Ca2+ channels at the plasma membrane of stomatal guard cells are activated by hyperpolarization and abscisic acid.
In stomatal guard cells of higher-plant leaves, abscisic acid (ABA) evokes increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) by means of Ca(2+) entry from outside and release from intracellular stores. The mechanism(s) for Ca(2+) flux across the plasma membrane is poorly understood. Because [Ca(2+)](i) increases are voltage-sensitive, we suspected a Ca(2+) channel at the guard cell plasma membrane that activates on hyperpolarization and is regulated by ABA. We recorded single-channel currents across the Vicia guard cell plasma membrane using Ba(2+) as a charge-carrying ion. Both cell-attached and excised-patch measurements uncovered single-channel events with a maximum conductance of 12.8 +/- 0.4 pS and a high selectivity for Ba(2+) (and Ca(2+)) over K(+) and Cl(-). Unlike other Ca(2+) channels characterized to date, these channels rectified strongly toward negative voltages with an open probability (P(o)) that increased with [Ba(2+)] outside and decreased roughly 10-fold when [Ca(2+)](i) was raised from 200 nM to 2 microM. Adding 20 microM ABA increased P(o), initially by 63- to 260-fold; in both cell-attached and excised patches, it shifted the voltage sensitivity for channel activation, and evoked damped oscillations in P(o) with periods near 50 s. A similar, but delayed response was observed in 0.1 microM ABA. These results identify a Ca(2+)-selective channel that can account for Ca(2+) influx and increases in [Ca(2+)](i) triggered by voltage and ABA, and they imply a close physical coupling at the plasma membrane between ABA perception and Ca(2+) channel control.
Hamilton
D W
DW
Laboratory of Plant Physiology and Biophysics, Imperial College of Science, Technology, and Medicine, Wye, Kent TN25 5AH, England.
Hills
A
A
Kohler
B
B
Blatt
M R
MR
D000040:Abscisic Acid / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002462:Cell Membrane / Q000502:physiology; D007887:Fabaceae / Q000166:cytology / Q000502:physiology*; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000502:physiology; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D010946:Plants, Medicinal; D011523:Protoplasts / Q000502:physiology
D015220:Calcium Channels; D000040:Abscisic Acid