Capacity and Plasticity of Potassium Channels and High-Affinity Transporters in Roots of Barley and Arabidopsis1[C][W]

The role of potassium (K+) transporters in high-and low-affinity K+ uptake was examined in roots of intact barley (Hordeum vulgare) and Arabidopsis (Arabidopsis thaliana) plants by use of K-42 radiotracing, electrophysiology, pharmacology, and mutant analysis. Comparisons were made between results from barley and five genotypes of Arabidopsis, including single and double knockout mutants for the high-affinity transporter, AtHAK5, and the Shaker-type channel, AtAKT1. In Arabidopsis, steady-state K+ influx at low external K+ concentration ([K+](ext) = 22.5 mu M) was predominantly mediated by AtAKT1 when high-affinity transport was inhibited by ammonium, whereas in barley, by contrast, K+ channels could not operate below 100 mu M. Withdrawal of ammonium resulted in an immediate and dramatic stimulation of K+ influx in barley, indicating a shift from active to passive K+ uptake at low [K+](ext) and yielding fluxes as high as 36 mu mol g (root fresh weight)(-1)h(-1) at 5 mM [K+](ext), among the highest transporter-mediated K+ fluxes hitherto reported. This ammonium-withdrawal effect was also established in all Arabidopsis lines (the wild types, atakt1, athak5, and athak5 atakt1) at low [K+](ext), revealing the concerted involvement of several transport systems. The ammoniumwithdrawal effect coincided with a suppression of K+ efflux and a significant hyperpolarization of the plasma membrane in all genotypes except athak5 atakt1, could be sustained over 24 h, and resulted in increased tissue K+ accumulation. We discuss key differences and similarities in K+ acquisition between two important model systems and reveal novel aspects of K+ transport in planta.