Mechanisms of cell wall expansion and elongation
Lesson Prepared Under MHRD project “National Mission
on Education Through ICT”
Discipline: Botany
Paper: Plant Physiology
National Coordinator: Prof. S.C. Bhatla
Lesson: Mechanism of cell wall expansion and elongation
Lesson Developer: Niharika Bharti
Department/College: Department of Botany, University of Delhi
Lesson Reviewer: Prof. S.C. Bhatla
Department/College: Department of Botany, University of Delhi
Language Editor: Ms. Vinee Khanna, Research Scholar
Department/College: Department of Genetics, University of Delhi, South
Campus
Lesson Editor: Dr. Rama Sisodia, Fellow in Botany ILLL
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Mechanisms of cell wall expansion and elongation
Table of contents
Mechanisms of Cell Expansion and Elongation
• Introduction
•
The primary cell wall is a network of cellulose
microfibrils embedded in a matrix of pectins.
•
Secondary walls contain more cellulose and lignin
making the wall rigid
• Cell wall expansion and elongation
•
How can the wall be strong, yet still allowed for
expansion?
• New
wall synthesis is needed to maintain wall
thickness during expansion
• Direction of growth is determined by the orientation
of cellulose microfibrils
• Cortical microtubules determine the orientation of
newly deposited cellulose microfibrils
• Mechanisms of cell wall expansion and elongation
•
Stress relaxation of the cell wall predisposes cell to
water influx and volume expansion
•
The multinet-growth hypothesis
•
The
acid-growth
hypothesis-
auxin-dependent
acidification of the cell wall promotes cell wall
expansion thereby leading to cell growth
•
Several proteins participate in controlled „loosening‟
of cell wall
• Cessation of wall expansion and elongation
• Summary
• Exercise/ Practice
• Glossary
• References/ Bibliography/ Further Reading
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Mechanisms of cell wall expansion and elongation
Learning outcomes
The student will be able to
•
Understand the basic structure and composition of cell wall
•
Know how cellulose microfibrils and cortical microtubules determine direction of
cell growth
•
Explain how turgor pressure, stress relaxation, and wall loosening events occur
in a coordinated manner to achieve wall expansion
•
Describe terms like stress relaxation, cell wall yield, yield threshold and wall
extensibility
•
Differentiate between growing and non-growing cells
•
Explain multinet growth hypothesis
•
Explain acidification of cell wall caused by auxin action, leading to cell growth
(acid-growth hypothesis)
•
Describe the role of wall loosening enzymes and proteins (expansins) in process
of cell wall expansion
•
Become familiar with the process of cessation of cell growth that leads to
rigidification of cell wall
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Mechanisms of cell wall expansion and elongation
Introduction
Unlike animal cells, plant cells encase themselves within a thin but mechanically strong
and rigid cell wall. Cell walls in plants primarily comprise of a complex mixture of
polysaccharides (such as, cellulose, hemicelluloses, pectin) and other polymers which
are assembled into an organized network. These polysaccharides provide strength and
flexibility to the cell wall. Cell wall also contains a small amount of structural proteins,
enzymes, phenolic polymers, and other materials which alter its physical and chemical
properties. Plant cell wall varies in composition and structure in different cell types. It
may be flexible or rigid, permeable or impermeable, packed in layers to form fibers or
porous in nature.
Cell walls are commonly classified into two major types: primary walls and secondary
walls. Primary walls are formed in the growing cells and are relatively unspecialized and
similar in composition in all cell types. However, secondary walls are the cell walls that
are formed after cell growth and expansion has ceased. These may be highly specialized
in structure and composition.
Primary cell wall is a network of cellulose microfibrils embedded
in a matrix of pectins.
•
Cellulose is the skeletal substance of the cell wall, and is the most abundant
substance in the plant kingdom. It is a polymer of β-D glucose units joined by 1, 4
covalent linkages to form a ribbon-like structure. Several linear chains of cellulose
form intramolecular hydrogen bonds causing them to adhere strongly to one
another in overlapping parallel arrays. These highly ordered, tightly packed
crystalline aggregates are called cellulose microfibrils. Cellulose microfibrils are of
indeterminate length and usually 5-12 nm wide. They are relatively stiff structures
and give tensile strength to the cell wall.
•
Hemicelluloses are the cross-linking glycans. They are a heterogeneous group of
flexible, branched polysaccharides which form tethers and cross-link cellulose
microfibrils together into a complex network. They bind tightly to the surface of
each microfibril by hydrogen bonds.
•
Pectins are a diverse group of polysaccharides which contain many negatively
charged galacturonic acid units and form a highly hydrated gel matrix in which the
cellulose-hemicellulose network is embedded. They help in preventing aggregation
and collapse of the cellulose network.
•
In addition to these polysaccharides, certain structural proteins may also be present
in the wall. Although the precise role of these cell wall proteins is uncertain, they
may help in providing mechanical strength to the wall and may assist in the proper
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Mechanisms of cell wall expansion and elongation
assembly of other cell wall components. Many of these proteins are enzymes,
responsible for wall turnover and remodeling, particularly during cell growth.
The primary wall is generally composed of approximately 25% cellulose, 25%
hemicelluloses, 35% pectins, and about 1 to 8% structural proteins. However, large
deviations from these values may be observed.
