Update on Genetic Identities of Plant Calcium Channels Plant Calcium-Permeable Channels 1

The transient elevation of free calcium ion concentration in the cytosol and organelles of plant cells is a wellestablished phenomenon. As discussed in this Focus issue on calcium signaling, these elevations occur in response to a range of biotic and abiotic stimuli (McAinsh and Pittman, 2009; Dodd et al., 2010). Recently, transcriptomic analyses coupled with calcium determinations have demonstrated that a specific stimulus (membrane voltage or ozone) can cause a specific free calcium ion transient, leading to a specific transcriptional response (Whalley et al., 2011; Short et al., 2012; Whalley and Knight, 2013). It is also clear that free cytosolic Ca ([Ca]cyt) has a role to play in regulating the polar growth of root hairs and pollen tubes (Monshausen et al., 2008; Michard et al., 2011). What has yet to be determined fully is the mechanistic basis of these calcium signaling “signatures” and the oscillations of [Ca]cyt that occur during polar growth. There would be sufficient energy in transmembrane Ca gradients to drive passive Ca flux into the cytosol from the apoplast or internal stores (such as the vacuole or endoplasmic reticulum). Pharmacological intervention has implicated such Ca influx in elevating free cytosolic [Ca]cyt in a variety of Ca 2+ signatures (response to environment, stress, hormones, immunity) and during polar growth. With the prediction of passive Ca transport comes the hypothesis that Ca-permeable channel proteins in plasma membranes and endomembranes would be responsible for [Ca]cyt elevation. This, in turn, has driven the electrophysiological characterization of these channel proteins and the search for the encoding genes (for review, see Demidchik and Maathuis, 2007; Wheeler and Brownlee, 2008; Dietrich et al., 2010; Dodd et al., 2010; Jammes et al., 2011; Hedrich, 2012; Kurusu et al., 2013). The electrophysiological approach has seen a relatively small band of laboratories overcome the technical barriers presented by plant cells to reveal distribution and regulatory properties of plasma membrane and endomembrane channels. The search for genes has been stimulated, but also to some extent stymied, by the advent of genome sequencing. For while plant genomes harbor two families of putative channel subunit genes that bear some relation to animal counterparts (the cyclic nucleotide-gated channels [CNGCs] and glutamate receptor-like [GLR]), they reveal no compelling candidates for the majority of channels or ionotropic receptors discovered by electrophysiology. For example, at this point, genes coding for the receptorlike channel activated by extracellular nucleotides and for the endomembrane inositol 1,4,5-trisphosphateand cyclic ADP ribose-activated channels remain unknown (Allen et al., 1995; Demidchik et al., 2011). With a limited pharmacological arsenal with which to capture channel proteins (despite some early advances with a verapamilbinding protein from maize [Zea mays]; Harvey et al., 1989) and difficulties with the characterization of recombinant channel proteins in heterologous expression systems, the Ca channel field has understandably lagged behind that of K and Cl channels and other transporters in terms of gene identification and characterization of the gene product (Hedrich, 2012). There have been several excellent reviews recently that have summarized gene families, evolution, channel activity, and possible functions (Demidchik and Maathuis, 2007; Wheeler and Brownlee, 2008; Dietrich et al., 2010; Dodd et al., 2010; Matzke et al., 2010; Jammes et al., 2011; Peiter, 2011; Hedrich, 2012; Stael et al., 2012; Kurusu et al., 2013). Beyond a brief introduction to the various channels for the benefit of readers new to the area, this Update will focus on the latest breakthroughs not addressed by those reviews, their implications, and the renewed efforts now evident in Ca channel research to find those genes. Before going on, a note on terminology. The relevant channels characterized in plant membranes so far do a really good job of transporting K and other cations in addition to Ca. The permeability ratio of Ca to K can be as low as 0.5 or as high as 15 (Véry and Davies, 2000; Demidchik and Maathuis, 2007), and this has led many researchers to use terms such as “Capermeable (nonselective) cation channel. ” Here, the term “calcium channel” will be used liberally, because in the signaling context of this Focus, even a spark or puff of Ca emanating from these proteins could be enough to trigger the next event. The active transporters involved in terminating Ca signals will not be addressed.