Interaction of replicase components between Cucumber mosaic virus and Peanut stunt virus

Three-dimensional reconstruction and comparison of vacuolar membranes in response to viral infection

The vacuole is a unique plant organelle that plays an important role in maintaining cellular homeostasis under various environmental stress conditions. However, the effects of biotic stress on vacuole structure has not been examined using three-dimensional (3D) visualization. Here, we performed 3D electron tomography to compare the ultrastructural changes in the vacuole during infection with different viruses. The 3D models revealed that vacuoles are remodeled in cells infected with cucumber mosaic virus (CMV) or tobacco necrosis virus A Chinese isolate (TNV-AC ), resulting in the formation of spherules at the periphery of the vacuole. These spherules contain neck-like channels that connect their interior with the cytosol. Confocal microscopy of CMV replication proteins 1a and 2a and TNV-AC auxiliary replication protein p23 showed that all of these proteins localize to the tonoplast. Electron microscopy revealed that the expression of these replication proteins alone is sufficient to induce spherule formation on the tonoplast, suggesting that these proteins play prominent roles in inducing vacuolar membrane remodeling. This is the first report of the 3D structures of viral replication factories built on the tonoplasts. These findings contribute to our understanding of vacuole biogenesis under normal conditions and during assembly of plant (+) RNA virus replication complexes.

Heterologous Replicase from Cucumoviruses can Replicate Viral RNAs, but is Defective in Transcribing Subgenomic RNA4A or Facilitating Viral Movement

Interspecific exchange of RNA1 or RNA2 between the cucumoviruses cucumber mosaic virus (CMV) and tomato aspermy virus (TAV) was reported to be non-viable in plants previously. Here we investigated viability of the reassortants between CMV and TAV in Nicotiana benthamiana plants by Agrobacterium-mediated viral inoculation. The reassortants were composed of CMV RNA1 and TAV RNA2 plus RNA3 replicated in the inoculated leaves, while they were defective in viral systemic movement at the early stage of infection. Interestingly, the reassortant containing TAV RNA1 and CMV RNA2 and RNA3 infected plants systemically, but produced RNA4A (the RNA2 subgenome) at an undetectable level. The defect in production of RNA4A was due to the 1a protein encoded by TAV RNA1, and partially restored by replacing the C-terminus (helicase domain) in TAV 1a with that of CMV 1a. Collectively, exchange of the replicase components between CMV and TAV was acceptable for viral replication, but was defective in either directing transcription of subgenomic RNA4A or facilitating viral long-distance movement. Our finding may shed some light on evolution of subgenomic RNA4A in the family Bromoviridae.

Complete Genome Sequences of Seven Peanut Stunt Virus Strains from Japan

We present the complete genomic RNA sequences of seven isolates of Peanut stunt virus discovered in diseased legume plants in various regions in Japan. These sequences and the published viral sequences were compared with respect to nucleotide percentages. Their phylogenetic analysis showed that all the isolates belong to subgroup IA.

A trio of viral proteins tunes aphid-plant interactions in Arabidopsis thaliana

Background

Virus-induced deterrence to aphid feeding is believed to promote plant virus transmission by encouraging migration of virus-bearing insects away from infected plants. We investigated the effects of infection by an aphid-transmitted virus, cucumber mosaic virus (CMV), on the interaction of Arabidopsis thaliana, one of the natural hosts for CMV, with Myzus persicae (common names: ‘peach-potato aphid’, ‘green peach aphid’).

Methodology/Principal Findings

Infection of Arabidopsis (ecotype Col-0) with CMV strain Fny (Fny-CMV) induced biosynthesis of the aphid feeding-deterrent 4-methoxy-indol-3-yl-methylglucosinolate (4MI3M). 4MI3M inhibited phloem ingestion by aphids and consequently discouraged aphid settling. The CMV 2b protein is a suppressor of antiviral RNA silencing, which has previously been implicated in altering plant-aphid interactions. Its presence in infected hosts enhances the accumulation of CMV and the other four viral proteins. Another viral gene product, the 2a protein (an RNA-dependent RNA polymerase), triggers defensive signaling, leading to increased 4MI3M accumulation. The 2b protein can inhibit ARGONAUTE1 (AGO1), a host factor that both positively-regulates 4MI3M biosynthesis and negatively-regulates accumulation of substance(s) toxic to aphids. However, the 1a replicase protein moderated 2b-mediated inhibition of AGO1, ensuring that aphids were deterred from feeding but not poisoned. The LS strain of CMV did not induce feeding deterrence in Arabidopsis ecotype Col-0.

