Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu |
Cell Walls - Structure & Function
I. Functions of the cell wall: The cell wall serves a variety of purposes including:
II. Wall Components - Chemistry
The main ingredient in cell walls are polysaccharides (or
complex carbohydrates or complex sugars) which are built from monosaccharides
(or simple sugars). Eleven different monosaccharides are common in these polysaccharides
including glucose and galactose. Carbohydrates are good building blocks
because they can produce a nearly infinite variety of structures. There
are a variety of other components in the wall including protein, and lignin.
Let's look at these wall components in more detail:
A. Cellulose
β1,4-glucan
(structure provided in class). Made of as many as
25,000 individual glucose molecules. Every other molecule (called residues) is
"upside down". Cellobiose (glucose-glucose disaccharide) is the basic building
block. Cellulose readily forms hydrogen bonds with itself (intra-molecular H-bonds) and
with other cellulose chains (inter-molecular H-bonds). A cellulose chain will form
hydrogen bonds with about 36 other chains to yield a microfibril. This is somewhat
analogous to the formation of a thick rope from thin fibers. Microfibrils are
5-12 nm
wide and give the wall strength - they have a tensile strength equivalent to steel. Some
regions of the microfibrils are highly crystalline while others are more
"amorphous".
B. Cross-linking glycans (=Hemicellulose)
Diverse group of carbohydrates that used to be called
hemicellulose. Characterized by being soluble in strong alkali. They are linear (straight), flat, with a
β-1,4 backbone and relatively short side chains.
Two common types include xyloglucans and glucuronarabinoxylans. Other less
common ones include glucomannans, galactoglucomannans, and galactomannans.
The main feature of this group is that they dont aggregate with themselves - in other
words, they dont form microfibrils. However, they form hydrogen bonds with cellulose
and hence the reason
they are called "cross-linking glycans". There
may be a fucose
sugar at the end of the side chains which may help keep the molecules planar by
interacting with other regions of the chain.
C. Pectic polysaccharides
These are extracted from the wall with hot water or dilute
acid or calcium chelators (like EDTA). They are the easiest constituents to remove from the wall. They form gels (i.e., used in jelly making).
They are also a diverse group of polysaccharides and are particularly rich in galacturonic acid (galacturonans = pectic acids).
They are polymers of
primarily
β 1,4 galacturonans
(=polygalacturonans) are called homogalacturons (HGA) and are particularly
common. These are helical in
shape. Divalent cations, like calcium, also form cross-linkages to join
adjacent polymers creating a gel. Pectic polysaccharides can also be cross-linked by
dihydrocinnamic or diferulic acids. The HGA's (galacturonans) are initially secreted
from the golgi as methylated
polymers; the methyl groups are removed by pectin methylesterase to initiate calcium binding.
Other pectic acids include Rhamnogalacturonan II (RGII) which
features rhamnose and galacturonic acid in combination with a large diversity of
other sugars in varying linkages. Dimers of RGII can be cross-linked by
boron atoms linked to apiose sugars in a side chain.
Although most pectic polysaccharides are acidic, others are
composed of neutral sugars including arabinans and galactans. The pectic
polysaccharides serve a variety of functions including determining wall
porosity, providing a charged wall surface for cell-cell adhesion - or in other
words gluing cells together (i.e,. middle
lamella), cell-cell recognition, pathogen recognition and others.
D. Protein
Wall proteins are typically glycoproteins (polypeptide backbone with
carbohydrate side chains). The proteins are particularly rich in the amino acids
hydroxyproline (hydroxyproline-rich glycoprotein, HPRG), proline (proline-rich
protein, PRP),
and glycine (glycine-rich protein, GRP). These proteins form rods (HRGP,
PRP) or beta-pleated sheets (GRP). Extensin is a well-studied HRGP.
HRGP is induced by wounding and pathogen attack. The wall proteins
also
have a structural role since: (1) the amino acids are characteristic of other structural
proteins such as collagen; and (2) to extract the protein from the wall
requires destructive conditions. Protein appears to be cross-linked to pectic substances
and may have sites for lignification. The proteins may serve as the scaffolding used to
construct the other wall components.
