References

192 M. Gilliham et al.
apex (Zhang et al. 2001), blocks Al 3+ -induced Em depolarisation and Ca 2+ -
dependent microtubule depolymerisation but does not block these in response
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 EK 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 glutamateinduced
Em depolarisation, supporting the hypothesis that ligand treatment
affects membrane permeability to multiple ions. Taken together with
theputativeroleofAtGLR in de-etiolation it is tempting to suggest an
involvement of AtGLR in the blue-light induced Em depolarisation of the
hypocotyl (Cho and Spalding 1996).
13.3
Roles of AtGLR
13.3.1
Expression
Individual AtGLRs show complex spatio-temporal expression patterns during
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, typically
with low messenger RNA (mRNA) abundance. RT-PCR indicated
that most AtGLRs 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 AtGLR 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 AtGLR heteromeric proteins,
analogous to NMDAR1 amongst NMDA receptors. Other AtGLRs
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