EFFECT OF IAA ON SYNTHESIS AND ACTIVITY OF THE PLASMA-MEMBRANE H+-ATPASE OF SUNFLOWER HYPOCOTYLS, IN RELATION TO IAA-INDUCED CELL ELONGATION AND H+ EXCRETION

The effect of indole-3-acetic acid (IAA) on the amount and the activity of the plasma membrane (PM) H+-ATPase (EC 3.6.1.35) of sunflower (Helianthus annuus L.) hypocotyls, in relation to IAA-induced cell elongation and H+ excretion, was investigated. IAA increased the elongation and the H+ excretion of hypocotyl segments about five times over the control within 2 h. Protein synthesis inhibitor (cycloheximide), RNA synthesis inhibitor (cordycepin), calcium-channel blocker (verapamil), and secretion inhibitors (monensin and brefeldin A) considerably reduced the IAA-induced elongation and H+ excretion. When these inhibitors were added 60 min after IAA treatment, in which a maximal IAA-induced elongation rate is maintained, they all inhibited elongation with a short lag time (less than 10 min). In protein slot blots, using an antibody against Arabidopsis thaliana PM H+-ATPase, IAA did not increase the amount of PM H+-ATPase in the extracted plasma membrane of sunflower hypocotyl segments. The above inhibitors, moreover, did not decrease the PM H+-ATPase amount up to 2 h after the treatments, which means that the enzyme is not subject to rapid turnover. IAA, when added to both the isolated membranes or the segments, had a little effect on the PM H+-ATPase activity. These results show that the IAA-induced elongation and H+ excretion may need some factors with rapid turnover that exist in the plasma membrane or are related to the secretory pathway, but PM H+-ATPase does not belong to these factors.