Evidence for the acid-growth theory of fusicoccin action

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Abstract
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.