Actin Dynamics Regulates Voltage-Dependent Calcium-Permeable Channels of the Vicia faba Guard Cell Plasma Membrane

Free cytosolic Ca²⁺ ([Ca²⁺]_cyt) is an ubiquitous second messenger in plant cell signaling, and [Ca²⁺]_cyt elevation is associated with Ca²⁺-permeable channels in the plasma membrane and endomembranes regulated by a wide range of stimuli. However, knowledge regarding Ca²⁺ channels and their regulation remains limited in planta . A type of voltage-dependent Ca²⁺-permeable channel was identified and characterized for the Vicia faba L. guard cell plasma membrane by using patch-clamp techniques. These channels are permeable to both Ba²⁺ and Ca²⁺, and their activities can be inhibited by micromolar Gd³⁺. The unitary conductance and the reversal potential of the channels depend on the Ca²⁺ or Ba²⁺ gradients across the plasma membrane. The inward whole-cell Ca²⁺ (Ba²⁺) current, as well as the unitary current amplitude and NP_o of the single Ca²⁺ channel, increase along with the membrane hyperpolarization. Pharmacological experiments suggest that actin dynamics may serve as an upstream regulator of this type of calcium channel of the guard cell plasma membrane. Cytochalasin D, an actin polymerization blocker, activated the NPo of these channels at the single channel level and increased the current amplitude at the whole-cell level. But these channel activations and current increments could be restrained by pretreatment with an F-actin stabilizer, phalloidin. The potential physiological significance of this regulatory mechanism is also discussed.