Plant cell wall
(Lecture 12)
Cell walls consist of 3 types of layers
Middle lamella: This is the first layer formed during cell division. It makes up the outer wall of
the cell and is shared by adjacent cells.
It is composed of
pectic compounds and protein
.
Primary wall: This is formed after the middle lamella and consists of a rigid skeleton of cellulose
microfibrils embedded in a gel-like matrix composed of :
pectic compounds, hemicellulose, and glycoproteins
.
Secondary wall: formed after cell enlargement is completed. The secondary wall is extremely
rigid and provides compression strength.
It is made of
cellulose, hemicellulose and lignin
. The secondary wall is often layered.
Primary/secondary cell wall
The plant cell wall is a dynamic compartment that changes throughout the life of the cell.
The
new primary cell wall
is born in the cell plate during cell division and rapidly
increases in surface area during cell expansion.
The middle lamella forms the interface between the primary walls of neighboring cells.
At differentiation, many cells elaborate within the primary wall a secondary cell wall.
The plant cell wall is a highly organized composite of many different polysaccharides,
proteins, and aromatic substances.
Specialized functions of the cell wall
1.Structural:
1.Fibers
2.Cross-linked matrix
2.Molecules affecting developmental pattern
3.Molecules defining cell’s position within the plant.
4.Molecules of cell-cell and cell-nucleus communication
5.Defense against pathogens (impregnation with lignin)
6.Recognition of symbiotic nitrogen-fixing bacteria
7.Recognition of self
Macromolecules of the cell wall
1.Cellulose
– the most abundant plant polysacharide (15-30% of the dry mass of the cell
wall)
Microfibrills – several dozen of (1à4) b-D-glucan chains
Cellulose polymers associate through H-bonds.
The H-bonding of many cellulose molecules to each other results in the formation of micro fibers
and the micro fibers can interact to form fibers.
Certain cells, like those in cotton ovules, can grow cellulose fibers of enormous lengths.
2. Callose
Differ from cellulose in consisting of (1à3) b-D-glucan chains that can form helical duplexes.
Callose is made in a few cells at specific stages of development (growing pollen tubes, cell plates
of dividing cells).
It is made in response to wounding or to penetration by invading fungal hyphae.
3.
Pectic acid
-
polymer of around 100 galacturonic acid molecules
- very hydrophilic and soluble - become very hydrated
- forms salts and salt bridges with Ca++ and Mg++ that are insoluble gels
- major component of middle lamella but also found in primary walls
galacturonic acid Pectic acid with salt bridges
4.
Pectin
-
polymer of around 200 galacturonic acid molecules
- many of the carboxyl groups are methylated (COOCH3)
- less hydrated then pectic acid but soluble in hot water
- another major component of middle lamella but also found in primary walls
Cross-linking glycans
Make hydrogen-bond to cellulose.
•XyGs – xyloglucans
•GAXs - glucuronoarabinoxylans
Macromolecules of the cell wall
1.Structural proteins of the cell wall
ii. Proline-rich glycoproteins (PRGPs)
iii. Glycine-rich proteins (GRPs)
iv. AGPs (mainly consist of carbohydrate)
v. All mRNAs for cell wall proteins encode signal peptides that target the proteins to the
secretory pathway.
Like the cell wall carbohydrates, glycoproteins are hydrophilic and can form H-bonds and
salt bridges with cell wall polysaccharides.
In addition to hydroxyproline, cell wall proteins are often high in the amino acids proline
and lysine. The NH3+ on lysine provides positive charges along the peptide backbone.
The positive charges residues can associate with negatively charged groups on pectic
acids, etc.
In addition to electrostatic interactions, H-bonds also form between amino acid side
chains and cell wall carbohydrates.
Another type of structural cell wall protein, called
extensin
, can form covalent bonds with
other extensin proteins through the amino acid tyrosine.
In extensin, the tyrosines are evenly spaced and when they bond with tyrosine on another
extensin molecule, they can wrap around other cell wall constituents "knitting" the wall
together.
The amount of extensin changes with development. Cells that have thick, hard walls are
often rich in extensin (i.e., sclerids and fibers).
The amount of extensin produced is dependent on mechanical wounding, infection and
these responses are mediated by plant hormones.
