The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 195

injection of any At GLRs. Additionally, ABA, BMAA, GABA, NMDA, and
glutamine have not activated any currents in any heterologous system con-
taining any AtGLRs as tested so far (Cheffings 2001; Lacombe et al. 2001b;
Meyerhoff et al. 2005; M. Gilliham, unpublished), although incubation of
the At GLR3. 4 injected oocytes with DNQX did reduce the Na + -permeable
“leak” current (M. Gilliham, unpublished).
Although at least At GLR3.4 is expressed at the plasma membrane in
heterologous systems (D. Becker, unpublished), it is possible that At GLRs
may require plant-specific processing for proper function. It is possible that
the activity of some At GLRsismodifiedbyendogenoussubunitsinoocytes
and other heterologous systems, resulting in activation of atypical currents
or loss of At GLR channel function (Green et al. 2002; Anantharam et al.
2003). Other proteins or cofactors may also be needed for proper activity
of At GLR in heterologous systems, for instance, SOL-1, a CUB-like protein,
colocalises at the neuron cell surface with GLR1 in Caenorhabditis elegans
and is required for ligand-gated GLR1 activity (Zheng et al. 2004).
In planta evidence for ion channel activity of glutamate receptors is lim-
ited. Kim et al. (2001) constitutively overexpressed At GLR3. 2, which led to
signs of Ca 2+ deficiency and a hypersensitivity to K + and Na + .Although
total tissue Ca 2+ was the same in wild-type plants and plants overexpressing
At GLR3.2, medium supplemented with Ca 2+ could rescue the mutant phe-
notype. On the basis of these findings At GLR3.2, which is most abundantly
expressed in vascular tissues, has a proposed role in Ca 2+ distribution
within the plant (Kim et al. 2001; Turano et al. 2002). Antisense At GLR1.1
plants did not show hypersensitivity to K + and Na + compared with wild-
type plants, but root growth was more inhibited by high levels of Ca 2+ (Kang
and Turano 2003). In unpublished work, Qi et al. (2004) have reported that
AtGLR T-DNA insertional mutants have aberrant growth rate responses to
glutamate or light. Knockouts of AtGLR3.3 also displayed reduced depo-
larisations in the root and hypocotyl associated with glutamate or glycine,
whereas knockouts of AtGLR3.4 showed reduced response to glycine only
(Qi et al. 2005).
13.3.4
C:N Signalling
Phenotypic analysis of antisense At GLR1.1 ( antiAtGLR1.1 )plantshasim-
plicated a role for At GLR1.1 in C:N perception and signalling (Kang and Tu-
rano 2003; Kang et al. 2004). antiAtGLR1.1 seed germination was inhibited
by sucrose, but rescued by supplemental nitrate. Compared with wild-type,
seed germination of antiAtGLR1.1 plants was also more susceptible to inhi-
bition by DNQX, possibly because antiAtGLR1.1 suppressed At GLR activity