Auxin augments conductance of K + inward rectifier in maize coleoptile protoplasts

Abstract

Potassium is taken up by maize (Zea mays L.) coleoptile cells via a typical plant inward rectifier (K ir ). Sufficient conductance of this channel is essential in order to maintain auxin-stimulated cell elongation. It was therefore investigated whether the activity of this channel is subject to direct or indirect control by this growth hormone. Patch-clamp measurements of whole coleoptile protoplasts revealed no appreciable effect of externally applied 10 μM or 100 μM α-naphthaleneacetic acid (NAA) on the activity of K ir over test periods of ≥ 18 or ≥ 8 min, respectively. When, however, K ir was recorded in the cell-attached configiuration and 10 μM NAA administered to the bath medium, the conductance of K ir increased significantly in 13 out of 18 protoplasts over the control. This rise occurred at a fixed protoplast voltage after a lag period of less than 10 min and exhibited no voltage dependency. The absence of response to NAA of protoplasts in the whole-cell configuration indicates that auxin perception and channel control is linked via a soluble cytoplasmic factor and that this mediator is washed out or modified upon perfusion of the cytoplasm with pipette solution. To search for this expected diffusible factor the K ir current was recorded before and after elevation of Ca2+ and H+ in the cytoplasm. In the whole-cell configuration the increase in Ca2+ from a nanomolar value to >1 μM by means of Ca2+-release from the caged precursor Na2-DM-nitrophen left K ir unaffected. The whole-cell K ir conductance was also not affected upon addition of 10 mM Na+-acetate to the bath medium, an operation used to lower the cytoplasmic pH. This excludes a primary role for the known auxin-evoked rise in cytoplasmic Ca2+ and H+ in K ir activity. We postulate that another, as yet unknown, mechanism mediates the auxin-evoked stimulation of the number of active K ir channels in the plasma membrane.