A Bunyavirus-Inducible Ubiquitin Ligase Targets RNA Polymerase IV for Degradation during Viral Pathogenesis in Rice

Abstract Ubiquitin-proteasome system (UPS) is an important post-translational gene regulation mechanism that controls many cellular functions in eukaryotic cells. Here we show that stable expression of P3 protein encoded by Rice grassy stunt virus (RGSV), a negative-strand RNA virus in the Bunyavirales, caused developmental abnormities such as dwarfing and excess tillering in transgenic rice plants similar to the disease symptoms of RGSV. We found that both transgenic expression of P3 and RGSV infection induce ubiquitination and UPS-dependent degradation of rice NUCLEAR RNA POLYMERASE D1a (OsNRPD1a), one of two orthologues of the largest subunits of plant-specific RNA polymerase IV (Pol IV) involved in the pathway of RNA-directed DNA methylation (RdDM). We identified a P3-inducible U-box type E3 ubiquitin ligase, designated as P3-inducible protein 1 (P3IP1), which interacted with OsNRPD1a and mediated its ubiquitination and UPS-dependent degradation in vitro and in vivo. Notably, both OsNRPD1 knockdown and P3IP1 over-expression in rice plants induced similar RGSV symptom-like developmental phenotypes. Together, our findings reveal a novel virulence mechanism of plant pathogens by targeting the host RNA polymerase IV for UPS-dependent degradation. Our results also provide the first evidence for RdDM as a potential target of UPS.