The plant cell wall is part of a
dynamic compartment, changing throughout the life of the cell. When cells of
the meristem enlarge and elongate, new material is continuously integrated into
the primary cell wall, which expands and increases in surface area, in some
cases over a 1,000‐fold. New walls are born in the cell plate during cell
division and fuse with the existing side walls. A middle lamella forms the
interface between the primary walls of neighboring cells, and material is also
added to this interface as their primary walls increase in area. In older
cells, the pectin‐rich material in the cell corners is sometimes digested by hydrolytic
enzymes, and an air space forms. Finally, at differentiation, many cells, such
as tracheids and fibers, elaborate within the primary wall a distinct secondary
cell wall, 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 crosslinked matrix analogous to the glass fibers and
plastic matrix in fiberglass. The molecular composition and arrangements of wall
polymers differ among species, tissues of a single species, individual cells,
and even regions of the wall around a single protoplast.
Some cell 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 wall polysaccharides may
elicit secretion of defense molecules, and the wall may become impregnated with
protein and lignin to armor it against invading fungal and bacterial pathogens.
Cell wall surface molecules also allow plant cells to recognize their own kind
in pollen‐style interactions.
Polysaccharides
are long chains of sugar molecules covalently linked at various positions, and
some are decorated with side‐chains of various lengths. Familiarity with the
chemistry and nomenclature of sugars greatly facilitates understanding of the
many biological functions of polysaccharides. Sugars are polyhydroxy aldehydes
(aldoses) and ketones (ketoses) that can be grouped according to their chemical
formula, configuration, and stereochemical conformation.
A developing
cell constructs its wall architecture to provide characteristic shapes. Some
examples: The spongy parenchyma of a Zinnia elegans leaf minimizes cell contact
and maximizes cell surface for gas exchange. Water droplets bead on the waxy
lower epidermis but not on the spongy mesophyll cells. An Arabidopsis trichome
is an exquisitely branched, modified epidermal cell. The thickened and
elaborated inner wall of a guard‐cell pair provides the physical form needed to
control aperture size of a stoma. The cuticle, a special wall outgrowth of waxy
and phenolic substances, minimizes evaporative loss of water from the exterior
surface. Transfer cells elaborate a highly fenestrated wall to enhance surface
area of the plasma membrane for sugar transport. The end wall (sieve plate) of
the sieve element is perforated with many canals as a result of selective wall
hydrolysis. The specialized shapes of these epidermal cells reflect light to
enrich the colors of a snapdragon (Antirrhinum sp.) petal. Pollen grains
elaborate outer walls of many forms.
Sugars that adopt
a five‐membered ring configuration (four carbons and an oxygen) are called
furanoses, and those forming six‐membered rings (five carbons and an oxygen)
are pyranoses. The ring conformation a sugar adopts is not defined by the
number of carbons: both pentoses and hexoses can occur in either ring form. The
cell plate forms during cytokinesis and undulates until it contacts the mother
wall. Coincident with partitioning of the mother cell into two daughter cells,
the cell plate changes from a wavy appearance to a firm, flattened wall. Values
in the corners of each plate are minutes from the onset of prophase. Primary
walls of cells are capable of expansion. Adhesion between certain cells is
maintained by the middle lamella, and the cell corners are often filled with
pectin‐rich polysaccharides. The middle lamella forms during cell division and
grows coordinately with the primary walls.
Cells often
alter their cell wall in response to environmental stimuli and potential
pathogens and symbionts. In response to attempted invasion by fungal hyphae of
Colletotrichum, a maize (Zea mays) cell produces a wall apposition called a
papilla. Largely composed of callose, the papilla also accumulates lignin, as
shown by staining with phloroglucinol and by staining with syringaldehyde stain
for laccase activity. In its response to Colletotrichum invasion, an infected
cell of sorghum (Sorghum bicolor) accumulates phytoalexins in inclusion bodies,
and neighboring cells armor their walls with reddish phenylpropanoids
(arrowheads). During sugar polymerization, the anomeric carbon of one sugar
molecule is joined to the hydroxyl group of another sugar, sugar alcohol,
hydroxylamino acid, or phenylpropanoid compound in a glycosidic linkage. Polysaccharides
are named after the principal sugars that constitute them. Most polysaccharides
have a backbone structure, and the composition of this structure is indicated by
the last sugar in the polymer’s name.
What makes sugar
subunits such versatile building materials is their ability to form linkages at
multiple positions. With 11 different sugars commonly found in plant cell walls,
four different linkage positions, and two configurations with respect to the
oxygen atom, the permutations of pentasaccharide structure zoom to over 5
billion! Glucose alone can form almost 15,000 different pentameric structures. Multiple
bonding positions also make possible the formation of branched polysaccharides,
enormously increasing the number of possible structures. Cellulose is the most
abundant plant polysaccharide, accounting for 15–30% of the dry mass of all
primary cell walls and a much larger percentage of secondary walls. Cellulose
exists in the form of microfibrils.
Crosslinking
glycans are a class of polysaccharides that can hydrogen bond to cellulose
microfibrils: they may coat microfibrils, but are also long enough to span the
distance between microfibrils and link them together to form a network. Crosslinking
glycans are often called “hemicelluloses,” a widely used term for all materials
extracted from the cell wall with molar concentrations of alkali, regardless of
structure.
