Arabidopsis HAK5 under low K+ availability operates as PMF powered high-affinity K+ transporter
Tobias Maierhofer, Sönke Scherzer, Armando Carpaneto, T. Müller, José M. Pardo, Inga Hänelt, Dietmar Geiger, Rainer Hedrich
Abstract
Abstract Plants can survive in soils of low micromolar potassium (K + ) concentrations. Root K + intake is accomplished by the K + channel AKT1 and KUP/HAK/KT type high-affinity K + transporters. Arabidopsis HAK5 mutants impaired in low K + acquisition have been identified already more than two decades ago, the molecular mechanism, however, is still a matter of debate also because of lack of direct measurements of HAK5-mediated K + currents. When we expressed AtHAK5 in Xenopus oocytes together with CBL1/CIPK23, no inward currents were elicited in sufficient K + media. Under low K + and inward-directed proton motive force (PMF), the inward K + current increased indicating that HAK5 energetically couples the uphill transport of K + to the downhill flux of H + . At extracellular K + concentrations above 25 μM, the initial rise in current was followed by a concentration-graded inactivation. When we replaced Tyr450 in AtHAK5 to Ala the K + affinity strongly decreased, indicating that AtHAK5 position Y450 holds a key for K + sensing and transport. When the soil K + concentration drops toward the range that thermodynamically cannot be covered by AKT1, the AtHAK5 K + /H + symporter progressively takes over K + nutrition. Therefore, optimizing K + use efficiency of crops, HAK5 could be key for low K + tolerant agriculture.