleus (see Figure 1.15).
(B) As the chromosomes align at metaphase, the microtubules in the preprophase band have disassembled,
leaving in place an actin-free zone at the cortex throughout mitosis. (C) Actin persists elsewhere in the cell and
helps guide the extending cell plate during cytokinesis to the correct site at the mother cell wall. On completion of
cytokinesis, the cortical array of microtubules is reestablished. (D–F) Cutaway diagrams showing the division of an
elongated and vacuolate cambium
cell.
These show the position of the preprophase band of microtubules (D), the
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1A5.2/1.17
extension of the cell plate following nuclear division (E), and the completed division (F) in which the cell plate has
fused with the mother cell wall at the predicted site to create two new cambial cells.
pores called plasmodesmata (singular, plasmodesma). Each pore is lined by
plasma membrane that is continuous from cell to cell and usually contains
a rodlike structure derived from the endoplasmic reticulum that is called
the desmotubule. While many plasmodesmata are formed during cell plate
formation, it is important to realize that they may also be formed de novo
in more established cell walls. Although their diameter is on the order of
50 nm or so, the aqueous channel between the plasma membrane and the
desmotubule that extends from cell to cell is rather tightly regulated and in
general will allow the free passage of only small molecules. (Plasmodesmata
are discussed in more detail in Concept 1C9.)
The presence of abundant plasmodesmata means that all of the cells in a
higher plant, with very few exceptions (stomatal guard cells and germ cells),
share, topologically speaking, a single plasma membrane and thus the cells
are cytoplasmically coupled to each other. This cytosolic continuity within
the plant creates a single, extended topological space, bounded by shared
plasma membrane, that is called the symplast (Figure 1.19). The remainder
of the plant, which includes everything outside the symplast, also forms a
continuous space, occupied by the cell walls, the intercellular spaces, and
the contents of such dead cells as xylem vessels that have lost their plasma
membrane. This continuum is called the apoplast (Figure 1.19) and is
broadly equivalent to what we know as the extracellular matrix.
cellulose
microfibrils
mother cell wall
plasma
new cross
membrane
wall
(A)
middle lamella
(B)
wall dissolution
(C)
new 3-way
junction
(D)
Figure 1.18 Steps in completing a new
cross wall and generating two new
daughter cells. After the membrane
around the cell plate has completely fused
with the plasma membrane of the mother
cell (A), cellulose begins to be deposited
within the plate and a new middle lamella
region is created in the central region
(B). (C) Wall hydrolytic enzymes aid the
controlled dissolution of a region of the
mother cell wall that allows the cutting
and pasting of the new wall into the
old. (D) The middle lamellae become
continuous, the two daughter cells acquire
their own complete walls, and a new
three-way junction is created.
12
Chapter 1
Cell Walls and Plant Anatomy
Figure 1.19 Apoplast and symplast.
Most but not all cells in the plant
are connected to their neighbors by
plasma membrane–lined cytoplasmic
channels called plasmodesmata. These
connections create two discrete topological
compartments within the plant: the
apoplast, consisting of everything outside
the cell’s plasma membrane, including the
dead xylem vessels and their contents; and
the symplast, which consists of everything
inside the plasma membrane—the plant’s
collective cytoplasm. As shown in the inset,
the cytoplasm of cell A is continuous with
that of cell B via the (deliberately simplified)
plasmodesma.
cytoplasm
(SYMPLAST)
plasmodesmata
cell wall
(APOPLAST)
cytoplasm of cell A
(SYMPLAST)
plasmodesma
A
cell wall
plasma
membrane
B
plasma
membrane
cytoplasm of cell B
(SYMPLAST)
xylem vessels
(APOPLAST)
Both apoplast and symplast can provide routes for the local transport of
water and solutes. Water, ions,pcw
and1A7.1/1.19
small signaling molecules, for example,
may be transported through the apoplast. The same molecules, together
with sugars, amino acids, and many other small organic molecules, may be
transported from cell to cell within the symplast. The plasma membrane,
which forms the boundary between apoplast and symplast throughout
the plant, has an important function in regulating the transport of solutes
between the two compartments. In some cases the movement of materials
may be restricted to either the apoplast or the symplast. Symplastic restriction is exemplified by stomatal guard cells (see Concept 1C2). These become
symplastically isolated from their neighbors early in their differentiation,
when all of the plasmodesmata connecting them to adjacent cells are
removed. Apoplastic restriction can be seen in the waterproof, suberinized
wall layer, known as the casparian band, which acts as a barrier between the
endodermal cells of the root to the movement of water and solutes through
the wall (see Concept 1C4). The idea of apoplast and symplast helps focus
our attention on the three-dimensional organized continuity of both cells
and their extracellular matrix.
B. Walls in Cell Growth and Differentiation
1. Cells become organized at an early stage into three
major tissue systems.ref8
In the previous section we discussed how and where new cells and new
walls arise. In this section we examine how cells grow and differentiate into
a variety of cell types and how these cell types are assembled into the tissues, tissue systems, and organs of the mature plant.
It is convenient, at this stage, to have a descriptive framework to make some
sense of the complex variety of cell patterns we find in sections of different plant parts. A simple hierarchical system of anatomy is widely used in
which plants are first broken down into organs (for example, root, stem, or
leaf), each of which in turn is built from different arrangements of three
tissue systems, the dermal system, the ground system, and the vascular
system (Panel 1.1). The dermal tissue system comprises the outer covering
of the plant, that is, the epidermis and its more complex replacements in
older organs such as bark. The ground tissue system consists of supportive
tissues (for example, parenchyma, collenchyma, and sclerenchyma, which
help support and protect the vascular tissue system) together with storage