C1 protein encoded in geminivirus satellite concertedly targets MKK2 and MPK4 to counter host defense

Plant viruses have evolved multiple strategies to overcome host defense to establish an infection. Here, we identified two components of a host mitogen-activated protein kinase (MAPK) cascade, MKK2 and MPK4, as bona fide targets of the C1 protein encoded by the betasatellite of tomato yellow leaf curl China virus (TYLCCNV). C1 interacts with the kinase domain of MKK2 and inhibits its activity. In vivo, C1 suppresses flagellin-induced MAPK activation and downstream responses by targeting MKK2. Furthermore, C1 also interacts with MPK4 and inhibits its kinase activity. TYLCCNV infection induces the activation of the MAPK cascade, mutation in MKK2 or MPK4 renders the plant more susceptible to TYLCCNV, and can complement the lack of C1. This work shows for the first time that a plant virus both activates and suppress a MAPK cascade, and the discovery of the ability of C1 to selectively interfere with the host MAPK activation illustrates a novel virulence function and counter-host defense mechanism of geminiviruses. Author summary We identified a plant MAPK cascade that contributes to the defense against geminiviruses. Rapid activation of MAPKs was observed upon geminivirus infection. The C1 protein of geminivirus betasatellite interacts with MKK2 and MPK4 and inhibits their kinase activity. Also, C1 suppresses flagellin-induced MAPK activation and downstream responses. Knocking out two components of this cascade, MKK2 or MPK4 in tobacco renders the plant more susceptible to geminivirus infection and virus infection of geminivirus without betasatellite induced typical virus symptom in MKK2 or MPK4 knockout tobacco, indicating that MKK2 or MPK4 mutant can complement the lack of C1 in virus infection. This study unveils MAPK cascades as novel players in anti-viral defense and illustrates how plant viruses have in turn evolved to suppress this pathway.