K+ channels of stomatal guard cells: bimodal control of the K+ inward-rectifier evoked by auxin.

The influence of the auxins indole-3-acetic acid (IAA) and 1-napthylene acetic acid (NAA) on K+ channels and their control was examined in stomatal guard cells of Vicia faba L. intact guard cells were impaled with multibarrelled microelectrodes to record membrane potentials and to monitor K+ channel currents under voltage clamp during exposures to 0.1-100 microM IAA and NAA. Following impalements, challenge with either IAA or NAA in the presence of 10 mM KCl resulted in the concerted modulation of at least four different currents with distinct kinetic characteristics and concentration dependencies. Equivalent concentrations of benzoic acid were wholly without effect. Most striking, current carried by inward-rectifying K+ channels (IK,in) exhibited a bimodal response to both IAA and NAA which was reversed on washing the auxins from the bathing medium. The steady-state current was augmented 1.3- to 2-fold at concentrations between 0.1 and 10 microM and antagonized at concentrations near 30 microM and above. Auxin agonism of IK,in was time- and voltage-independent. By contrast, IK,in inactivation at the higher auxin concentrations was marked by a voltage-dependence and slowing of the kinetics for current activation. Inactivation of IK,in by the auxins was relieved when cytoplasmic pH (pHi) was clamped near 7.0 in the presence of 30 mM Na(+)-butyrate. In addition to the control of IK,in, current carried by a second class of (outward-rectifying) K+ channels rose in a monotonic and largely voltage-independent manner with auxin concentrations about 10 microM and above, and IAA and NAA also activated an inward-going current with a voltage dependence characteristic of guard cell anion channels. Further changes in background current were consistent with a limited activation of the H(+)-ATPase. Over the concentration range examined, the auxins evoked membrane hyperpolarizations and depolarizations of up to +/- 12-19 mV, depending on the free-running membrane potential prevailing before auxin additions. Prolonging exposures to 100 microM auxin beyond 3-5 min frequently elicited rapid transitions to voltages near EK as well as regenerative action potentials. However, in every case the voltage response was a predictable consequence of auxin action on the K+ channels and, at 100 microM auxin, on the anion current.(ABSTRACT TRUNCATED AT 400 WORDS)