GSTU7 affects growth performance and acts as an antagonist of oxidative stress induced by methyl viologen

Plant glutathione S-transferases (GSTs) are glutathione-dependent enzymes with versatile functions, mainly related to detoxification of electrophilic xenobiotics and peroxides. The Arabidopsis genome codes for 53 GSTs, divided into seven subclasses, however understanding of their precise functions is limited. A recent study showed that class II TGA transcription factors TGA2, TGA5 and TGA6 are essential for tolerance of UV-B-induced oxidative stress and that this tolerance is associated with an antioxidative function of cytosolic tau-class GSTUs. Specifically, TGA2 controls the expression of several GSTUs under UV-B light and constitutive expression of GSTU7 in the tga256 triple mutant is sufficient to revert the UV-B-susceptible phenotype of tga256. To further study the function of GSTU7, we characterized its role in mitigation of oxidative damage caused by the herbicide methyl viologen (MV). Under non-stress conditions, gstu7 null mutants were smaller than wild-type (WT) plants and delayed in the onset of the MV-induced antioxidative response, which led to accumulation of hydrogen peroxide and diminished seedling survival. Complementation of gstu7 by constitutively expressed GSTU7 rescued these phenotypes. Furthermore, live monitoring of the glutathione redox potential in intact cells with the fluorescent probe Grx1-roGFP2 revealed that GSTU7 overexpression completely abolished the MV-induced oxidation of the cytosolic glutathione buffer compared to WT plants. GSTU7 was found to act as a glutathione peroxidase able to complement the lack of peroxidase-type GSTs in yeast. Together, these findings show that GSTU7 is crucial in the antioxidative response by limiting oxidative damage and thus protecting cells from oxidative stress.