Abstract
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.