In their excellent Tansley Review, Martin McAinsh & Jon Pittman (2009) describe how the cellular complement of Ca2+ channels, Ca2+-ATPases and Ca2+/H+-antiporters can interact to produce defined spatial and temporal changes in cytosolic Ca2+ concentration ([Ca2+]cyt) in response to specific environmental and developmental stimuli. Here, I describe how the unique pharmacological characteristics of specific depolarization-activated calcium channels (DACCs) in the plasma membrane of plant cells have provided evidence for their involvement in shaping the changes in [Ca2+]cyt in response to low-temperature stress.
Pharmacological studies indicate that acclimation to low temperatures requires Ca2+ influx across the plasma membrane and a transient increase in [Ca2+]cyt (White & Broadley, 2003). Cooling plant cells rapidly to low, non-freezing temperatures elicits an electrical response, termed the slow action potential (SAP), which is thought to result from the successive opening of DACCs, Ca2+-dependent Cl− channels and outward rectifying K+ channels. The opening of DACCs during the SAP would facilitate Ca2+ influx, raise [Ca2+]cyt and initiate acclimation. Two observations implicate a DACC called the ‘maxi cation channel’ or ‘VDCC1’ in the generation of [Ca2+]cyt perturbations in root cells in response to cooling (White, 2004). First, the voltage-dependent kinetics of Ca2+ influx through the maxi cation channel are consistent with it catalysing a transient Ca2+ influx during the SAP. It has been observed that the magnitude of both the SAP (Minorsky & Spanswick, 1989) and the increase in [Ca2+]cyt (Plieth et al., 1999) are related to the rate, extent and duration of cooling. Calcium influx through the maxi cation channel is correspondingly proportional to the rate, extent and duration of plasma membrane depolarization (White, 2000). Second, the pharmacology of Ca2+ influx through the maxi cation channel, which is inhibited by 100 µm La3+ and insensitive to 100 µm verapamil, 100 µm nifedipine and 100 µm diltiazem (White, 2000), parallels the pharmacology of both the SAP and the [Ca2+]cyt perturbations observed when roots are cooled, which are both inhibited by La3+ but not by verapamil, nifedipine or diltiazem (Minorsky & Spanswick, 1989; Knight et al., 1996).
To explore further the links between the activity of the maxi cation channel and the [Ca2+]cyt perturbations in root cells that are elicited by cooling, a modeling approach has been adopted (White, 2004). A simple model of a root cell lacking a net Ca2+ flux across the tonoplast was assumed (Fig. 1a). To recreate [Ca2+]cyt perturbations resembling those induced by cooling it was necessary to postulate the presence of three types of Ca2+ transporter in the root plasma membrane: VDCCs, Ca2+-ATPases and Ca2+-permeable voltage-independent cation channels (VICCs). The VICCs are responsible for maintaining [Ca2+]cyt homeostasis in the resting cell and are thought to be encoded by members of the CNGC and/or GLR gene families (White & Broadley, 2003; Demidchik & Maathuis, 2007). Fluxes through these Ca2+ transporters were calculated for five contrasting temperature perturbations based on explicit models of their temperature and voltage-dependent transport activities (Fig. 1b). The model predicted that:
- • brief cooling followed by rewarming produces a transient increase in [Ca2+]cyt, whose magnitude is proportional to the drop in temperature;
- • rapid cooling increases [Ca2+]cyt, whereas slow cooling does not;
- • prolonged cooling by a few degrees elicits a transient increase in [Ca2+]cyt similar to that observed upon brief cooling and rewarming; and
- • prolonged cooling at an injurious temperature elicits a sustained increase in [Ca2+]cyt (Fig. 1b).
These predictions are all consistent with the changes in [Ca2+]cyt concentrations observed in plant roots subjected to comparable low-temperature treatments (Plieth et al., 1999), which suggests that voltage-dependent modulation of DACC activity produces the distinct temporal changes in [Ca2+]cyt elicited by specific temperature perturbations.
Leaf cells possess DACCs with similar properties to those of root cells (White, 2000; Miedema et al., 2008), and a DACC activated directly by rapid cooling has been observed in leaf mesophyll-cell protoplasts (Carpaneto et al., 2007). It is possible that the transient activation of this DACC upon rapid cooling triggers an immediate Ca2+ influx and plasma membrane depolarization, resulting in the voltage-dependent activation of other Ca2+-permeable DACCs and an augmented [Ca2+]cyt signal.
In summary, both the pharmacological and electrophysiological characteristics of plasma membrane DACCs are consistent with them mediating the changes in [Ca2+]cyt observed when plant cells are cooled and (presumably) also the acclimatory responses of plants to low-temperature stress.