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Plant Cell Growth and
Elongation
Secondary article
Article Contents
. Introduction
Maureen C McCann, John Innes Centre, Norwich, UK
Keith Roberts, John Innes Centre, Norwich, UK
Nicholas C Carpita, Purdue University, West Lafayette, Indiana, USA
. Cell Expansion Can Be Uniform (Isodiametric) or
Directional
. New Wall Synthesis is Needed for Cell Expansion
. The Water Potential Provides the Driving Force for Cell
Expansion
The plant cell wall is a strong material that resists expansion when water is taken up by the
protoplast; for cells to expand irreversibly, the existing wall architecture must be loosened
to permit the insertion of newly synthesized wall polymers. Biochemical approaches have
identified changes in wall composition that occur during growth and some of the enzymes
involved in wall loosening, whilst the study of mutants is beginning to identify genes
involved in plant cell growth and elongation.
Introduction
Plants enlarge by the coordinated expansion of individual
cells. In some cases, such as the large storage cells in
potatoes or metaxylem elements, the final cell size can be
100 000 or 1 000 000 times greater than the original size of
the cell, when born in the meristem. Most of the volumetric
increase in the protoplast is accounted for by expansion of
the vacuole (up to 95% cell volume). The existing cell wall
architecture must change during cell expansion to incorporate new material, increasing the surface area of the cell
by as much as 10 000 times and inducing water uptake by
the protoplast. As the rate of cell wall expansion is ratelimiting for growth, plant form derives from two processes:
first, establishing the planes of division in cells, thus
determining the positions in which new walls are created,
and second, targeting new cell wall material to particular
areas of the cell surface. The primary cell wall is born in the
cell plate during cell division and is capable of cell
expansion and elongation. At differentiation, many cells
elaborate, on the membrane side of the primary wall, a
separate, secondary cell wall containing complex structures uniquely suited to the cell’s function. Many mature
cells, then, have both a primary and a distinct secondary
cell wall.
In plants, the expansion must be coordinated among all
neighbouring cells. Organ size, and thus plant stature and
form, is controlled genetically, but entrained by environmental factors. Leaf size may be equal between wild-type
and mutant maize plants, but the number of cells may
differ. However, while the final size of leaf may be
genetically predetermined, whether a plant attains its
potential will depend upon extrinsic factors. The supply of
water and nutrients through vascularized tissue may limit
organ size. Plants must be exquisitely responsive to
environmental signals, such as light, water stress and
nutrient status, which all modulate the pattern of cell
. Proteins Participate in Controlled Cell Wall Loosening
. How Does Growth Stop?
. Mutations That Alter Cell Growth and Elongation Are
Beginning to Tell Us More About the Genes Involved
growth and ultimate cell size. The developmental pathways
to various cell types involve several growth factors and
their dependent signal transduction mechanisms. Thus the
cells that comprise plant organs must be able to integrate
complex networks of intracellular and extracellular signals
to start growth, expand, and then to stop growth in a
coordinated fashion.
Plant cell growth is a cellular process that integrates the
‘loosening’ of the cell wall, the outward push of the
protoplast driven by osmotic forces, and the deposition
and intercalation of new wall to maintain thickness as the
existing wall extends. The orientation of cellulose microfibrils spooled on the inner surface of the wall establishes
the eventual direction of cell expansion and the cytoskeleton is an essential determinant of this orientation. The
direction of cell growth may be influenced by cortical
microtubules whereas the rate of growth is regulated by
wall loosening and wall deposition.
The cessation of growth is a mysterious process but
equally as important. Cell size is coupled to chromosome
copy number: chromosome doubling, endoreduplication
or polyteny may be required to support a specific volume of
cytoplasm. Arabidopsis leaf trichome increases from 16n to
128n during its growth to its final cell size (Traas et al.,
1998). At the end of growth, various crosslinking mechanisms in the wall are activated that lock the cell into its
specific size and shape.
