Transaminase that degrades gamma-amino butyric acid (GABA) and uses pyruvate or glyoxylate as amino-group acceptor, but not 2-oxoglutarate. The pyruvate-dependent activity is reversible while the glyoxylate-dependent activity is irreversible. Cannot use beta-alanine, ornithine, acetylornithine, serine, glycine, asparagine, glutamine, glutamate, valine, leucine, isoleucine, methionine, phenylalanine, histidine, lysine, arginine, aspartate, threonine, tyrosine, tryptophan, proline, or cysteine as amino donors. Modulates steady-state GABA levels in diploid pistil cells and the haploid pollen tube. Involved in the formation of a gradient of GABA along the pollen tube path. Involved in the maintenance of the shoot apical meristem (SAM) structure and subsequent adaxial-abaxial axis-dependent development of cotyledons and leaves (PubMed:21690177). 4-aminobutanoate + pyruvate = L-alanine + succinate semialdehyde 4-aminobutanoate + glyoxylate = glycine + succinate semialdehyde Inhibited by gamma-vinyl-gamma-aminobutyrate (vigabatrin), beta-alanine and ornithine. Optimum pH is 9.0 in the forward reaction. Expressed in flowers, siliques, stems, leaves, shoots and roots. Detected in reproductive tissues, in the stigma, style, abscission zone of siliques, stamens and pollen. Not found in pollen tubes or the transmitting tract. In vegetative tissues, found in dark-grown hypocotyls, leaves, guard cells and primary roots, including the root tips and the elongation zones. Not found in the division zone of the root. Expressed in the outermost layer of the shoot apical meristem (SAM) (PubMed:21690177). Not induced by hypoxia. Up-regulated by salt stress. No vegetative phenotype. Oversensitivity to ionic stress but not to osmotic stress. Sustained roots growth upon treatment with E-2-hexenal. Increased gamma-amino butyric acid (GABA) in leaves and flowers and defects in pollen tube growth, guidance and fertility. Cell elongation defects. Suppresses partially the ENF1 disruption pleiotropic developmental phenotypes (including abaxialized and adaxialized leaves). Enlarged expressing region of FIL, thus leading to an increase in the size of leaf abaxial region associated with altered GABA and SucA levels in shoots; however, the enf1 gabat1 double mutant has a restored almost normal FIL expression pattern (PubMed:21690177). Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family.