The Structure and Functions of Xyloglucan

Xyloglucan is a polysaccharide found in the primary cell walls of all higher plants examined. Its cellulose-like backbone, which is about 015 to 1-5 ?un long, consists of 300 to 3000 /3-(l ^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-a-1 ->, D-galactopyranosyl-^-(l ^2)-D-xylopyranosyl-a-l L-arabinofuranosyl-(l 2)-D-xylopyranosyl-a-1 and (except in grasses) L-fucopyranosyl-a-(l-»2)-D-galactopyranosyl-j3-(l-^-2)-D-xylopyranosyl-a-lThere 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 ~ 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 ~ 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.