Identification of the amino acid residues and domains in the cysteine-rich protein of Chinese wheat mosaic virus that are important for RNA silencing suppression and subcellular localization

TaWRKY50-TaSARK7 module-mediated cysteine-rich protein phosphorylation suppresses the programmed cell death response to Chinese wheat mosaic virus infection

WRKY transcription factors are widely involved in plant responses to biotic and abiotic stresses. However, there is currently a limited understanding of the regulation of viral infection by WRKY transcription factors in wheat (Triticum aestivum). The WRKY transcription factor TaWRKY50 in group IIb wheat exhibited a significant response to Chinese wheat mosaic virus infection. TaWRKY50 is localized in the nucleus and is an activating transcription factor. Interestingly, we found that silencing TaWRKY50 induces cell death following inoculation with CWMV. The protein kinase TaSAPK7 is specific to plants, whereas NbSRK is a closely related kinase with high homology to TaSAPK7. The transcriptional activities of both TaSAPK7 and NbSRK can be enhanced by TaWRKY50 binding to their promoters. CRP is an RNA silencing suppressor. Furthermore, TaWRKY50 may regulate CWMV infection by regulating the expression of TaSAPK7 and NbSRK to increase CRP phosphorylation and reduce the amount of programmed cell death (PCD).

Zn2+-dependent association of cysteine-rich protein with virion orchestrates morphogenesis of rod-shaped viruses

The majority of rod-shaped and some filamentous plant viruses encode a cysteine-rich protein (CRP) that functions in viral virulence; however, the roles of these CRPs in viral infection remain largely unknown. Here, we used barley stripe mosaic virus (BSMV) as a model to investigate the essential role of its CRP in virus morphogenesis. The CRP protein γb directly interacts with BSMV coat protein (CP), the mutations either on the His-85 site in γb predicted to generate a potential CCCH motif or on the His-13 site in CP exposed to the surface of the virions abolish the zinc-binding activity and their interaction. Immunogold-labeling assays show that γb binds to the surface of rod-shaped BSMV virions in a Zn2+-dependent manner, which enhances the RNA binding activity of CP and facilitates virion assembly and stability, suggesting that the Zn2+-dependent physical association of γb with the virion is crucial for BSMV morphogenesis. Intriguingly, the tightly binding of diverse CRPs to their rod-shaped virions is a general feature employed by the members in the families Virgaviridae (excluding the genus Tobamovirus) and Benyviridae. Together, these results reveal a hitherto unknown role of CRPs in the assembly and stability of virus particles, and expand our understanding of the molecular mechanism underlying virus morphogenesis.

Identification of a negative-strand RNA virus with natural plant and fungal hosts

The presence of viruses that spread to both plant and fungal populations in nature has posed intriguingly scientific question. We found a negative-strand RNA virus related to members of the family Phenuiviridae, named Valsa mali negative-strand RNA virus 1 (VmNSRV1), which induced strong hypovirulence and was prevalent in a population of the phytopathogenic fungus of apple Valsa canker (Valsa mali) infecting apple orchards in the Shaanxi Province of China. Intriguingly, VmNSRV1 encodes a protein with a viral cell-to-cell movement function in plant tissue. Mechanical leaf inoculation showed that VmNSRV1 could systemically infect plants. Moreover, VmNSRV1 was detected in 24 out of 139 apple trees tested in orchards in Shaanxi Province. Fungal inoculation experiments showed that VmNSRV1 could be bidirectionally transmitted between apple plants and V. mali, and VmNSRV1 infection in plants reduced the development of fungal lesions on leaves. Additionally, the nucleocapsid protein encoded by VmNSRV1 is associated with and rearranged lipid droplets in both fungal and plant cells. VmNSRV1 represents a virus that has adapted and spread to both plant and fungal hosts and shuttles between these two organisms in nature (phyto-mycovirus) and is potential to be utilized for the biocontrol method against plant fungal diseases. This finding presents further insights into the virus evolution and adaptation encompassing both plant and fungal hosts.

