Plant Physiology Preview
Published on September 29, 2006; 10.1104/pp.106.088989
OPEN ACCESS ARTICLE
Received August 29, 2006
Accepted September 20, 2006
Calcium Entry Mediated by GLR3.3, an Arabidopsis Glutamate Receptor with a Broad Agonist Profile
Zhi Qi , Nicholas R. Stephens , and Edgar P. Spalding *
Department of Botany, University of Wisconsin, 430 Lincoln Drive, Madison, Wisconsin 53706, USA; Donald Danforth Plant Science Center, 975 North Warson Road, St. Louis, Missouri 63132, USA
Department of Botany, University of Wisconsin, 430 Lincoln Drive, Madison, Wisconsin 53706, USA
* Corresponding author; email: spalding{at}wisc.edu.
The amino acids glutamate and glycine trigger large, rapid rises in cytosolic Ca2+ concentration and a concomitant rise in membrane potential (depolarization) in plants. The possibility that plant homologs of neuronal ionotropic glutamate receptors mediate these neuron-like ionic responses was tested in Arabidopsis thaliana seedlings using a combination of Ca2+ measurements, electrophysiology, and reverse genetics. The membrane depolarization triggered by glutamate was greatly reduced or completely blocked in some conditions by mutations in GLR3.3, one of the 20 GLR genes in Arabidopsis. The same mutations completely blocked the associated rise in cytosolic Ca2+. These results genetically demonstrate the participation of a glutamate receptor in the rapid ionic responses to an amino acid. The GLR3.3-independent component of the depolarization required glutamate concentrations above 25 µM, did not display desensitization, and was strongly suppressed by increasing extracellular pH. It is suggested to result from H+-amino acid symport. Six amino acids commonly present in soils (glutamate, glycine, alanine, serine, asparagine, and cysteine) as well as the tripeptide glutathione ( -glutamyl-cysteinyl-glycine) were found to be strong agonists of the GLR3.3-mediated responses. All other amino acids induced a small depolarization similar to the non-GLR, putative symporter component and in most cases evoked little or no Ca2+ rise. From these results it may be concluded that sensing of six amino acids in the rhizosphere and perhaps extracellular peptides is coupled to Ca2+ signaling through a GLR-dependent mechanism homologous to a fundamental component of neuronal signaling.
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