Figure: Arrangement of various components of primary cell wall
Source:http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Plant_cell_wall_dia
gram.svg/2000px-Plant_cell_wall_diagram.svg.png (CC)
Secondary wall contains more cellulose and lignin thereby providing it
rigidity
Once cell growth and wall expansion stops, a rigid secondary cell wall is formed by the
deposition of new layers on the inner side. The secondary wall provides mechanical
support to the plant and is comprised primarily of cellulose and lignin. Secondary walls
are often quite thick and can be distinguished into three distinct layers, S1, S2 and S3 which differ in the orientation, or direction, of the cellulose microfibrils. The cellulose
microfibrils are oriented more neatly parallel to each other in secondary walls. Secondary
walls are often (but not always) impregnated with lignin.
Lignin is a phenolic polymer of aromatic alcohol subunits joined together with a complex,
irregular pattern of linkages. As lignin is deposited in the cell wall, it displaces water
from its matrix and forms a hydrophobic network which binds tightly to cellulose and
prevents, further enlargement of the cell wall.
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Mechanisms of cell wall expansion and elongation
Figure: Structure of secondary cell wall
Source: http://www.mdpi.com/1420-3049/15/12/8641 (CC)
Table I: Major components of cell wall
POLYMER
COMPOSITION
FUNCTIONS
Cellulose
Linear polymer of β-Dglucose
Confer tensile strength
Hemicellulose
xyloglucan,
glucuronoarabinoxylan,
and mannans
Cross-link cellulose microfibrils
Pectin
homogalacturonans and
rhamnogalacturonans
Forms negatively charged, highly
hydrated matrix; cell-cell
adhesion
Lignin
cross-linked coumaryl,
coniferyl, and sinapyl
alcohols
Proteins and
glycoproteins
enzymes,
hydroxyproline-rich
proteins
Forms strong waterproof
polymer; provides strength;
defend against pathogen
Responsible for wall turnover,
remodeling; mechanical strength
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Mechanisms of cell wall expansion and elongation
Source: http://www.ncbi.nlm.nih.gov/books/NBK26928/table/A3611/?report=objectonly
Plant cell wall not only determines the mechanical strength of plant body but is also
essential
for many
processes
in plant growth, development,
maintenance, cell
differentiation, intercellular communication, reproduction, water movement and defense.
Plant morphogenesis majorly depends on the control of cell wall properties because the
expansive growth of plant cells is restricted principally by the ability of the cell wall to
expand.
Cell wall expansion and elongation
Plant cells expand many folds in volume before reaching maturity. However, xylem
vessel elements enlarge more than 10,000-fold in volume.
Plant growth and shape is controlled by wall expansion of individual cells. Growth, which
is an irreversible increase in cell volume, can occur by either by expansion (increase in
cell size in more than one dimension) or by elongation (expansion in only one
dimension). Cell expansion involves extensive changes in the cell wall architecture,
including both mass and composition of the cell wall. The main driving force for cell
expansion is osmotic or turgor pressure exerted by the protoplast, however the rate and
direction of expansion are controlled by the mechanical properties of the cell wall. The
extracellular fluid in the plant cell wall is hypotonic in comparison with the interior of the
cell. Due to this osmotic imbalance, the cell develops a large internal hydrostatic
pressure, or turgor pressure that pushes outward on the cell wall. Turgor pressure
creates an outward-directed force, equal in all directions. The turgor pressure increases
until the cell is in osmotic equilibrium, with no net influx of water. This pressure is of
great importance to plants because it is the driving force for cell expansion. It also
provides rigidity and strength to the living plant tissues.
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Mechanisms of cell wall expansion and elongation
Figure: Turgor pressure in plant cells
Source:http://commons.wikimedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.s
vg (CC)
When growth occurs, the wall has to allow extension by yielding to turgor pressure and,
at the same time, new wall material has to be deposited. Individual cellulose microfibrils
are unable to stretch because of their crystalline nature. Thus, stretching or deformation
of the cell wall must involve either the sliding of microfibrils past one another, the
separation of adjacent microfibrils, or both.
How can the cell wall be strong, yet allow for expansion?
Many factors influence the rate of cell wall expansion. It depends on the following
factors:
1. Developmental factors such as, cell type and age. Cell wall should be capable of
expansion. Cells with only primary walls are capable of growth. Secondary walls
do not allow expansion because of the orientation of microfibrils. In addition to
cellulose microfibril orientation, mature walls apparently lose their ability to
expand because the wall components become resistant to loosening-activities.
2. Cell wall should permit loosening (stress relaxation): Wall loosening is considered
as one of the major factors which determine cell expansion. The wall components
are linked by strong (covalent) and weak (hydrogen bonds) bonds which resist
expansion. When the wall is loosened, weak bonds are temporarily broken to
allow the wall components to glide over one another and allow insertion of new
cell wall material for expansion.
3. Cell wall synthesis: As the cell grows, cell wall synthesis needs to occur to
maintain relatively uniform wall thickness throughout the cell growth.
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Mechanisms of cell wall expansion and elongation
4. Enhanced solute synthesis: The solute concentration of the cell needs to be
maintained high to allow water uptake.
