Mitogen-Activated Protein Kinase Kinase OsMEK2 Positively Regulates Ca2+ Influx and Ferroptotic Cell Death during Rice Immune Responses

Mitogen-activated protein (MAP) kinase (MAPK) signaling pathway is important in plant immune responses, involved in iron- and reactive oxygen species (ROS)-dependent ferroptotic cell death mediated by Ca²⁺. High Ca²⁺ influx triggered iron-dependent ROS accumulation, lipid peroxidation, and subsequent hypersensitive response (HR) cell death in rice (<i>Oryza sativa</i>). Apoplastic Ca²⁺ chelation by EGTA during avirulent <i>Magnaporthe oryzae</i> infection altered Ca²⁺, ROS, and Fe²⁺ accumulation, increasing rice susceptibility to infection. By contrast, acibenzolar-<i>S</i>-methyl (ASM), a plant defense activator, significantly enhanced Ca²⁺ influx, and H₂O₂ accumulation, triggering rice ferroptotic cell death during virulent <i>Magnaporthe oryzae</i> infection. Here, we report a novel role of the MAPK signaling pathway in regulating cytoplasmic Ca²⁺ increase during ferroptotic cell death in rice immunity, using the <i>ΔOsmek2</i> knockout mutant rice. The knockout of rice <i>OsMEK2</i> impaired the ROS accumulation, lipid peroxidation, and iron accumulation during avirulent <i>M. oryzae</i> infection. This study has shown that OsMEK2 could positively regulate iron- and ROS-dependent ferroptotic cell death in rice by modulating the expression of <i>OsNADP-ME</i>, <i>OsRBOHB</i>, <i>OsPLC</i>, and <i>OsCNGC</i>. This modulation indicates a possible mechanism for how OsMEK2 participates in Ca²⁺ regulation in rice ferroptotic cell death, suggesting its broader role in plant immune responses in response to <i>M. oryzae</i> infection.