Overexpression of IbGAD4 enhances salt tolerance in sweetpotato by modulating the GABA pathway.

γ-Aminobutyric acid (GABA) is a non-proteinogenic amino acid that plays a pivotal role in plant development and adaptation to environmental stressors. Although sweetpotato (Ipomoea batatas) is a nutritionally valuable crop, the regulatory mechanisms governing the GABA biosynthetic pathway and its role in salt stress tolerance remain poorly understood. In this study, we identified and functionally characterized IbGAD4, a glutamate decarboxylase gene from sweetpotato. IbGAD4 was predominantly expressed in roots and developmentally regulated, with expression peaking at 90 days in storage roots. Meanwhile, IbGAD4 expression was induced by salt stress. Overexpression of IbGAD4 in transgenic sweetpotato significantly elevated endogenous GABA levels and upregulated key genes in the GABA shunt and tricarboxylic acid (TCA) cycle. Under salt stress conditions, IbGAD4 overexpression enhanced plant tolerance, as evidenced by improved growth, increased antioxidant enzyme activity, reduced malondialdehyde (MDA) levels, and transcriptional activation of salt-responsive genes, including components of the SOS pathway and ABA biosynthesis. Yeast one-hybrid screening using fragments of the IbGAD4 promoter identified IbMYB44 as a transcriptional repressor of IbGAD4. Notably, IbMYB44 expression was downregulated by salt stress, suggesting a stress-responsive regulatory circuit. This study provides a foundation for developing sweetpotato cultivars with improved resilience to abiotic stress by modulating the GABA pathway.