Reexamination of the Acid Growth Theory of Auxin Action1

Some crucial arguments against the acid growth theory of auxin action (U Kutschera, P Schopfer [1985] Planta 163: 483-493) have been reinvestigated by simultaneous measurements of proton fluxes and growth of maize (Zea mays L.) coleoptiles. Special care was taken to obtain a mild, effective, and reproducible abrasion of the cuticle. Proton secretion rates were determined in a computer-controlled pH-stat. In some experiments, equilibrium pH was measured. Growth rates were determined simultaneously in the same vessel using a transducer-type auxanometer. It was found that (a) the timing of auxin and fusicoccin-induced (FC) proton secretion and growth matches well, (b) the equilibrum external pHs in the presence of IAA and FC are lower than previously recorded and below the so-called 'threshold-pH,' (c) neutral or alkaline unbuffered solutions partially inhibit FC and IAA-induced growth in a similar manner, (d) the action of pH, FC, and IAA on growth are not additive. It is concluded that the acidgrowth-theory correctly describes incidents taking place in the early phases of auxin-induced growth. The acid growth theory of auxin action (2, 9) states IAAinduced apoplastic acidification (3, 4, 19) causes an increase in extensibility of the cell wall (6). Extensibility limits hydraulic extension driven by turgor pressure. Four predictions for the validity of this theory and the function of an acidic pH in growth control have been critically inspected by Kutschera and Schopfer (1 1, 12) as follows: 1. Auxin must cause the cells to excrete protons quantitatively related to growth. The cell wall pH must decrease prior to the change in growth rate. Kutschera and Schopfer (11, 12), however, found no correlation in the kinetics between the elongation of segments and medium acidification. 2. Exogenous protons must be able to substitute for auxin causing cell wall loosening and growth, but Kutschera and Schopfer found the elongation elicited by external buffers pH 4.5 to 5.0 was significantly less than by IAA. 3. Neutral or alkaline solutions infiltrated into the cell wall must inhibit auxin-induced growth, but no influence of alkaline buffers on the auxin-induced component of growth was detected (1 1, 12). 4. The fungal toxin FC,2 known to be a potent stimulator of proton secretion, promotes growth similar to auxin (16, Funding from the Deutsche Forschungsgemeinschaft (grant No. BO-537/1) is gratefully acknowledged. 2Abbreviation: FC, fusicoccin; SEM, scanning electron micros-