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apex (Zhang et al. 2001), blocks Al
3+
-induced
Em
depolarisation and Ca
2+
-
dependent microtubule depolymerisation but does not block these in re-
sponse to L-glutamate. An alternative sequence of events in response to
glutamate is suggested by plant action potentials. The action potential of
giant algal cells is dependent on three phases: (1) an initial small Ca
2+
influx
that (2) stimulates a substantial Cl
−
efflux to below
E
K
which (3) stimulates
an efflux of K
+
repolarising
Em
(Shepherd et al. 2002). It is entirely possible
that the depolarisation induced by glutamate could involve a similar chain of
events. Qi et al. (2005) have reported that, in contrast to lone applications,
a combination of NPPB and 1,2-bis(
o
-aminophenoxy)ethane-
N
,
N
,
N
,
N
-
tetraacetic acid (BAPTA) (a Ca
2+
chelator) can decrease the glutamate-
induced
Em
depolarisation, supporting the hypothesis that ligand treat-
ment affects membrane permeability to multiple ions. Taken together with
theputativeroleof
At
GLR in de-etiolation it is tempting to suggest an
involvement of
At
GLR in the blue-light induced
Em
depolarisation of the
hypocotyl (Cho and Spalding 1996).
13.3
Roles of AtGLR
13.3.1
Expression
Individual
At
GLRs show complex spatio-temporal expression patterns dur-
ing plant development and a diversity of responses to stresses (Sect. 13.3.5).
Data from multiple sources (RT-PCR, promoter:reporter fusions, expressed
sequence tags, microarrays) indicate widespread
AtGLR
transcription, typ-
ically with low messenger RNA (mRNA) abundance. RT-PCR indicated
that most
AtGLR
s were transcribed in root, shoot, flower, and silique in
8-week-old plants, although transcripts of several clade 2 genes (2.1-2.3,
2.9) were detected roots only (Chiu et al. 2002). At the single-cell level,
individual mature leaf epidermal and mesophyll cells contained three to
nine
At
GLR transcripts, with no consistent pattern except that
AtGLR3.7
was ubiquitous (Roy et al. 2004). The putative promoter of
AtGLR3.7
was
fused to the
β
-glucuronidase (GUS) reporter and indicated widespread
transcription in all tissues in seedlings and in root vasculature, leaves,
siliques (but not seeds), and flowers of mature plants (Essah 2002). The
near ubiquity of
AtGLR3.7
transcripts raises the possibility that
AtGLR3.7
could be a key subunit involved in a variety of
At
GLR heteromeric pro-
teins, analogous to NMDAR1 amongst NMDA receptors. Other
AtGLR
s
may be more confined in their expression, for instance, localisation of
At-
GLR3.2
by promoter:GUS fusion, RT-PCR, and an antibody raised to the
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