The nanovirus U2 protein suppresses RNA silencing via three conserved cysteine residues

Nanoviruses have multipartite, circular, single-stranded DNA genomes and cause huge production losses in legumes and other crops. No viral suppressor of RNA silencing (VSR) has yet been reported from a member of the genus Nanovirus. Here, we demonstrate that the nanovirus U2 protein is a VSR. The U2 protein of milk vetch dwarf virus (MDV) suppressed the silencing of the green fluorescent protein (GFP) gene induced by single-stranded and double-stranded RNA, and the systemic spread of the GFP silencing signal. An electrophoretic mobility shift assay showed that the U2 protein was able to bind double-stranded 21-nucleotide small interfering RNA (siRNA). The cysteine residues at positions 43, 79 and 82 in the MDV U2 protein are critical to its nuclear localization, self-interaction and siRNA-binding ability, and were essential for its VSR activity. In addition, expression of the U2 protein via a potato virus X vector induced more severe necrosis symptoms in Nicotiana benthamiana leaves. The U2 proteins of other nanoviruses also acted as VSRs, and the three conserved cysteine residues were indispensable for their VSR activity.

Large-scale phosphoproteome analysis in wheat seedling leaves provides evidence for extensive phosphorylation of regulatory proteins during CWMV infection

BACKGROUND

Chinese wheat mosaic virus (CWMV) often causes severe damage to wheat (Triticum aestivum L.) growth and yield. It is well known that a successful infection in plants depends on a complex interaction between the host plant and the pathogen. Post-translational modification (PTM) of proteins is considered to be one of the main processes that decides the outcome of the plant-pathogen arms race during this interaction. Although numerous studies have investigated PTM in various organisms, there has been no large-scale phosphoproteomic analysis of virus-infected wheat plants. We therefore aimed to investigate the CWMV infection-induced phosphoproteomics changes in wheat by high-resolution liquid chromatography-tandem mass spectroscopy (LC-MS/MS) using affinity-enriched peptides followed by comprehensive bioinformatics analysis.

RESULTS

Through this study, a total of 4095 phosphorylation sites have been identified in 1968 proteins, and 11.6% of the phosphorylated proteins exhibited significant changes (PSPCs) in their phosphorylation levels upon CWMV infection. The result of Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that most of the PSPCs were associated with photosynthesis, plant-pathogen interactions, and MAPK signaling pathways. The protein-protein interaction (PPI) network analysis result showed that these PSPCs were mainly participated in the regulation of biosynthesis and metabolism, protein kinase activities, and transcription factors. Furthermore, the phosphorylation levels of TaChi1 and TaP5CS, two plant immunity-related enzymes, were significantly changed upon CWMV infection, resulting in a significant decrease in CWMV accumulation in the infected plants.

CONCLUSIONS

Our results indicate that phosphorylation modification of protein plays a critical role in wheat resistance to CWMV infection. Upon CWMV infection, wheat plants will regulate the levels of extra- and intra-cellular signals and modifications of enzyme activities via protein phosphorylation. This novel information about the strategies used by wheat to resist CWMV infection will help researchers to breed new CWMV-resistant cultivars and to better understand the arms race between wheat and CWMV.

Wheat adaptation to environmental stresses under climate change: Molecular basis and genetic improvement

Wheat (Triticum aestivum) is a staple food for about 40% of the world's population. As the global population has grown and living standards improved, high yield and improved nutritional quality have become the main targets for wheat breeding. However, wheat production has been compromised by global warming through the more frequent occurrence of extreme temperature events, which have increased water scarcity, aggravated soil salinization, caused plants to be more vulnerable to diseases, and directly reduced plant fertility and suppressed yield. One promising option to address these challenges is the genetic improvement of wheat for enhanced resistance to environmental stress. Several decades of progress in genomics and genetic engineering has tremendously advanced our understanding of the molecular and genetic mechanisms underlying abiotic and biotic stress responses in wheat. These advances have heralded what might be considered a "golden age" of functional genomics for the genetic improvement of wheat. Here, we summarize the current knowledge on the molecular and genetic basis of wheat resistance to abiotic and biotic stresses, including the QTLs/genes involved, their functional and regulatory mechanisms, and strategies for genetic modification of wheat for improved stress resistance. In addition, we also provide perspectives on some key challenges that need to be addressed.

