An apoplastic Ca2+ sensor regulates internal Ca2+ release in aequorin-transformed tobacco cells.
Removal of Ca(2+) from tobacco suspension cell medium has two immediate effects on cytosolic Ca(2+) fluxes: (i) externally derived Ca(2+) influx (occurring in response to cold shock or hypo-osmotic shock) is inhibited, and (ii) organellar Ca(2+) release (induced by a fungally derived defense elicitor, caffeine, or hypo-osmotic shock) is elevated. We show here that the enhanced release of internal Ca(2+) is likely due to increased discharge from a caffeine-sensitive store in response to a signal transduced from an extracellular Ca(2+) sensor. Thus, chelation of extracellular Ca(2+) in the absence of any other stimulus directly activates release of intracellular Ca(2+) into the cytosol. Evidence that this chelator-activated Ca(2+) flux is dependent on a signaling pathway includes its abrogation by prior treatment with caffeine, and its inhibition by protein kinase inhibitors (K252a and staurosporine) and anion channel blockers (niflumate and anthracene-9-carboxylate). An unexpected characteristic of tobacco cell adaptation to low external Ca(2+) was the emergence of a new Ca(2+) compartment that was inaccessible to external EGTA, yet responsive to the usual stimulants of extracellular Ca(2+) entry. Thus, cells that are exposed to EGTA for 20 min lose sensitivity to caffeine and defense elicitors, indicating that their intracellular Ca(2+) pools have been depleted. Surprisingly, these same cells simultaneously regain their ability to respond to stimuli that usually activate extracellular Ca(2+) influx even though all external Ca(2+) is chelated. Because this gradual restoration of Ca(2+) influx can be inhibited by the same kinase inhibitors that block EGTA-activated Ca(2+) release, we propose that chelator-activated Ca(2+) release from internal stores leads to deposition of this Ca(2+) into a novel EGTA- and caffeine-insensitive compartment that can subsequently be activated by stimulants of extracellular Ca(2+) entry.
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Biochemistry and Molecular Biology Program, Department of Chemistry, Purdue University, West Lafayette, Indiana 47904, USA.
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D000331:Aequorin / Q000378:metabolism*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D017136:Ion Transport / Q000187:drug effects; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D010947:Plants, Toxic; D015398:Signal Transduction / Q000187:drug effects; D013696:Temperature; D014026:Nicotiana
D002614:Chelating Agents; D010726:Phosphodiesterase Inhibitors; D002110:Caffeine; D000331:Aequorin; D004533:Egtazic Acid; D002118:Calcium