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.

泛素-蛋白酶体系统（UPS）是一种重要的翻译后调控机制，可控制真核生物的许多细胞功能。在这里，我们示出了通过编码P3蛋白的稳定表达稻草矮病毒（RGSV），在负链RNA病毒Bunyavirales，导致类似于引起RGSV疾病症状发展的异常现象，如矮化和过量的分蘖，在转基因水稻植物。我们发现P3的转基因表达和RGSV感染会诱导水稻NUCLEAR RNA聚合酶D1a（OsNRPD1a）的泛素化和UPS依赖性降解，这是植物特异性RNA聚合酶IV（Pol IV）的最大亚基的直系同源基因之一，这是RNA定向DNA甲基化所必需的（ RdDM）。此外，我们确定了一种P3诱导型U盒E3型泛素连接酶，称为P3诱导蛋白1（P3IP1），它与OsNRPD1a相互作用并介导其泛素化和在体外和体内依赖UPS的降解。值得注意的是，这两个击倒OsNRPD1和表达P3IP1在水稻植物中诱导的发育表型类似于RGSV疾病症状。综上所述，我们的发现揭示了一种新颖的毒力机制，植物病原体以此目标针对宿主RNA Pol IV进行UPS依赖性降解以诱导疾病症状。我们的研究还确定了一种E3泛素连接酶，其针对RdDM成分NRPD1的UPS介导的水稻降解。