Several model systems have been employed to examine
the physiological mechanisms of cell expansion, including
the grass coleoptile and the dicot epicotyl and hypocotyl.
Additionally, several Arabidopsis thaliana developmental
mutants have contributed new molecular and genetic tools
to probe cell expansion, from genes that determine cell
identity to those that constitute the biochemical and
physiological mechanisms of expansion and differentiation.
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Plant Cell Growth and Elongation
Cell Expansion Can Be Uniform
(Isodiametric) or Directional
Plant cell growth, an irreversible increase in cell volume,
can occur by expansion (increase in cell size in two or three
dimensions) or by elongation (expansion constrained to
one dimension). Variety in cell shape may result if either of
these two processes occur only at specific regions of the cell
surface. A few plant cells, such as root hairs and pollen
tubes, grow by tip growth, in which the mechanisms of wall
loosening and deposition of new wall material occur only
at the tip of an elongating cell. However, these two
mechanisms operate along the entire wall in the vast
majority of plant cells. A variation of elongation growth,
called intrusive growth, has the appearance of tip growth.
This type of growth describes the tapering elongation at the
tips of some types of vascular cells, such as inter- and
intraxylary fibre cells, as they extend through the middle
lamellae of their neighbours above and below their zone of
elongation.
Cell expansion involves widespread changes to cell wall
architectures, in both mass and composition. The osmotic
pressure exerted by the protoplast is necessary to drive cell
expansion, but because growth can begin and end with
almost imperceptible changes in turgor pressure, wall
loosening is considered as the primary determinant of cell
expansion. The cell wall architecture must be extensible,
that is, biochemical loosening of the cell wall matrix
permits insertion of newly synthesized polymers. Cells may
extend their original length several orders of magnitude
with little change in wall thickness. Thus, loosening and the
continued cutting and pasting of new material into the
texture of the wall must be tightly integrated events.
Cellulose forms the main scaffolding framework of the
cell wall and accounts for 20% to 30% of the dry mass of
elongating cell walls. Cellulose forms microfibrils, paracrystalline assemblies of several dozen (1!4)b-d-glucan
chains hydrogen bonded to one another along their length
(Figure 1). During cell growth, the orientation of inextensible cellulose microfibrils controls the direction of
elongation. In cells that grow by isodiametric expansion,
the microfibril orientation within successive lamellae is
relatively random, but in cells that grow by elongation,
microfibrils in each lamella align in a net transverse
orientation to the axis of elongation. Plant cells possess
arrays of cortical microtubules underlying and connected
to the plasma membrane. As the orientation of the cortical
microtubule array often precedes new cellulose microfibril
deposition, it is thought by many to act as a template for
the orientation of newly synthesized microfibrils (Baskin,
2000). Cellulose synthesis is catalysed at multimeric
enzyme complexes located in the plasma membrane.
Microtubules may define tracks in which the cellulose
synthase complexes are constrained to move through the
plasma membrane, or more direct connections, proteins
2
Figure 1 The fast-freeze, deep-etch, rotary-shadowed replica technique
is used to image cell wall architecture without the use of chemical fixatives
and dehydrants. Pectins have been extracted from this onion parenchyma
cell wall to expose the cellulose–xyloglucan network. One cellulose
microfibril is drawn out. Cellulose microfibrils are paracrystalline arrays of
several dozen (1!4)b-D-glucan chains that tightly hydrogen bond to each
other, both side-to-side and top-to-bottom. The arrangement of the
glucan chains in a cross-section of a single microfibril, and the arrangement
of atoms in the unit structure of the microfibril core, are shown. The glucan
chains in the core of the microfibril have a precise spacing as determined by
X-ray diffraction. (From R. J. Preston (1974) Physical Biology of the Plant Cell
Wall, Chapman and Hall, London.) From studies involving solid-state
nuclear magnetic resonance (NMR) spectroscopy, glucan chains at the
surface of the microfibril are thought to adopt a slightly different alignment
from 1808. (Courtesy of M. Jarvis)
linking the complexes to microtubules, have been proposed. Treatment with the cellulose synthesis inhibitor
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Plant Cell Growth and Elongation
isoxaben results in disorganized microtubules in elongating cells.
