Functional Characterization of RNA Silencing Suppressor P0 from Pea Mild Chlorosis Virus

New persistent plant RNA virus carries mutations to weaken viral suppression of antiviral RNA interference

Persistent plant viruses are widespread in natural ecosystems. However, little is known about why persistent infection with these viruses may cause little or no harm to their host. Here, we discovered a new polerovirus that persistently infected wild rice plants by deep sequencing and assembly of virus-derived small-interfering RNAs (siRNAs). The new virus was named Rice tiller inhibition virus 2 (RTIV2) based on the symptoms developed in cultivated rice varieties following Agrobacterium-mediated inoculation with an infectious RTIV2 clone. We showed that RTIV2 infection induced antiviral RNA interference (RNAi) in both the wild and cultivated rice plants as well as Nicotiana benthamiana. It is known that virulent virus infection in plants depends on effective suppression of antiviral RNAi by viral suppressors of RNAi (VSRs). Notably, the P0 protein of RTIV2 exhibited weak VSR activity and carries alanine substitutions of two amino acids broadly conserved among diverse poleroviruses. Mixed infection with umbraviruses enhanced RTIV2 accumulation and/or enabled its mechanical transmission in N. benthamiana. Moreover, replacing the alanine at either one or both positions of RTIV2 P0 enhanced the VSR activity in a co-infiltration assay, and RTIV2 mutants carrying the corresponding substitutions replicated to significantly higher levels in both rice and N. benthamiana plants. Together, our findings show that as a persistent plant virus, RTIV2 carries specific mutations in its VSR gene to weaken viral suppression of antiviral RNAi. Our work reveals a new strategy for persistent viruses to maintain long-term infection by weak suppression of the host defence response.

Systematic mutagenesis of Polerovirus protein P0 reveals distinct and overlapping amino acid functions in Nicotiana glutinosa

The protein P0 serves as the viral suppressor of RNA silencing (VSR) for poleroviruses, but elicits the hypersensitive response (HR) in specific Nicotiana species. We subjected P0 proteins from turnip yellows virus (P0^Tu) and potato leafroll virus (P0^PL) to serial deletion and performed extensive site-directed mutagenesis of P0^Tu. Most deletions of the N-terminus and many substitution mutations disrupted both HR elicitation and VSR activity. Two conserved blocks of amino acid residues were found to be associated with HR. A double lysine to arginine substitution in HR-specific block 1 caused P0^Tu to elicit a more robust HR. Conversely, deletion or mutation of block 2 in the C-terminus preserved VSR activity, but impaired HR elicitation, allowing virus escape from Nicotiana glutinosa resistance when expressed in the heterologous potato virus X vector. Our observations suggest that P0 residues responsible for suppressing RNA silencing and eliciting HR have overlapping, but distinct functions.

The Carboxyl Terminal Regions of P0 Protein Are Required for Systemic Infections of Poleroviruses

P0 proteins encoded by poleroviruses Brassica yellows virus (BrYV) and Potato leafroll virus (PLRV) are viral suppressors of RNA silencing (VSR) involved in abolishing host RNA silencing to assist viral infection. However, other roles that P0 proteins play in virus infection remain unclear. Here, we found that C-terminal truncation of P0 resulted in compromised systemic infection of BrYV and PLRV. C-terminal truncation affected systemic but not local VSR activities of P0 proteins, but neither transient nor ectopic stably expressed VSR proteins could rescue the systemic infection of BrYV and PLRV mutants. Moreover, BrYV mutant failed to establish systemic infection in DCL2/4 RNAi or RDR6 RNAi plants, indicating that systemic infection might be independent of the VSR activity of P0. Partially rescued infection of BrYV mutant by the co-infected PLRV implied the functional conservation of P0 proteins within genus. However, although C-terminal truncation mutant of BrYV P0 showed weaker interaction with its movement protein (MP) when compared to wild-type P0, wild-type and mutant PLRV P0 showed similar interaction with its MP. In sum, our findings revealed the role of P0 in virus systemic infection and the requirement of P0 carboxyl terminal region for the infection.

Comparative Analysis of Biological Characteristics among P0 Proteins from Different Brassica Yellows Virus Genotypes

Simple Summary

Polerovirus P0 proteins are multifunctional proteins. Besides their viral suppressor of RNA silencing (VSR) functions, several P0 proteins can induce a cell death phenotype within the infiltrated region of Nicotiana benthamiana or Nicotiana glutinosa. Recently, the Brassica yellows virus (BrYV) genotype A P0 protein (P0^BrA) was identified as a strong viral suppressor of RNAi. In this study, we compared the features of the P0 proteins encoded by different genotypes of BrYV and revealed their difference in inducing cell death in N. benthamiana. Key residues in P0^BrA for inducing cell death were also identified. We also showed that all three BrYV genotypes had synergistic interaction with PEMV 2 in N. benthamiana. This study provides theoretical guidance for controlling the viral disease caused by poleroviruses in the future.

Abstract

Brassica yellows virus (BrYV) is a tentative species of the genus Polerovirus, which has at least three genotypes (A, B, and C) in China. The P0 protein of BrYV-A (P0^BrA) has been identified as a viral suppressor of RNA silencing (VSR), which can also induce cell death in infiltrated Nicotiana benthamiana leaves. In this study, we demonstrated that the cell death induced by P0^BrA was accompanied by the accumulation of reactive oxygen species (ROS) and increased Pathogenesis-related protein genes-1 (PR1) expression. Meanwhile, this cell death phenotype was delayed by salicylic acid (SA) pretreatment. Biological function comparison of the three P0 proteins showed that transiently expressed P0^BrB or P0^BrC induced a significantly delayed and milder cell death response compared with P0^BrA. However, like P0^BrA, they also suppressed local and systemic RNA silencing. Six residues of P0^BrA essential for inducing cell death were identified by comparative analysis and amino acid substitution assay. We also show that all three BrYV genotypes have synergistic interactions with pea enation mosaic virus 2 (PEMV 2) in N. benthamiana. This study provides theoretical guidance for controlling the viral disease caused by poleroviruses in the future.

Impact of viral silencing suppressors on plant viral synergism: a global agro-economic concern

Plant viruses are known for their devastating impact on global agriculture. These intracellular biotrophic pathogens can infect a wide variety of plant hosts all over the world. The synergistic association of plant viruses makes the situation more alarming. It usually promotes the replication, movement, and transmission of either or both the coexisting synergistic viral partners. Although plants elicit a robust antiviral immune reaction, including gene silencing, to limit these infamous invaders, viruses counter it by encoding viral suppressors of RNA silencing (VSRs). Growing evidence also suggests that VSRs play a driving role in mediating the plant viral synergism. This review briefly discusses the evil impacts of mixed infections, especially synergism, and then comprehensively describes the emerging roles of VSRs in mediating the synergistic association of plant viruses. KEY POINTS: • Synergistic associations of plant viruses have devastating impacts on global agriculture. • Viral suppressors of RNA silencing (VSRs) play key roles in driving plant viral synergism.