References

13
The Arabidopsis thaliana
Glutamate-like Receptor Family (AtGLR)
Matthew Gilliham, Malcolm Campbell, Christian Dubos,
Dirk Becker, Romola Davenport
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 Ca 2+ influxintocells;inoneiGluRsubfamilyglycinealsoisrequiredasacoagonist.In
Arabidopsis, both glutamate and glycine have been demonstrated to depolarise the plasma
membrane and increase [Ca 2+ ]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.
13.1
Introduction
Twenty genes in the Arabidopsis thaliana genome encode subunits of
glutamate-likereceptors(AtGLRs) (Lametal.1998;Lacombe2001a).AtGLR
subunits are so named owing to their similarity in amino acid coding sequence,
and predicted secondary structure, to animal ionotropic glutamate
receptor (iGluR) subunits (Lam et al. 1998; Fig. 13.1). Phylogenetic analysis
suggests that the evolution of the AtGLR subunits predates the divergence
of plants and animals (Chiu et al. 1999, 2002). It is believed that their evolutionary
precursor evolved from the insertion of an inverted K + -selective
ion channel into an amino acid binding protein (Wo and Oswald 1995). The
discovery of a functional glutamate-activated K + -selective channel (GluR0)
in the prokaryotic cyanobacteria Synechocystis provides compelling support
for both hypotheses (Chen et al. 1999). To date, the functional roles of
AtGLR subunits have not been clearly defined.
Communication in Plants
F. Baluška, S. Mancuso, D. Volkmann (Eds.)
© Springer-Verlag Berlin Heidelberg 2006