Cell walls also contain functional proteins. Enzymatic activities in cell walls include:
•Oxidative enzymes - peroxidases•Hydrolytic enzymes - pectinases,
cellulases•"Expansins" - enzymes that catalyze cell wall "creep" activity
General functions of cell wall enzymes
include:
protection against pathogens,
cell expansion,
cell wall maturation.
Cell wall architecture
The primary cell wall is made up of two, sometimes three, structurally independent but
interacting networks.
The
framework of cellulose and crosslinking glycans
lies embedded in a second
The third independent network consists of the
structural proteins or phenylpropanoid
network
.
Type I
cell wall.
The walls of
most dicots
and the
noncommelinoid monocots
contain about equal
amounts of XyGs and cellulose
Type II
cell walls
Occurs in
commelinoid monocots
.
Instead of XyG the principal polymers are GAXs
•XyGs – xyloglucans
•GAXs - glucuronoarabinoxylans
Cell expansion involves extensive changes in the mass and composition of the cell wall.
During elongation or expansion, existing cell wall architecture must change to incorporate
new material, increasing the surface area of the cell and inducing water uptake by the
protoplast.
Wall loosening
and continued
deposition of new material
- primary determinant of
rates of cell expansion.
The wall-localized event, called
stress relaxation
, serves as the fundamental difference
between growing and nongrowing cells.
The yield threshold, defines the pressure potential that must be exceeded before
expansion can occur.
Postulates that auxin-dependent acidification of the cell wall promotes wall extensibility
and cell growth.
The acid-growth proposes that auxin activates a plasma-membrane proton pump, which
acidifies the cell wall.
The low pH activates apoplast-localized growth-specific hydrolases, which cleave the
load-bearing bonds that tether cellulose microfibrils to other polysaccharides.
Cleavage of these bonds results in loosening of the cell wall, and the water potential
difference causes uptake of water.
Relaxation of the wall passively leads to an increase in cell size.
Enzymes with wall-loosening acitivities
Xyloglucan endotransglycosylase
(XET) –
transglycosylation of XyG in which one chain of XyG is cleaved and reattached to the
nonreducing terminus of another XyG chain.
Other proteins catalyze wall extension in vitro without any detectable hydrolytic or
tansglycolytic events.
Called
expansins
, these proteins probably catalyze breakage of hydrogen bonds
between cellulose and the load-bearing cross-linking glycans.
Such an activity could disrupt the tethering of cellulose.
Expansins are the only proteins shown to produce wall expansion in vitro.
They appear ubiquitous in growing tissues of all flowering plants, and they appear to
break hydrogen bonds between cellulose and cross-linking glycans.
1) Wall acidification - H+ATPase in plasma membrane 'pumps" H+ from cytoplasm into
cell wall.
The pH of the wall drops and carboxylic acids become protonated and 'salt bridges"
are broken.
2) In addition, the enzyme "expansin" is activated and causes cellulose micro fibers to slip
(mechanism of expansin action is unknown).
This results in cell wall "creep".
3) Hydrolytic enzymes like cellulase and pectinase, "degrade" cell walls by breaking polymers
into smaller subunits or by breaking crosslinks.
Cell differentiation
Within a single wall there are zones of different architectures – the middle lamella,
plasmodesmata, thickenings, channels, pitfields, and the cell corners.
The size of such microdomains in the wall, compared with the size of the polymers that
must fit in these domains, implies that mechanisms must exist for packaging and
positioning of large molecules.
Changes of cell wall architecture are involved in fruit-ripening
Most fruits in which the pericarp or endocarp softens during ripening develop thickened primary walls (pectic substances)
The texture of the ripe fruit pulp is governed by the extent of wall degradation and loss of cell-cell adhesion.
Pectin often constitute more than 50% of the fruit wall.
The softening process in tomato parenhyma tissue is associated with loss of methyl esters of HGA (pectic polysaccharide homogalacturonan).
The deesterified HGA backbone then is susceptible to the activity of Pgase
(peptidoglutaminase).PGase solubilizes pectins from the cell wall, facilitating progressive hydrolysis.
Pectin modification within the wall during ripening is a tightly regulated process.
Using antisense inhibition of PGase, researchers were able almost completely preventing pectic depolymerization in transgenic fruit.
However, little or no reduction in softening was achieved.
