Vanadate inhibition of auxin-enhanced H + secretion and elongation in pea epicotyls and oat coleoptiles

Abstract

The acid secretion theory of auxin action was investigated using Na orthovanadate, a selective inhibitor of a plasma membrane-associated H+-pumping ATPase in Neurospora. At 1 mM, vanadate inhibited auxin-enhanced medium acidification by pea epicotyl segments within 5 min, whether added 0.5 or 2.5 h after auxin. Inhibition of acidification was total after 10-15 min but was reversed within 10 min after vandate removal. When given as a 40 min pretreatment, vanadate completely prevented any auxin-enhanced acidification. Vanadate inhibition of medium acidification by oat coleoptile segments was total and reversible, but both inhibition and reversal occurred after longer lag times than in pea. Inhibitory effects of vanadate on elongation in pea and oat tissue closely paralleled its effects on acidification, and the inhibitory effect of vanadate on elongation was reversed by an acidic buffer. Vanadate did not inhibit respiration or protein synthesis in pea epicotyl segments, but it strongly inhibited L-[14C]leucine uptake. The importance of cell wall acidification for short- and long-term auxin-enhanced growth was indicated. The participation in wall acidification of a plasma membrane-associated ATPase acting as an H+ pump was suggested.