CalciumattheCrossroadsofSignaling..docxVIP

The Plant Cell, S401–S417, Supplement 2002, ? 2002 American Society of Plant BiologistsCalcium at the Crossroads of SignalingDale Sanders,a,1 Jér?me Pelloux,a Colin Brownlee,b and Jeffrey F. Harper ca Biologyb MarineDepartment, University of York, York YO10 5YW, United KingdomBiological Association, The Laboratory, Citadel Hill, Plymouth PL1 2PB, United Kingdomc Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037INTRODUCTIONCalcium Signals: A Central Paradigm inStimulus–Response CouplingCells must respond to an array of environmental and devel-opmental cues. The signaling networks that have evolved togenerate appropriate cellular responses are varied and arenormally composed of elements that include a sequence ofreceptors, nonprotein messengers, enzymes and transcrip-tion factors. Receptors are normally highly specific for thephysiological stimulus, and therefore are disparate in theiridentities. Likewise enzymes and transcription factors tendtoward specificity, and this fact is reflected in abundance atthe 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. Theyinclude cyclic nucleotides (Newton et al., 1999), hydrogenions (Guern et al., 1991), active oxygen species (VanBreusegem 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 apparentduring the transduction of a very wide variety of abiotic andbiotic signals. The spectrum of stimuli that evokes rapidchanges in [Ca2 ]c has been cataloged in a number of re-cent reviews (Sanders et al., 1999; Knight, 2000; Anil andRao, 2001; Knight and Knight, 2001; Rudd and Franklin-Tong, 2001). Abiotic stimuli include light—with red, blue,and UV/B irradiation each acting via different receptors andlead