Auxin augments conductance of K+ inward rectifier in maize coleoptile protoplasts |
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Authors: | Gerhard Thiel Ralf Weise |
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Institution: | Albrecht-von-Haller Institut für Pflanzenwissenschaften, Biophysik der Pflanzen, Universit?t G?ttingen, Untere Karspüle 2, D-37073 G?ttingen, Germany, DE
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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.
Received: 13 May 1998 / Accepted: 9 November 1998 |
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Keywords: | : Auxin Calcium Coleoptile growth K+-inward rectifier (channel) pH Zea (coleoptile protoplasts) |
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