Xyloglucans from plant cell-walls

Xyloglucans are classified as gum when they are extractable with hot water from seed endosperm cell walls, such as the tamarind seed xyloglucan, and as hemicelluloses because they are alkali-extractable from the cell walls of vegetative plant tissues where they are closely associated with cellulose [2]. Also /3-glucans with mixed linkages appear under the name gum as well as hemicellulose in the literature. 

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]).

The plant cell wall biology studies require more sensitive and specific probes to target individual wall components. Traditionally, antibodies have been the primary workhorses for the spatial localization of cell wall polysaccharides. Currently, nearly 30 monoclonal antibodies directed toward specific arabinan, galactan, xylan, galacturonan, fucosylated xyloglucan, and cell wall glycoprotein epitopes are available, from academic and commercial sources [172], Nevertheless, CBMs may be used for this purpose, since they present intrinsic specificity for individual carbohydrates. 

Hemicelluloses have traditionally been defined by extraction procedures, i. e., hemicelluloses are those polysaccharides extracted by alkaline solutions from plant tissues after removal of low-molecular-weight substances with hot aqueous alcohol, removal of waxes and other lipid-soluble substances, delignification, and removal of pectin with an aqueous solution of a calcium ion chelator. The most abundant hemicelluloses in the primary cell walls of dicotyledons are xyloglucans. Xyloglucans have a backbone chain whose chemical stmcture is identical to that of cellulose. About 75% of the 8-D-glucopyranosyl units in that chain are substituted... 

From the results of cellulase digestion and the characterization of a number of oligosaccharides, an improved partial structure for the runner bean cell wall xyloglucan was proposed (O Neill and Selvendran, 1985b). It would appear that the xyloglucan does not contain a simple repeating unit but is composed of a block-type structure that is commonly encountered in plant polysaccharides (Stephen, 1983). 

The term hemicelluloses refers to substances which occupy the spaces between the cellulose fibrils within the cell walls of plants. Various studies, e. g., on apples, potatoes, and beans, show that xyloglucans dominate in the class Dicotyledoneae. A section of the structure of a xyloglucan from runner beans is presented in Formula 4.161. 

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