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

The ubiquitin-proteasome system(UPS)is an important post-translational regulatory mechanism that controls many cellular functions in eukaryotes.Here,we show that stable expression of P3 protein encoded by Rice grassy stunt virus(RGSV),a negative-strand RNA virus in the Bunyavirales,causes developmental abnormities similar to the disease symptoms caused by RGSV,such as dwarfing and excess tillering,in transgenic rice plants.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 orthologs of the largest subunit of plant-specific RNA polymerase IV(Pol IV),which is required for RNA-directed DNA methylation(RdDM).Furthermore,we identified a P3-inducible U-box type E3 ubiquitin ligase,designated as P3-inducible protein 1(P3IP1),which interacts with OsNRPD1a and mediates its ubiquitination and UPS-dependent degradation in vitro and in vivo.Notably,both knockdown of OsNRPD1 and overexpression of P3IP1 in rice plants induced developmental phenotypes similar to RGSV disease symptomss.Taken together,our findings reveal a novel virulence mechanism whereby plant pathogens target host RNA Pol IV for UPS-dependent degradation to induce disease symptoms.Our study also identified an E3 ubiquitin ligase,which targets the RdDM compotent NRPD1 for UPS-mediated degradation in rice.

Arabidopsis ENOR3 regulates RNAi-mediated antiviral defense

Viruses can infect host plants to cause severe diseases and substantial agricultural loss, while plants have evolved RNA interference （RNAi） strategy to defend against viral infection. Despite enormous efforts, only a few host proteins in RNAi pathway were shown to mediate antiviral defense, including RNA-dependent RNA polymerase I （RDRI）, RDR6, DICER-LIKE 2 （DCL2） and DCL4. In this study, we carried out a genetic screen for antiviral factors of RNAi pathway in Arabidopsis rdr6 background via inoculation with a 2b- deficient Cucumber Mosaic Virus （CMV-△2b）. We identified a mutant susceptible to CMV-△2h, referred to as enhancer o ojrdr6 （enor） 3-1 rdr6, and found that ENOR3 encodes a functionally unknown protein with high homology to the mammalian Non Imprinted in Prader-Willi/Angelman （NIPA） magnesium transporters. ENOR3 inhibits accumulation of CMV-△2b and acts additively with RDR1, RDR6, DCL2 and DCL4 in antivira/ defense. These results uncover that ENOR3 is a key component in antiviral RNAi Dathwav, and provide new insights into antiviral immunity.