The multinet growth hypothesis was developed to
explain how cellulose microfibrils in the cotton fibre are
deposited in a shallow helical orientation net transverse to
the elongation axis but are displaced longitudinally as
elongation progresses. New microfibrils deposited in
lamellae on the inner surface of the wall in a generally
transverse orientation functionally replace older microfibrils. Reorientation of the helical microfibrils occurs
primarily in the inner layers of the wall close to the plasma
membrane, and the older microfibrils to the exterior of the
wall are passively reoriented in a longitudinal direction as
the cell elongates. The driving force for wall extension is
generally viewed to be the turgor generated by the
protoplast, but it is the tension created on the microfibrils
908 to the outward push of the protoplast that leads to
separation of the microfibrils. A turgor pressure of a few
atm (about 100 kPa) can generate several thousand atm of
tensile forces in the wall because the internal pressure
exerted by an increase in volume of a relatively large
protoplast is resisted by a very thin cell wall (Figure 2).
The multinet growth hypothesis describes wall growth in
many cells, but microfibrils do not necessarily reorient
axially, and significant extension is possible with little
reorientation if many lamellae contribute to the expansion.
Cellulose microfibrils woven in a shallow helix around the
cell prevent the growing cell from becoming spherical. By
analogy, the springlike toy Slinky1 or Flexi1 stretches
easily along its axis, but resists attempts to increase its
diameter. Similarly, helical cellulose microfibrils would
offer little resistance to cell stretching in the longitudinal
axis – once crosslinks between adjacent turns are weakened
– but withstand an enormous tension across this axis.
When stretched, the spring extends substantially with only
a small change in the helical angle, but with a wide
separation of the coils. Extension of a cell wall might be
viewed as a series of tightly interacting concentric Slinkys
in both right-handed and left-handed orientations that
reorient at crossed angles during separation. Given the
estimated thickness of the primary wall and the dimensions
of the matrix components, only about four to ten lamellae
make up the wall. Microfibrils from inner lamellae must
move into outer lamellae as the wall is remodelled, to fill in
the gaps (Figure 3).
New Wall Synthesis is Needed for Cell
Expansion
Biochemically, there are three classes of polymers that
constitute nearly independent determinants of strength in
elongating cells: (1) the microfibrils arranged in the
transverse axis, (2) the crosslinking glycans in the longitudinal axis, and (3) networks involving structural
Figure 2 The original multinet growth hypothesis explains that as walls
stretch during growth, the microfibrils reorient passively from a transverse
direction on the inner wall to a longitudinal direction at the outer wall. The
hydrostatic pressure developed by the protoplasm is resisted by a relatively
thin cell wall, and the tensile force pulling the microfibrils apart is several
orders of magnitude higher than cell turgor pressure. For example, a
spherical cell with a radius (t) of 50 mm and 10 atm of turgor (P), enveloped
by a cell wall only 0.1 mm thick (t), develops 2500 atm of tension in the wall
(s1). This enormous tension changes as the cell geometry changes. When
this cell begins to elongate and become cylindrical, the tension increases to
5000 atm tangentially (s2) simply because of the change in cell dimension.
Whereas the Slinky is difficult to pull outward because of the orientation of
the coils, it is easily pulled longitudinally. Hence, cell shape is controlled in
plants similarly. Altering the interaction between the tethering crosslinking
glycans and cellulose is the principal determinant of cell expansion.
proteins or phenylpropanoid compounds, or elements of
the pectin network. As well as new cellulose microfibrils,
other wall polymers must be synthesized and deposited
during growth. A direct relationship exists between the
amount of new wall material made and deposited by a cell,
and the extent of corresponding cell growth. The Golgi
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Plant Cell Growth and Elongation
Figure 3 Wall loosening and incorporation of new wall polymers is
integrated so that wall thickness is maintained during cell expansion. As the
walls are only a few strata thick, loosening of the wall with no insertion of
new wall material would very quickly thin the wall during growth and cause
rupture. In contrast, deposition without loosening would increase wall
thickness, because the walls would not expand.
stacks are the sites of synthesis for all noncellulosic
polysaccharides, which are packaged in secretory vesicles
and exported to the cell surface where they are assembled
onto and around the growing cellulose microfibrils
(Figure 4) (Samuels et al., 1995).
