Cf-4- and Cf-5-triggered plant immunity: Similarities and differences
Plant immunity is, amongst others, mediated by receptor like proteins (RLPs), which are localized on the plasma membrane. RLPs recognize extracellular immunogenic patterns (ExIPs) originating from pathogens or derived from the host itself, which leads to extracellularly triggered immunity (ExTI). Cf proteins, which are well-known RLPs of tomato (Solanum lycopersicum) confer resistance against the fungal pathogen Fulvia fulva. Cf-9, Cf-4, Cf-2 and Cf-5 are well-known examples of Cf proteins, mediating recognition of the matching ExIPs Avr9, Avr4, Avr2 and Avr5, respectively, which are secreted effectors of F. fulva and trigger hypersensitive response (HR)-related cell death in tomato plants carrying these Cf proteins. Although all these Cf proteins confer proper resistance to the fungus, Cf-9 and Cf-4 trigger a stronger and faster cell death than Cf-5 and Cf-2. It is unknown whether these phenotypical differences arise from variations in the molecular mechanism of the cellular immune response that is initiated by the Cf proteins, and whether this phenotypic difference correlates with varying degrees in the intensity and timing of the triggered immune responses and robustness of the resistance. To try to answer these questions, in this study the immune responses triggered by Cf-4 and Cf-5 were compared. Cf-4 and Cf-5 share the same core upstream signaling components to trigger HR-related cell death in Nicotiana benthamiana. In tomato, both receptors induce rapid MAPK activation, which is more sustained for the Cf-5/Avr5 combination. Both Avr4 and Avr5 induce an apoplastic burst of reactive oxygen species (ROS), independently of the presence of their matching receptors, while remaining dependent on RBOHB for this ROS burst. Full transcriptome analysis at 3 and 7 hours after immune activation revealed a large shared set of differentially expressed genes, alongside qualitative and quantitative differences, with the Cf-5/Avr5 combination inducing a broader transcriptional reprogramming. Despite these differences, Cf-4 and Cf-5 confer a comparable level of resistance to F. fulva. These results demonstrate that Cf-4 and Cf-5 share conserved immune initiation mechanisms, but diverge in downstream signaling dynamics, and that the intensity and timing of the HR-related cell death do not affect the robustness of the resistance.