Genome-Wide Identification and Functional Analysis of NAP1 in Triticum aestivum

As a main molecular chaperone of histone H2A-H2B, nucleosome assembly protein 1 (NAP1) has been widely researched in many species. However, there is little research investigating the function of NAP1 in Triticum aestivum. To understand the capabilities of the family of NAP1 genes in wheat and the relationship between TaNAP1 genes and plant viruses, we performed comprehensive genome-wide analysis and quantitative real-time polymerase chain reaction (qRT-PCR) for testing expression profiling under hormonal and viral stresses. Our results showed that TaNAP1 was expressed at different levels in different tissues, with higher expression in tissues with high meristematic capacity, such as roots. Furthermore, the TaNAP1 family may participate in plant defense mechanisms. This study provides a systematic analysis of the NAP1 gene family in wheat and lays the foundation for further studies on the function of TaNAP1 in the response of wheat plants to viral infection.

Coat protein of rice stripe virus enhances autophagy activity through interaction with cytosolic glyceraldehyde-3-phosphate dehydrogenases, a negative regulator of plant autophagy

Viral infection commonly induces autophagy, leading to antiviral responses or conversely, promoting viral infection or replication. In this study, using the experimental plant Nicotiana benthamiana, we demonstrated that the rice stripe virus (RSV) coat protein (CP) enhanced autophagic activity through interaction with cytosolic glyceraldehyde-3-phosphate dehydrogenase 2 (GAPC2), a negative regulator of plant autophagy that binds to an autophagy key factor, autophagy-related protein 3 (ATG3). Competitive pull-down and co-immunoprecipitation (Co-IP)assays showed that RSV CP activated autophagy by disrupting the interaction between GAPC2 and ATG3. An RSV CP mutant that was unable to bind GAPC2 failed to disrupt the interaction between GAPC2 and ATG3 and therefore lost its ability to induce autophagy. RSV CP enhanced the autophagic degradation of a viral movement protein (MP) encoded by a heterologous virus, citrus leaf blotch virus (CLBV). However, the autophagic degradation of RSV-encoded MP and RNA-silencing suppressor (NS3) proteins was inhibited in the presence of CP, suggesting that RSV CP can protect MP and NS3 against autophagic degradation. Moreover, in the presence of MP, RSV CP could induce the autophagic degradation of a remorin protein (NbREM1), which negatively regulates RSV infection through the inhibition of viral cell-to-cell movement. Overall, our results suggest that RSV CP induces a selective autophagy to suppress the antiviral factors while protecting RSV-encoded viral proteins against autophagic degradation through an as-yet-unknown mechanism. This study showed that RSV CP plays dual roles in the autophagy-related interaction between plants and viruses.

Central Role of Ubiquitination in Wheat Response to CWMV Infection

Ubiquitination is a major post-translational modification (PTM) involved in almost all eukaryotic biological processes and plays an essential role in plant response to pathogen infection. However, to date, large-scale profiling of the changes in the ubiquitome in response to pathogens, especially viruses, in wheat has not been reported. This study aimed to identify the ubiquitinated proteins involved in Chinese wheat mosaic virus (CWMV) infection in wheat using a combination of affinity enrichment and high-resolution liquid chromatography-tandem mass spectroscopy. The potential biological functions of these ubiquitinated proteins were further analyzed using bioinformatics. A total of 2297 lysine ubiquitination sites in 1255 proteins were identified in wheat infected with CWMV, of which 350 lysine ubiquitination sites in 192 proteins were differentially expressed. These ubiquitinated proteins were related to metabolic processes, responses to stress and hormones, plant–pathogen interactions, and ribosome pathways, as assessed via Gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses. Furthermore, we found that the ubiquitination of Ta14-3-3 and TaHSP90, which are essential components of the innate immune system, was significantly enhanced during CWMV infection, which suggested that ubiquitination modification plays a vital role in the regulatory network of the host response to CWMV infection. In summary, our study puts forward a novel strategy for further probing the molecular mechanisms of CWMV infection. Our findings will inform future research to find better, innovative, and effective solutions to deal with CWMV infection in wheat, which is the most crucial and widely used cereal grain crop.

