Hemicelluloses are polysaccharides in plant cell walls that have ß-

(1¿4)-linked backbones with an equatorial configuration. Hemicelluloses

include xyloglucans, xylans, mannans and glucomannans, and

ß-(1¿3,1¿4)-glucans. These types of hemicelluloses are present in

the cell walls of all terrestrial plants, except for ß-(1¿3,1¿4)-glucans,

which are restricted to Poales and a few other groups. The detailed

structure of the hemicelluloses and their abundance vary widely between

different species and cell types. The most important biological

role of hemicelluloses is their contribution to strengthening the cell wall

by interaction with cellulose and, in some walls, with lignin. These features

are discussed in relation to widely accepted models of the primary

wall.

Hemicelluloses are synthesized by glycosyltransferases located in the

Golgi membranes. Many glycosyltransferases needed for biosynthesis

of xyloglucans and mannans are known. In contrast, the biosynthesis

of xylans and ß-(1¿3,1¿4)-glucans remains very elusive, and recent

studies have led to more questions than answers.