Xyloglucan

A Metabolically Dynamic Polysaccharide

Abstract

Xyloglucan is a major structural polysaccharide of the primary cell walls of higher plants. It is an extended, relatively rigid polysaccharide, with a β-(1→4)-D-glucan backbone approximately 150-1, 500nm in total contour length. Although pure xyloglucan is water-soluble, this polysaccharide may contribute to wall architecture e.g. by hydrogen-bonding to, and thus tethering, pairs of adjacent cellulosic microfibrils. Treatments, e.g. with auxin or H⁺, that promote plant cell expansion often result in in vivo xyloglucan depolymerisation. This may be partly caused by an induction of cellulase, which hydrolyses xyloglucan. In addition, however, a new xyloglucan-cleaving enzyme activity has recently been discovered—xyloglucan endotransglycosylase (XET)—which cuts a xyloglucan chain and then transfers the newly-formed (potentially reducing) terminus on to the non-reducing terminus of a neighbouring xyloglucan chain. By the action of this enzyme, intermicrofibrillar “tethers” could be broken, allowing incremental cell expansion, and then re-formed, restoring the original strength of the cell wall. The possible role of XET in wall assembly is discussed.