Relaxation in a high-stress environment: the molecular bases of extensible cell walls and cell enlargement.

SUMMARY AND PERSPECTIVE The enlargement of plant cells involves the coordinate con- trol of wall synthesis and expansion, solute and water trans- port, membrane synthesis, Golgi secretion, ion transport, and many other processes. In this review, I have focused on the wall because it is the major control point for cell enlarge- ment. Some of the key processes that may be involved in wall enlargement are summarized in Figure 4. I offer the following speculative picture as a tentative working model for the control of wall expansion. The primary wall is initially secreted and assembled in a form that is me- chanically tough yet has “hot spots” where expansin can weaken microfibril-matrix adhesion. Expansin activity, which is modulated both by secretion of the protein to the wall and by changes in the pH and redox potential of the wall, in- duces the stress relaxation and polymer creep needed for wall enlargement and water uptake by the cell. By altering Synthesis & secretion secretion of wall polysaccharides and proteins