ABSTRACT
This chapter deals with a working model for the rapid regulation of γ-aminobutyrate (GABA) synthesis in response to external stimuli. It presents some possibilities on how longer-term regulation of GABA synthesis under stress might be achieved. The possible important role of plant L-glutamate decarboxylase (GAD) in nitrogen-carbon balance may explain the need for rapid regulation of this enzyme by calcium-calmodulin (CaM) and possibly by other regulators. The chapter discusses the evidence from both in vitro and in vivo studies indicating that plant GAD is a calcium-CaM-regulated enzyme and that CaM is a component of the multisubunit GAD complex. These data are in accord with the observations of rapid activation of GAD in responses to certain stresses. After prolonged stress situations GAD gene expression appears to be involved in modulating the rate of GABA synthesis. The occurrence of mechanisms for rapid and slow modulation of metabolic pathways in response to stresses is likely a general phenomenon not confined to GAD regulation.
