Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is a significant
component of the free amino acid pool in most prokaryotic and eukaryotic organisms. In
plants, stress initiates a signal-transduction pathway, in which increased cytosolic Ca21 activates
Ca21/calmodulin-dependent glutamate decarboxylase activity and GABA synthesis.
Elevated H1 and substrate levels can also stimulate glutamate decarboxylase activity. GABA
accumulation probably is mediated primarily by glutamate decarboxylase. However, more
information is needed concerning the control of the catabolic mitochondrial enzymes (GABA
transaminase and succinic semialdehyde dehydrogenase) and the intracellular and intercellular
transport of GABA. Experimental evidence supports the involvement of GABA synthesis
in pH regulation, nitrogen storage, plant development and defence, as well as a compatible
osmolyte and an alternative pathway for glutamate utilization. There is a need to identify the
genes of enzymes involved in GABA metabolism, and to generate mutants with which to
elucidate the physiological function(s) of GABA in plants