Antioxidant Gene Overexpression Enhances Salt Tolerance in Transgenic Peppermint (Mentha × piperita L.)

High-menthol mint varieties are traditionally grown in regions characterized by a continental climate, characterized by pronounced seasonality with cold winters and hot, usually dry summers. Climate change means that this plant is increasingly grown in extreme conditions, where primary and secondary soil salinization is the norm. However, by its origin, mint is a typical glycophyte, that is, a plant that is very sensitive to the effects of salt, suspending the gain of biomass under conditions of increasing salinity and limited available water. We demonstrate an attempt to desensitize and increase tolerance of peppermint to high-salinity conditions corresponding to 100 mM and 150 mM NaCl. To maintain productivity under these uncharacteristic conditions, it was necessary to modify plant metabolism in such a way as to effectively utilize the non-specific potential inherent in plant genetics for resistance to stress factors characteristic of other abiotic stresses, such as cold or drought, which cause oxidative and osmotic damage to metabolism and growth. By obtaining peppermint plants transgenic for Fe-SOD and codA genes, we intended to induce mild internal stress, inducing protective mechanisms, without damaging the systems that ensure resistance due to specific anti-stress systems. An in vitro experiment confirmed that with this strategy, despite a significant decrease in growth and biomass, a decrease in the pool of photosynthetic pigments, and changes in the morphology of epidermal cells, the parameters recording plant viability were significantly higher in transgenic lines than in the original peppermint genotype. We also confirmed the absence of significant growth inhibition in transgenic lines under optimal conditions. This method of inducing non-specific resistance will be quite effective, but only if the salt concentration is subsequently reduced, since it is difficult to achieve a fundamental shift, manifested in the complete removal of the effects of stress in such a highly sensitive glycophyte as mint.