Identification and Functional Characterization of Glutamate Decarboxylase (GAD) Genes in Potato (Solanum tuberosum L.) and Analysis of Their Expression under Drought and Salt Stress Conditions

Gamma-aminobutyric acid (GABA) plays a critical role in plant stress responses and development by regulating osmotic balance, mitigating oxidative stress, and maintaining cellular homeostasis. Glutamate decarboxylase (GAD) is the first and key enzyme in the GABA biosynthesis pathway. In this study, three distinct GAD genes—StGAD1, StGAD2, and StGAD3—were identified, each distributed on separate chromosomes, indicating non-redundant functional roles. Expression profiling revealed that StGAD1 is the primary stress-responsive gene, with significant upregulation in both roots and leaves under drought stress, promoting GABA accumulation to enhance water-use efficiency and reduce oxidative damage. In contrast, StGAD2 showed limited involvement, maintaining basal GABA levels but displaying minimal stress-induced expression. Protein-protein interaction analysis highlights functional relationships among the GAD proteins, with StGAD1 and StGAD2 sharing significant homology. The findings suggest that GABA metabolism, primarily driven by StGAD1, plays a more prominent role in drought tolerance than salt stress adaptation, where other regulatory mechanisms such as ion homeostasis may be more critical. This study provides foundational insights into the molecular mechanisms of GAD-mediated stress responses in potatoes, offering potential avenues for enhancing crop resilience through targeted genetic strategies.