5. Water uptake and turgor pressure
6. Hormones such as, auxin and gibberellin.
7. Environmental conditions such as light and water availability
These internal and external factors help in loosening the cell wall so that it yields
(stretches irreversibly).
New cell wall synthesis is needed to maintain its thickness during
expansion
For irreversible expansion of plant cells, new cell wall polymers must be continuously
synthesized and deposited to re-establish the mechanical resistance of the cell wall to
pressure. Mostly growth and deposition of new material occur uniformly along the entire
expanding wall. The amount of new wall material synthesised is directly proportional to
the extent of cell growth. During plant cell expansion, the existing cell wall architecture
becomes loosened to allow the newly synthesized polymers to get inserted. Biochemical
approaches have helped in identifying various changes in wall composition which occur
during growth.
Wall expansion may be highly localized and restricted to the tip of the cell (tip growth) or
it may be evenly distributed over the entire cell wall surface (diffuse growth). Root hairs
and pollen tubes exhibit tip growth. However, most of the other plant cells exhibit diffuse
growth. Fibers, some sclereids, and trichomes grow in a pattern intermediate between
diffuse and tip growth.
Plant cell expansion, thus, involves ‘loosening’ of the cell wall, the outward push of the
protoplast due to turgor pressure, and the synthesis and introduction of new wall
material so that the to thickness of the existing wall remains the same.
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Mechanisms of cell wall expansion and elongation
Figure: Wall loosening and incorporation of new wall polymers
to allow wall expansion
Source: http://www.mdpi.com/1422-0067/15/3/5094/htm (CC)
Direction of growth is determined by the orientation of cellulose
microfibrils
The direction of cell growth and expansion in response to turgor depends on the
orientation of the inextensible cellulose microfibrils attached on the inner surface of the
primary wall. The cells of the meristem are at first isodiametric. Cellulose microfibril
orientation controls whether cells expand or elongate and determines the plane of
elongation. If the orientation of cellulose microfibril is relatively random (isotropic) within
the successive lamellae present in the primary cell wall, the cell would expand equally in
all directions to generate a sphere (cell expansion). However, the orientation of cellulose
microfibrils is anisotropic (non random) in most plant cell walls thus, resulting in variable
expansion in different directions (cell elongation). The microfibril strengthening elements
of a typical primary cell wall align in a transverse or helical orientation to the axis of
elongation, rendering cell wall structurally and mechanically anisotropic. For this reason,
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Mechanisms of cell wall expansion and elongation
growing plant cells elongate, and exhibit minimal increase in girth. Cells, therefore,
predict their future morphology by controlling the orientation of microfibrils deposited in
the wall. Cellulose synthase, a
plasma membrane bound enzyme complex, spins out
long cellulose molecules by utilizing UDP-glucose supplied from the cytosol. On the outer
surface of membrane chains of cellulose chains assemble spontaneously into microfibrils
forming a lamella, wherein all the microfibrils are more or less aligned in similar manner.
As the synthesis continues, each new lamella forms inside the earlier one, resulting in
the formation of concentrically arranged lamellae in the wall, with the first formed on the
outside. The newly deposited microfibrils in elongating cells lie at 900 to the cell
elongation axis. Thus, it is the orientation of wall lamellae which determine the direction
of cell expansion.
But, what determines the orientation of microfibrils? The orientation of the microfibrils
further depends on the arrangement of cortical microtubules. The orientation of cortical
microtubules precedes the deposition of cellulose microfibrils. Thus, microtubules are
thought to guide the orientation of newly synthesized microfibrils by directing the
cellulose synthesizing enzymes to the plasma membrane.
Figure: Orientation of cellulose microfibrils greatly influences the direction of cell
elongation. Different orientations of microfibrils (A) and (B) causes the cell to
elongate
in different directions thus, leading to different cell shapes
Source:http://www.ncbi.nlm.nih.gov/books/NBK26928/figure/A3614/?report=objectonly
Cell wall loosening and deposition of new wall material control the rate of cell growth
while the orientation of cortical microtubules regulate the direction of cell growth. The
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Mechanisms of cell wall expansion and elongation
main driving force for cell wall expansion is the turgor pressure generated by the
protoplast.
Cortical
microtubules
determine
the
orientation
of
newly
deposited cellulose microfibrils
The cytoskeleton is a central player in the co-ordination of cell wall expansion and is a
crucial factor that determines microfibril orientation. Plant cells possess arrays of cortical
microtubules lying beneath the cell cortex which are held there by proteins and connected to
the plasma membrane. These intracellular microtubular arrays often influence the
microfibril orientation in the cell wall. It has been observed that the orientation of the
microtubules in the cortical cytoplasm usually mirrors that of the cellulose microfibrils
which are being deposited in the cell wall in that region. Both are usually coaligned in
the transverse direction, at right angles to the axis of polarity. In some cell types, such
as tracheids, the microfibrils alternate between transverse and longitudinal orientations,
and in such cases the microtubules are parallel to the microfibrils of the most recently
deposited wall layer. The congruent alignment of the cortical array of microtubules
(inside the plasma membrane) and cellulose microfibrils (outside the plasma membrane)
is observed in many plant cells and is present during the deposition of both primary and
secondary cell walls, suggesting a causal relationship between the two.