Coat protein of Chinese wheat mosaic virus upregulates and interacts with cytosolic glyceraldehyde-3-phosphate dehydrogenase, a negative regulator of plant autophagy, to promote virus infection

Autophagy is an intracellular degradation mechanism involved in antiviral defense, but the strategies employed by plant viruses to counteract autophagy-related defense remain unknown for the majority of the viruses. Herein, we describe how the Chinese wheat mosaic virus (CWMV, genus Furovirus) interferes with autophagy and enhances its infection in Nicotiana benthamiana. Yeast two-hybrid screening and in vivo/in vitro assays revealed that the 19 kDa coat protein (CP19K) of CWMV interacts with cytosolic glyceraldehyde-3-phosphate dehydrogenases (GAPCs), negative regulators of autophagy, which bind autophagy-related protein 3 (ATG3), a key factor in autophagy. CP19K also directly interacts with ATG3, possibly leading to the formation of a CP19K-GAPC-ATG3 complex. CP19K-GAPC interaction appeared to intensify CP19K-ATG3 binding. Moreover, CP19K expression upregulated GAPC gene transcripts and reduced autophagic activities. Accordingly, the silencing of GAPC genes in transgenic N. benthamiana reduced CWMV accumulation, whereas CP19K overexpression enhanced it. Overall, our results suggest that CWMV CP19K interferes with autophagy through the promotion and utilization of the GAPC role as a negative regulator of autophagy.

Phosphorylated viral protein evades plant immunity through interfering the function of RNA-binding protein

Successful pathogen infection in plant depends on a proper interaction between the invading pathogen and its host. Post-translational modification (PTM) plays critical role(s) in plant-pathogen interaction. However, how PTM of viral protein regulates plant immunity remains poorly understood. Here, we found that S162 and S165 of Chinese wheat mosaic virus (CWMV) cysteine-rich protein (CRP) are phosphorylated by SAPK7 and play key roles in CWMV infection. Furthermore, the phosphorylation-mimic mutant of CRP (CRP^S162/165D) but not the non-phosphorylatable mutant of CRP (CRP^S162/165A) interacts with RNA-binding protein UBP1-associated protein 2C (TaUBA2C). Silencing of TaUBA2C expression in wheat plants enhanced CWMV infection. In contrast, overexpression of TaUBA2C in wheat plants inhibited CWMV infection. TaUBA2C inhibits CWMV infection through recruiting the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to induce cell death and H₂O₂ production. This effect can be supressed by CRP^S162/165D through changing TaUBA2C chromatin-bound status and attenuating it’s the RNA- or DNA-binding activities. Taken together, our findings provide new knowledge on how CRP phosphorylation affects CWMV infection as well as the arms race between virus and wheat plants.

Chinese wheat mosaic virus (CWMV) causes a damaging disease in cereal plants. However, CWMV interacts with host factors to facilitate virus infection is not clear yet. Here, we found that S162 and S165 of CWMV cysteine-rich protein (CRP) are phosphorylated by SAPK7 in vivo and in vitro. Mutational analyses have indicated that these two phosphorylation sites of CRP (CRP^S162/165D) promoting CWMV infection in plants, due to the supressed cell death and H₂O₂ production. Further investigations found the CRP^S162/165D can interact with TaUBA2C, while the non-phosphorylatable mutant of CRP (CRP^S162/165A) does not. Futhermore, we have determined that CRP^S162/165D and TaUBA2C interaction inhibited the formation of TaUBA2C speckles in nucleus to attenuate its RNA- and DNA-binding activity. We also showed that TaUBA2C recruit the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to up-regulated these genes expressions and then induce cell death and H₂O₂ production in plant. This effect can be supressed by the expression of CRP^S162/165D, in a dose-dependent manner. Taken together, our discovery may provide a new sight for the arms race between virus and its host plants.

Enrichment of beneficial rhizosphere microbes in Chinese wheat yellow mosaic virus-resistant cultivars

The microbial community within the root system, the rhizosphere closely connected to the root, and their symbiotic relationship with the host are increasingly seen as possible drivers of natural pathogen resistance. Resistant cultivars have the most effective strategy in controlling the Chinese wheat yellow mosaic disease, but the roles of the root and rhizosphere microbial interactions among different taxonomic levels of resistant cultivars are still unknown. Thus, we aimed to investigate whether these microbial community composition and network characteristics are related to disease resistance and to analyze the belowground plant-associated microflora. Relatively high microbial diversity and stable community structure for the resistant cultivars were detected. Comparison analysis showed that some bacterial phyla were significantly enriched in the wheat root or rhizosphere of the resistant wheat cultivar. Furthermore, the root and rhizosphere of the resistant cultivars greatly recruited many known beneficial bacterial and fungal taxa. In contrast, the relative abundance of potential pathogens was higher for the susceptible cultivar than for the resistant cultivar. Network co-occurrence analysis revealed that a much more complex, more mutually beneficial, and a higher number of bacterial keystone taxa in belowground microbial networks were displayed in the resistant cultivar, which may have been responsible for maintaining the stability and ecological balance of the microbial community. Overall, compared with the susceptible cultivar, the resistant cultivar tends to recruit more potential beneficial microbial groups for plant and rhizosphere microbial community interactions. These findings indicate that beneficial rhizosphere microbiomes for cultivars should be targeted and evaluated using community compositional profiles. KEY POINTS: • Different resistance levels in cultivars affect the rhizosphere microbiome.. • Resistant cultivars tend to recruit more potential beneficial microbial groups. • Bacteria occupy a high proportion and core position in the microflora network.

