K+ transport by Arabidopsis root hairs at low pH

Abstract

Mechanisms underlying changed K ⁺ uptake by plantsat low pH need to be deciphered. One possibility is that K⁺ acquisition is under the strict control of plasmamembrane potential (E _m ), which,in turn, is affected by external pH. To test this hypothesis, we used themicroelectrode ion flux measurement (MIFE ) technique tostudy net K ⁺ and H ⁺fluxes near Arabidopsis root hairs at different externalpH, KCl concentrations and clamped E_m . Lowering the solution pH led to strong H⁺ influx, K ⁺ efflux andsignificant E _m depolarisation.Addition of K ⁺ to the bathing media causedsignificant net K ⁺ uptake when external pH wasover the range 5.5–6.0. At external pH below 5.0, however, correlationbetween K ⁺ availability and net K⁺ uptake was negative. To explain this apparentparadox, measurements of net K ⁺ and H⁺ fluxes from the root hair surface were performedconcurrently with E m clamped at different values above and below the restingpotential (approx. –180 mV). Our data revealed a strong dependence ofnet K ⁺ flux on the clamping voltage. Clamping atvalues more negative than the resting potential caused a significant increasein K ⁺ uptake into the root hair; clamping at lessnegative values (–20 and 0 mV) caused significant net K⁺ efflux from the cell. Qualitatively similarresults were observed for net H ⁺ flux. Ourobservations indicate direct control of K ⁺ flux bychanging E _m , and suggest thatE _m depolarisation could be themain reason for the observed K ⁺ efflux at low pH.