Cellulose synthesis is catalyzed at large, multimeric cellulose synthase complexes which
are integral plasma membrane bound proteins. As the synthesis of long cellulose
molecules and their assembly into microfibrils proceeds at the outer face of the
membrane, the distal ends of the stiff microfibrils form indirect cross-links with the
previous layer of wall material and become integrated into the texture of the wall. Their
elongation at the proximal end pushes the synthesizing complex along in the plane of
the membrane. Thus, each layer of microfibrils would tend to be spun out from the
membrane in the same orientation as the previously laid down layer.
The orientation of the microfibrils can be disconcerted by genetic mutations and
treatment with microtubule- depolymerizing drugs. It has been observed that treatment
of growing roots with oryzalin (a drug known to disrupt cytoplasmic microtubules), leads
to the formation of bulbous and tumor-like root. This disrupted growth is due to the
isotropic expansion of the cells; i.e., they enlarge like a sphere instead of elongating.
The drug-induced disruption of microtubule orientation in the growing cells also deforms
the transverse orientation of cellulose microfibrils. Cellulose microfibrils are deposited
randomly (isotropically) in the absence of microtubules and cells expand equally in all
directions. Since the microtubule depolymerizing drugs specifically target microtubules,
these results suggest that microtubules act as primary determinants and guide the
orientation of cellulose microfibril deposition. Also treatment with isoxaben (an inhibitor
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Mechanisms of cell wall expansion and elongation
of cellulose synthesis) leads to disorganization of microtubules in the elongating cells
suggesting a relationship between orientation of microfibrils and microtubules, and
ultimately cell expansion.
Figure: Model depicting how orientation of cortical microtubules determines the
orientation of newly deposited cellulose microfibrils
Source:http://www.ncbi.nlm.nih.gov/books/NBK26928/figure/A3616/?report=objectonly
Plant cells can control their direction of expansion by changing the orientation of the
cortical array of microtubules. The morphology of a multicellular plant depends on the
coordinated and highly patterned control of cortical microtubule orientation during
plant development. It has also been shown that cortical microtubules can reorient rapidly
in response to extracellular stimuli.
Mechanisms of cell wall expansion and elongation
The strength and rigidity of the cell wall largely determines the ability of a plant cell to
grow (increase in size). How can the strength and rigidity of cell wall be modified to
permit expansion and elongation of cell? During elongation or expansion, existing cell
wall architecture needs to be altered to enable integration of new material, thus,
increasing the surface area of the cell and allowing water uptake by the protoplast. The
turgor pressure exerted by the protoplast usually remains a relatively constant driving
force for expansion. The cell wall architecture must be extensible; that is, mechanisms
must exist which allow cell wall matrix to undergo loosening, permitting microfibril
separation and insertion of newly synthesized polymers. Thus, wall loosening and
continued deposition of new material into the wall must be tightly integrated events to
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Mechanisms of cell wall expansion and elongation
maintain a constant wall thickness as the cells expand to tens, hundreds, or even
thousands of times their original length.
Growing cell walls are generally less rigid than walls of non-growing cells, and under
appropriate conditions they undergo irreversible stretching, or yielding, which is lacking
or nearly lacking in non-growing walls.
Figure: Model of cell wall expansion
Source:
http://plantsinaction.science.uq.edu.au/edition1//?q=files/imagecache/figure-
thumb/Fig%204.19.png
Stress relaxation of the cell wall predisposes cells to water influx
and volume expansion
Unlike animal cells, plant cells exhibit directionally controlled cell expansion under
pressure. Inflow of water results in cell expansion (growth) as the primary cell wall
stretches (yields) to accommodate water uptake. Cell walls are, however, not infinitely
extensible. Cell wall yielding or irreversible stretching is of great significance in plant
growth and expansion because plant cells are enclosed in a matrix of wall polymers
which resists expansion sufficiently to generate pressure within the cell contents but at
the same time, yields sufficiently to allow cell expansion in growing regions. It is
considered likely to be the primary event in growth than osmotically driven water inflow.
Water uptake is a passive process which takes place spontaneously in response to a
water potential difference, with no expenditure of energy. The rate of water uptake
depends on the surface area of the cell, the permeability of the plasma membrane to
water (also known as hydraulic conductance, which is a measure of how readily water
can permeate through the membrane), turgor pressure of the cell, and osmotic potential
gradient between the cell and its surroundings. The rate of cell expansion is a rate of
water uptake in cells. As water flows in, the cell volume increases resulting in an
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Mechanisms of cell wall expansion and elongation
increased cell turgor pressure as the protoplast pushes harder against the rigid cell wall.
Since cell wall is composed of interwined and cross-linked chains of cellulose and other
polysaccharides,
turgor
pressure
develops
because
these
entanglements
resist
deformation due to expanding protoplast. The force of the expanding protoplast pushing
against the wall generates stress within the wall. In non-growing cells, the turgor
pressure rises to a value equal to that of the cell’s osmotic potential and water potential
of the cell increases and quickly reaches zero. Thus, water uptake ceases. In nongrowing cells, stress relaxation does not occur.
