Overexpression of SIMK in Menadione-Treated Alfalfa Enhances Antioxidant Machinery and Leads to Oxidative Stress Resistance

Mitogen-activated protein kinases (MAPKs) transduce stress and developmental signals related to the production of reactive oxygen species (ROS). Alfalfa (Medicago sativa L.) is a valuable forage and human nutrition crop, however, the involvement of MAPKs in plant resistance to oxidative stress is poorly understood in this species. Therefore, we elucidated the role of STRESS-INDUCED MAPK (SIMK) in alfalfa response to menadione, a compound inducing ROS generation, exploiting transgenic alfalfa lines with contrasting SIMK abundance. SIMK was activated by short-term menadione treatment and relocated from the nucleus to the cytoplasm. Proteomic analysis revealed that menadione caused changes in the abundance of proteins involved in metabolism, oxidative stress, biotic stress response, detoxification of carbonyl species, glutathione homeostasis, chloroplast protein turnover, photosynthesis, and membrane trafficking. Genetic manipulations of SIMK altered the abundance of proteins involved in mitochondrial and chloroplast protein import and processing, as well as GLUTATHIONE S-TRANSFERASES (GSTs). Increased GST abundance and activity in roots, and modifications in mitochondrial and chloroplast protein turnover might be responsible for the elevated oxidative stress resistance of alfalfa line overexpressing SIMK. This was supported by the reduced ROS levels in this line. These results reveal a complex nature of plant stress response and suggest a new role of SIMK in the alfalfa resistance to menadione-induced oxidative stress.