Evidence for the acid-growth theory of fusicoccin action.

Three predictions of the acid-growth theory of fusicoccin (FC) action in inducing cell elongation were reinvestigated using abraded segments of maize (Zea mays L.) coleoptiles. i) Quantitative comparison of segment elongation and medium-acidification kinetics measured in the same sample of tissue shows that these FC-induced processes are strictly correlated in time and respond coordinately to cations present in the medium. ii) Fusicoccin (1 μmol l(-1)) induces a rapid acidification of the cell-wall solution, reaching a final level of pH 3.8-4.0. Exogenous protons are able to substitute quantitatively for FC in causing segment elongation at pH 3.8-4.0. At pH 4, FC has no additional effect on cell elongation. iii) Neutral buffers (pH 7) completely abolish the FC-mediated growth response. iv) Cycloheximide (10 mg l(-1)) inhibits both FC-induced and acid-buffer(pH 4)-induced elongation after a lag of 40-45 min, and FC-induced H(+) excretion after a lag of 2 h. Under the same conditions, indole-3-acetic acid-induced elongation and H(+) excretion are inhibited without detectable lag. It is concluded that these results are fully compatible with the acid-growth theory of FC action.