Binding between elongation factor 1A and the 3'-UTR of Chinese wheat mosaic virus is crucial for virus infection

The Chinese wheat mosaic virus (CWMV) genome consists of two positive-strand RNAs that are required for CWMV replication and translation. The eukaryotic translation elongation factor (eEF1A) is crucial for the elongation of protein translation in eukaryotes. Here, we show that silencing eEF1A expression in Nicotiana benthamiana plants by performing virus-induced gene silencing can greatly reduce the accumulation of CWMV genomic RNAs, whereas overexpression of eEF1A in plants increases the accumulation of CWMV genomic RNAs. In vivo and in vitro assays showed that eEF1A does not interact with CWMV RNA-dependent RNA polymerase. Electrophoretic mobility shift assays revealed that eEF1A can specifically bind to the 3'-untranslated region (UTR) of CWMV genomic RNAs. By performing mutational analyses, we determined that the conserved region in the 3'-UTR of CWMV genomic RNAs is necessary for CWMV replication and translation, and that the sixth stem-loop (SL-6) in the 3'-UTR of CWMV genomic RNAs plays a key role in CWMV infection. We conclude that eEF1A is an essential host factor for CWMV infection. This finding should help us to develop new strategies for managing CWMV infections in host plants.

Autophagy Inhibits Intercellular Transport of Citrus Leaf Blotch Virus by Targeting Viral Movement Protein

Autophagy is an evolutionarily conserved cellular-degradation mechanism implicated in antiviral defense in plants. Studies have shown that autophagy suppresses virus accumulation in cells; however, it has not been reported to specifically inhibit viral spread in plants. This study demonstrated that infection with citrus leaf blotch virus (CLBV; genus Citrivirus, family Betaflexiviridae) activated autophagy in Nicotiana benthamiana plants as indicated by the increase of autophagosome formation. Impairment of autophagy through silencing of N. benthamiana autophagy-related gene 5 (NbATG5) and NbATG7 enhanced cell-to-cell and systemic movement of CLBV; however, it did not affect CLBV accumulation when the systemic infection had been fully established. Treatment using an autophagy inhibitor or silencing of NbATG5 and NbATG7 revealed that transiently expressed movement protein (MP), but not coat protein, of CLBV was targeted by selective autophagy for degradation. Moreover, we identified that CLBV MP directly interacted with NbATG8C1 and NbATG8i, the isoforms of autophagy-related protein 8 (ATG8), which are key factors that usually bind cargo receptors for selective autophagy. Our results present a novel example in which autophagy specifically targets a viral MP to limit the intercellular spread of the virus in plants.

Genome-wide identification and characterization of UBP gene family in wheat (Triticum aestivum L.)

Ubiquitination is essential for plant growth and development. Deubiquitination cooperates with ubiquitination to regulate the ubiquitination levels of target proteins. The ubiquitin-specific protease (UBP) family is the largest group of deubiquitinases (DUBs), which perform extensive and significant roles in eukaryotic organisms. However, the UBP genes in wheat (TaUBPs) are not identified, and the functions of TaUBPs are unknown. The present study identified 97 UBP genes in the whole genome of T. aestivum. These genes were divided into 15 groups and non-randomly distributed on chromosomes of T. aestivum. Analyses of evolutionary patterns revealed that TaUBPs mainly underwent purification selection. The studies of cis-acting regulatory elements indicated that they might be involved in response to hormones. Quantitative real-time PCR (qRT-PCR) results showed that TaUBPs were differentially expressed in different tissues. Besides, several TaUBPs were significantly up-regulated when plants were treated with salicylic acid (SA), implying that these DUBs may play a role in abiotic stress responses in plants and few TaUBPs displayed differential expression after viral infection. Furthermore, TaUBP1A.1 (TraesCS1A02G432600.1) silenced by virus-induced gene silencing (VIGS) facilitates Chinese wheat mosaic virus (CWMV) infection in wheat, indicating that TaUBP1A.1 may be involved in a defense mechanism against viruses. This study comprehensively analyzed the UBP gene family in wheat and provided a basis for further research of TaUBPs functions in wheat plant response to viral infection.

