Tributylbenzylammonium (TBBA+)-dependent fusicoccin (FC)-induced H+ extrusion in maize roots: relationship between the stimulating effects of TBBA+ on H+ extrusion and on Cl- efflux

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Abstract
The mechanism of the stimulating effect of lipophilic cations on H+ extrusion in maize root segments (Zea mays L.) has been investigated. The measurement of the uptake of [3H]tributylbenzylammonium ([3H]TBBA+), the most active lipophilic cation on H+ extrusion, indicated that although a relevant fraction of TBBA+ taken up by the tissue is adsorbed to cell surfaces, a fraction of the cation enters the cells. However no correlation was observed between the rate of TBBA+ uptake and that of H+ extrusion. On the other hand, the lipophilic cations active on H+ extrusion (TBBA+ and dibenzyldimethylammonium (DDA+)), in the presence of fusicoccin (FC), induced under the same conditions an efflux of Cl-, while tetramethylammonium (TMA+), inactive on H+ extrusion, did not. The stimulation of Cl- efflux by TBBA+ was independent of the anion present in the medium and was inhibited by Na-orthovanadate, an inhibitor of plasma membrane ATPase and of TBBA+-induced H+ extrusion. These results suggest that the stimulation of H+ extrusion by TBBA+ depends on its effect on Cl- efflux rather than on its penetration into the cells.