Schematic representation of Ca2+– and voltage-dependent regulation of S-type anion channels in Nicotiana tabacum guard cells. Ion channels are represented by butterfly motifs, transporters by filled circles and the membrane potential by plus or minus symbols. The arrows through the motifs indicate the direction of ion flow under physiological conditions. The lines connecting the symbols represent regulatory interactions: stimulation and inhibition are indicated by arrowheads and perpendicular line ends, respectively. Proven interactions are given in black, and possible regulatory events are shown as grey lines. The scheme shows that hyperpolarization can have a twofold effect on the activity of S-type anion channels: S-type anion channels are inhibited by hyperpolarization, but stimulated by a hyperpolarization-induced increase in the cytosolic Ca2+ level. In addition, the cytosolic free Ca2+concentration feeds back on the Ca2+ transporters and possibly on Ca2+ channels as well.
Ca2+-dependent activation of guard cell anion channels, triggered by hyperpolarization, is promoted by prolonged depolarization
M. Rob G. Roelfsema
The Plant Journal, 2010, 62, 2, 265-276 –
Rapid stomatal closure is driven by the activation of S-type anion channels in the plasma membrane of guard cells. This response has been linked to Ca2+ signalling, but the impact of transient Ca2+ signals on S-type anion channel activity remains unknown.
In this study, transient elevation of the cytosolic Ca2+ level was provoked by voltage steps in guard cells of intact Nicotiana tabacum plants. Changes in the activity of S-type anion channels were monitored using intracellular triple-barrelled micro-electrodes. In cells kept at a holding potential of −100 mV, voltage steps to −180 mV triggered elevation of the cytosolic free Ca2+concentration. The increase in the cytosolic Ca2+ level was accompanied by activation of S-type anion channels.
Guard cell anion channels were activated by Ca2+ with a half maximum concentration of 515 nm (SE = 235) and a mean saturation value of −349 pA (SE = 107) at −100 mV. Ca2+ signals could also be evoked by prolonged (100 sec) depolarization of the plasma membrane to 0 mV. Upon returning to −100 mV, a transient increase in the cytosolic Ca2+ level was observed, activating S-type channels without measurable delay.
These data show that cytosolic Ca2+elevation can activate S-type anion channels in intact guard cells through a fast signalling pathway.
Furthermore, prolonged depolarization to 0 mV alters the activity of Ca2+ transport proteins, resulting in an overshoot of the cytosolic Ca2+ level after returning the membrane potential to −100 mV.