Calcium at the crossroads of signaling.

Cells must respond to an array of environmental and developmental cues. The signaling networks that have evolved to generate appropriate cellular responses are varied and are normally composed of elements that include a sequence of receptors, nonprotein messengers, enzymes and transcription factors. Receptors are normally highly specific for the physiological stimulus, and therefore are disparate in their identities. Likewise enzymes and transcription factors tend toward specificity, and this fact is reflected in abundance at the genome level. The Arabidopsis genome, for example, potentially encodes in the region of 1000 protein kinases, 300 protein phosphatases, and 1500 transcription factors. By contrast, nonprotein messengers are relatively few. They include cyclic nucleotides (Newton et al., 1999), hydrogen ions (Guern et al., 1991), active oxygen species (Van Breusegem et al., 2001), lipids (Ng and Hetherington, 2001; Nurnberger and Scheel, 2001; Munne-Bosch and Alegre, 2002), and, above all, calcium. Changes in cytosolic free calcium ([Ca 2 ] c ) are apparent during the transduction of a very wide variety of abiotic and biotic signals. The spectrum of stimuli that evokes rapid changes in [Ca 2 ] c has been cataloged in a number of recent reviews (Sanders et al., 1999; Knight, 2000; Anil and Rao, 2001; Knight and Knight, 2001; Rudd and FranklinTong, 2001). Abiotic stimuli include light—with red, blue, and UV/B irradiation each acting via different receptors and leading to distinct developmental responses (Shacklock et al., 1992; Baum et al., 1999; Frohnmeyer et al., 1999), low and high temperature, touch, hyperosmotic stress, and oxidative stress. Biotic stimuli include the hormones abscissic acid (ABA) and gibberellin, fungal elicitors, and nodulation (Nod) factors. The Specificity Question