For many cells, the differentiation process is associated with formation of a secondary wall. When cells stop growing, the wall is crosslinked into its ultimate shape. At that point, deposition of the secondary wall begins.
Cotton fiber – 98% cellulose at maturity.
May contain additional noncellulosic polysaccharides, proteins, and aromatic substances such as lignin.
The walls of many cells function long after the cells that produced them are dead.
Secondary cell walls of the cotyledon and the endosperm of the developing seeds contain little or no cellulose.
These secondary walls serve two functions:
•Provide a strong wall to protect the embryo or impose mechanical dormancy;
•Contain specialized storage carbohydrates that are digested during germination and converted to sucrose for transport to the growing seedling.
Approximately 1000 gene products involved in cell wall biosynthesis, assembly, and turnover are known.
Arabidopsis mutants with altered carbohydrate components in the primary wall have been detected by gas chromatography of alditol acetates of their neutral sugars.
More than three dozen mutants have been classified by mapping to 11 different loci. Mur1 has been identified as a GDP-mannose-4,6-dehydratase;
Mur2 as a fucosyltransferase ; Mur4 – C-4 epimerase.
Rsw1 - prymary wall cellulose synthase.
Cell walls as food, feed, and fibers
Cell walls directly affect the raw material quality of human and animal food, textiles, wood, and paper and may play a role in human medicine.
The food industry uses isolated arabinogalactan proteins (AGPs) and pectins as gums and gelling agents.
Fungal and bacterial wall hydrolases are used to adjust food textures and states.
The cell walls of fruits and vegetables are now recognized as important dietary components and may protect against cancer of the colon, coronary heart disease, diabetes.
b-Glucans are the causal agents in the ability of oat and barley brans to lower serum cholesterol and reduce the insulin demand of people with diabetes.
Some pectins may have antitumor activities, possibly by stimulating immune system. Agricultural researches are investigating particular enzymes involved in cell wall metabolism in hopes of producing crops with desired characteristics by enhancing commercially valuable traits (fiber production in flax, cotton, ramie and sisal).
The pulp and paper industry processes trees into cellulose and livestock industry, which depends on the transformation of the cell wall into muscle tissue, are striving to reduce the lignin content in their respective sources of fiber and fodder.
Reducing lignin content would reduce organochlorine wastes and cut costs tremendously for the paper industry, which currently uses chemical extractions to purify cellulose from the wood. Lignin-carbohydrate interactions influence the digestibility of forage crops by animals. Hence, the mutants with altered lignin type may yield new forage crops that exhibit greater digestibility with remaining strengthening function of lignin to the water-conducting cells of the plant.
Unfortunately, the results of attempts to change only one parameter of wall metabolism are rarely those predicted, because of the complexity of the wall and the ability of plant cells to adapt to change.
Summary
1.The primary wall of the cell is extensible but constrains the final size and shape of every cell 2.In some cells, secondary walls are deposited on the inner surface of the primary wall after growth has stopped.
3.Cell wall becomes specialized for the function of the approximately 40 cell types that plants comprise.
4.The cellulose microfibrills form the scaffold of all cell walls and are tethered together by crosslinking glycans; this framework is embedded in a gel of pectic substances.
5. There are two types of primary cell walls:
•Type I – xyloglucan-cellulose networks embedded in pectin-rich matrix that is further cross-linked with a network of structural proteins.
•Type II – have glycoronoarabinoxylan – cellulose networks in a relatively pectin-poor matrix that is crosslinked by ferulate esters and aromatic substances.
6.Cellulose microfibrills are synthesized at the surface of the plasma membrane at terminal complexes called particle rosettes. All noncellulosic crosslinking glycans and pectic substances are made at the Golgi apparatus and secreted.
7.All cell wall sugars are synthesized de novo from interconversion of nucleotide sugars 8.Cell enlargement depends on the activities of endoglycosidase, endotransglycosylase, or expansin, or some combination of these; cell shape is largely governed by the pattern of cellulose deposition.
9. Termination of cell growth is accompanied by crosslinking reactions involving proteins and aromatic substances.
10. Transgenic plants with altered cell wall structures will become an important factor in crop and biomass improvement.
Part of the material was modified from
•
http://www.landfood.unimelb.edu.au/research/Biotech/research.html
•
http://www.agrevo.com/is_agrevo/biotech/ps/ps_sl_sl1.htm
