Highway or byway: the metabolic role of the GABA shunt in plants Aaron Fait 1 , Hillel Fromm 2 , Dirk Walter 3 , Gad Galili 4 and Alisdair R. Fernie 1 1 Department Willmitzer, Max Planck-Institut fu ¨ r Molekulare Pflanzenphysiologie, Am Mu ¨ hlenberg 1, 14476 Potsdam-Golm, Germany 2 Department of Plant Sciences, the Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel 3 Bioinformatics Group, Max Planck-Institut fu ¨ r Molekulare Pflanzenphysiologie, Am Mu ¨ hlenberg 1, 14476 Potsdam-Golm, Germany 4 Department of Plant Sciences, the Weizmann Institute of Science, 76100 Rehovot, Israel Much of the recent work on the g-aminobutyrate (GABA) shunt in plants has concentrated on stress/pest-associ- ated and signalling roles. However, fifty years after the structural elucidation of the pathway, aspects of its regu- lation and even of its biological significance remain large- ly obscure. Here, we assess the importance of GABA metabolism in plants, reviewing relevant biological cir- cumstances and taking advantage of high-throughput data accessibility and computational approaches. We discuss the premise that GABA metabolism plays a major role in carbon and nitrogen primary metabolism. We further evaluate technological developments that will likely allow us to address the quantitative importance of this shunt within the biological processes to which it contributes. The GABA shunt 60 years on In plants, the g-aminobutyrate (GABA) shunt (Figure 1a) was first reported more than half a century ago in potato (Solanum tuberosum) tuber [1], however, its functional significance is still not fully understood. The pathway starts with the decarboxylation of glutamate (Glu) to pro- duce GABA and CO 2 in the cytosol. GABA is then pre- sumably transported to the mitochondria by an as yet unidentified GABA transporter, where it is converted to succinic semialdehyde (SSA). Subsequently, SSA is con- verted either to succinate (Figure 1a) or 4-hydroxybutyrate (GHB) (Figure 1e). The metabolite GABA is proposed to be involved in a legion of cellular processes ranging from neuronal inhibition in animals [2] to pollen-tube develop- ment in Arabidopsis thaliana (Arabidopsis) [3]. Hypothe- ses on its role in plants have flourished, largely owing to its rapid accumulation in response to biotic and abiotic stres- ses [4] and its high concentration in various tissues [5–8]. Postulates to explain such alterations in GABA metab- olism include roles in signalling, herbivore deterrence, pH regulation, redox regulation, energy production and main- tenance of carbon/nitrogen (C/N) balance (see Ref. [9] and references therein). However, when changes in GABA are looked at in the context of broader changes of metabolism, interesting patterns emerge. For example, the GABA shunt has been shown to be activated by light, during developmental phases and in an N status-dependent manner [7,10–12], as well as in parallel to other changes that occur in central metabolism during plant growth [13]. Given that the published scientific literature on the role of the shunt in signalling and in herbivore deterrence has been recently expertly reviewed [9,14,15], this article will focus on the involvement of the GABA shunt in central metabolic processes. Moreover, we will attempt to place changes in GABA metabolism within a global context through the analysis of publicly available functional genomics datasets of Arabidopsis and tomato (Solanum lycopersicum). Configurations of the GABA shunt and its intimately associated pathways To comprehend fully the myriad of processes that the GABA shunt is potentially involved in, it seems reasonable to start with a description of the possible pathway struc- tures to which it can contribute. The GABA shunt itself The classic depiction of the GABA shunt involves three main reactions catalyzed by glutamate decarboxylase (GAD), GABA transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH), respectively (Figure 1a). The regulation of these enzymes is relatively well characterized. In plants, the cytosolic decarboxylation of Glu to GABA, catalyzed by GAD, is generally controlled by Ca 2+ /calmodu- lin [16]. Removal of the GAD calmodulin-binding domain results in altered Glu and GABA metabolism and abherrant plant development in tobacco (Nicotiana tabacum) [17].A rice (Oryza sativa) GAD that, intriguingly, does not depend on Ca 2+ /calmodulin for activation has recently been isolated [18]. GABA transaminase can use either pyruvate or 2- oxoglutarate (2OG) as amino acceptor to catalyze the con- version of GABA to SSA. Use of the former leads to alanine (Ala) production, whereas the latter leads to Glu formation and thus would potentially set up a futile cycle, since at least part of the Glu recycled by the transamination of GABA would eventually feed back into the GABA shunt [19,20]. This might serve to maintain the mitochondrial GABA/Glu balance as depicted in Figure 1b. Irrespective of the affinity nature of GABA-T, a coordinated regulation of gene expres- sion of the GABA shunt might thus represent a key regu- latory factor in carbon and nitrogen partitioning, linking Opinion Corresponding author: Fait, A. (fait@mpimp-gom.mpg.de). 14 1360-1385/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2007.10.005 Available online 21 December 2007