Role of gAminobutyrate and gHydroxybutyrate in Plant Communication

Barry J. Shelp, Wendy L. Allan, and Denis Faure

Abstract The neurotransmitters gamma-aminobutyrate (GABA) and gamma-hydroxy-butyrate (GHB) are found in virtually all prokaryotic and eukaryotic organisms. The physiological roles of these metabolites in plants are not yet clear, but both readily accumulate in response to stress through a combination of biochemical and transcriptional processes. GABA accumulation has been associated with the appearance of extracellular GABA, and evidence is available for a role of extracellular GABA in communications between plants and animals, fungi, bacteria or other plants, although the mechanisms by which GABA functions in communication appear to be diverse. As yet there is no evidence from plants of GHB receptors, GHB signaling or extracellular GHB, although the level of the quorum-sensing signal in Agrobacterium is known to be modulated by GHB.

1 Introduction g-Aminobutyrate (GABA), a nonprotein amino acid, and g-hydroxybutyrate (GHB), a short-chain fatty acid that closely resembles GABA (Fig. 1), are found in virtually all prokaryotic and eukaryotic organisms. They are endogenous constituents of the mammalian nervous system, wherein GABA plays a role in neural transmission and development, and functions through interactions with specialized receptors (GABAA, GABAB, GABAC) and transporters, and GHB serves as a neurotransmitter or neuromodulator postulated to act via a GABAb receptor or an independent GHB-specifc receptor (see review by Fait et al. 2006). When administered, GABA does not cross

Department of Plant Agriculture, Bovey Bldg., Rm 4237, University of Guelph, Guelph, ON, Canada N1G2W1

e-mail: [email protected], [email protected] D. Faure

Institut des Sciences du Végétal, Centre National de la Recherche Scientifique, Gif-sur-Yvette, 91 198, France e-mail: [email protected]

F. Baluska (ed.), Plant-Environment Interactions, Signaling and Communication in Plants, 73 DOI: 10.1007/978-3-540-89230-4_4, © Springer-Verlag Berlin Heidelberg 2009

Fig. 1 a Alternative pathways for GABA metabolism via succinic semialdehyde. b Glyoxylate reductase reaction. Enzymes are in italics. GAD, glutamate decarboxylase; GABA-T, GABA transaminase; GR glyoxylate reductase; SSADH, succinic semialdehyde dehydrogenase; SSR, succinic semialdehyde reductase. (Adapted from Hoover et al. 2007a)

Fig. 1 a Alternative pathways for GABA metabolism via succinic semialdehyde. b Glyoxylate reductase reaction. Enzymes are in italics. GAD, glutamate decarboxylase; GABA-T, GABA transaminase; GR glyoxylate reductase; SSADH, succinic semialdehyde dehydrogenase; SSR, succinic semialdehyde reductase. (Adapted from Hoover et al. 2007a)

the blood-brain barrier, whereas GHB does so with ease, penetrating the brain and producing diverse neuropharmacological and neurophysiological effects. For further details on the roles of GABA and GHB in animals, refer to reviews by Mamelak (1989) and Fait et al. (2006).

Evidence for the existence of GABA receptors in plants and the notion that GABA serves as a signaling molecule is emerging: (1) the growth of Stellaria longipes and duckweed is sensitive to GABA, GABA isomers, and GABA antagonists or agonists (Kathiresan et al. 1998; Kinnersley and Lin 2000); (2) the N-terminal regions of the superfamily of ionotropic glutamate receptors are highly homologous to members of the GABAb receptors (Lacombe et al. 2001; Bouché et al. 2003a, b); (3) a GABA gradient is required for the guidance of the pollen tube through the apoplastic spaces within the Arabidopsis pistil to the female gametophyte (Palanivelu et al. 2003); (4) proteins capable of transporting GABA are present in the plasma membrane of Arabidopsis (Meyer et al. 2006); (5) GABA binding sites are found on the protoplast membrane of both pollen and somatic cells of tobacco, and these sites are involved in the regulation of endogenous Ca2+ level (Yu et al. 2006); (6) Arabidopsis 14-3-3 expression is regulated by GABA in a calcium-dependent manner (Lancien and Roberts 2006); (7) £-2-hexanal responses in Arabidopsis are mediated by GABA (Mirabella et al. 2008); (8) GABA is translocated in phloem, and changes in phloem GABA are positively correlated with nitrate influx during nitrogen deprivation and over the growth cycle of rape (Bown and Shelp 1989; Beuve et al. 2004), and; (9) extracellular GABA induces expression of a plasma membrane-located nitrate transporter and stimulates 15NO3 influx by the root system (Beuve et al. 2004). To date, there is no direct evidence for GHB receptorsor GHB signaling in plants.

While the physiological roles of GABA and GHB in plants are not yet clear, evidence indicates that both metabolites readily accumulate in response to stress (Shelp et al. 1999; Allan et al. 2008). GABA accumulation has been associated with the appearance of extracellular GABA, either in the apoplast or external medium (Secor and Schrader 1985; Chung et al. 1992; Crawford et al. 1994; Solomon and Oliver 2001; Bown et al. 2006). Herein, the evidence for and the mechanisms involved in the accumulation of GABA and GHB are reviewed. This is followed by a description of evidence for their role in communication between plants and other organisms.

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