Ca2+ channels at the plasma membrane of stomatal guard cells are activated by hyperpolarization and abscisic acid

AUTOR(ES)

Hamilton, David W. A.

FONTE

The National Academy of Sciences

RESUMO

In stomatal guard cells of higher-plant leaves, abscisic acid (ABA) evokes increases in cytosolic free Ca2+ concentration ([Ca2+]i) by means of Ca2+ entry from outside and release from intracellular stores. The mechanism(s) for Ca2+ flux across the plasma membrane is poorly understood. Because [Ca2+]i increases are voltage-sensitive, we suspected a Ca2+ 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 Ba2+ 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 Ba2+ (and Ca2+) over K+ and Cl−. Unlike other Ca2+ channels characterized to date, these channels rectified strongly toward negative voltages with an open probability (Po) that increased with [Ba2+] outside and decreased roughly 10-fold when [Ca2+]i was raised from 200 nM to 2 μM. Adding 20 μM ABA increased Po, 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 Po with periods near 50 s. A similar, but delayed response was observed in 0.1 μM ABA. These results identify a Ca2+-selective channel that can account for Ca2+ influx and increases in [Ca2+]i triggered by voltage and ABA, and they imply a close physical coupling at the plasma membrane between ABA perception and Ca2+ channel control.

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