Conclusions/Significance

Inhibition of AGO1 by the 2b protein could act as a booby trap since this will trigger antibiosis against aphids. However, for Fny-CMV the interplay of three viral proteins (1a, 2a and 2b) appears to balance the need of the virus to inhibit antiviral silencing, while inducing a mild resistance (antixenosis) that is thought to promote transmission. The strain-specific effects of CMV on Arabidopsis-aphid interactions, and differences between the effects of Fny-CMV on this plant and those seen previously in tobacco (inhibition of resistance to aphids) may have important epidemiological consequences.

Comparative analysis of RNA silencing suppression activities between viral suppressors and an endogenous plant RNA-dependent RNA polymerase

RNA silencing is an evolutionarily conserved system that functions as an antiviral mechanism in eukaryotes, including higher plants. To counteract this, several plant viruses express silencing suppressors that inhibit RNA silencing in host plants. Here, we show that both 2b protein from peanut stunt virus (PSV) and a hairpin construct (designated hp-RDR6) that silences endogenous RNA-dependent RNA polymerase 6 (RDR6) strongly suppress RNA silencing. The Agrobacterium infiltration system was used to demonstrate that both PSV 2b and hp-RDR6 suppressed local RNA silencing as strongly as helper component (HC-Pro) from potato virus Y (PVY) and P19 from tomato bush stunt virus (TBSV). The 2b protein from PSV eliminated the small-interfering RNAs (siRNAs) associated with RNA silencing and prevented systemic silencing, similar to 2b protein from cucumber mosaic virus (CMV). On the other hand, hp-RDR6 suppressed RNA silencing by inhibiting the generation of secondary siRNAs. The small coat protein (SCP) of squash mosaic virus (SqMV) also displayed weak suppression activity of RNA silencing. Agrobacterium-mediated gene transfer was used to investigate whether viral silencing suppressors or hp-RDR6 enhanced accumulations of green fluorescence protein (GFP) and β-glucuronidase (GUS) as markers of expression in leaf tissues of Nicotina benthamiana. Expression of both GFP and GUS was significantly enhanced in the presence of PSV 2b or CMV 2b, compared to no suppression or the weak SqMV SCP suppressor. Co-expression with hp-RDR6 also significantly increased the expression of GFP and GUS to levels similar to those induced by PVY HC-Pro and TBSV P19.

Impact of a defective RNA 3 from cucumber mosaic virus on helper virus infection dynamics

D RNA 3Yalpha (D3Yalpha), a defective (D) RNA 3 derived from the Y strain of cucumber mosaic virus (CMV-Y), was further characterized in combination with different helper viruses in the genus Cucumovirus. Interestingly, Nicotiana benthamiana plants inoculated with CMV-D8 and D3Yalpha developed systemic symptoms which were different from those induced by CMV-D8. To elucidate the potential effects of D RNA 3 on virus infection on the basis of the original combination of CMV-Y and D3Yalpha, a point mutation was made in the coat protein gene, which determined symptoms, of either CMV-Y RNA 3 (Y3) or D3Yalpha. Symptoms induced on N. benthamiana and N. tabacum plants, and semi-quantitative RT-PCR revealed that the ratio of RNA 3 to D RNA 3 was associated with the differences of symptoms in the leaf tissues. Furthermore, analysis of in situ hybridization suggested that there were spatial effects between coat proteins of Y3 and D3Yalpha in the infected leaves.

Peanut stunt virus 2b cistron plays a role in viral local and systemic accumulation and virulence in Nicotiana benthamiana

To analyze the role of the 2b protein (2bP) of Peanut stunt virus (PSV) in the viral infection cycle, we constructed PSV mutants that express either no 2bP or N-terminal-truncated 2bP. The accumulation of wild-type and mutant viruses in tobacco protoplasts indicated that the 2b cistron is not essential for viral replication. Viral accumulation in Nicotiana benthamiana plants suggested that the 2b cistron is responsible for viral accumulation in inoculated and upper leaves and has a role in virulence. The involvement of eight N-terminal amino acids of 2bP in these functions is discussed.

Regulation of positive-strand RNA virus replication: the emerging role of phosphorylation

Protein phosphorylation is a reversible post-translational modification that plays a fundamental role in the regulation of many cellular processes. Phosphorylation can modulate protein properties such as enzymatic activity, stability, subcellular localization or interaction with binding partners. The importance of phosphorylation of the replication proteins of negative-strand RNA viruses has previously been documented but recent evidence suggests that replication of positive-strand RNA viruses – the largest class of viruses, including significant human, animal and plant pathogens – may also be regulated by phosphorylation events. The objective of this review is to summarize current knowledge regarding the various regulatory roles played by phosphorylation of nonstructural viral proteins in the replication of positive-strand RNA viruses.