Genome-Wide Identification and Characterization of Long Noncoding RNAs Involved in Chinese Wheat Mosaic Virus Infection of Nicotiana benthamiana

Simple Summary

Recent studies have shown that a large number of long noncoding RNAs (lncRNAs) can regulate various biological processes in animals and plants. However, the roles of long non-coding RNAs (lncRNAs) in the interaction between plants and viruses is unclear, particularly for the Chinese wheat mosaic virus (CWMV) interaction. In this study, we used a deep RNA sequencing strategy to profile lncRNAs involved in the response to CWMV infection in Nicotiana benthamiana and analyzed differentially expressed lncRNAs that responded to CWMV infection, using a bioinformatics method. We identified 1175 new lncRNAs in N. benthamiana infected with CWMV, with 65 lncRNAs showing differential expression. These lncRNAs were mainly enriched in plant hormone signal transduction and other pathways according to GO and KEGG pathway enrichment analyses. In addition, differential expression of XLOC_006393 after CWMV infection may be the precursor of NbmiR168c, which can respond to CWMV infection by modulating the expression of its target gene NbAGO1. We believe that our study makes a significant contribution to the literature because these results provide a valuable resource for studying lncRNAs involved in CWMV infection and improving the understanding of the molecular mechanism of CWMV infection.

Abstract

Recent studies have shown that a large number of long noncoding RNAs (lncRNAs) can regulate various biological processes in animals and plants. Although lncRNAs have been identified in many plants, they have not been reported in the model plant Nicotiana benthamiana. Particularly, the role of lncRNAs in plant virus infection remains unknown. In this study, we identified lncRNAs in N. benthamiana response to Chinese wheat mosaic virus (CWMV) infection by RNA sequencing. A total of 1175 lncRNAs, including 65 differentially expressed lncRNAs, were identified during CWMV infection. We then analyzed the functions of some of these differentially expressed lncRNAs. Interestingly, one differentially expressed lncRNA, XLOC_006393, was found to participate in CWMV infection as a precursor to microRNAs in N. benthamiana. These results suggest that lncRNAs play an important role in the regulatory network of N. benthamiana in response to CWMV infection.

Comparative proteomic analysis of Nicotiana benthamiana plants under Chinese wheat mosaic virus infection

BACKGROUND

Chinese wheat mosaic virus (CWMV) is a severe threat to winter wheat and is transmitted by Polymyxa graminis. The mechanisms of interactions between CWMV and plants are poorly understood. In this study, a comparative proteomics analysis based on nanoliquid chromatography mass spectrometry (MS)/MS was conducted to characterize proteomic changes in plants responding to CWMV infection.

RESULTS

In total, 2751 host proteins were identified, 1496 of which were quantified and 146 up-regulated and 244 down-regulated proteins were identified as differentially expressed proteins (DEPs). Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that DEPs were most strongly associated with photosynthesis antenna proteins, MAPK signaling plant and glyoxylate and dicarboxylate metabolism pathways. Subcellular localization analysis predicted that more than half of the DEPs were localized in the chloroplast, an organelle indispensable for abscisic acid (ABA) synthesis. Our results suggest that CWMV infection interrupts normal chloroplast functions and decreases ABA concentrations in Nicotiana benthamiana. Further analysis showed that the ABA pathway was suppressed during CWMV infection and that ABA treatment induced plant hosts defenses against CWMV.

CONCLUSIONS

We identified several candidate proteins expressed during CWMV infection, and the ABA pathway was strongly associated with responses to CWMV infection in N. benthamiana.