However, in a growing cell, water potential is prevented from reaching zero and cell
growth is initiated by wall-loosening events which cause load-bearing elements (crosslinks and entanglements) in the wall to yield, leading to relaxation of stress generated in
the wall. This wall-localized event is called stress relaxation, and it is the fundamental
difference between growing and non-growing cells. Cell wall yielding (stretching), stress
relaxation (which tends to reduce turgor pressure) and water absorption (which tends to
increase turgor pressure) occur in a coordinated manner in growing cells and might be
considered as partners which sustain cell growth.
Stress relaxation in the wall is a crucial concept and plays a key role in the process of
cell growth. In growing plant cells, turgor pressure is typically between 0.3 and 1.0 MPa.
Turgor pressure causes cell wall to stretch and, in return, a counterbalancing stress or
tension develops in the wall. This wall tension is equivalent to 10 to 100 MPa. Stress
relaxation is a decrease in wall stress with nearly no change in wall dimensions. As a
consequence of wall stress relaxation, there is a simultaneous and proportionate
reduction in the turgor pressure generated thereby, leading to a decrease in the water
potential of the cell. This is followed by passive uptake of water causing a measurable
extension of the cell wall and increase in cell surface area and volume. Both wall stress
and turgor pressure get restored to their equilibrium values. The process of cell growth
is thus a continuous adjustment of turgor pressure through cell wall yielding and stress
relaxation. Wall stress relaxation plays a crucial role in plant cell growth by helping
reduce their turgor and water potentials, and thus, enabling them to absorb water and
expand. Without stress relaxation, wall synthesis would only cause thickening of the wall
and not expansion.
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Mechanisms of cell wall expansion and elongation
Figure: Turgor pressure and the cell wall.
Source: http://www.mdpi.com/1422-0067/15/3/5094/htm (CC)
In growing cells, reduction in turgor pressure due to an increase in external osmotic
pressure causes wall relaxation and ultimately growth slows down. Therefore, wall
relaxation and expansion depend on turgor pressure. Growth usually ceases before
turgor reaches zero. The turgor value at which growth ceases is called the yield
threshold. It is the minimum pressure potential that is necessary for expansion to occur.
The increase in change of rate of growth above the yield threshold depends on another
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Mechanisms of cell wall expansion and elongation
factor, wall extensibility, which is the quantitative measure of the capacity of the wall to
irreversibly increase its surface area. Under conditions of steady-state growth, the
increase in cell volume is equal to the volume of water uptake. The two processes of wall
expansion and water uptake show opposing reactions to a change in turgor. For
example, an increase in turgor increases wall extension but reduces water uptake. Under
normal conditions, the turgor is dynamically balanced in a growing cell.
An increase in plant cell volume is thus, achieved through coordination of many events:
cell walls yield to generated turgor pressure, solute and water influx are initiated,
membranes surrounding the vacuoles and cytoplasm expand and finally new wall and
membrane components are synthesized.
The multinet growth hypothesis
Cell wall deposition continues as cells enlarge. From the studies of developing cotton
fibers, the multinet growth hypothesis has been developed to explain how cellulose
microfibrils deposited in a transverse or slightly helical orientation to the elongation axis
displace axially during growth of the cell wall. Cellulose microfibrils are woven in a
shallow helix around the cell. The cellulose microfibrils prevent the growing cell from
becoming spherical by withstanding enormous tension across the axis. During cell
expansion, new microfibrils are deposited on the inner surface of the wall in a transverse
orientation. As the cell elongates, each successive wall layer is stretched and thinned, so
the older microfibrils are pushed into the outer layers of the wall and become passively
reoriented in the longitudinal direction—that is, in the direction of growth. Successive
layers of microfibrils thus show a gradation in their degree of reorientation across the
thickness of the wall. Because of thinning and fragmentation, the outer wall layers have
much less influence on the direction of cell expansion than the newly deposited inner
layers. The inner one-fourth of the wall bears nearly all the stress due to turgor pressure
and determines the directionality of cell expansion.
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Mechanisms of cell wall expansion and elongation
Figure: Cell wall polysaccharide deposition and reorientation
Source: http://journal.frontiersin.org/article/10.3389/fpls.2012.00089/full (CC)
The acid-growth hypothesis- auxin-dependent acidification of the
cell wall promotes cell wall expansion, thereby leading to cell
growth
Since the discovery in the 1930s that the plant hormone auxin has the ability to rapidly
induce cell expansion and elongation in oat and grass coleoptiles, scientists have tried to
determine the mechanism by which auxin can control cell expansion. According to the
acid growth hypothesis, auxin causes acidification of the cell wall by stimulating proton
secretion into the cell wall via activation of plasma membrane bound ATPases (proton
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Mechanisms of cell wall expansion and elongation
pumps). As a result of this, the pH of the cell wall gets lowered near this region of the
plasma membrane. In turn, the low pH either lowers the yield threshold of the wall or
optimizes the activity of apoplast-localized cell wall-loosening proteins (e.g., hydrolases,
expansions) which cleave the load bearing bonds which bind the cellulose microfibrils to
other polysaccharides and loosen the wall. Wall Relaxation (i.e., separation of
microfibrils) thus, results in an increase in cell size. Thus, auxin-induced localized
loosening of the primary cell wall, which leads to cell expansion in a particular direction.
With the rigidity of the wall reduced, the cell can elongate. Cell wall acidification (due to
extrusion of protons across the membrane) is one mechanism that causes wall stress
relaxation and wall yielding. Cell wall loosening is enhanced at acidic pH.
