SUITABILITY OF ARABIDOPSIS FOR STUDIES ON INTRACELLULAR PH REGULATION - CORRELATION BETWEEN H+ PUMP ACTIVITY, CYTOSOLIC PH AND MALATE LEVEL IN WILD-TYPE AND IN A MUTANT PARTIALLY INSENSITIVE TO FUSICOCCIN

Data are presented on the suitability of Arabidopsis thaliana seedlings for studies on intracellular pH regulation. In this material, grown in the dark in liquid medium, the determination of weak acid distribution at equilibrium provides an adequate method for calculating cytosolic pH values, in spite of the failure of benzylamine as a vacuolar pH probe. The stimulation of the H+ pump by K+ or K+ and fusicoccin (FC) is associated with a marked alkalinization of both cytosol and cell sap, and with a strong increase in malate level, whereas its inhibition by erythrosin B (EB) leads to the opposite effects. A good quantitative correlation is evident between the changes in net H+ extrusion and those in intracellular pH and malate content, in particular, with FC+K+. Cell sap buffer capacity is strongly influenced by the different treatments, its changes being substantially accounted for by changes in malate level. A comparison between the values of intracellular pH and malate level in wt and in the 5-2 mutant shows that in the mutant the cytosolic pH is always more acidic, and the intracellular alkalinization induced by FC+K+ and also by K+ alone is significatively lower. These results support the view that the partial insensitivity of 5-2 to FC is due to a reduced functionality of the H+-extruding system on which FC acts, and that the depression of the H+ pump activity in the mutant does not depend on a possible regulation by constitutively higher cytosolic pH values.