The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 190

as compatible solutes under salt and water stress in Arabidopsis (Chen
and Murata 2002; Kaplan et al. 2004). GABA is a key neurotransmitter
in animals and is synthesised rapidly in plants in response to biotic and
abiotic stresses, including anoxia, oxidative stress, and mechanical stress
(Scott-Taggart et al. 1999). Changes in GABA levels are also implicated in
plant development, and a gradient of GABA directs pollen tube formation
( Palanivelu et al. 2003). β -Aminobutyric acid (BABA) potentiates plant

defences against pathogen attack (Zimmerli et al. 2000). Glycine is also in-
volved in the movement response in Mimosa pulvinus cells (Otsiogo-Oyabi
and Roblin 1985).

13.2.2
Glutamate and Glycine as Signalling Molecules

Recent evidence supports the hypothesis that glutamate and glycine func-
tion as signalling molecules in Arabidopsis (Dubos et al. 2003). Aside from
their obvious effects on plant growth and development glutamate can in-
duce an atypical depolarisation of the electrical potential gradient across
the plasma membrane ( Em )of Arabidopis root, hypocotyl, and leaf meso-
phyll cells when compared with other amino acids (Dennison and Spalding
2000; Sivaguru et al. 2003; Meyerhoff et al. 2005) and new data suggest that
this occurs with glycine as well (Qi et al. 2004; M. Gilliham, unpublished).
The depolarisation is multiphasic, consisting of an initial rapid, substantial
transient with duration in the order of tens of seconds followed by a smaller
secondary phase that plateaus until removal of the agonist. The secondary
phase seen in the presence of other amino acids probably reflects the ac-
tivity of proton-coupled amino acid symporters (Kinraide and Etherton
1980; de Jong and Borstlap 2000), whereas the primary transient is proba-
bly due, at least in part, to the passage of Ca 2+ across the plasma membrane.
The primary depolarisation by glutamate is, at least initially, coincidental
with a transient rise in [Ca 2+ ] cyt , as indicated by whole seedlings consti-
tutively expressing the chemiluminescent Ca 2+ -binding protein aequorin,
with both phenomena being abolished by the non-specific calciumchannel
blocker La 3+ (Dennison and Spalding 2000; Sivaguru et al. 2003; Meyerhoff
et al. 2005).
Both the glutamate-induced primary Em transientandtherisein[Ca 2+ ] cyt
appear to be concentration-dependent, first detectable around 50 µM and
saturate in the low millimolar range (Demidchik et al. 2004; Meyerhoff et
al. 2005). As has been observed in animals, glutamate and glycine applied
together synergistically decrease the amount of agonist needed to increase
[Ca 2+ ] cyt to 10 µM (Dubos et al. 2003). Subsequent applications of agonist
decrease both these responses (Meyerhoff et al. 2005; Qi et al. 2005; M.