Figure: Model depicting acid-growth hypothesis. 1. Auxin stimulates proton pumps,
leading to acidification of cell walls. 2. As a result, cell walls become more plastic
(stretch more easily). 3. Water moves into cell, pushing out on and expanding cell wall.
Source: http://www.mdpi.com/2223-7747/2/4/650/htm (CC)
Studies of the fungal compound fusicoccin provide additional evidence for the acidgrowth
hypothesis. Like auxin, fusicoccin
also induces
rapid
cell
elongation
by
stimulating pumping of protons and localized wall loosening. The action of fusicoccin or
auxin can be blocked by introducing buffers into the cell wall which will prevent the
extracellular pH from falling.
However, there are certain problems with acid-growth hypothesis:
1. No enzymes have been found which hydrolyze cell wall cross-linking glycans
exclusively at pH lower than 5.0
2. No reasonable explanation exists for control of growth once hydrolases are
activated.
3. There have been no hydrolases which after being extracted from the wall do not
cause extension upon addition to isolated tissue sections.
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Mechanisms of cell wall expansion and elongation
Acid growth is the phenomenon wherein extension occurs much faster at acidic pH than
at neutral pH. Acid growth is a characteristic of growing cells. It is evident treatment of
growing cells with acid buffers or with the drug fusicoccin leads to acidification of the cell
wall due to activation of H+-ATPase pumps in the plasma membrane thereby resulting in
cell elongation. Acid-induced growth has been observed in the initiation of root hair
where the wall pH lowers considerably at the time when the epidermal cell begins to
bulge outward. The acid-induced creep is characteristic of growing cells, but it is not
observed in mature (nongrowing) walls. The term creep refers to a time-dependent
irreversible extension of the wall as a result of slippage of wall polymers over each other.
Auxin-induced growth (by cell wall acidification) is a pH-dependent mechanism of wall
extension that appears to be an evolutionarily conserved process common to all land
plants and involved in a variety of growth processes.
Several proteins participate in controlled „loosening‟ of cell wall
Enzymes and proteins having possible wall-loosening activities leading to modification of
the matrix have been identified. A special class of wall-loosening proteins called
expansins is involved in pH-dependent wall loosening and stress relaxation. Expansins
appear to catalyze hydrogen bonds between cellulose and several kinds of load-bearing
cross-linking glycans regardless of their chemical structure, causing the laminate
structure of the cell wall to loosen. Expansins are the only proteins that have been
shown to cause wall expansion in vitro. They are ubiquitously present in growing tissues
of all flowering plants and are effective in catalytic amounts (about 1 part protein per
5000 parts wall, by dry weight). Expansins were discovered and purified using a novel
biochemical assay on pure cellulose paper. Plant cell wall extracts were tested for their
ability to weaken paper at different pH. It was found that paper gets weakened at pH
values between 3.0 and 5.0, but not at pH 7. The probable expansin-triggered loosening
of the wall was found to be reversed when the pH is raised back to 7.0, suggesting that
expansin does not break covalent bonds.
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Mechanisms of cell wall expansion and elongation
Figure: Role of various enzymes and a special class of proteins, expansins in cell wall
loosening.
Source: http://www.glycoforum.gr.jp/science/word/saccharide/SA-A04E.html
Evidence from various studies indicate that expansins lead to cell wall expansion by
loosening of non-covalent adhesion (hydrogen bonds) between wall polysaccharides.
Xyloglucan endotransglycosylase (XET) has also been suggested as a probable wallloosening enzyme. XET carries out a transglycosylation of xyloglucan (XyG) using XyG as
substrate. However, some XETs may exhibit hydrolytic functions. XET may also cause
XyG chains in different lamellae to get realigned during growth. XET helps integrate
newly secreted xyloglucan into the existing wall structure, but its function as a wallloosening agent is still speculative. In some cases, the correlation between XET activity
and growth is not clear.
Several types of experiments have implicated the role of wall hydrolytic enzymes such
as, glucanases and hydrolases in cell wall loosening. Studies indicate the expression of
(1, 4) β-D-glucanases in growing tissues and application of glucanases to cells in vitro
may stimulate growth. Such results support the idea that glucanase activity in plant cell
wall results in stress relaxation and expansion. However, most glucanases and other wall
hydrolases do not cause cell wall expansion in the same way as expansins do. Instead,
treatment of walls with glucanases or pectinases may enhance the subsequent extension
response to expansins. These results suggest that wall hydrolytic enzymes such as (1, 4)
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Mechanisms of cell wall expansion and elongation
β-D-glucanases are not the principal catalysts of wall expansion, but they may act
indirectly by modulating expansin-mediated wall expansion.
Figure: Mechanism of cell wall expansion in plant cells. Plant cell elongation is achieved
by coordination of various events.
Source: http://journal.frontiersin.org/article/10.3389/fpls.2014.00771/full (CC)
Cessation of wall expansion and elongation
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Mechanisms of cell wall expansion and elongation
As the cell approaches its maximum dimensions and matures, its rate of growth
diminishes and finally stops. Growth cessation is an irreversible process and is typically
accompanied by a reduction in wall extensibility and ‘locking’ of wall into a rigid,
hardened conformation. Many structural and physical changes occur in the wall during
and after cessation of growth.
