The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 191

Gilliham, unpublished), possibly indicating a desensitisation of response
(Everts, et al. 1999; Sun et al. 2002). Unpublished results of Qi et al (2005)
suggestthatglutamatedesensitisedtheresponsetoglycineinthehypocotyl,
but not vice versa, whereas no such specificity in desensitisation response
has been observed in roots (M. Gilliham, unpublished). De novo synthesis
of proteins may be required for full recovery of the response, since recovery
is inhibited by cycloheximide (Meyerhoff et al. 2005).
Neither response is initiated by GABA, NMDA, AMPA or KA, support-
ing the divergence of At GLRandiGluRbeforetheydevelopeduniqueligand
specificity (Chiu et al. 1999). However, DNQX and 6-cyano-7-nitroquinoxal-
ine-2,3-dione (CNQX) can abolish the glutamate-induced and/or the gly-
cine-induced [Ca 2+ ] cyt increase (Dubos et al. 2003; Meyerhoff et al. 2005).
The cold-shock-induced rise in [Ca 2+ ] cyt is unaffected by DNQX (or previ-
ous application of glutamate) (Dubos et al. 2003), indicating separate Ca 2+
response pathways but also specificity in the CNQX/DNQX antagonism.
TheNMDA/glutamatesitebindingiGluRantagonistAP-5alsoblocksboth
Em depolarisation and the Ca 2+ -dependent microtubule depolymerisation
induced by glutamate (Sivaguru et al. 2003), however, its effect on [Ca 2+ ] cyt
has not been measured directly. Interestingly NMDA also depolymerises
microtubules despite not inducing a [Ca 2+ ] cyt or an Em response (Sivaguru
et al. 2003), suggesting that caution must be exercised when using iGluR ag-
onists/antagonists as indication of At GLR involvement in these processes.
It should also be noted that NMDA-type iGluRs, the only iGluR subtype to
bind glycine, are not inhibited by CNQX/DNQX which bind at the glutamate
binding site of AMPA/KA iGluR.
L-glutamatealsoinducedtheinfluxofNa + and Ca 2+ in a small percent-
age of patch-clamped protoplasts derived from Arabidopsis root cells by
activating 'spiky' inward currents at hyperpolarised potentials with exter-
nal Ca 2+ or less erratic currents with Na + (Demidchik et al. 2004). Glycine
can also activate an instantaneous current in patch-clamped protoplasts
derived from wheat root cells (M. Gilliham, unpublished). The effect of
glutamate on unidirectional influx of Na + into Arabidopsis roots was in-
conclusive (Demidchik et al. 2004). Because Na + influx is mainly via NSCCs,
which are permeable to other monovalent cations, the movement of Na + ,
K + ,Cs + ,andNH 4 , although not dependent upon a glutamate-induced
depolarisation may contribute to it.
Sivaguru et al. (2003) have suggested that At GLRs are involved in root
responses to toxic Al 3+ .TheyproposethatAl 3+ , known to induce organic
anion release in some species (Ma and Furukawa 2003), stimulates gluta-
mate release, which activates Ca 2+ influx via plasma-membrane-localised
At GLR, leading to Em depolarisation and microtubule depolymerisation.
They observed that 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB),
known to inhibit the Al 3+ -activated organic anion channel of the root