Geminiviruses are a group of DNA viruses that infect important crops to cause serious damages in agriculture and lead to a great economic loss worldwide. Betasatellite (DNA β) is a class of single-stranded circular DNA molecules that is associated with monopartite geminiviruses to induce devastating diseases. So far, it has been recognized that DNA β encodes a viral protein βC1 on the complementary strand, which is a pathogenic factor that interferes multiple plant development and defense processes.

On April 18^th, a collaborate work from Dr. Xueping Zhou’s team (Zhejiang University, CHN) and Dr. Xiuren Zhang’s team (Texas A&M university) was published on the PLoS Pathogens，entitled “βC1 protein encoded in geminivirus satellite concertedly targets MKK2 and MPK4 to counter host defense” (https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007728). This work revealed a βC1 protein of geminivirus betasatellite targets plant MKK2 and MPK4 to interfere host MAPK cascade mediated defense.

The MAPK cascade serves as an important line of defense system in perception of microbial pathogens in both animals and plants. A typical MAPK cascade is fulfilled through three sequential kinase reactions that involve in each of MAPK kinase kinases (MEKKs), MAPK kinases (MKKs), and MAPKs. The βC1 protein encoded by the betasatellite of tomato yellow leaf curl China virus (TYLCCNV) was identified to interact with Arabidopsis MKK2 protein and inhibits its activity. In vivo, βC1 suppresses flagellin-induced MAPK activation and downstream responses by targeting MKK2. Also, βC1 could interact with MPK4 and inhibits its kinase activity.

Furthermore, TYLCCNV infection induces a rapid activation of the MAPK cascade, indicating that plant MAPK cascade contributes to the defense against geminivirus. Knocking out MKK2 or MPK4 renders the plant more susceptible to virus infection. And virus infection of geminivirus without betasatellite could induce typical virus symptom in MKK2 or MPK4 knockout tobacco, indicating that mutation in MKK2 or MPK4 renders the plant more susceptible to TYLCCNV, and can complement the lack of βC1. These results suggested that MKK2-MPK4 cascade is one line of defense system against geminivirus in plants, and targeting MKK2-MPK4 to inhibit MAPK activation by βC1 helps symptom induction.

This work shows for the first time that a plant virus both activates and suppresses a MAPK cascade, and the discovery of the ability of βC1 to selectively interfere with the host MAPK activation illustrates a novel virulence function and counter-host defense mechanism of geminiviruses.