Complexity of potassium acquisition: how much flows through channels?

The involvement of potassium (K(+))-selective, Shaker-type channels, particularly AKT1, in primary K(+) acquisition in roots of higher plants has long been of interest, particularly in the context of low-affinity K(+) uptake, at high K(+) concentrations, as well as uptake from low-K(+) media under ammonium (NH₄(+)) stress. We recently demonstrated that K(+) channels cannot mediate K(+) acquisition in roots of intact barley (Hordeum vulgare L.) seedlings at low (22.5 µM) external K(+) concentrations ([K(+)](ext)) and in the presence of high (10 mM) external NH₄(+), while the model species Arabidopsis thaliana L. utilizes channels under comparable conditions. However, when external NH₄(+) was suddenly withdrawn, a thermodynamic shift to passive (channel-mediated) K(+) influx was observed in barley and both species demonstrated immediate and dramatic stimulations in K(+) influx, illustrating a hitherto unexplored magnitude and rapidity of K(+)-uptake capacity and plasticity. Here, we expand on our previous work by offering further characterization of channel-mediated K(+) fluxes in intact barley, with particular focus on anion effects, root respiration and pharmacological sensitivity and highlight key additions to the current model of K(+) acquisition.