References

13 The Arabidopsis thaliana Glutamate-like Receptor Family(AtGLR) 189
and Seeburg 1995; Dingledene et al. 1999). In addition to glutamate, NMDA
receptors require glycine as a coagonist (Dingledine et al. 1999). AtGLRs
have been divided into three clades according to sequence (AtGLR1.1–1.4;
2.1–2.9; 3.1–3.7) (Lacombe et al. 2001a; Chiu et al. 2002).
This brief review will concentrate on recent developments in the field,
specifically exploring the roles and effects of glutamate and glycine, and related
metabolites, in plant physiology relative to potential roles for AtGLRs.
It will then examine the progress made toward defining the functions of
particular AtGLRs and will conclude by recommending potentially fruitful
future avenues of research.
13.2
Roles (and Effects) of Glutamate, Glycine
and Interrelated Amino Acids in Plants
13.2.1
Effects of Amino Acids on Plant Development
Plant development is sensitive both to light and to tissue C:N, which determines
the allocation of biomass between root and shoot (Thum et al. 2003).
The molecules that signal this ratio, and the receptors that detect it, are
largely unknown, but it is thought that certain amino acids signal nitrogen
status (Thum et al. 2003). Attempts to relate AtGLRs to C:N signalling have
shown that iGluR agonists and inhibitors affect hypocotyl development.
6,7-Dinotroquinoxaline 2,3-(1H,4H)-dione (DNQX) caused an etiolated
phenotype in seedlings grown in light but not those grown in the dark
(Lam et al. 1998) and this was reversed by glutamate and/or glycine (Dubos
et al. 2003). S(+)-β-Methyl-α,β-diaminopropionic acid (BMAA) also
caused etiolation in light but inhibited hypocotyl elongation in the dark,
and both effects were reduced by glutamate and glutamine (Brenner et al.
2000).
Rhizosphere amino acids may signal the location of nutrient-rich organic
matter (Filleur et al. 2005). Both glutamate and glycine have been
demonstrated to modify root elongation. Micromolar levels of glutamate
induced root bending toward a glutamate source, whereas greater concentrations
inhibited primary root growth (Filleur et al. 2005). Glycine has also
been demonstrated to exert positive (White 1939; Fries 1953; Skinner and
Street 1953) and negative (Skinner and Street 1953) effects on root growth.
Amino acids also function in developmental and stress responses both
as signals and as osmoprotectants. For instance, the glutamate-derived γaminobutyric
acid (GABA) and glycine-derived glycine betaine function