Introduction
Newly obtained evidence from electron microscopy
suggests that cellulose for cell walls is synthesized by a protein complex
in the outer plasmamembrane. These have been called particle
or rosette complexes. It is thought that these units contain
many units of cellulose synthase, the enzyme that puts glucoses
together into cellulose. Glucoses bound to uridine diphosphate (UDP-glucose)
are transferred to the enzyme complex (which can bind two UDP-glucoses
at one time) and then they are attached to the growing chain of cellulose.
The glucoses themselves probably come from sucrose. The fact that
the enzyme complex attaches two glucoses at once may be the reason that
the basic unit of cellulose is a disaccharide (cellobiose).
As the microfibrils are produced (see Fig. 15.8 from Taiz and Zeiger, 1998) hemicelluloses and pectins begin to attach and bind to them. This give them their characteristic shape and strength and orientation within the cell wall. The most common hemicellulose is xyloglucan (see previous carbohydrate lecture). This compound is similar to cellulose, in that the backbone chain is (1-4) linked, but the side chains differ, containing xylose and galactose. These side chains interfere with the crystallizing process that occurs with cellulose, and thus these compounds are more flexible.
Pectins are much more complex, and often form highly irregular complexes of many sugars. One pectin, called by the very long name of rhamnogalacturonon II (RGII) contains ten different sugars! Pectins contain many carboxyl groups (COO-) and these are linked together with Ca+2. This is why calcium is so important for cell wall formation. Other pectins may be linked via ester bridges to methyl and acetyl groups. This allows the carboxyl groups to remain free and charged, and may result in attraction of various ions in the cell wall, and activation of certain wall containing enzymes.
Walls also contain proteins. The best studied
group is called the hydroxyproline-rich proteins (HPRPs).
As their name implies, they are enriched in the amino acid proline.
These proteins are often produced in response to wounding or pathogen attack,
and may somehow be involved in protecting the cell. This is still
an area of ongoing investigation.
Cell Wall Formation
Cells contain primary cell walls, and in certain
cases, secondary cell walls, along with a middle lamella. We will
discuss each in turn. Primary cell walls are synthesized just as
cell division ceases. When plant cells divide, they form a cell
plate, and the new wall material is synthesized on this first.
The general process of wall formation is:
Synthesis -->> Secretion -->> Assembly -->> Expansion (in growing cells) -->>Cross-linking and Seconary Wall Formation
After synthesis of the cell wall polymers (cellulose, hemicellulose, pectins) they must assemble in some order. The currently accepted hypothesis is that self-assembly occurs, that is, the molecules arrange themselves in the most stable configuration without using enzymes to structure themselves. For example, if you isolate cellulose all by itself, and then spin it, it will self-assemble into the product we know as rayon! However, there are some enzymes that work to put the wall together, and so a combination of self-assembly and enzymatic ordering is the most likely scenario.
Secondary cell walls are synthesized after the primary wall has been laid down. This secondary wall resides to the inside of the primary wall, that is, it is located between the plasmamembrane and the primary wall (see Figure 1.11 in your book). Secondary walls can be quite thick, and are common in tracheids, vessels, and fibers and sclereids. They add great strength to cell walls. This is important for cells that must withstand either pressure or tension, and prevents them from collapsing in response to those stresses. These secondary walls usually contain 3 layers, called S1, S2, and S3, with S1 being the outermost wall material, and S1 adjacent to the plasmamembrane.
Secondary cell walls differ in structure from the primary wall - they contain xylans, rather than xyloglucans, and a higher proportion of cellulose (less hemicellulose). The microfibrils are often laid down nearly parallel to each other within each S layer of the secondary wall, whereas they are much more loosely aligned in the primary wall. This divergence in orientation of the microfibrils in the secondary wall prevents cellular deformation in much the same way that plywood layers, which are laid down perpendicular to each other, prevent plywood from deforming in one dimension or the other.
Embedded among the microfibrils in the secondary cell wall is a compound called lignin. Lignin is a phenolic polymer (made up of many phenol molecules) in a complex, and highly irregular arrangement. Lignin is linked together by alcohol subunits (coumaryl, coniferyl alcohols). As it forms, it displaces water from the wall, and tightly binds around the cellulose microfibrils. This also adds a great deal of strength to the cell, makes it more resistant to pathogens, less digestible, and prevents further changes in cell size.
How Cells and Cell Walls Expand
For cells with only primary cell walls, and that
are growing, new cell wall material must be synthesized and incorporated
amidst the already existing cell wall material. How this happens
is only now being understood, and involves many complex reactions, including
mechanical ones (suitable for study by engineers), hormones, enzymes, and
water relations. We will concentrate at this point on just how new
material is laid down, and come back and discuss hormonal relations later.
The one main point to remember here is that cells will not expand without positive turgor pressure. Without turgor pressure, there is no force to cause the cell wall to expand, and the cell will not grow in size. This is why plants are stunted when subjected to drought.
The orientation of the primary cell wall microfibrils determines how a cell will expand. If the microfibrils are randomly oriented, then expansion occurs equally over the entire surface of the cell, and the cell will be spherical in shape.
If the microfibrils are mostly parallel to one dimension (say length), then the cell will expand at a right angles to the length (see Figure 15.17 from Taiz and Zeiger, 1998).
Most cells increase in size right after cell division by factors ranging from 10-100X in volume before reaching maturity. Some xylem vessel elements may expand by 10,000X their initial size!! These are very large changes in size, and require a lot of new cell wall material to be laid down. For cell walls to expand, they must first be loosened, then the microfibrils slide apart under pressure from inside the cell (turgor pressure), and then new cell wall material is laid down in the new open spaces.\\
We'll come back to this process when we begin discussin
hormones, particularly auxins, since auxins are integral in cell wall expansion.
Active Processes Occurring in the Cell Wall
As mentioned earlier, a lot of physiology goes on
in the cell wall. Walls are the first line of defense against pathogens,
particularly bacteria and fungi. In addition, by degrading the middle
lamella, cells become unglued from each other, and plants can shed certain
parts if necessary (abscissing leaves in the fall, for example).
Certain enzymes may break down the hemicelluloses,
causing the cell wall to become softer, as in ripening fruit. Enzymes
like pectin methyl esterase hydrolyzes methyl esters from pectin.
This makes them susceptible to attack by pectinases, which degrade pectins,
and cause the cell walls to become softer, and for the cells to become
unglued from each other. If you take unglued cells (like fruit juices)
and add back pectins that have been isolated from plants, you can stick
them together in a gel, and this is how you make jelly and jam.
When plants are attacked by pathogens they often
elicit an "oxidative burst", which produces hydrogen peroxide, superoxide
radicals, and so on. These reactive oxygen species may attack the
membranes of the attacking organism, killing it. These oxygen radicals
may also cause cross-links to form in the plant cell wall, effectively
barring the attacking organism from penetrating to the cell membrane.
Compounds called oligosaccharins may stimulate the cell to produce defensive compounds, such as ethylene, and enzymes like chitnase and glucanase, which can break down fungal cell walls. These oligasaccharins are produced by the fungus as it breaks down cell walls, and these fragments stimulate the defensive reactions detailed herein.
See the handout from Science that I gave to you for
more information on how active and dynamic plant cell walls are!
The Cuticle