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

The rate of acid-induced wall extension depends not only onthe pH of the wall solution, but on the capacity of the wallsto undergo acid-induced wall loosening (CAWL). CAWL has beenevaluated by incubating sections in vivo under various conditions,freezing them, and then measuring the rate of acid-induced extensionwhen the thawed sections are subjected to a constant stressand a constant acidic pH. CAWL varies in response to auxin andfusicoccin (FC). While the control CAWL declines steadily, auxincauses the CAWL to increase slowly so that after 10 h auxin-treatedsections undergo acid-induced wall loosening at twice the rateof the controls, given the same pH and stress. This increasein CAWL is unlikely to be a response to the elongation becauseno such increase occurs in the presence of FC. With FC, boththe CAWL and the growth rate decrease sharply after about 2h, suggesting that in this case the changes in CAWL might bethe primary factor controlling the growth rate. The CAWL maybe a function of the proteins present in the wall rather thanthe polysaccharides, since the protein synthesis inhibitor cycloheximidecauses the CAWL to decrease while sucrose has no effect on theCAWL. Key words: Avena sativa, Acid-growth, Auxin, Coleoptile, Wall loosening