Evidence for interaction between the 2a polymerase protein and the 3a movement protein of Cucumber mosaic virus

The genome of Cucumber mosaic virus consists of three single-stranded RNA molecules, RNAs 1, 2 and 3. RNAs 1 and 2 encode the 1a and 2a proteins, respectively, which are necessary for replication of the viral genome and have been implicated in movement of the viral RNAs in plants. The 3a movement protein (MP), encoded by RNA 3, is essential for transferring the RNA genomes from infected cells to adjacent cells across the plasmodesmata. Far-Western analysis demonstrated that bacterially expressed 2a polymerase protein directly interacted with the MP. Interaction was confirmed in a yeast two-hybrid assay, and co-immunoprecipitation analysis showed that the MP interacted only with the 2a polymerase protein. A yeast three-hybrid assay showed that the 1a–2a protein interaction relevant for replicase complex formation was not affected by the MP. Although the MP has no affinity for the 1a protein, it interacted indirectly with the 1a protein via the 2a polymerase protein. These results suggest that the replicase complex may be involved in movement through its interaction with the MP.

Kinetics and functional studies on interaction between the replicase proteins of Tomato Bushy Stunt Virus: requirement of p33:p92 interaction for replicase assembly

The assembly of the functional replicase complex via protein:protein and RNA:protein interactions among the viral-coded proteins, host factors and the viral RNA on cellular membranes is a key step in the replication process of plus-stranded RNA viruses. In this work, we have characterized essential interactions between p33:p33 and p33:p92 replication proteins of Tomato bushy stunt virus (TBSV), a tombusvirus with a non-segmented, plus-stranded RNA genome. Surface plasmon resonance (SPR) measurements with purified recombinant p33 and p92 demonstrate that p33 interacts with p92 in vitro and that the interaction requires the S1 subdomain, whereas the S2 subdomain plays lesser function. Kinetic SPR analyses showed that binding of S1 subdomain to the C-terminal half of p33 takes place with moderate binding affinity in the nanomolar range whereas S2 subdomain binds to p33 with micromolar affinity. Using mutated p33 and p92 proteins, we identified critical amino acid residues within the p33:p92 interaction domain that play essential role in replication and the assembly of the tombusviral replicase. In addition, we show that interaction takes place between replication proteins of TBSV and the closely related Cucumber necrosis virus but not between TBSV and the more distantly related Turnip crinkle virus, suggesting that selective protein interactions might prevent the assembly of chimeric replicases carrying replication proteins from different viruses during mixed infections.

Interaction between the replicase proteins of Tomato Bushy Stunt virus in vitro and in vivo

Tomato bushy stunt virus (TBSV), a tombusvirus with a non-segmented, plus-stranded RNA genome, codes for p33 and p92 replicase proteins. The sequence of p33 overlaps with the N-terminal domain of p92, which also contains the signature motifs of RNA-dependent RNA polymerases (RdRps) in its non-overlapping C-terminal portion. In this research, we demonstrate in vitro interactions between p33:p33 and p33:p92 using surface plasmon resonance analysis with purified recombinant p33 and p92. The sequence in p33 involved in the above protein-protein interactions was mapped to the C-terminal region, which also contains an RNA-binding site. Using the yeast two-hybrid assay, we confirmed that two short regions within p33 could promote p33:p33 and p33:p92 interactions in vivo. Mutations in either p33 or p92 within the short regions involved in p33:p33 and p33:p92 interactions decreased the replication of a TBSV defective interfering RNA in yeast, a model host, supporting the significance of these protein interactions in tombusvirus replication.

Compatibility of the movement protein and the coat protein of cucumoviruses is required for cell-to-cell movement

For the cell-to-cell movement of cucumoviruses both the movement protein (MP) and the coat protein (CP) are required. These are not reversibly exchangeable between Cucumber mosaic virus (CMV) and Tomato aspermy virus (TAV). The MP of CMV is able to function with the TAV CP (chimera RT), but TAV MP is unable to promote the cell-to-cell movement in the presence of CMV CP (chimera TR). To gain further insight into the non-infectious nature of the TR recombinant, RNA 3 chimeras were constructed with recombinant MPs and CPs. The chimeric MP and one of the CP recombinants were infectious. The other recombinant CP enabled virus movement only after the introduction of two point mutations (Glu-->Lys and Lys-->Arg at aa 62 and 65, respectively). The mutations served to correct the CP surface electrostatic potential that was altered by the recombination. The infectivity of the TR virus on different test plants was restored by replacing the sequence encoding the C-terminal 29 aa of the MP with the corresponding sequence of the CMV MP gene or by exchanging the sequence encoding the C-terminal 15 aa of the CP with the same region of TAV. The analysis of the recombinant clones suggests a requirement for compatibility between the C-terminal 29 aa of the MP and the C-terminal two-thirds of the CP for cell-to-cell movement of cucumoviruses.