|
ScienceWeek
PLANT BIOLOGY: A SYSTEMS APPROACH TO PLANT CELL WALLS
The following points are made by C. Somerville et al (Science 2004 306:2206):
1) Plant cell walls are complex and dynamic structures composed mostly of polysaccharides with high molecular weights [1 4], highly glycosylated proteins, and lignin. As a measure of the complexity, the Arabidopsis genome contains more than 730 genes encoding putative glycosyltransferases or glycosyl hydrolases [5] and several hundred additional genes encoding other types of proteins implicated in wall biosynthesis or function. Although their general catalytic activity can often be inferred from sequence, the precise enzymatic function and biological role of most of these proteins are unknown [2]. For example, genetic analysis has identified the specific biological role for only two of the more than 170 gene products with similarity to pectin-degrading enzymes.
2) Faced with the prospect of analyzing the function of 1000 or more genes that may contribute to the synthesis and remodeling of cell walls, the authors explored the idea that a systems approach may provide a useful framework for defining the hierarchy of essential questions. The concept of systems biology has recently emerged as a way of envisioning how multifactorial biological processes operate as a whole. The concept is usually applied to understanding networks of genes or gene products but is more broadly applicable. Kitano (2002) defines four key elements in a system: the design principles, system structure, the control method, and the system dynamics. The authors attempted to evaluate the current state of knowledge about the polysaccharide components of dicotyledonous plant cell walls in the context of these elements.
3) The body plan of a higher plant is essentially like a building made of "osmotic bricks". Each cell is osmotically pressurized to between 0.1 and 3.0 MPa (1 MPa is approximately 145 pounds per square inch). The pressure rigidifies the cells by creating tension in the cell walls. Each cell is glued to adjacent cells by pectic polysaccharides that normally prevent sliding of the cells under large strains. However, cell walls are also capable of controlled modifications that allow cells to expand in a polarized fashion during growth. Because each cell wall is attached to adjoining cell walls, coordinated expansion is necessary. It has been proposed that the role of the brassinosteroid hormones is to coordinate cell expansion.
4) Plant cell division involves the biogenesis and integration of new walls at the plane of division. In this process, two opposing walls form within the mother cell, and then the new walls integrate with the existing wall, and the plasma membrane repositions to form the daughter cells. Certain cell types, such as the fiber cells in wood, are subject to mechanical stress and undergo additional cell wall synthesis after the cells have finished dividing and are fully expanded. This "secondary cell wall" is deposited interior to the "primary cell wall." Thus, the fundamental design principles include strength, expandability, and modularity.
5) Cell walls also provide a barrier to infection by pathogens. Exogenous application of cell wall fragments to uninfected plants triggers defensive reactions, indicating the existence of glycan-activated signal transduction chains. It has been proposed that some of the structural complexity in plant cell wall composition reflects the presence of latent signal molecules, which trigger defensive responses when they are released during the cell wall degradation that accompanies pathogenesis. Several lines of evidence have also implicated cell wall polysaccharide fragments and proteoglycans in developmental processes. For example, deglycosylation inactivated a proteoglycan named xylogen that mediates intercellular interactions required for xylem differentiation in cultured Zinnia cells. Thus, the design principles of cell walls cannot be understood solely in the context of mechanical properties.
6) In summary: One of the defining features of plants is a body plan based on the physical properties of cell walls. Structural analyses of the polysaccharide components, combined with high-resolution imaging, have provided the basis for much of the current understanding of cell walls. The application of genetic methods has begun to provide new insights into how walls are made, how they are controlled, and how they function. However, progress in integrating biophysical, developmental, and genetic information into a useful model will require a system-based approach.
References (abridged):
1. N. C. Carpita, D. M. Gibeaut, Plant J. 3, 1 (1993)
2. S. C. Fry, New Phytol. 161, 641 (2004)
3. M. C. McCann et al., Phytochemistry 57, 811 (2001)
4. B. L. Ridley, M. A. O'Neill, D. A. Mohnen, Phytochemistry 57, 929 (2001)
5. B. Henrissat, P. M. Coutinho, G. J. Davies, Plant Mol. Biol. 47, 55 (2001)
Science http://www.sciencemag.org
--------------------------------
Related Material:
ON PLANT CELL WALLS
The following points are made by Herman Hoefte (Science 2001 294:795):
1) More than 300 years ago, Robert Hooke (1635-1703) pointed his primitive microscope at a slice of cork and discovered the cellular basis of organisms. Unfortunately, since then, plant cell walls, which formed the compartments he actually observed, have never been considered particularly entertaining structures. Indeed, the word "wall" itself evokes something dull and rigid, built only to enclose, support, divide, and protect. However, a closer look reveals just how erroneous this view is. Walls of growing plant cells are extremely sophisticated composite materials made of dynamic networks of polysaccharides, protein, and phenolic compounds. Cellulose microfibrils with a tensile strength comparable to that of steel provide the plant with a load-bearing framework. These microfibrils are rigid wires made of crystalline arrays of beta-1,4-linked chains of glucose residues, which are extruded from little hexameric spinnerets in the plant cell plasma membrane and which surround the growing cell like the hoops around a barrel.
2) Because cellulose microfibrils constrain turgor-driven cell expansion in one preferential direction, they control the shape of plant cells and ultimately the shapes of the plants themselves. Hemicelluloses, such as xyloglucans, are tethered by hydrogen bonds to cellulose and form cross-links that may control the separation of the cellulose microfibril hoops. The cellulose-hemicellulose network is embedded in a matrix of complex galacturonic acid-rich pectic polysaccharides that form a hydrated gel inside the wall, providing a dynamic operating environment for cell wall processes.
