SYNERGISTIC ACTIVATION OF PLASMA-MEMBRANE H-ATPASE IN ARABIDOPSIS-THALIANA CELLS BY TURGOR DECREASE AND BY FUSICOCCIN()

The regulation of the H+-ATPase of plasma membrane is a crucial point in the integration of transport processes at this membrane. In this wo rk the regulation of H+-ATPase activity induced by changes in turgor p ressure was investigated and compared with the stimulating effect of f usicoccin (FC). The exposure of cultured cells of Arabidopsis thaliana L. (ecotype Landsberg 310-14-2) to media containing mannitol (0.15 or 0.3 M) or polyethylene glycol 6000 (PEG) (15.6% or 22% w/v) resulted in a decrease in the turgor pressure of the cells and in a strong stim ulation of H+ extrusion in the incubation medium. The osmotica-induced H+ extrusion was (1) inhibited by the inhibitor of plasma membrane H-ATPase, erythrosin B (EB), (2) dependent on the external K+ concentra tion, (3) associated with a net K+ influx, and (4) lead to an increase of cellular malate content. These results show that the reduction of external osmotic potential stimulates the activity of plasma membrane H+-ATPase. The effect of mannitol was only partially inhibited by trea tments with cycloheximide (CH) and cordycepin, which block protein and mRNA synthesis, respectively. All the effects of osmotica were qualit atively and quantitatively similar to those induced by 5 muM FC. Howev er, when FC and mannitol (or PEG) were fed together, their effects on H+ extrusion appeared synergistic, irrespective of whether FC was pres ent at suboptimal or optimal concentrations. This behaviour suggests t hat the modes of action of FC and of the osmotica on H+-ATPase activit y differ at least in some step(s).