At least two types of primary cell walls are
made by angiosperms
‘Type I’ walls, which are found in dicots and the noncommelinoid monocots, contain about equal amounts of
cellulose and crosslinking xyloglucans (XyGs), polymers
with a glucan backbone decorated with xylose-containing
side-chains (Carpita and Gibeaut, 1993). XyGs occur in
two distinct locations in the wall: binding tightly to
exposed faces of glucan chains in the cellulose microfibrils,
and spanning the distance between adjacent microfibrils or
simply linking to other XyGs to space and lock the
microfibrils into place.
The cellulose–XyG framework is embedded in a pectin
matrix that controls, among other physiological properties, wall porosity. The pectic polysaccharides that
comprise the gel matrix are some of most complex
polysaccharides known and fall into two major classes.
4
Figure 4 Biosynthesis of the wall requires a coordination of the synthesis
of cellulose microfibrils at the plasma membrane surface, with the synthesis
and glycosylation of proteins and wall-modifying enzymes at the rough
endoplasmic reticulum and the synthesis of all noncellulosic
polysaccharides at the Golgi apparatus. Material destined for the cell wall is
packaged into secretory vesicles, transported to the cell surface and
integrated with the newly synthesized microfibrils. It is estimated that
assembly of the new wall stratum begins when no more than 10 glucose
residues of a cellulose chain are made.
The two major pectins are homogalacturonans (HGs),
polymers with a backbone of galacturonic acid residues,
and rhamnogalacturonan I (RG-I), polymers with a
backbone of alternating rhamnose and galacturonic acid
residues. HGs are thought to be secreted as highly methyl
esterified polymers, and the enzyme pectin methylesterase
(PME) located in the cell wall removes some of the methyl
groups to initiate binding of the carboxylate ions to Ca2 1 .
The helical chains of HGs can condense by crosslinking
with Ca2 1 to form gels. The extent of methyl esterification
may remain high in the walls of some cells, and a type of gel
may form with highly esterified parallel chains of HGs.
Some HGs and RGs are crosslinked by ester linkages to
pectins or other polymers held more tightly in the wall
matrix and can only be released from the wall by deesterifying agents. Neutral polymers (arabinans or galactans) are pinned at one end to the pectic backbone, but
extend into, and are highly mobile in, the wall pores. Some
type I walls contain several types of structural proteins.
Maize and other commelinoid monocots possess a
different kind of primary wall, a ‘type II’ wall (Carpita
and Gibeaut, 1993). They contain cellulose microfibrils of
the same structure as those of the type I wall, but
glucuronoarabinoxylans (GAXs) are the principal polymers that interlock the microfibrils. Unbranched GAXs
can hydrogen bond to cellulose or to each other. The
attachment of arabinose and glucuronic acid side groups to
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Plant Cell Growth and Elongation
the xylan backbone of GAXs prevents the formation of
hydrogen bonds, diminishing the extent of crosslinking
between two unbranched GAX chains or GAX to
cellulose. In general, grasses are pectin-poor, but the
pectins they do contain are similar in structure to those of
dicots. When grass cells begin to elongate, they accumulate
mixed-linked b-glucans in addition to GAX. Grasses,
which have very little structural protein compared with
dicots and non-commelinoid monocots, can have extensive
interconnecting networks of phenylpropanoids that generally form after cells stop expanding.