A Conserved Helix in the C-Terminal Region of Watermelon Silver Mottle Virus Nonstructural Protein S Is Imperative For Protein Stability Affecting Self-Interaction, RNA Silencing Suppression, and Pathogenicity

In orthotospovirus, the nonstructural protein S (NSs) is the RNA-silencing suppressor (RSS) and pathogenicity determinant. Here, we demonstrate that a putative α-helix, designated H8, spanning amino acids 338 to 369 of the C-terminal region of the NSs protein, is crucial for self-interaction of watermelon silver mottle virus NSs protein and that the H8 affects RSS function. Co-immunoprecipitation, yeast two-hybrid, and bimolecular fluorescence complementation analyses revealed that the triple point mutation (TPM) of H8 amino acids Y338A, H350A, and F353A resulted in NSs protein self-interaction dysfunction. Transient expression of H8-deleted (ΔH8) and TPM NSs proteins in Nicotiana benthamiana plants by agroinfitration indicated that these proteins have weaker RSS activity and are far less stable than wild-type (WT) NSs. However, an electrophoretic mobility assay revealed that small interfering RNA (siRNA) binding ability of TPM NSs protein is not compromised. The pathogenicity assay of WT NSs protein expressed by the attenuated turnip mosaic virus vector restored severe symptoms in recombinant-infected N. benthamiana plants but not for ΔH8 or TPM proteins. Taken together, we conclude that the H8 helix in the C-terminal region of NSs protein is crucial for stabilizing NSs protein through self-interaction to maintain normal functions of RSS and pathogenicity, but not for NSs-siRNA binding activity.

Chinese wheat mosaic virus-derived vsiRNA-20 can regulate virus infection in wheat through inhibition of vacuolar- (H+ )-PPase induced cell death

Vacuolar (H⁺ )-PPases (VPs), are key regulators of active proton (H⁺ ) transport across membranes using the energy generated from PPi hydrolysis. The VPs also play vital roles in plant responses to various abiotic stresses. Their functions in plant responses to pathogen infections are unknown. Here, we show that TaVP, a VP of wheat (Triticum aestivum) is important for wheat resistance to Chinese wheat mosaic virus (CWMV) infection. Furthermore, overexpression of TaVP in plants induces the activity of PPi hydrolysis, leading to plants cell death. A virus-derived small interfering RNA (vsiRNA-20) generated from CWMV RNA1 can regulate the mRNA accumulation of TaVP in wheat. The accumulation of vsiRNA-20 can suppress cell death induced by TaVP in a dosage-dependent manner. Moreover, we show that the accumulation of vsiRNA-20 can affect PPi hydrolysis and the concentration of H⁺ in CWMV-infected wheat cells to create a more favorable cellular environment for CWMV replication. We propose that vsiRNA-20 regulates TaVP expression to prevent cell death and to maintain a weak alkaline environment in cytoplasm to enhance CWMV infection in wheat. This finding may be used as a novel strategy to minimize virus pathogenicity and to develop new antiviral stratagems.

Passion Fruit Green Spot Virus Genome Harbors a New Orphan ORF and Highlights the Flexibility of the 5'-End of the RNA2 Segment Across Cileviruses

Passion fruit green spot and passion fruit sudden death are two reportedly distinct viral diseases that recurrently affect passion fruit (Passiflora spp.) groves in Brazil. Here we used a systematic approach that interconnects symptoms, transmission electron microscopy, RT-PCR detection assays followed by Sanger sequencing, and high-throughput sequencing of the RNA of affected passion fruit plants to gain insights about these diseases. Our data confirmed not only the involvement of cileviruses in these two pathologies, as previously suggested, but also that these viruses belong to the same tentative species: passion fruit green spot virus (PfGSV). Results revealed that PfGSV has a positive-sense RNA genome split into two molecules of approximately 9 kb (RNA1) and 5 kb (RNA2), which share about 50–70% nucleotide sequence identity with other viruses in the genus Cilevirus. Genome sequences of five PfGSV isolates suggest that they have more conserved RNA1 (<5% of nucleotide sequence variability) compared to RNA2 (up to 7% of variability) molecules. The highest nucleotide sequence divergence among PfGSV isolates and other cileviruses is in the genomic segment covering from the 5′-end of the RNA2 until the 5′-end of the open reading frame (ORF) p61, which includes the ORF p15 and the intergenic region. This genomic stretch also harbors a novel orphan ORF encoding a 13 kDa protein presenting a cysteine-rich domain. High variability of 5′-end of the RNA2 in cileviruses is discussed in an evolutionary context assuming that they share putative common ancestors with unclassified arthropod-infecting single-strand positive RNA viruses, including mosquito-specific viruses of the group Negevirus (clades Nelorpivirus and Sandwavirus), and other viruses in the family Kitaviridae.

