GABA and GHB Neurotransmitters in Plants and Animals - Communication in Plants: Neuronal Aspects of Plant Life - page 171

12 GABA and GHB Neurotransmitters

in Plants and Animals
Aaron Fait, Ayelet Yellin, Hillel Fromm

γ Abstract -Aminobutyric acid (GABA) is a four-carbon non-protein amino acid conserved

from bacteria to plants and vertebrates. In the latter it is mainly known as a neurotrans-
mitter. The enzymes that synthesize and catabolize GABA constitute a metabolic pathway
known as the GABA shunt which bypasses two steps of the tricarboxylic acid cycle. Func-
tional genomics tools using Arabidopsis as a model system revealed that the GABA shunt
is imperative for normal plant development and for response to stress, and suggest roles
for GABA as an important metabolite as well as a potential signaling molecule. Moreover,
γ

-hydroxybutyrate, a by-product of the GABA shunt and a neurotransmitter in animals, was
recently discovered in plants. Here we discuss the possible roles of these two neurotrans-
mitters in plants with focus on components that underlie their roles as signaling molecules.

12.1
Introduction
γ -Aminobutyricacid(GABA)wasdiscoveredinplantsoverhalfacentury

ago (Steward et al. 1949), and shortly after it was discovered in vertebrates.
Interest in GABA shifted to animals when it was revealed that GABA occurs
at high levels in the brain, where it plays a major role in neurotransmission.
Interest in the GABA shunt in plants emerged later, following observations
that GABA is largely and rapidly produced in response to biotic and abi-
otic stresses (reviewed by Shelp et al. 1999; Snedden and Fromm 1999;
Kinnersley and Turano 2000). A breakthrough in studying GABA in plants
came from molecular studies, particularly following the cloning of the
Ca 2+ /calmodulin-regulated GABA-synthesizing enzyme glutamate decar-
boxylase (GAD; Baum et al. 1993), and later by employing transgenic plants
(Baum et al. 1996) and functional genomics tools in Arabidopsis to inves-
tigate mutants of the GABA metabolic pathway (Bouché et al. 2003, 2004;
Palanivelu et al. 2003; Fait et al. 2005).
GABA is mainly metabolized via a short pathway known as the GABA
shunt,whichbypassestwostepsofthetricarboxylicacid(TCA)cycle
(Fig. 12.1), and is composed of three enzymes: the cytosolic-localized
GAD, and the mitochondrial-localized GABA trans -aminase (GABA-T),
and succinic semialdehyde dehydrogenase (SSADH). Owing to the fact that
most of the knowledge about GABA as a signaling molecule comes from
mammalian studies, we will briefly review the subject of GABA signaling
in mammals as a basis for discussing aspects of neurotransmitter signaling