The Primary Cell-Wall

The biological role of the cell wall was discussed in the previous Section, and it is therefore obvious that a knowledge of its structure, biosynthesis, and degradation is of profound interest and importance. 

The biochemistry of plant cell-walls is still at the stage of identifying and elucidating the covalent structures of the macromolecular components of the primary cell-wall. The secondary, tertiary, and quarternary structures of the polysaccharides therein have received only scant attention.29-32 The ultrastructural distribution of polymers within the wall, the integration of newly synthesized macromolecules into the wall, and 

Lamport40 considered the primary cell-wall to be a single, bagshaped macromolecule having a coherent, cross-linked structure, with bonds both between the hydroxy-L-proline-rich, wall protein extensin and wall polysaccharides, and between individual polysaccharides. 

These pioneering studies have been further developed by other lead- 

Significant technical problems face those who wish to study the structure of primary cell-walls. An important consideration is the purity of the cell-wall preparations. Often, cell-wall studies have been carried out on walls obtained from heterogeneous, differentiated tissues containing 

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Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

The primary cell walls of most higher plant species contain XGs of the XXXG type, which bear trisaccharide side chains (8) on the backbone [247]. The seeds of many plants contain XXXG-type XGs, in which about 30% of the xylose units possess a /3-D-Galp residue attached to position 2. Several plant species produce XGs that lack fucose and galactose, and have a-L-Ara/ attached to 0-2 of some of the Xylp side-chains, such as XG isolated from olive fruit [262] and soybean (Glycine maxima) meal [263]. However, a-L-Ara/ residues occur also 2-linked directly to some of the Glcp residues of the backbone [154]. 

Rhamnogalacturonan 11 (RG-11) is a structurally complex, pectic polysaccharide that is present in the primary cell-walls of higher plants. It is composed of 60 glycosyl residues, and is a very complex molecule indeed. For example, on acid hydrolysis, at least ten different monosaccharides are formed, including the novel aceric acid (30), which is the only branched-... 

An Hypothesis The Same Six Polysaccharides are Components of the Primary Cell Walls of All Higher Plants... 

We hypothesize that the fundamental processes of cell wall expansion are conserved in all higher plants, that is, growth of the cells of all higher plants requires the synthesis and insertion of the same polysaccharides by the same procedures. If this hypothesis is correct, then all primary cell walls have a common set of stmctural polysaccharides. The commonality of the primary cell wall polysaccharides hypothesis does not require that (i) the common polysaccharides be present in all cell walls in the same proportions, (ii) the polysaccharides be... 

Other polysaccharides of primary cell walls.-A complex mixture of enzymes including endopolygalacturonase, pectin methylesterase, and/or pectin lyase solubilizes a mixture of polysaccharides from the primary cell walls of fruits [57-64]. Food scientists have referred for some 15 years to this mixture of polysaccharides as the hairy region to describe the highly branched character of the polysaccharides in the fraction and to emphasize the contrast to unbranched homogalacturonan. The recent discovery of rhamnogalacturonan hydrolase [65,66], which selectively cleaves the backbone of RG-I, led to the realization that the hairy... 

By combining our images of these walls and their constituent polymers during chemical extraction, with data from immunogold labelling of thin-sectioned material, we were able to construct a simple structural model of the primary cell wall of onion (Fig 3). 

McCann, M.C. and Roberts, K. (1991) Architecture of the primary cell wall. In The Cytoskeletal Basis of Plant Growth and Form, edited by C.W. Lloyd, pp. 109-129. Academic Press, London. 

METHYL-ESTERIFICATION, DE-ESTERIFICATION AND GELATION OF PECTINS IN THE PRIMARY CELL WALL. 

McNeil,M, Darvill, A.G., Fry, S.C. and Albersheim, P. 1984 Structure and function of the primary cell walls of plants. Annu. Rev. Biochem. 53 625-663... 

Cosgrove DJ. Assembly and enlargement of the primary cell wall in plants. Ann Rev Cell Dev Biol 1997a 13 171-201. 

Fry SC, Miller JG. Toward a working model of the growing plant cell wall. Phenolic cross-linking reactions in the primary cell walls of dicotyledons. American Chemical Society, Washington, DC, 1989. 

These protruding structures may form catalytic sites or binding sites for small molecules. Remarkably, some polysaccharide modification enzymes of pathogenic bacteria are similar to enzymes of their host cells. For example, during cell development, pectin, a principal component in the primary cell wall of plants, is modified by its own pectin methylesterases that have /1-solenoid structures (Johansson et al, 2002) and in this respect, resemble pectin lyases secreted by bacteria to break down these structures (Lietzke et al., 1996). 

Refining is the most important of all the processes to which fibres are subjected, in terms of developing pulp suspension characteristics and final sheet properties, and a great deal of research has been carried out into understanding the process more fundamentally. Whilst there is still much controversy about certain aspects of the refining process and its effects upon the fibres, a number of things are widely accepted. Firstly the primary cell wall, which does not... 

Phenolic Cross-Linking Reactions in the Primary Cell Walls... 

In order to think about the nature and consequences of cell wall polymer phenolic cross-linking, we need a working model of the mode of assembly and the final structure of the primary cell wall. Unfortunately, there is no universally acceptable model that proposed by Albersheim and co-workers (3) is not now widely accepted because the postulated interpolysaccharide glycosidic bonds have not been demonstrated (4) and the warp-weft model of Lamport (5) rests on the assumptions that extensin (i) forms a defined-porosity network (not proven) (ii) is orientated anti-clinally to the cell surface [some evidence against (6)] and (iii) is a major component of all primary cell walls (not true). 

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