ABSTRACT
Elucidation of the biochemical mechanism of cell wall polysaccharide biosynthesis has attracted considerable attention over the past 30 years (1 — 5 ), The earliest studies, focusing on the characterization of polysaccharide biosynthesis in isolated membrane preparations, raised an important and longstanding question. Namely, is callose synthase activity (1 ,3 - B-D-glucan synthase) measured in vitro cat alyzed by the same enzyme complex responsible for cellulose bio synthesis ( 1,4 -$-D-glucan) in vivo, i . e . , is it a component of the particle complexes demonstrated morphologically to be associated with microfibril deposition? I f so, what factors cause cellulose biosynthe sis to switch over to callose biosynthesis upon the disruption of in tact plant cells? A lternatively, i f cellulose and callose synthesis are mediated by separate enzymes, each specific for one linkage type, why does cellulose biosynthesis cease to occur in vitro? The fact that these questions have persisted for so many years represents a serious gap in our understanding of plant growth and development.
