Cellulose xyloglucan network

Figure 3.5 A simplified model of the molecular architecture of a primary cell wall rich in pectic polysaccharides, such as a potato cell wall. Two co-extensive, but independent polysaccharide networks are shown a cellulose-xyloglucan network and a pectic-polysaccharide network. The middle lamella is located between the primary cell walls of adjacent cells and is responsible for cell-cell adhesion. Reprinted with permission from McCann and Roberts (1991).

M. Pauly, P. Albersheim, A. Darvill, W.S. York. A model for the cellulose/xyloglucan network in the cell walls of higher plants. Plant J, 2000, in press... 

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J.K.C. Rose, A.B. Bennett. Copperative dissasembly of the cellulose-xyloglucan network of plant cell walls parallels between cell expansion and fruit ripening. Trends Plant Sci, 1999, 4, 176-183... 

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Xyloglucans probably contribute to the cross-linking of each cellulose microfibril network in the walls of growing plant cells [3]. The binding of adjacent microfibrils probably gives cell wall its rigidity. The cross-linking between perpendicular fibrils may function as a bracket, and that between parallel fibrils as a beam. 

The interaction between cellulose microfibrils and the hemlceUulose-xyloglucan network is believed to represent the major load-bearing structure in the primary ceU waU. In physical terms, cell shape and size are governed by the mechanics of the ceU waU. CeU expansion occurs via strictly regulated reorientation of the waU components and several enzymes play a rule in this mechanism [121]. 

In the absence of suitable cell wall mutants, DCB-adapted tomato cells provide an opportunity to characterise the pectin network of the plant cell wall. It should be noted that synthesis and secretion of hemicellulose is not inhibited but, in the absence of a cellulose framework for it to stick to, most of the xyloglucan secreted remains in soluble form in the cells culture medium (9, 10) while other non-cellulosic polysaccharides and other uronic-acid-rich polymers predominate in the wall. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

In Section III, it was mentioned that cell wall is a complex structure formed by different polysaccharides connected to glycoproteins. Hydroxy-L-proline-rich glycoproteins, such as extensin, have been found in almost all plants surveyed, and in some algae.203,281 A network of protein, pectic polymers, and xyloglucan, serving to cross-link the cellulose fibers of the cell wall, has been proposed.282,283 However, covalent links between the different components have not been demonstrated moreover, some of them can be extracted separately,284 and some associations may be artificial.285 Nevertheless, results are consistent with interactions through dipole-dipole (such as hydrogen bonds) or hydrophobic bonds. 

The primary cell wall of dicotyledonous plants consists of cellulose microfibrils dispersed within a matrix of predominantly non-cellulosic polysaccharides, including xyloglucans and pectic polysaccharides. The xyloglucans are neutral polysaccharides which bind to the cellulose microfibrils through secondary interactions, and have the ability to crosslink the fibrillar cellulose network. This fibrillar network is then dispersed in a network of the pectic polysaccharides.1 The pectic polysaccharide network also forms the middle lamella in dicotyledons and is responsible for cell-cell adhesion. 

Figure 3. Segment of a plant cell wall (schematic drawing). Bundles of cellulose microfibers in near crystalline packing (thick bundles) are crosslinked by hemicelluloses (drawn as double-lined intercalated network) and pectins (fine lines with egg-box interaction sites). Carbohydrate-carbohydrate interaction occurs between individual cellulose chains, between cellulose and xyloglucan (a hemi-cellulose) and between different hemicelluloses and pectins. Besides different forms of non-covalent interactions (Ca + mediated egg-box structures as well as ion-independent forms of molecular interactions) also covalent ester linkages between carbohydrate chains are found (adapted from [23]).

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