The Arabidopsis thaliana glutamate-like receptors (AtGLR)

Abstract

The 20 genes that encode the Arabidopsis thaliana glutamate-like receptor family (AtGLR) share significant similarity in amino acid coding sequence and predicted secondary structure with animal ionotropic glutamate receptor (iGluR) subunits. In animals, iGluR subunits form glutamate-gated non-selective cation channels (NSCCs) catalysing Na+ and/or Ca2+ influx into cells; in one iGluR subfamily glycine also is required as a coagonist. In Arabidopsis, both glutamate and glycine have been demonstrated to depolarise the plasma membrane and increase [Ca2+]cyt, and iGluR antagonists blocked these effects. AtGLRs are therefore predicted to function in an analogous manner to iGluR. Attempts to functionally characterise AtGLRs in heterologous expression systems have proved inconclusive with no ligand-gated activity detected. Research into the glutamate receptor-like family has been hindered by the lack of phenotypes associated with the AtGLR genes but several phenotypes associated with AtGLR overexpression and knockout have recently given hints as to their function. AtGLR have been implicated in light and C:N signalling, hypocotyl detiolation, root growth, abscisic acid (ABA) metabolism, stress responses, and general ion transport. This review will concentrate on recent developments in the AtGLR field, including the roles and effects of glutamate and glycine and related metabolites in plant physiology relative to potential roles for AtGLRs. It will examine progress made toward defining the functions of particular AtGLRs and will conclude by recommending potentially fruitful avenues of future research.