Biochemistry and Molecular Biology of Cellulose Biosynthesis in Plants: Prospects for Genetic Engineering

Abstract
Cellulose is a major component of the plant cell wall, and understanding the mechanism of synthesis of this polysaccharide is a major challenge for plant biologists. Cellulose microfibrils are synthesized and assembled by membrane-localized protein complexes that are visualized as rosettes by freeze-fracture electron microscopy. Cellulose synthase is required for cellulose synthesis. So far only this enzyme has been localized to these cellulosesynthesizing complexes. Although it has not been possible to purify and fully characterize cellulose synthase activity from plants, it has been possible to obtain cellulose synthesis in vitro using membranes and detergent-solubilized membrane fractions. Cellulose synthase uses uridine 5'-diphosphate (UDP)- glucose as a substrate and polymerizes glucose residues into long β-1,4-linked glucan chains in a single-step reaction. Cellulose synthases are encoded by genes belonging to a superfamily, and each plant synthesizes a number of different cellulose synthases. Genetic analysis suggests that each cellulosesynthesizing complex contains at least three nonredundant cellulose synthases and mutation in any one of these cellulose synthases results in cellulose deficiency. More interestingly, different cellulose synthases perform cellulose synthesis in the primary cell wall and the secondary cell wall.Apart from the cellulose synthases, a number of other proteins have been suggested to play a role in cellulose synthesis, but so far their functions are not clearly understood. Genetic manipulation of cellulose synthesis in plants will therefore require not only a complete understanding of the different cellulose synthases but also other proteins that regulate the temporal and spatial synthesis and assembly of this very important polysaccharide.

Key Words: Cellulose, Cellulose biosynthesis, Cellulose synthase, Cellulose synthase-like, CesA, Csl, Arabidopsis, Cotton, Acetobacter xylinum, Genetic manipulations.