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

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.