BIOSYNTHESIS OF PLANT-CELL WALL POLYSACCHARIDES

The cell wall is the principal structural element of plant form. Cellu lose, long crystals of several dozen glucan chains, forms the microfib rillar foundation of plant cell walls and is synthesized at the plasma membrane. Except for callose, all other noncellulosic components are secreted to the cell surface and form a porous matrix assembled around the cellulose microfibrils. These diverse noncellulosic polysaccharid es and proteins are made in the endomembrane system. Many questions ab out the biosynthesis and modification within the Golgi apparatus and i ntegration of cell components at the cell surface remain unanswered. T he lability of synthetic complexes upon isolation is one reason for sl ow progress. However, with new methods of membrane isolation and analy sis of products in vitro, recent advances have been made in purifying active synthases from plasma membrane and Golgi apparatus. Likely synt hase polypeptides have been identified by affinity-labeling techniques , but we are just beginning to understand the unique features of the c oordinated assembly of complex polysaccharides. Nevertheless, such pro gress renews hope that the first gene of a synthase for a wall polysac charide from higher plants is within our grasp.