Metabolism and Functions of 4-Aminobutyrate (GABA) in Plants: Regulation of Glutamate Decarboxylases

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The non-proteinogenic amino acid 4-aminobutyrate (GABA) is being revealed as an important metabolite and signal with multiple and diverse roles in plant life. The primary production of GABA is associated with multiple glutamate decarboxylases (GAD). The flux of carbon through the GABA metabolic pathway (i.e., the GABA shunt) generally increases under abiotic and biotic stress, but the contributions of the various GAD isoforms remain unclear. Most evidence suggests that plant GADs are cytosolic, and unlike bacterial and mammalian GADs, they are regulated by Ca2+/calmodulin, as well as pH. Consequently, stress-induced increases in cytosolic H+ or Ca2+ can cause plant GABA to accumulate. In contrast, mammalian GAD65/67 can be phosphorylated, reside in different subcellular locations (cytosol vs membrane), and respond oppositely to phosphorylation. For example, phosphorylation of threonine95 in GAD65 stimulates activity, whereas phosphorylation of multiple serine residues in its N-terminal region is proposed to regulate unknown aspects of GAD65 function in the synaptic vesicle membrane. Notably, Arabidopsis thaliana GAD1/4 possess the same five Ser residues in their N terminal domain, and AtGAD1/2/4 possess a threonine93-94 doublet; phosphorylation of these residues could potentially modify their location/activity, and membrane association could facilitate interaction between GABA and GABA transporters/GABA-activated anion channels. Recent research with maize seedlings and Arabidopsis suspension cells demonstrated that salinity and inorganic phosphorus (Pi) supply, respectively, increases the phosphorylation of multiple serine residues in the N-terminal region of the single GADs under consideration. Furthermore, our research showed that expression of AtGAD4, but not AtGAD1 and AtGAD2, is rapidly induced in Arabidopsis by salinity, and this is preceded or accompanied by expression of several transcription factors. The proposed research will compare the seemingly contrasting roles of GAD phosphorylation under salinity versus Pi resupply, and investigate the potential involvement of candidate transcription factors in GAD4 induction.