The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 187

13 The Arabidopsis thaliana

Glutamate-like Receptor Family ( At GLR)

Matthew Gilliham, Malcolm Campbell, Christian Dubos,
Dirk Becker, Romola Davenport

Abstract The 20 genes that encode the Arabidopsis thaliana glutamate-like receptor family
( At GLR) share significant similarity in amino acid coding sequence and predicted sec-
ondary 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. At GLRs are
therefore predicted to function in an analogous manner to iGluR. Attempts to functionally
characterise At GLRs 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 At GLR genes but several phenotypes
associated with At GLR overexpression and knockout have recently given hints as to their
function. At GLR 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 At GLR field, including the roles
and effects of glutamate and glycine and related metabolites in plant physiology relative to
potential roles for At GLRs. It will examine progress made toward defining the functions
of particular At GLRs and will conclude by recommending potentially fruitful avenues of
future research.

13.1
Introduction

Tw e nt y g e n e s i n t h e Arabidopsis thaliana genome encode subunits of
glutamate-like receptors ( At GLRs) (Lam et al. 1998; Lacombe 2001a). At GLR
subunits are so named owing to their similarity in amino acid coding se-
quence, 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 At GLR subunits predates the divergence
of plants and animals (Chiu et al. 1999, 2002). It is believed that their evo-
lutionary 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 sup-
port for both hypotheses (Chen et al. 1999). To date, the functional roles of
At GLR subunits have not been clearly defined.