Several modifications in the maturing wall contribute to rigidification of the wall as
growth cease.
•
Recently synthesised polysaccharides may be modified to form tighter complexes
with cellulose or other wall polymers
•
Removal of mixed-link β-D-glucans and de-esterification of pectins is associated
with cessation of growth in plant cell walls.
•
Exocytosis of molecules with defined roles in cessation into cell walls. Crosslinking of phenolic groups in the wall (such as tyrosine residues in HRGPs, ferulic
acid residues attached to pectins, and lignin) generally coincides with wall
maturation and is believed to be mediated by peroxidase.
•
Degradation of enzymes responsible for cleaving of polymers.
Summary
Plant cells are enclosed within a thin, rigid and mechanically strong cell wall that is made
up of complex polymers. Cell wall plays important roles in plant growth, cell
differentiation, intercellular communication, water movement and defence and provides
mechanical strength to the cell. In growing cells, the primary wall is a thin, flexible layer
which is primarily made up of complex polysaccharides and some structural proteins.
The primary wall is composed of cellulose microfibrils which are embedded in a hydrated
matrix made of pectin. The cellulose microfibrils form a strong network which gives
shape to the underlying protoplast. The plant cell wall is dynamic in nature and plays a
major role in cell expansion and elongation. Plant cells grow by coordinated and
controlled expansion of their cell walls. The main driving force for cell expansion is
osmotic or turgor pressure exerted by the protoplast. As a result of turgor pressure, the
protoplast pushes hard and generates wall stress. This results in the stretching of wall
polymers and provides the mechanical energy for cell wall expansion. The rate of cell
expansion and growth is a function of rate of water uptake by the cell leading to an
irreversible extension of the pre-existing cell wall. As the cell wall yields, new polymers
are integrated into the wall simultaneously, to prevent it becoming thinner and weaker.
Stress relaxation is, however, the major player in driving cell expansion. The direction of
cell growth is determined by the orientation of cellulose microfibrils which in turn
depends on the orientation of cortical microtubules. Thus, cytoskeleton plays a key role
determining cell shape and size. Theories have been proposed suggesting the possible
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Mechanisms of cell wall expansion and elongation
mechanisms of cell wall expansion explaining. According to multinet growth hypothesis,
the older microfibrils get axially deformed while the inner wall layer made up of recently
deposited microfibrils oriented transversely to the elongation axis determine direction of
cell growth. According to acid-growth hypothesis, auxin-induced cell wall acidification
due to proton pumping activates various wall-loosening enzymes and proteins that help
in cell wall expansion. Various wall loosening enzymes (glucanases, XET and hydrolases)
and a special class of proteins; expansins have been characterized and shown to play
role in cell wall loosening via different mechanisms. Wall loosening leads to stress
relaxation and allows deposition and new wall material thus, causing cells to expand. As
the cell reaches its maximum size and matures, cell growth ceases. A rigid secondary
cell wall is formed which is primarily composed of cellulose and lignin. Cell wall
expansion and elongation is, thus, a highly coordinated and complex process involving
various events which take place in a synchronized manner.
Exercises
Question Number
Type of question
1
True or False
Question
a.
b.
c.
d.
e.
Mature cell walls become resistant to loosening activities.
Cytokinin and ABA play important role in cell wall loosening.
Growth is a reversible process.
Orientation of microfibrils determines the direction of cell growth.
The rate of cell expansion is a function of rate of water uptake in cells.
Correct Answer /
Option(s)
a.
b.
c.
d.
e.
True
False
False
True
True
Justification/ Feedback for the correct answer
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Mechanisms of cell wall expansion and elongation
a. As the cell matures, lignin is deposited which forms a hydrophobic network and
binds tightly to cellulose thus, preventing the cell wall from further expansion.
b. Auxin plays important role in cell wall loosening by inducing acidification of cell
wall ultimately leading to cell growth.
c. Growth is an irreversible process because it involves changes in cell wall
architecture and deposition of new wall material.
d. The direction of cell growth and expansion in response to turgor depends on the
orientation of the inextensible cellulose microfibrils spooled on the inner surface
of the primary wall.
e. Cell expansion depends on the rate of water uptake in cells which leads to
increase in turgor pressure and increase in cell volume.
Question Number
Type of question
2
Match the following
Question
1.
2.
3.
4.
Cellulose synthase
Isoxaben
Oryzalin
Hydrolases
5. Auxin
Correct Answer
a. Microtubule depolymerising drug
b. Cell wall loosening
c. Acid-growth hypothesis
d. Multimeric membrane bound enzyme
complex
e. Inhibitor of cellulose synthesis
1.(d), 2.(e), 3.(a), 4.(b), 5.(c)
Question Number
Type of question
3
Fill in the blanks
Question
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Mechanisms of cell wall expansion and elongation
a.
b.
c.
d.
e.
Cellulose is a linear polymer of ________
Plant growth occur either by _________ or _________
The main driving force for cell expansion is _________
Orientation of newly deposited cellular microfibrils depends on ____________
_________, ___________ and ___________ occur in a coordinated manner
in growing cells and might be considered as partners which sustain cell
growth.
