K+ transport by Arabidopsis root hairs at low pH

Mechanisms underlying changed K+ uptake by plants at low pH need to be deciphered. One possibility is that K+ acquisition is under the strict control of plasma membrane potential (E-m), which, in turn, is affected by external pH. To test this hypothesis, we used the microelectrode ion flux measurement (MIFE(R)) technique to study net K+ and H+ fluxes near Arabidopsis root hairs at different external pH, KCl concentrations and clamped E-m. Lowering the solution pH led to strong H+ influx, K+ efflux and significant E-m depolarisation. Addition of K+ to the bathing media caused significant net K+ uptake when external pH was over the range 5.5-6.0. At external pH below 5.0, however, correlation between K+ availability and net K+ uptake was negative. To explain this apparent paradox, measurements of net K+ and H+ fluxes from the root hair surface were performed concurrently with E-m clamped at different values above and below the resting potential (approx. -180 mV). Our data revealed a strong dependence of net K+ flux on the clamping voltage. Clamping at values more negative than the resting potential caused a significant increase in K+ uptake into the root hair; clamping at less negative values (-20 and 0 mV) caused significant net K+ efflux from the cell. Qualitatively similar results were observed for net H+ flux. Our observations indicate direct control of K+ flux by changing E-m, and suggest that E-m depolarisation could be the main reason for the observed K+ efflux at low pH.