Another group of wall proteins are heavily
glycosylated with arabinose and galactose. These arabinogalactan proteins, or AGP's, seem
to be tissue specific and may function in cell signaling. They may be important in
embryogenesis and growth and guidance of the pollen tube.
E. Lignin
Polymer of phenolics, especially phenylpropanoids. Lignin is
primarily a strengthening agent in the wall. It also resists fungal/pathogen attack.
F. Suberin, wax, cutin
A variety of lipids are associated with the wall for
strength and waterproofing.
G. Water
The wall is largely hydrated and comprised of between 75-80% water.
This is responsible for some of the wall properties. For example, hydrated walls have
greater flexibility and extensibility than non-hydrated walls.
III. Morphology of the Cell Wall - there are three major regions of the wall:
IV. Tire analogy for the cell wall
The wall is similar to a tire
that has a series of steel belts or cords embedded in an amorphous matrix of rubber. In
the plant cell wall, the "cords" are analogous to the cellulose microfibrils and
they provide the structural strength of the wall. The matrix of the wall is analogous to
the rubber in the tire and is comprised of non-cellulosic wall components.
How are the various wall polymers arranged? It appears that:
V. Wall Formation
The cell wall is made during cell division when the cell plate is formed between
daughter cell nuclei. The cell plate forms from a series of vesicles produced by the golgi
apparatus. The vesicles migrate along the cytoskeleton and move to the cell equator. The
vesicles coalesce and dump their contents. The membranes of the vesicle become the new
cell membrane. The golgi synthesizes the non-cellulosic polysaccharides. At first, the
golgi vesicles contain mostly pectic polysaccharides that are used to build the middle lamella. As the
wall is deposited, other non-cellulosic polysaccharides are made in the golgi and
transported to the growing wall.
Cellulose is made at the cell surface. The process is catalyzed by the enzyme cellulose synthase that occurs in a rosette complex in the membrane. Cellulose synthase, which is initially made in by the ribosomes (rough ER) and move from the ER → vesicles → golgi → vesicle → cell membrane. The enzyme apparently has two catalytic sites that transfer two glucoses at a time (i.e., cellobiose) from UDP-glucose to the growing cellulose chain. Sucrose may supply the glucose that binds to the UDP. Wall protein is presumably incorporated into the wall in a similar fashion.
Remember that the wall is made from the outside in. Thus, as the wall gets thicker the lumen (space within the wall) gets smaller.
Exactly how the wall components join together to form the wall once they are in place is not completely understood. Two methods seem likely:
VI. Strong Wall/Cell Expansion Paradox
(dont you just love a
good paradox?)
How can the wall be strong (it must withstand pressures of 100 MPa!), yet still allow
for expansion? Good question, eh? The answer requires that the wall:
A. Be capable of expansion
In other words, only cells with primary walls are
capable of growth since the formation of the secondary wall precludes further expansion
of the cell. The sequence of microfibril orientation changes during development. Initially
the microfibrils are laid down somewhat randomly (isotropically). Such a cell can expand in
any direction. As the cell matures, most microfibrils are laid down laterally, like the hoops of a
barrel, which restricts lateral growth but permits growth in length. As
the cell elongates the microfibrils take on an overlapping cross-hatched pattern, similar to fiberglass. This occurs because the cell
expands like a slinky - the width of the cell doesn't change by the microfibrils
become aligned in the direction of growth just like the spring. This overlapping
of microfibrils, which is strong and
lightweight, prohibits further expansion.
But, what determines the orientation of the microfibrils? They are correlated with the direction of the microtubules in the cell. Evidence: treating a cell with colchicine or oryzalin (which inhibit microtubule formation) destroys the orientation of the microfibrils. The microtubules apparently direct the cellulose synthesizing enzymes to the plasma membrane.
In addition to cellulose microfibril orientation, mature walls apparently loose their ability to expand because the wall components become resistant to loosening-activities. This would occur if there were increased cross-linking between wall components during maturation. This would result from:
B. Loosening (or Stress relaxation) the wall at the appropriate time
Even
though the microfibrils may be in the proper position to permit loosening, the wall is
still rather strong. Recall that our wall model proposed strong (covalent) and weak
(hydrogen bonds) links between the
wall components. When the wall is loosened, weak bonds are
temporarily broken to allow the wall components to slide or creep past one another. So,
how is the wall temporarily loosened?
