Some characteristics of anion transport at the tonoplast of oat roots, determined from the effects of anions on pyrophosphate-dependent proton transport

The effects of anions on inorganicpyrophosphate-dependent H+-transport in isolated tonoplast vesicles from oat (Avena sativa L.) roots were determined. Both fluorescent and radioactive probes were used to measure formation of pH gradients and membrane potential in the vesicles. Pyrophosphate hydrolysis by the H+-translocating pyrophosphatase was unaffected by anions. Nonetheless, some anions (Cl-, Br- and NO3-) stimulated H+-transport while others (malate, SO42- and iminodiacetate) did not. These differential effects were abolished when the membrane potential was clamped at zero mV using potassium and valinomycin. Stimulation of H+-transport by Cl- showed saturation kinetics whereas that by NO3- consisted of both a saturable component and a linear phase. For Cl- and NO3-, the saturable phase had a Km of about 2 mol·m-3. The anions that stimulated H+-transport also dissipated the membrane potential (Δψ) generated by the pyrophosphatase. It is suggested that the stimulatory anions cross the tonoplast in response to the positive Δψ generated by the pyrophosphatase, causing dissipation of Δψ and stimulation of ΔpH, as expected by the chemiosmotic hypothesis. The work is discussed in relation to recent studies of the effects of anions on ATP-dependent H+-transport at the tonoplast, and its relevance to anion accumulation in the vacuole in vivo is considered.