References

13 The Arabidopsis thaliana Glutamate-like Receptor Family(AtGLR) 195
injection of any AtGLRs. Additionally, ABA, BMAA, GABA, NMDA, and
glutamine have not activated any currents in any heterologous system containing
any AtGLRs as tested so far (Cheffings 2001; Lacombe et al. 2001b;
Meyerhoff et al. 2005; M. Gilliham, unpublished), although incubation of
the AtGLR3. 4 injected oocytes with DNQX did reduce the Na + -permeable
“leak” current (M. Gilliham, unpublished).
Although at least AtGLR3.4 is expressed at the plasma membrane in
heterologous systems (D. Becker, unpublished), it is possible that AtGLRs
may require plant-specific processing for proper function. It is possible that
the activity of some AtGLRsismodifiedbyendogenoussubunitsinoocytes
and other heterologous systems, resulting in activation of atypical currents
or loss of AtGLR channel function (Green et al. 2002; Anantharam et al.
2003). Other proteins or cofactors may also be needed for proper activity
of AtGLR 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 limited.
Kim et al. (2001) constitutively overexpressed AtGLR3. 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
AtGLR3.2, medium supplemented with Ca 2+ could rescue the mutant phenotype.
On the basis of these findings AtGLR3.2, 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 AtGLR1.1
plants did not show hypersensitivity to K + and Na + compared with wildtype
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 In unpublished work, Qi et al.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 depolarisations
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 AtGLR1.1 (antiAtGLR1.1) plantshasimplicated
a role for AtGLR1.1 in C:N perception and signalling (Kang and Turano
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 inhibition
by DNQX, possibly because antiAtGLR1.1 suppressed AtGLR activity