Strategies and tools for studying the metabolism and function of γ-aminobutyrate in plants. I. Pathway structure

γ-Aminobutyrate (GABA) is a ubiquitous four-C, nonprotein, amino acid that has been linked to stress, signaling, and storage in plants. In this paper, we discuss analytical, enzyme-linked, and colorimetric methods for analyzing GABA and related metabolites, and review tracer evidence for the derivation of GABA from glutamate and its subsequent catabolism to succinic semialdehyde and either succinate or γ-hydroxybutyrate. Also, we describe biochemical, complementation, bioinformatic, recombinant, and modelling strategies for identifying genes and investigating properties of the encoded proteins responsible for transport and metabolism of GABA. For Arabidopsis, evidence supports the involvement of a plasma membrane GABA transporter, a mitochondrial GABA permease, a cytosolic Ca 2+ /calmodulin- and pH-regulated cytosolic glutamate decarboxylase, a pyruvate- and glyoxylate-regulated mitochondrial GABA transaminase, a redox-regulated mitochondrial succinic semialdehyde dehydrogenase, and redox-regulated glyoxylate/succinic semialdehyde reductases located in both cytosol and plastid, respectively. This simple biochemical model does not account for species and tissue differences in the isoform complement of GABA pathway enzymes or transcriptional control of the pathway. In a companion review, we provide a more integrated view of GABA metabolism and function.