Enhancement of γ-Aminobutyric Acid (GABA) Content in Germinated Soybeans through Glutamate and Cold Stress Treatments

This study examined how exogenous glutamate and cold stress enhance γ-aminobutyric acid (GABA) accumulation in germinated soybeans and regulate key enzymes of the GABA shunt. Treatment with 1.5 mmol L⁻¹ glutamate or 2 h of cold stress (4 °C) increased GABA levels by 10.22 and 10.81%, respectively. Both interventions upregulated glutamate decarboxylase (GAD) while suppressing GABA transaminase. Glutamate directly activated GAD via substrate provision, whereas cold stress enhanced GABA synthesis through polyamine degradation. Metabolomics profiling revealed increased levels of amino acids, phenolic acid, and flavonoids in response to abiotic stress. Flavonoids drove antioxidant activity in glutamate-treated samples, while proline and arginine strongly correlated with antioxidants in cold-stressed groups. KEGG pathway analysis highlighted glutamate metabolism and polyamine degradation as core routes regulating GABA biosynthesis. These findings highlight distinct regulatory networks governing GABA accumulation under stress conditions, offering insights for optimizing bioactive compound production in functional foods.