The Water Potential Provides the
Driving Force for Cell Expansion
When plant growth regulators, such as auxin and
gibberellin, change the dimension of growth, they do so
through changes in the orientation of cortical microtubules
and cellulose microfibrils. When they change the rate of
growth, their mechanisms include dissociation or breakage
of the tethering molecules, XyGs or GAXs, between
microfibrils. A mathematical formula that describes the
growth rate of plant cells allows us to define the cell wall
properties that must be modified to permit growth (Taiz,
1984).
James Lockhart was one of the pioneers in efforts to
apply such a biophysical approach to characterize the rate
of plant cell enlargement. The driving force for water
uptake in a symmetrical elongating cell can be quantified
by the equation:
dl/dt 5 Lp (DCw)
where dl/dt is the change in length per unit time, Lp is
hydraulic conductivity, i.e. the rate at which water can flow
across the membrane, and DCw is the water potential
difference between the cell and the external medium. The
difference in water potential is the driving force for water
movement and comprises two components, Cp 1 Cp,
which are the osmotic potential and pressure potential
(turgor), respectively. The equation is revised to include
any type of growth by:
dV/dt 5 A Lp (DCw)
where growth is reflected as a change in volume per unit
time, and dependent on the surface area (A) of the plasma
membrane available for water uptake. Thus, in this
equation, the rate of growth is proportional to membrane
surface area, the conductivity of the membrane, and the
water potential difference driving water uptake. However,
the equation fails to account fully for some biophysical
properties of the wall. In nongrowing cells (dV/dt 5 0), DC
is zero, because the rigid cell wall prevents an increase in
cell volume and the turgor pressure rises to a value equal to
that of the cell’s osmotic potential. As we have seen earlier
(Figure 2), small changes in turgor pressure can result in the
generation of enormous tensions on the microfibrils.
However, in growing cells the DC does not quite reach
zero because the wall tethers have been loosened and the
volume increases irreversibly. This event is called stress
relaxation; it is a wall-localized event that serves as the
fundamental difference between growing and nongrowing
cells.
Actually, when turgor is reduced in growing cells by an
increase in the external osmotic potential, growth ceases
before turgor reaches zero. This value is called the yield
threshold. Lockhart noted that the increment of growth
rate change above the yield threshold was dependent not
only on turgor but also on a factor ‘m’, called wall
extensibility, which is the slope of a general equation:
rate 5 m(Cp 2 Y)
where Y is the yield threshold. Much of the work that
remains to be done is to assign biochemical determinants of
yield threshold and extensibility. As we shall see next, some
likely candidates have been identified.
Proteins Participate in Controlled Cell
Wall Loosening
The extraction of several kinds of polysaccharide hydrolases from the cell walls of tissues rich in growing cells
raised the possibility that the regulation of these enzymes
was a mechanism by which auxin could cause wall
expansion. A major breakthrough came with the discovery
that the growth-promoting activity of auxin could be
substituted by mere H 1 , and, further, auxin caused an
acidification of the culture medium in which elongating
tissue sections were bathed. From these observations
sprang the acid-growth hypothesis, the essence of which
is that auxin activates a proton pump in the plasma
membrane that acidifies the apoplast, and there, growthspecific hydrolases are activated that cleave load-bearing
bonds of polysaccharides that tether the cellulose microfibrils. Cleavage of these bonds results in loosening of the
wall, and the water potential difference causes uptake of
water. Relaxation of the wall, i.e. separation of the
microfibrils, passively leads to an increase in cell size. The
basic tenets have stood the test of time, but two problems
persist. First, no enzymes have been found that hydrolyse
wall crosslinking glycans exclusively at pH lower than 5.0,
and second, no reasonable explanation exists for how
growth is kept in check once the hydrolases are activated.
Furthermore, no hydrolases extracted from the wall and
added back to the isolated tissue sections, regardless of
external pH, cause extension in vitro.
