Plant cell wall (Lecture 12)

Transcription

1 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.

2 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).

3 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 i. Hydroxyproline-rich glycoproteins (HRGPs) Extensin

4 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 network of matrix pectic polysaccharides.

5 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. Cell wall loosening can occur by at least 3 mechanisms:

6 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. Elaborate specialization of secondary walls

7 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.

8 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

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