Multiple endo-1,4-beta-D-glucanase (cellulase) genes in Arabidopsis

The plant cell wall is modified in coordination with almost all plant developmental processes. Modifications in the cell wall are thought to be mediated by cell wall hydrolases, including those encoded by a large family of genes specifying endo-1,4-beta-D-glucanases (EC 3.2.1.4), which participate in the breakdown of beta-1,4 glucosidic linkages. The enzymes expected to modify cellulose, commonly referred to as cellulases, are encoded by members of this gene family. In Arabidopsis the endo-1,4-beta-D-glucanase (EGase) gene family is extensive (more than 12 members) and encompasses structurally different classes of genes encoding proteins with contrasting enzyme functions. Within the family there are enzymes located at the plasma membrane that are presumed to act at the innermost layers of the cell wall, and enzymes that are secreted and are presumed to act at any stratum within the cell wall, including the outermost layer. Both structural gene groups are members of the glycosyl hydrolase gene Family 9. Evidence suggests that EGases anchored in the plasma membrane play a role in cell wall biosynthetic processes, presumably by editing cellulose synthesis or during the assembly of the cellulose-hemicellulose network. Those EGases that are extracellular play specific roles in cell wall catabolic processes and their activity ranges from partial and localized to massive and catastrophic. This range in activity is linked to processes such as cell growth and cell death, respectively. For all Arabidopsis EGases nothing is known about their true in vivo substrate, mode of action, or to what extent they can act on cellulose or other beta-1,4 glucans. The study of the EGase gene family is in its infancy, and because of the possible agronomic implications this group of genes deserves continued attention.