There are two new candidates for wall-loosening
enzymes. An enzyme activity was discovered that, under
conditions of excess XyG as substrate, could carry out a
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Plant Cell Growth and Elongation
transglycosylation rather than hydrolysis. Hence, one
chain of XyG can be cleaved and reattached to the
nonreducing terminus of another XyG chain. The enzyme
is called xyloglucan endotransglycosylase (XET) (Nishitani and Tominaga, 1992). In such a mechanism, microfibrils
could undergo a transient slippage but overall tensile
strength of the wall would not diminish (Figure 5). XETs
may also function in the realignment of XyG chains in
different lamellae during growth, and in the assembly of the
wall as newly synthesized XyGs are incorporated. In some
cases, the correlation between growth and XET activity is
not clear, and some XETs appear to function hydrolytically.
Other proteins catalyse wall extension in vitro without
any detectable hydrolytic or transglycolytic events (McQueen-Mason et al., 1992). Called expansins, these proteins
allow extension of paper strips under tension, indicating
that they probably catalyse breakage of hydrogen bonds.
Such an activity could disrupt the tethering of cellulose by
XyGs in type I walls and by GAXs and b-glucans in type II
walls.
XET and expansin may not be the only wall-loosening
agents, and work continues to determine the role, if any, of
the hydrolases. Recognition of the differences in wall
composition between the grasses and all other flowering
plant species has also partitioned the studies appropriately
to the xyloglucanase, which cleaves XyG selectively, and
the exo- and endo-b-d-glucanases, which hydrolyse the
grass b-d-glucans to glucose. Addition of purified exo- and
endoglucanases to heat-killed coleoptiles cannot induce
extension. However, when antibodies directed against
these enzymes are added to enzyme-active walls, they
inhibit growth.
Biochemical changes in the cell wall
accompany cell enlargement
If XET and expansin activities are indeed growth-inducing
factors in the cell wall, then how are they regulated? The
cellulose/crosslinking glycan network lies embedded in a
network of pectins which may control access of these
enzymes to their substrates. The self-hydrolysis of isolated
walls by nascent enzymes, termed autolysis, yields substantial amounts of Ara and Gal from type I walls,
suggesting that changes in the neutral sugar side-branches
of RG-I or arabinogalactan proteins (AGPs) occur during
growth. From biochemical analyses, the most marked
change is the increase in methyl esterification of the wall as
newly synthesized pectins are deposited. This seems to be
succeeded by a de-esterification event when growth stops.
The wall Ca2 1 content of the meristem and elongation
zones is actually quite low, and intermediate types of esterfree acid gel can also form between partially de-esterified
HGA chains in the absence of Ca2 1 . Thus, Ca2 1 –HG
junction zones are structures more likely to be found in
6
Figure 5 Stress-relaxation is considered to be the underlying basis of cell
expansion. When an elongating cell is stretched by turgor, the longitudinal
stress is borne more or less equally by the glycans tethering the cellulose
microfibrils. If some of the tethers are dislodged from the microfibrils or
hydrolysed, then they temporarily ‘relax’ and the yield threshold is
breached because the other tethers are strained. Water uptake results in
expansion of the microfibrils, which attach to the relaxed glycans, and they
again are placed under tensile stress. Microfibril separation driven by
osmotic pressure of the cell is facilitated by loosening of the crosslinking
glycans that tether them. This may be accomplished by coordinate action
of expansins, which break the steric interactions between the crosslinking
glycans and cellulose, and xyloglucan endotransglycosylase (XET), which
hydrolyses a glycan and reattaches one part of the chain to the
nonreducing terminus of another. This action by XET may also function in
forming new tethers as microfibrils from inner lamellae merge with
microfibrils of the outermost lamellae as they are pulled apart during wall
extension.
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Plant Cell Growth and Elongation
cells after cell elongation has stopped. Runs of HG at the
ends or within RG could link these two types of polymers,
but the Rha units of RG-I and their side-chains interrupt
the Ca2 1 junctions. In addition to the Ca2 1 -binding
junction zones, pectins in some species may be crosslinked
to other pectins and noncellulosic polysaccharides by ester
linkages with dihydroxycinnamic acids such as diferulic
acid. Location of the de-esterification of HG, the size and
frequency of junction zones, and the size and conformation
of the side-chains attached to the RG could all influence the
porosity of the pectin gel to the extent that movement and
activity of enzymes for wall metabolism could be
controlled (Goldberg et al., 1996).
