GABA and GHB Neurotransmitters in Plants and Animals - Communication in Plants: Neuronal Aspects of Plant Life - page 174

studies of Arabidopsis mutants revealed that disruption of the GABA shunt
by functional knockout of the SSADH gene causes the accumulation of
high levels of reactive oxygen intermediates, necrosis, growth retardation,
and hypersensitivity to environmental stresses, including UV radiation
and heat (Bouché et al. 2003; Fait et al. 2005). However, from these studies
alone it is not clear if GABA functions only as a metabolite en route to
theTCAcycleand/orasasignalingmoleculebyinteractingwithspecial-
ized receptors, and possibly by being transported to specific intracellular
compartments or translocated throughout the plant. Further evidence for
the possible role of GABA as a signaling molecule in plants came from
observations that a gradient of GABA concentration along the pathway of
the pollen tube from the stigma to the ovule is imperative for the proper
guidance of pollen tubes to the ovule, hence for reproduction (Palanivelu
et al. 2003). An Arabidopsis mutant of GABA-T (designated pop2 )showed
disruption of this GABA gradient and the concomitant inability of pollen
tubes to be directed to the ovule, hence causing infertility. This suggests
that GABA plays a role in intercellular signaling in plants, possibly sim-
ilar to its role in the immature brain of animals. If this is the case, it
would imply that GABA interacts with specialized plant receptors, and
that GABA-mediated cell-cell communication involves specialized GABA
transporters. Moreover, GABA, and known GABA agonists and antagonists
affect growth in duckweed (Kinnersley and Lin 2000) and Stellaria longipes
(Kathiresan et al. 1998). GABA activates arginine decarboxylase in soybean
(Turano et al. 1997) and induces gene expression in S. longipes (Kathiresan
et al. 1998). However, the possible existence of GABA receptors in plants
remains elusive. In this regard, genes that are highly homologous to the
animal GABA receptors are not found in the Arabidopsis genome. How-
ever, Arabidopsis has a family of 20 genes encoding putative ionotropic
glutamate receptors (AtGLRs) (Lacombe et al. 2001) based on their ex-
tensive homology with mammal ionotropic glutamate receptors (iGluRs).
Interestingly, iGluRs of mammals contain a domain (LIVBP-like domain)
which shows structural homology with several receptors, including GABA B
receptors. Therefore, it is tempting to speculate that GABA could bind to
this domain and modulate the activity of some of the AtGLRs (Bouché et
al. 2003).
In conclusion, there are clues supporting the notion that GABA is a sig-
naling molecule in plants though much remains to be discovered. Further
clues about the role of GABA could be obtained by studying the precise
dynamic distribution and transport of GABA within plant cells as well as
its long-distance translocation in the plant. The issue of neurotransmitter
transport in plant and nonplant organisms is discussed in the following.