GABA and GHB Neurotransmitters in Plants and Animals - Communication in Plants: Neuronal Aspects of Plant Life - page 172

γ Fig. 12.1. Overview of the -aminobutyric acid ( GABA ) shunt and relevant metabolic

branches in plants and other organisms. GABA is metabolized via a short pathway known
as the GABA shunt, which bypasses two steps of the tricarboxylic acid cycle ( TCA ), and
is composed of three enzymes: the cytosolic-localized Ca 2+ -calmodulin ( CaM )-regulated
glutamate decarboxylase ( GAD ), and the mitochondrial-localized GABA trans -aminase
( GABA-T ) and succinic semialdehyde dehydrogenase ( SSADH ). The SSADH substrate
succinic semialdehyde ( SSA )isalsocatabolizedto γ -hydroxybutyrate ( GHB )byGHB-

dehydrogenase ( GHBDH ), which is cytosolic in mammals. GHB catabolism occurs in mam-
mals either in the reverse direction by GHBDH/semialdehyde reductase (see text) or by
the mitochondrial-localized L-glucuronate reductase ( GR ; EC 1.1.1.19). In plants, GHB
catabolism has not been characterized. Chemical structures are given for GABA-shunt
substrates, intermediates, and products. Unknown functions and transport systems are
depicted as dotted arrows and question marks . Glu glutamate, α α

-KG

-ketoglutarate, GABA-

α

TK / P -ketoglutaric acid/pyruvate-dependent GABA-T

γ in plants. We also address the occurrence, in plants, of -hydroxybutyrate

(GHB), a by-product of the GABA shunt (Fig. 12.1) and a neurotransmitter,
which was only recently discovered in plants (Allan et al. 2003; Breitkreuz et
al. 2003; Fait et al. 2005). For further details on the different roles of GABA
in plants the readers are referred to two recent reviews (Bouché et al. 2003;
Bouché and Fromm 2004) and to earlier reviews by Shelp et al. (1999) and
Kinnersley and Turano (2000).