Voltage-dependent calcium-permeable channels in the plasma membrane of a higher plant cell

Numerous biological assays and pharmacological studies on various higher plant tissues have led to the suggestion that voltage-dependent plasma membrane Ca²⁺ channels play prominent roles in initiating signal transduction processes during plant growth and development. However, to date no direct evidence has been obtained for the existence of such depolarization-activated Ca²⁺ channels in the plasma membrane of higher plant cells. Carrot suspension cells (Daucus carota L.) provide a well-suited system to determine whether voltage-dependent Ca²⁺ channels are present in the plasma membrane of higher plants and to characterize the properties of putative Ca²⁺ channels. It is known that both depolarization, caused by raising extracellular K⁺, and exposure to fungal toxins or oligogalacturonides induce Ca²⁺ influx into carrot cells. By direct application of patch-clamp techniques to isolated carrot protoplasts, we show here that depolarization of the plasma membrane positive to -135 mV activates Ca2⁺-permeable channels. These voltagedependent ion channels were more permeable to Ca²⁺ than K⁺, while displaying large permeabilities to Ba²⁺ and Mg²⁺ ions. Ca2+-permeable channels showed slow and reversible inactivation. The single-channel conductance was 13 pS in 40 mM CaCl₂. These data provide direct evidence for the existence of voltagedependent Ca²⁺ channels in the plasma membrane of a higher plant cell and point to physiological mechanisms for plant Ca²⁺ channel regulation. The depolarization-activated Ca2+-permeable channels identified here could constitute a regulated pathway for Ca²⁺ influx in response to physiologically occurring stimulus-induced depolarizations in higher plant cells.