3) In all higher plants, one of the apparently essential polysaccharides in this hydrated gel is a mysterious polysaccharide known as rhamnogalacturonan II (RGII), which is believed to be the most complex polysaccharide on Earth. It is composed of 11 kinds of sugar monomers, and apparently at least 21 enzymes are dedicated to the construction of all the linkages between the sugar residues.
Science http://www.sciencemag.org
--------------------------------
Related Material:
PLANT CELL WALLS AND EXPANSINS
Notes by ScienceWeek:
Plant and bacterial cells are almost always surrounded by a rigid non-living "cell wall" that both protects and contains the cell. The presence of a cell wall allows the cell to withstand osmotic and mechanical stresses that would almost certainly rupture cells without such a wall (e.g., animal cells, which do not have cell walls).
The walls that surround plant cells resemble bacterial cell walls only in that they define shape and confer rigidity. Chemically, there is little similarity between plant cell walls and bacterial cell walls. Plant cell walls consist of rigid microfibrils of cellulose molecules embedded in a non-cellulose gel-like matrix in a manner that has been likened to the embedding of metal rods in concrete to reinforce the concrete and give it added strength.
Cellulose is an unbranched polymer of glucose, with beta-1,4 glycosidic bonds, and as the principal structural material of plants, cellulose is the most abundant organic compound on Earth. The fibrous matrix in which the cellulose microfibrils are embedded contains hemicellulose, pectin, lignin, and protein. Despite the name, hemicellulose bears no chemical resemblance to cellulose at all, but is in fact a pentose polymer. Pectin is a polymer of hexuronic acid, and lignin is a complex organic polymer and major component of wood.
The walls that surround growing plant cells are called "primary cell walls", and they are characterized by an extensibility that allows them to expand in response to cell growth under the influence of the plant hormone auxin. Once a plant cell has achieved its final size and shape, it may deposit a "secondary cell wall" on the inner surface of the primary wall. The secondary wall usually contains more cellulose than the primary wall and often has a high lignin content as well, rendering the wall inextensible and thus definitively specifying the ultimate size and shape of the cell.
The following points are made by Daniel J. Cosgrove (Nature 2000 407:321):
1) The author points out that for plants, the cell wall is an important structure that determines cell shape, glues cells together, provides essential mechanical strength and rigidity, and acts as a critical barrier against pathogens. Secreted by growing cells, the primary wall is a polymeric network of crystalline cellulose microfibrils embedded in a hydrophilic matrix of hemicellulose and pectins. Unlike the bacterial cell wall, which may form one giant covalently linked macromolecule, the polysaccharides of the growing plant cell wall are mostly separate long-chained polymers that form a cohesive network through non-covalent lateral associations and physical entanglements.
2) In plant cells, cellulose microfibrils are synthesized by large complexes in the plasma membrane. Newly secreted cellulose then associates with matrix glycans (hemicelluloses and pectins), which are synthesized in the *Golgi apparatus of the cell and delivered to the cell wall by *secretory vesicles. The cellulose microfibril is a thin ribbon, approximately 5 nanometers in diameter and many microns in length. It consists of an ordered array of many parallel chains of an unbranched glucose polymer (1,4-beta-glucan). Hemicelluloses are generally branched polysaccharides characterized by a strong tendency to bind to cellulose, whereas pectins are generally acidic polysaccharides with a strong tendency to form ionic gels. A small amount of structural protein is also found on the plant cell wall, but its function is unclear.
3) The growing plant cell wall possesses a remarkable combination of strength and pliancy, enabling it to withstand the large mechanical forces that arise from cell *turgor pressure, while at the same time permitting a controlled polymer "creep" that distends the wall and creates space for the enlarging protoplast. Cellulose microfibrils themselves are effectively inextensible, and wall expansion occurs by slippage or rearrangement of the matrix polymers that coat the microfibrils and hold them in place. Until recently, this was thought to occur primarily by hydrolysis of matrix polysaccharides.
4) It has recently become apparent that during growth, plant cells secrete a protein called "expansin", a protein that unlocks the network of cell wall polysaccharides, permitting turgor-driven cell enlargement. The action of expansin has puzzling implications for plant cell-wall structure; in addition, the recent explosion of gene sequences and expression data has provided new hints of further biological functions for expansins. This class of proteins appears to have evolved specifically in the land plant lineage, with biological functions in cell growth and in other situations where the movement, adhesion, and enzymatic accessibility of cell wall polysaccharides are important. Expansins offer potential applications for bioengineering of cell walls, either to manipulate the growth and texture of plants or to modify the structure and physical properties of cell walls used in commercial products such as wood, textiles, and polymers.
Nature http://www.nature.com/nature
--------------------------------
Notes by ScienceWeek:
Golgi apparatus: The Golgi apparatus (Golgi complex) is a collection of organelles (Golgi bodies) in eukaryotic cells that essentially function as a collecting and packaging center for substances that the cell manufactures for export.
secretory vesicles: The term "secretory vesicles" refers to intracellular spheroids with sizes ranging from 30 nanometers to 2 microns in diameter and bounded by a bimolecular layer membrane, various vesicles containing various chemical messenger substances.
Turgor pressure: (wall pressure) In this context, the term "turgor" refers to the rigidity of a plant and its cells and organs resulting from hydrostatic pressure exerted on the cell walls. The term "turgor pressure" is sometimes replaced by "pressure potential".
ScienceWeek http://scienceweek.com
|