More obvious biochemical changes occur with the
crosslinking glycans of the type II walls of commelinoid
monocots. A highly substituted GAX (HS-GAX), with six
out of every seven Xyl units bearing an appendant group, is
associated with the maximum growth rate of coleoptiles. In
dividing and elongating cells, highly branched GAXs are
abundant, whereas after elongation and differentiation,
more and more unbranched GAX accumulates. Cleavage
of the Ara and other side groups from contiguously
branched Xyl units could yield runs of unbranched xylan
capable of binding to other unbranched xylans or to
cellulose microfibrils.
Type II walls are also notably poor in pectin. Chemically, pectins of the type II wall comprise both HG and RG,
but HS-GAX is closely associated with these pectins. The
HGs of maize pectins also contain nonmethyl esters, whose
formation and disappearance coincide with the most rapid
rate of cell elongation. We do not yet know the chemical
nature of the nonmethyl esters. Some arabinans, particularly the 5-linked arabinans, are found in the walls of
dividing cells but are not made during cell expansion.
When grass cells begin to elongate, they accumulate bglucans in addition to GAX (Carpita, 1996). b-Glucans are
unique to the Poaceae. b-Glucan is one of the few known
developmental stage-specific polysaccharides. Absent
from the meristems and dividing cells, b-glucans accumulate to almost 30% of the noncellulosic cell wall material
during the peak of cell elongation, and then are largely
hydrolysed by the cells during differentiation. The appearance of b-glucans during cell expansion and the acceleration of their hydrolysis by growth regulators all implicate
direct involvement of the polymer in growth.
How Does Growth Stop?
Once elongation is complete, the primary wall locks the cell
into shape. One component of the locking mechanism for
type I walls may be hydroxyproline-rich glycoproteins
(HRGPs). HRGP monomers accumulate early in the cell
cycle and become insoluble during cell elongation and
differentiation. But how HRGPs are crosslinked in the
wall, either in homo-networks or in hetero-networks with
other proteins, is not known. Type II walls contain a
threonine-rich protein with sequences reminiscent of the
typical HRGP structure. This protein is prevalent in
vascular tissue and in special, reinforced wall structures,
such as the pericarp. However, much of the crosslinking
function in type II walls probably rests with the esterified
and etherified phenolic acids, and formation of these
crosslinkages accelerates at the end of the growth phase.
Mutations That Alter Cell Growth and
Elongation Are Beginning to Tell Us
More About the Genes Involved
Cell elongation and differentiation are developmental
programmes that are under strong control by light
intensity and quality, and, in turn, by several growth
regulators. It is not surprising that mutations that alter
expression of phytochrome, cryptochrome, and blue light
photoreceptors greatly alter growth responses, from the
shortening of elongation responses to phototrophic bending. Likewise, mutations in the synthesis of growth
regulators such as auxin, gibberellin, cytokinin, abscisic
acid, ethylene and their putative receptors, lead to aberrant
growth, usually manifest in dwarfism or miniaturization.
Other than confirming the role of such growth regulators,
such mutations have not been exceptionally informative in
gaining an understanding of the biochemical or physical
mechanisms of growth. On the other hand, some dwarf
mutations have ultimately led to the discovery of an
entirely new class of growth regulators called brassinosteroids, whose action is epistatic to the action of the oldest
known growth regulator, auxin.
The de-etiolated (det) mutants are an interesting group
of mutants that undergo the first stages of phytochromeactivated morphogenesis in darkness. The defect of one of
these mutants was traced to a defect in a vacuolar protonpumping gene whose absence altered cytosolic pH. The pH
status, in fact, an acidification of the cytosolic compartment, has been postulated to be the primary signal for
auxin-induced elongation, and it appears that some
nominal support for this hypothesis has come from the
discovery of a positive-control mutant.