Correct Answer /
Option(s)
a.
b.
c.
d.
e.
β-D-glucose
elongation, expansion
osmotic or turgor pressure
orientation of cortical microtubules
cell wall yielding, wall stress
absorption
Question Number
Type of question
4
MCQ
relaxation,
water
Question
a.
Hormone responsible for acid-growth hypothesis is
1. Cytokinin
2. ABA
3. Auxin
4. Giberellic acid
b.
Transport of which ion is responsible for cell wall loosening
1. H+
2. Na+
3. K+
4. Ca2+
Correct Answer /
Option(s)
a. 3
b. 1
5. Write short notes on
a.
Role of cell wall
b. Factors which influence the rate of cell wall expansion
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Mechanisms of cell wall expansion and elongation
c.
Why developing plant cells have tendency to elongate rather than increasing
radially?
d. Stress relaxation
e.
Events associated with cell growth
f.
Multinet growth hypothesis
g. Proteins that participate in cell wall loosening
h. Modifications in maturing wall that contribute to wall rigidification and
cessation of growth
6. Describe the following
a.
Wall extensibility
b. Yield threshold
c.
Cell wall loosening
7. Distinguish between
a.
Primary and secondary cell wall
b. Tip growth and diffuse growth
c.
Growing and non-growing cells
8. Diagrammatically explain the following
a.
Acid-growth hypothesis
b. Events associated with cell wall expansion
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Mechanisms of cell wall expansion and elongation
Glossary
Acid growth: It is the ability of cells to elongate faster under acidic conditions.
Cellulose microfibril: It is a tough, inelastic fiber wrapped in layers (lamellae) within
the plant cell wall. Composed of linear chains (1, 4)-linked β-d-glucosyl residues.
Cellulose synthase: It is a large, multimeric complex bound to the plasma membrane
that spins out long cellulose molecules using UDP-glucose.
Cortical microtubules: Are slender, tubular structures that that form an array and lie
just beneath the plasma membrane
Creep: Slow, time-dependent, irreversible extension, in which the microfibrils and
associated matrix polysaccharides slowly slide within the wall, therefore increasing its
surface area.
Diffuse growth: Wall expansion evenly distributed over the entire wall surface.
Endotransglycosylases: Are the enzymes which cause cleavage of glycan and ligation
of one of the fragments to the free end of another polymer.
Elongation: It is the increase in cell size in only one dimension.
Expansins: are a special class of wall loosening proteins that induce stress relaxation of
cell wall in a pH-dependent manner.
Expansion: It is the increase in cell size in more than one dimension (two or three
dimensions).
Hemicelluloses: Are a group of complex polysaccharides which bind tightly to the
surface of cellulose and form cross-links.
Hydraulic conductance: It is a measure of how readily water can permeate through
the membrane
Lamellae: It is a thin layer formed by cellulose microfibrils.
Multinet growth: It is the steady state pattern postulated for the growth of cell walls
where in the microfibrils deposited in a transverse orientation become passively
reoriented and displace axially by the growth of cell.
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Mechanisms of cell wall expansion and elongation
Pectins: Group of complex polysaccharides that form a highly hydrated matrix. They
include homogalacturonan, rhamnogalacturonans I and II, galactans, arabinans and
other polysaccharides.
Primary cell wall: It is the flexible, rigid and strong extracellular layer that is deposited
while the cell is expanding.
Secondary cell wall: It is the rigid extracellular matrix that is deposited when
expansion ceases. Primarily composed of cellulose and lignin.
Tip growth: Wall expansion is highly localized and restricted to the tip of the cell.
Turgor pressure: It is the force generated as the protoplast pushes outward on the cell
wall due to water influx. It results in plant rigidity however, loss of turgor pressure
causes wilting.
Wall extensibility: It is the quantitative measure of the capacity of the wall to
irreversibly increase its surface area.
Wall loosening: It is the modification of the cell wall that enables it to expand in
response to the turgor generated wall stress.
Wall stress relaxation: It is the reduction in mechanical wall stress by slippage or
breakage of load-bearing polymers in the cell wall.
Wall yielding: It is the irreversible stretching of cell wall in response to turgor pressure.
Water Potential: It is the pressure that causes water to move across a membrane;
water always moves naturally from areas of higher water potential to those of lower
water potential.
Yield threshold: It is the minimum pressure potential that is necessary for expansion
to occur.
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Mechanisms of cell wall expansion and elongation
References/Further Reading
•
Introduction to Plant Physiology (2009) 4th ed. Hopkins WG and Huner. John Wiley
and Sons, Inc (USA)
•
Plant Physiology (2006) 4th ed. Taiz L and Zeiger E. Sinauer Associates Inc. (USA)
•
Biochemistry and Molecular Biology of Plants (2000) Buchanan BB, Gruissem W and
Jones RL. Courier Companies Inc. (USA)
Weblinks
http://www.mdpi.com/1422-0067/15/3/5094/htm
http://www.nature.com/ncomms/2013/130617/ncomms2967/full/ncomms2967.html
http://www.mdpi.com/2223-7747/2/4/650/htm
http://journal.frontiersin.org/article/10.3389/fpls.2013.00163/full
http://journal.frontiersin.org/article/10.3389/fpls.2012.00089/full
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