The Capacity for Acid-Induced Wall Loosening as a Factor in the Control ofAvenaColeoptile Cell Elongation

Abstract

The rate of acid-induced wall extension depends not only on the pH of the wall solution, but on the capacity of the walls to undergo acid-induced wall loosening (CAWL). CAWL has been evaluated by incubating sections in vivo under various conditions, freezing them, and then measuring the rate of acid-induced extension when the thawed sections are subjected to a constant stress and a constant acidic pH. CAWL varies in response to auxin and fusicoccin (FC). While the control CAWL declines steadily, auxin causes the CAWL to increase slowly so that after 10 h auxin-treated sections undergo acid-induced wall loosening at twice the rate of the controls, given the same pH and stress. This increase in CAWL is unlikely to be a response to the elongation because no such increase occurs in the presence of FC. With FC, both the CAWL and the growth rate decrease sharply after about 2 h, suggesting that in this case the changes in CAWL might be the primary factor controlling the growth rate. The CAWL may be a function of the proteins present in the wall rather than the polysaccharides, since the protein synthesis inhibitor cycloheximide causes the CAWL to decrease while sucrose has no effect on the CAWL.