Other kinds of dwarf or miniaturization mutants do not
appear to fall within the domain of growth regulators per
se, but the defects are closer to the mechanisms of cell
expansion. For example, lilliputian mutants appear to be
derived from defects in microtubule (MT) assembly, and
cells that cannot escape cell division stage may have
impaired MT organizing centres required for proper
ordering of the MTs at the plasma membrane. Other
cytoskeletal mutants appear to affect polarity rather than
cell growth proper.
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7
Plant Cell Growth and Elongation
Mutations that affect the cell wall directly have been
harder to track down. Polysaccharides are not primary
gene products, and it has proven difficult to analyse
genetically the role that each component (and its modifications) plays in the overall mechanical and functional
properties of the cell wall, or of the tissues that contain
them. Over 1000 gene products are probably involved in
cell wall biosynthesis, assembly and turnover, and genetically defined variation offers the most comprehensive
approach to addressing function. Arabidopsis mutants
with altered carbohydrate components in the primary wall
have now been defined. Over three dozen mutants have
been classified, mapping to 11 different loci in which one or
several specific sugars are over- or under-represented
compared with the sugar composition in wild-type plants
(Reiter et al., 1997). Of these, the mur1 defect has been
traced to a GDP-mannose 4,6 dehydratase, mur2 to a
fucosyl transferase, and mur4 to a C-4 epimerase.
However, many cell wall mutants have also been selected
on the basis of a growth or developmental phenotype. A
temperature-sensitive mutant in primary wall cellulose
synthase, rsw1, was selected by a root radial swelling
phenotype at restrictive temperatures, while a secondary
wall cellulose synthase mutant, irx3, was selected by a
collapsed xylem phenotype. A dwarf hypocotyl mutant,
korrigan, results from a lesion in a membrane-bound
endoglucanase, while another dwarf, acaulis, is an XET
mutant. The tch4 mutant is also an XET mutant but
remains elongated rather than becoming dwarfed, as wildtype plants do when they are mechanically stressed. When
the genes of many cell wall modifying enzymes and
structural proteins become available, the function of their
products will be tested in suppression and over-expression
studies, to identify the key players in determining the rate
of plant cell growth.
References
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Further Reading
Carpita NC and McCann MC (2000) The cell wall. In: Buchanan BB,
Gruissem W and Jones RL (eds) Biochemistry and Molecular Biology
of Plants, chap 2. Rockville, MD: American Society of Plant
Physiology.
Cassab GI (1998) Plant cell wall proteins. Annual Review of Plant
Physiology and Plant Molecular Biology 49: 281–309.
Cosgrove DJ (1997) Assembly and enlargement of the primary cell wall
in plants. Annual Review of Cell Development and Biology 13: 171–201.
Delmer DP (1999) Cellulose biosynthesis: Exciting times for a difficult
field of study. Annual Review of Plant Physiology and Plant Molecular
Biology 50: 245–276.
Gunning B and Steer M (1996) Plant Cell Biology. Structure and
Function. Sudbury, MA: Jones and Bartlett Publishers.
McCann MC and Roberts K (1991) Architecture of the primary cell wall.
In: Lloyd CW (ed.) The Cytoskeletal Basis of Plant Growth and Form,
pp. 109–129. London: Academic Press.
McCann MC, Wells B and Roberts K (1990) Direct visualization of
cross-links in the primary plant cell wall. Journal of Cell Science 96:
323–334.
Nishitani K (1995) Endo-xyloglucan transferase, a new class of
transferase involved in cell-wall construction. Journal of Plant
Research 108: 137–148.
Reiter W-D (1998) Arabidopsis thaliana as a model system to study
synthesis, structure, and function of the plant cell wall. Plant
Physiology and Biochemistry 36: 167–176.
Staehelin LA and Moore I (1995) The plant Golgi apparatus: structure,
functional organization and trafficking mechanisms. Annual Review of
Plant Physiology and Plant Molecular Biology 46: 261–288.
ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net