References

13 The Arabidopsis thaliana Glutamate-like Receptor Family(AtGLR) 197
AtGLR2.1 and AtGLR3.5 were downregulated. AtGLR1.3 and AtGLR1.1
seemed not to respond at all to this stress hormone despite the latter being
implicated in the regulation of ABA biosynthesis and antiAtGLR1.1 plants
exhibiting elevated ABA levels (Kang and Turano 2003; Sect. 3.3). Maathuis
et al. (2003) demonstrated that members of the AtGLR family showed timedependent
changes in transcription in response to Ca 2+ deprivation and
excess Na + ,butnotK + deprivation, supporting the prediction that they
function as NSCCs.
AnalysisofGUSexpressionundercontroloftheAtGLR3.4 promoter suggested
that AtGLR3.4 expression was induced by mechanical stress such as
wounding or touch and cold (Meyerhoff et al. 2005). The upregulation of
AtGLR3.4 transcription by wounding and during senescence could suggest
an involvement of methyljasmonate (Sect. 13.3.1), a common component
of both processes (Devoto and Turner 2005). Quantitative RT-PCR analyses
of AtGLR3.4 expression revealed that cold-induced transcription was fast,
reaching peak transcript levels after 30 min. Additionally, La 3+ inhibited
cold-induced AtGLR3.4 transcription, suggesting the involvement of Ca 2+ .
AtGLR1.2 and AtGLR2.5 are also upregulated by cold treatment (Zimmermann
et al. 2004).
13.4
Conclusions and Future Perspectives
Research into the glutamate receptor-like family has been hindered by the
lack of phenotypes associated with the AtGLR genes. The novel pore region
of the predicted proteins and the paucity of understanding of NSCC function
in vivo have made it difficult to predict function. However there are
now several phenotypes associated with AtGLR overexpression (Kim et al.
2001) and knockout (Kang and Turano 2003; Kang et al. 2004), and systematic
electrophysiological characterisation of single AtGLR knockout plants
is underway (Qi et al. 2004, 2005; Gilliham 2005). Despite recent progress,
several fundamental questions still remain regarding AtGLR function.
13.4.1
Expression
One promising approach to predicting AtGLR function is the study of
AtGLR gene expression in response to developmental and environmental
cues. In particular, very rapid changes in gene expression in response to environmental
signals (such as application of a ligand, or stresses) implicate
the gene product in specific responses to stimuli (Meyerhoff et al. 2004,