Cytidine-to-Uridine RNA editing factor NbMORF8 negatively regulates plant immunity to Phytophthora pathogens

Mitochondria and chloroplasts play key roles in plant-pathogen interactions. Cytidine-to-uridine (C-to-U) RNA editing is a critical post-transcriptional modification in mitochondria and chloroplasts that is specific to flowering plants. Multiple organellar RNA editing factors (MORFs) form a protein family that participates in C-to-U RNA editing, but little is known regarding their immune functions. Here, we report the identification of NbMORF8, a negative regulator of plant immunity to Phytophthora pathogens. Using virus-induced gene silencing (VIGS) and transient expression in Nicotiana benthamiana, we show that NbMORF8 functions through regulation of ROS production, SA signaling, and accumulation of multiple RXLR effectors of Phytophthora pathogens. NbMORF8 is localized to mitochondria and chloroplasts, and its immune function requires mitochondrial targeting. The conserved MORF box domain is not required for its immune function. Furthermore, we show that the preferentially mitochondrial localized NbMORF proteins negatively regulate plant resistance against Phytophthora, whereas the preferentially chloroplast localized ones are positive immune regulators. Our study reveals that the C-to-U RNA editing factor NbMORF8 negatively regulates plant immunity to the oomycete pathogen Phytophthora, and that mitochondrial and chloroplast localized NbMORF family members exert opposing effects on immune regulation.