References

190 M. Gilliham et al.
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 involved
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 function
as signalling molecules in Arabidopsis (Dubos et al. 2003). Aside from
their obvious effects on plant growth and development glutamate can induce
an atypical depolarisation of the electrical potential gradient across
the plasma membrane (Em)ofArabidopis root, hypocotyl, and leaf mesophyll
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this occurs with glycine as well (Qi et al. 2004; M. Gilliham, unpublished).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 activity
of proton-coupled amino acid symporters (Kinraide and Etherton
1980; de Jong and Borstlap 2000), whereas the primary transient is probably
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 constitutively
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).
Boththeglutamate-inducedprimary 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.