A virulence protein activates SERK4 and degrades RNA polymerase IV protein to suppress rice antiviral immunity

Rice, a major global food staple, is threatened by viral infections that hinder its growth and yield. We have recently shown that the virulence protein P3 of rice grassy stunt virus promotes pathogenesis by inducing proteasome-controlled degradation of the rice RNA polymerase IV (RNA Pol IV) protein NRPD1a controlled by the P3-interacting E3 ubiquitin ligase P3IP1. However, the underlying mechanisms remain elusive. In this study, we show that P3 acts as a virus-encoded transcription activator-like effector to upregulate transcription of somatic embryogenesis receptor kinase 4 (SERK4) by directly binding to its promoter. SERK4 phosphorylates P3IP1 and enhances RNA Pol IVa (NRPD1a) degradation following P3IP1-controlled ubiquitination, leading to attenuated antiviral defense in rice. Thus, our study finds a critical viral virulence strategy by encoding a transcription factor-like protein that activates a host kinase to promote proteasome-controlled degradation of NRPD1a, thereby disarming RNA-directed DNA methylation (RdDM) antiviral defense.