Glutamate occupies a central position in amino acid
metabolism in plants. The acidic amino acid is formed
by the action of glutamate synthase, utilizing glutamine
and 2-oxoglutarate. However, glutamate is also
the substrate for the synthesis of glutamine from
ammonia, catalysed by glutamine synthetase. The
a-amino group of glutamate may be transferred to
other amino acids by the action of a wide range of
multispecific aminotransferases. In addition, both the
carbon skeleton and a-amino group of glutamate form
the basis for the synthesis of g-aminobutyric acid,
arginine, and proline. Finally, glutamate may be
deaminated by glutamate dehydrogenase to form
ammonia and 2-oxoglutarate. The possibility that the
cellular concentrations of glutamate within the plant
are homeostatically regulated by the combined action
of these pathways is examined. Evidence that the well known
signalling properties of glutamate in animals
may also extend to the plant kingdom is reviewed. The
existence in plants of glutamate-activated ion channels
and their possible relationship to the GLR gene family
that is homologous to ionotropic glutamate receptors
(iGluRs) in animals are discussed. Glutamate signalling
is examined from an evolutionary perspective, and
the roles it might play in plants, both in endogenous
signalling pathways and in determining the capacity of
the root to respond to sources of organic N in the soil,
are considered.