Cell Wall Acidification and its Role in Auxin-Stimulated Growth

The role of cell wall acidification in auxin-stimulated growth was examined in abraded hypocotyl segments of etiolated Cucumis .sativus seedlings. Acidification of the medium by these segments was strongly inhibited by a pretreatment and the continued presence of 1·0 mol m−3 vanadate, widely used as an inhibitor of plasma membrane ATPase activity. Elongation of segments in pH 6·5 buffer was almost completely inhibited by such a treatment with vanadate, and the promotion of growth by indole-3-acetic acid (IAA) seen in the absence of vanadate was completely abolished. However, both inhibited and uninhibited segments showed a pronounced elongation in response to pH 4·0) buffer. In pH 4·0 buffer, in contrast to the results obtained at pH 6·5, IAA significantly promoted growth in both the presence and absence of vanadate. The results indicate that IAA can promote growth in the absence of endogenous acidification, but that an acid wall is necessary for wall loosening to occur.