Expansive growth of plant cell walls.

The enlargement of plant cell walls is a key determinant of plant morphogenesis. Current models of the cell wall are reviewed with respect to their ability to account for the mechanism of cell wall enlargement. The concept of primary and secondary wall loosening agents is presented, and the possible roles of expansins, xyloglucan endotransglycosylase, endo-1,4-beta-D-glucanase, and wall synthesis in the process of cell wall enlargement are reviewed and critically evaluated. Experimental results indicate that cell wall enlargement may be regulated at many levels.