File Name: structure and function of plant cell wall .zip
- The Structure and Function of a Cell Wall
- Plant Cell Wall Polymers: Function, Structure and Biological Activity of Their Derivatives
- Heterogeneity in the chemistry, structure and function of plant cell walls
Secondary cell walls SCWs are produced by specialized plant cell types, and are particularly important in those cells providing mechanical support or involved in water transport. As the main constituent of plant biomass, secondary cell walls are central to attempts to generate second-generation biofuels. Partly as a consequence of this renewed economic importance, excellent progress has been made in understanding how cell wall components are synthesized. SCWs are largely composed of three main polymers: cellulose, hemicellulose, and lignin. In this review, we will attempt to highlight the most recent progress in understanding the biosynthetic pathways for secondary cell wall components, how these pathways are regulated, and how this knowledge may be exploited to improve cell wall properties that facilitate breakdown without compromising plant growth and productivity.
The Structure and Function of a Cell Wall
Since protoplasts are invariably round, this is good evidence that the wall ultimately determines the shape of plant cells. However, remember that the wall is very porous and allows the free passage of small molecules, including proteins up to 60, MW. This burst produces hydrogen peroxide, superoxide and other active oxygen species that attack the pathogen directly or cause increased cross-links in the wall making the wall harder to penetrate.
Let's look at how this system works. Consider a pathogenic fungus like Phytophthora. In contact with the host plant the fungus releases enzymes such as pectinase that break down plant wall components into oligosaccharins. The oligosaccharins stimulate the oxidative burst and phytoalexin synthesis, both which will deter the advance of the fungus. In addition, the oligosaccharins stimulate chitinase and glucanase production in the plant.
These are released and begin to digest the fungal wall. Fragments of fungal wall also act as oligosaccharins in the plant to further induce phytoalexin synthesis. 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:. Made of as many as 25, 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 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". Characterized by being soluble in strong alkali. Two common types include xyloglucans and glucuronarabinoxylans.
Other less common ones include glucomannans, galactoglucomannans, and galactomannans. 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. They are the easiest constituents to remove from the wall. They form gels i.
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. 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.
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. Lignin is primarily a strengthening agent in the wall. This is responsible for some of the wall properties. For example, hydrated walls have greater flexibility and extensibility than non-hydrated walls.
Morphology of the Cell Wall - there are three major regions of the wall: Middle lamella - outermost layer, glue that binds adjacent cells, composed primarily of pectic polysaccharides. Primary wall - wall deposited by cells before and during active growth.
The primary wall of cultured sycamore cells is comprised of pectic polysaccharides ca. The actual content of the wall components varies with species and age.
All plant cells have a middle lamella and primary wall. Secondary Wall - some cells deposit additional layers inside the primary wall. This occurs after growth stops or when the cells begins to differentiate specialize. The secondary wall is mainly for support and is comprised primarily of cellulose and lignin. Often can distinguish distinct layers, S1, S2 and S3 - which differ in the orientation, or direction, of the cellulose microfibrils.
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? 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. The enzyme apparently has two catalytic sites that transfer two glucoses at a time i.
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:. How can the wall be strong it must withstand pressures of MPa! Good question, eh? The answer requires that the wall:. 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: producing wall polysaccharides in a form that makes tighter complexes with cellulose or other materials increasing the lignin in the wall would increase cross-links between polymers de-esterifying the pectic acids would increase calcium bridges; B.
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? Protons are the primary wall loosening factor Acid Growth Hypothesis. This idea was first proposed by David Rayle and R. Cleland in Some evidence:.
Plant Cell Wall Polymers: Function, Structure and Biological Activity of Their Derivatives
Since protoplasts are invariably round, this is good evidence that the wall ultimately determines the shape of plant cells. However, remember that the wall is very porous and allows the free passage of small molecules, including proteins up to 60, MW. This burst produces hydrogen peroxide, superoxide and other active oxygen species that attack the pathogen directly or cause increased cross-links in the wall making the wall harder to penetrate. Let's look at how this system works. Consider a pathogenic fungus like Phytophthora. In contact with the host plant the fungus releases enzymes such as pectinase that break down plant wall components into oligosaccharins.
The review concerns the newer aspects of plant cell wall construction and modification, including the structure and biosynthesis of basic components during the cell growth and differentiation, as well as their breakdown. The special interest is given to the enzymes incorporated into the cell wall and their specific activity in the biosynthesis and degradation processes, but also in the transfer of glycosyl fragments blocks , which is connected with its thickening, softening, constructing the channels a. New aspects of lignification and specialisation of particular wall fragments, playing various functions, such as fruit ripening, dropping down leaves, fruits and flowers, breaking the dormancy, and others, are also presented. This is a preview of subscription content, access via your institution. Rent this article via DeepDyve.
Heterogeneity in the chemistry, structure and function of plant cell walls
Plant cell walls represent the most abundant renewable resource on this planet. They are rich in mixed complex and simple biopolymers, which has opened the door to the development of wide applications in different technologic fields. In this regard the polymerization processes that allow the synthesis of the cell wall and their components in living models are relevant, as well as the properties of the polymers and their derivatives. Therefore this chapter outlines the basis of polymerization with a biological approach in the plant cell wall, highlighting the biological effects of plant cell wall derivatives and their current applications. Plant cell wall is a dynamic network highly organized which changes throughout the life of the cell.