The structure and functions of xyloglucan
1989
Fry, S.C.
Xyloglucan is a polysaccharide found in the primary cell walls of all higher plants examined. Its cellulose-like backbone, which is about 0.15 to 1.5 micrometer long, consists of 300 to 3000 beta-(1 linked to 4)-linked D-glucopyranose residues. About 60-75% (or, in grasses, about 30-40%) of the glucose residues have side-chains attached to position 6. The major side-chains are: D-xylopyranosyl-alpha-1 linkage, D-galactopyranosyl-beta-(1 linked to 2)-D-xylopyranosyl-alpha-1 linkage, L-arabinofuranosyl-(1 linked to 2)-D-xylopyranosyl-alpha-1 linkage, and (except in grasses) L-fucopyranosyl-alpha-(1 linked to 2)-D-galactopyranosyl-beta-(1 linked to 2)-D-xylopyranosyl-alpha-1 linkage. There is some regularity in the distribution of these side-chains along the backbone. Xyloglucan plays two very different roles in the control of cell growth: (a) as a major building material of the wall [concentration of xyloglucan in the wall in vivo equals approximately 10% (w/v)] it probably directly dictates wall extensibility and, therefore, the rate of cell expansion and (b) it can be broken down to a fucose-containing oligosaccharide which [at a concentration of approximately 0.0000001% (w/v)] exerts a hormone-like anti-auxin effect on growth. In addition, xyloglucan lacking fucose is used by certain dicotyledonous seeds as a food reserve which is mobilized after germination. Xyloglucan is, therefore, the subject of considerable current interest in several apparently disparate areas of botany.
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