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

Feb 23, 2020Molecular plant

A virus-triggered protein causes breakdown of a key RNA-making enzyme during rice infection

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Abstract

Stable expression of the P3 protein from Rice grassy stunt virus in transgenic rice plants leads to developmental abnormalities, including dwarfing and excess tillering.

Transgenic expression of P3 protein and RGSV infection both result in degradation of rice NUCLEAR RNA POLYMERASE D1a (OsNRPD1a) through the ubiquitin-proteasome system.
OsNRPD1a is crucial for RNA-directed DNA methylation, a process important for regulating gene expression in plants.
A novel E3 ubiquitin ligase, P3-inducible protein 1 (P3IP1), is identified as responsible for the ubiquitination and degradation of OsNRPD1a.
Knockdown of OsNRPD1 and overexpression of P3IP1 in rice also produce developmental phenotypes resembling those caused by RGSV.
These findings suggest a new mechanism by which plant pathogens may disrupt host cellular functions to induce disease.

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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.

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

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