Pectins perform
many functions: determining wall porosity and providing charged surfaces that
modulate wall pH and ion balance, regulating cell–cell adhesion at the middle lamella,
and serving as recognition molecules that alert plant cells to the presence of
symbiotic organisms, pathogens, and insects. Particular cell wall enzymes may
bind to the charged pectin network, constraining their activities to local
regions of the wall. By limiting wall porosity, pectins may affect cell growth,
thereby regulating access of wall‐loosening enzymes to their glycan substrates.
There are three
major classes of structural proteins named for their respectively enriched
amino acid: hydroxyproline‐ rich glycoproteins (HRGPs), proline‐rich proteins (PRPs),
and glycine‐rich proteins (GRPs). All of them are developmentally regulated,
with relative amounts varying among tissues and species. The extent of methyl
esterification may remain high in walls of some cells, and a type of gel may
form that contains highly esterified parallel chains of HGs[1].
Some HGs and RGs are crosslinked by ester linkages to other pectins or other polymers
held more tightly in the wall matrix and can only be released from the wall by
de‐esterifying agents. Other pectic polymers may separate sites of borate
di‐diester crosslinking by RG II dimers along the pectic backbone. In
boron‐deficient cell cultures walls swell and porosity.
Even in a single
cell, modifications occur that distinguish between transverse and longitudinal
walls. For example, preferential digestion of end walls occurs in sieve tube
elements. Some substances, such as waxes, are secreted only to a cell’s outer
epidermal face. Within a single wall there are zones of different
architectures, the middle lamella, plasmodesmata, thickenings, channels,
pit‐fields and cell corners, and there are domains within the thickness of a
wall in which degree of pectin esterification and abundance of RG I side‐chains
differ.
Unesterified
pectins can be as long as 700 nm, and yet in some cell types, are accommodated in
a middle lamella that is only 10–20 nm wideMicrodiversity within walls reveals
that the wall is not a homogeneous and uniform building material, but a mosaic
of different wall architectures in which various components contribute to the
multifunctional properties of the apoplast. Most fruits in which the pericarp
or endocarp softens during ripening develop thickened primary walls that are
markedly enriched in pectic substances, primarily HG and RG I. Texture of ripe
fruit pulp is governed by the extent of wall degradation and loss of cell–cell
adhesion. The walls of the apple (Malus domestica) cortex undergo little change
in rigidity and exhibit little separation, whereas walls of the peach (Prunus
persica) mesocarp and tomato (Solanum lycopersicum) pericarp soften
considerably through wall swelling and loss of cell adhesion. In tomato,
locules containing the seeds dissolve completely in a process called
liquefaction. For many cell types, the differentiation process is associated with
the formation of a distinct secondary wall on the plasmamembrane side of the primary wall. Regardless of chemical
composition, the primary wall is always defined as the structure that
participates in irreversible cell expansion. When cells stop growing, the wall
is crosslinked into its ultimate shape. At that point, deposition of the secondary
wall begins.
The cotton
(Gossypium hirsutum) fiber consists of nearly 98% cellulose at maturity. Some
thickened walls have a composition typical of a primary wall but simply contain
more strata. Guard cells and epidermal cells thicken the wall facing the environment
to a much greater degree than side walls or the inward‐facing wall. Pairs of
guard cells contain thickenings of radially arranged cellulose microfibrils,
especially in walls adjacent to the stoma, which are needed to withstand the enormous
turgor pressure generated by the cell during stomatal opening. Suberin is found in specific tissues and cell
types, notably the root and stem epidermis, cork cells of the periderm,
surfaces of wounded cells, and parts of the endodermis and bundle. The
polyester cutin and its associated waxes also are found on leaf and stem
surfaces, providing a strong barrier to diffusion of water. Waxes generally are
esters of long‐chain fatty acids and alcohols, but are better described as
complex mixtures of these hydrocarbon esters with ketones, phenolic esters,
terpenes, and sterols.
Another site of
diversity among plant species is the secondary walls of the cotyledon and
endosperm of developing seeds. These walls contain little or no cellulose but
rather consist of a single noncellulosic polysaccharide typically found in the
primary wall. These secondary walls serve two functions. First, they provide a
strong wall to protect the embryo or impose mechanical dormancy. Second, they
contain specialized storage carbohydrates that are digested during germination
and converted to sucrose for transport to the growing seedling.
Cell walls
contain molecular elements called receptor‐like kinases (RLKs), which
facilitate response mechanisms to biotic and abiotic stimuli as well as
developmental and positional signals.
[1]
Two fundamental constituents of pectins are homogalacturonan (HG) and
rhamnogalacturonan I (RGI). There are two kinds of structurally modified HGs,
xylogalacturonan and rhamnogalacturonan II (RG II). RG II is misnamed as it is
not structurally related to RG I, but was first identified as a
rhamnose‐containing pectin. RG II has the richest diversity of sugars and
linkage structures known, including apiose, aceric acid (3‐C′‐carboxy‐5‐deoxy‐ L‐xylose),
2‐O‐methyl fucose, 2‐O‐methyl xylose, Kdo (3‐ deoxy‐d‐manno‐2‐octulosonic
acid), and Dha (3‐deoxyd‐ lyxo‐2‐heptulosaric acid).
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