Abstract:

Current models of potassium acquisition and cytochemical processes in plants assume that potassium concentrations in the cytosol ([K+](cyt)) are maintained homeostatically at approximately 100 mM. Here, we use K-42 radiotracer data in the model plant species Hordeum vulgare L. (barley) to show that this assumption is incorrect. Our study reveals that [K+](cyt) in root cells of intact barley seedlings is held at a minimum of two physiological set points, coinciding with two fundamentally distinct modes of K+ transport, each of which is characterized by a unique network of fluxes to and from the cytosol, and reflects variations in mechanisms and energetics of K+ transport, cytosolic K+ turnover, flux partitioning, and sensitivity to NH4+. Increased external potassium or ammonium concentrations caused a substantial drop in [K+](cyt), as well as a switch from a transport mode dominated by high-affinity, energy-dependent, influx to a mode dominated by channel-mediated fluxes in both directions across the plasma membrane. Our study provides the first subcellular demonstration of the flexibility, rather than strict homeostasis, of cellular K+ maintenance, and of the dynamic interaction between plant membrane fluxes of the two major nutrient cations K+ and NH4+.