1. Protons are the primary wall loosening factor (Acid Growth Hypothesis). This idea was first proposed by David Rayle and R. Cleland in 1970. Some evidence:
- acid buffers stimulate elongation and rapid responses 5-15 min even in non-living tissues (Evans,1974);
- acid secretion is associated with sites of cell elongation (see Evans & Mulkey, 1981)
- Fusicoccin, a diterpene glycoside extracted from a fungus, stimulates proton secretion (activates a H+/K+ pump) and stimulates elongation.
2. Mechanism of proton action: Protons stimulate wall loosening by:
- disrupting acid-labile bonds such as H-bonds and calcium bridges; and
- enhancing the activity of enzymes that break wall cross-links including H-bonds and calcium bridges. Evidence for the enzyme involvement includes: (1) when primary walls are heated or treated with protein denaturing agents they can't be "loosened" by acid; and (2) adding proteins extracted from growing walls to heat-treated walls restores the acid response.
Expansins appear to be the primary wall-loosening enzymes. This class of proteins are activated by low pH and break the hydrogen bonds between cellulose and the cross-linking glycans. Other candidates for enzymes involved include: (1) pectin methyl esterase which would break the calcium bridges between pectins by esterifying the carboxyl groups; and (2) hydrolases which would hydrolyze the cross-linking glycans (hemicelluloses). For example, xyloglucan endotransglycosylase (XET) has been shown to cleave cross-linking glycans that could allow slippage of the wall components3. The acid effect is induced by indole-3-acetic acid (IAA, auxin), one of the major plant hormones. IAA stimulates proton excretion and cell growth/elongation. Evidence:
- peeled coleoptiles + IAA → medium acidic; peeled coleoptiles + water → not acidic; and
- flooding auxin-treated tissue with neutral buffers prevents the growth response.
4. Mechanism of Auxin Action How does auxin stimulate proton excretion and wall elongation? There are two ideas:
Hypothesis 1: Auxin activates pre-existing H+-ATPase pump proteins in the cell membrane. These proteins transport protons from the protoplast into the wall. Auxin probably first binds to a receptor molecule and this complex then actives the pump. This process is active - thus the pump requires ATP. Evidence: ATP stimulated acidification is observed soon after auxin treatment.Hypothesis 2: Auxin stimulates transcription and translation. Transcription/translation (protein synthesis) would be required to produce proton pump proteins (a wonderful alliteration), respiratory enzymes to provide ATP to power the process; and even enzymes for the synthesis of wall components and cell solutes (see C. & D. below). Evidence for the involvement of transcription/translation:
- Nooden (1968) found that artichoke disks increased in size when incubated with IAA but that the addition of antimycin (a protein synthesis inhibitor) prevented this response;
- soybean hypocotyls incubated with 2,4-D (an analog of IAA) produce at least 3 new polypeptides within three hours (Zurfluh & Guilfoyle, 1980);
- in vitro
translation of mRNA occurs within 15 minutes of IAA treatment- The proton effect is short-lived. Cell elongation stops 30-60 minutes after acidification. Continuous elongation requires longer term metabolic changes such as protein synthesis.
C. Wall synthesis occurs
As the cell grows, wall synthesis needs to
occur. Think about the color of a balloon as it is blown up - it gets
lighter in color as the balloon gets larger because the thickness of the balloon decreases
as it expands and stretches. Using this logic, we expect that plant cells should become
thinner as they expand. Right? Wrong - cell walls remain a relatively uniform thickness
throughout cell growth. Thus, we can conclude that new wall material must be made during
cell elongation.
D. Enhanced solute synthesis
The solute concentration of the cell remains
constant during cell enlargement. This suggests that solutes are being synthesized since
the volume of the cell is increasing. Maintaining a high solute concentration is necessary
to allow for water uptake.
E. Lock wall in place after
expansion is complete
Once wall elongation is completed, the cell
needs to "lock it" in place. This likely happens as the
temporary bonds that were broken reform, and due to increased interactions
(including enzymatic) between wall molecules.
F. Water Uptake/Pressure
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References
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Last updated:
09/30/2011 � Copyright by SG
Saupe