Mitochondrial ROS trigger interorganellular signaling and prime ER processes to establish enhanced plant immunity.

Reactive oxygen species (ROS) are key signaling molecules in plant development and immunity, but current understanding is primarily focused on apoplastic and chloroplastic ROS. Mitochondria are also a key source of intracellular ROS, yet their contribution to plant immunity is poorly characterized. Here, we studied mitochondrial ROS (mROS) function in plant-pathogen interactions, deploying genetically encoded sensors, assorted fluorescent markers, and genetic approaches to track mROS, specifically H2O2, dynamics and identify interorganelle contact sites. We unexpectedly found a mitochondria-endoplasmic reticulum (ER) ROS signal cascade functioning independently of apoplastic and chloroplastic ROS in plant immunity. mROS initiate immune responses induced by the oomycete pathogen Phytophthora parasitica and promote mitochondria-ER association. These enhanced mitochondria-ER membrane associations are required for transfer of mROS signals and initiation of extensive unfolded protein responses. We conclude that mROS transfer via mitochondria-ER membranes to the ER lumen is an underappreciated yet essential component in plant defense.