Body of Abstract: Glutathione S-transferases (GSTs) are ubiquitous multifunctional enzymes that detoxify xenobiotics and toxic endogenous metabolites. Plant GST families are extensive and commonly arrayed in tandem genes or large gene clusters in the genome due to gene duplication. This increases the chance of functional redundancy and makes their individual characterization highly challenging. Arabidopsis has 53 GSTs grouped into seven classes, of which the plant-specific class tau (GSTU) is the largest. GSTUs have two GSH-dependent enzymatic activities: (1) GSH-binding to the electrophilic center of potentially toxic compounds (GST activity) and (2) a GSH-dependent peroxidase function towards lipid peroxides (GPOX activity). Among all GSTUs, GSTU24 and GSTU25 stand out with the highest GST and GPOX activities, and the genes coding for these are the most highly induced genes under biotic and abiotic stress compared to other stress-related genes, suggesting a potential role in stress defense. Yet, their physiological role under oxidative stress is unknown, and no distinct phenotype reported in null mutants for these genes. We have isolated a mutant line for GSTU25, which shows accelerated growth and increased resistance towards methyl viologen (MV)-induced oxidative stress due to reciprocal genetic compensation through upregulation of other GSTUs. This work aims to dissect the critical functions of GSTU24 and GSTU25 under oxidative stress. As an experimental strategy, we generated variants of GSTU24 and GSTU25 affected in their GSH-dependent functions and evaluated them in a yeast strain deficient in GST activity and an Arabidopsis mutant lacking the transcription factors TGA2, TGA5, and TGA6, which is compromised in the expression of 12 GSTUs, including GSTU24 and GSTU25. The respective results provide valuable insight into the function of GSTs in plants and their potential applications in stress defense.