High-affinity potassium transport into wheat roots involves sodium - a role for HKT1?

Abstract

When a high-affinity K ⁺ transporter, HKT1, was cloned from the roots of K ⁺ -starved wheat and characterized, it seemed that the wheat high-affinity K ⁺ -uptake mechanism had been found. We review the present status of HKT1 as a putative component of the wheat root high-affinity mechanism, concluding that its role remains unclear, as it also does in barley. We describe a new attempt to find its role. High-affinity K ⁺ transport in K ⁺ -starved wheat seedling roots was studied by measuring K ⁺ -evoked depolarization. A single dominant transport system was shown to carry K ⁺ , Rb ⁺ and Cs ⁺ , with a binding site selectivity of about 1:1:0.15, respectively. We showed that the small effect of 1 mM Na ⁺ on K ⁺ -evoked depolarization could be inhibition under some conditions and stimulation under others. We confirmed that the effect of 1 mM Na ⁺ was to raise the K ⁺ affinity and to reduce transport velocity. In 0 Na ⁺ , high pH greatly reduced the K ⁺ -affinity, but in 1 mM Na ⁺ , high pH had no effect. So, in a given test, the effect of Na ⁺ was to increase or decrease K ⁺ -evoked depolarization, depending on the test [K ⁺ ] and the pH. We discuss simplified, but plausible, kinetic models for this interaction of pH and Na ⁺ . The simplest model includes a K ⁺ symporter driven by H ⁺ or Na ⁺ depending on their relative concentrations and affinities, with random binding order for the two possible driver ions. This symporter would differ from HKT1 in its selectivity for Rb ⁺ vs K ⁺ and in its inability to carry Na ⁺ alone. No role has been found for a symporter resembling HKT1 as it is presently characterized by heterologous expression.