Phytoplasma effector SWP1 induces witches' broom symptom by destabilizing the TCP transcription factor BRANCHED1

Phytoplasmas are insect-transmitted phytopathogenic bacteria, which secrete effector proteins that are often responsible for altering the plant morphology and behaviours of their vectors. Phytoplasma multifunctional effector proteins TENGU and SAP11 induce typical witches' broom symptoms, but their mode of action remains unknown. Previously, we have identified a SAP11-like effector from wheat blue dwarf phytoplasma, SWP1, which induces witches' broom symptoms in Nicotiana benthamiana. In this study, we observed that SWP1-expressing transgenic Arabidopsis thaliana plants showed typical witches' broom symptoms. On overexpression of SWP1 truncation mutants in N. benthamiana, we identified that the coiled-coil domain and nuclear localization were responsible for the induction of witches' broom symptoms. In addition, using yeast two-hybrid and bimolecular fluorescence complementation assays, we demonstrated that SWP1 interacts with A. thaliana transcription factor TCP18 (BRC1), the key negative regulator of branching signals in various plant species. Moreover, in planta co-expression analysis showed that SWP1 promotes the degradation of BRC1 via a proteasome system. These findings suggest that the phytoplasma effector SWP1 induces witches' broom symptoms through targeting of BRC1 and promoting its degradation.

p15 encoded by Garlic virus X is a pathogenicity factor and RNA silencing suppressor

Garlic virus X (GarVX) encodes a 15 kDa cysteine-rich protein (CRP). To investigate the function(s) of p15, its subcellular localization, role as a symptom determinant and capacity to act as a viral suppressor of RNA silencing (VSR) were analysed. Results showed that GFP-tagged p15 was distributed in the cytoplasm, nucleus and nucleolus. Expression of p15 from PVX caused additional systemic foliar malformation and led to increased accumulation of PVX, showing that p15 is a virulence factor for reconstructed PVX-p15. Moreover, using a transient agro-infiltration patch assay and a Turnip crinkle virus (TCV) movement complementation assay, it was demonstrated that p15 possesses weak RNA silencing suppressor activity. Removal of an amino acid motif resembling a nuclear localization signal (NLS) prevented p15 from accumulating in the nucleus but did not abolish its silencing suppression activity. This study provides the first insights into the multiple functions of the GarVX p15 protein.

Chinese Wheat Mosaic Virus-Induced Gene Silencing in Monocots and Dicots at Low Temperature

Virus-induced gene silencing (VIGS) is an important tool for functional genomics studies in plants. With this method, it is possible to target most endogenous genes and downregulate the messenger RNA (mRNA) in a sequence-specific manner. Chinese wheat mosaic virus (CWMV) has a bipartite, single-strand positive RNA genome, and can infect both wheat and Nicotiana benthamiana, and the optimal temperature for systemic infection in plants is 17°C. To assess the potential of the virus as a vector for gene silencing at low temperature, a fragment of the N. benthamiana or wheat phytoene desaturase (PDS) gene was expressed from a modified CWMV RNA2 clone and the resulting photo bleaching in infected plants was used as a reporter for silencing. Downregulation of PDS mRNA was also measured by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR). In experiments using fragments of PDS ranging from 500 to 1500 nucleotides, insert length influenced the stability and the efficiency of VIGS. The CWMV induced silencing system was also used to suppress miR165/166 and miR3134a through expression of miRNA target mimics. The relative expression levels of mature miR165/166 and miR3134a decreased whereas the transcript levels of their target genes increased. Interestingly, we also found the CWMV-induced silencing system was more efficient compare with the vector based on Barley stripe mosaic virus (BSMV) or Foxtail mosaic virus (FoMV) in wheat or the vector based on TRV in N. benthamiana at 17°C. In summary, the CWMV vector is effective in silencing endogenous genes and miRNAs at 17°C, thereby providing a powerful tool for gene function analysis in both N. benthamiana and wheat at low temperature.

N-terminal region of cysteine-rich protein (CRP) in carlaviruses is involved in the determination of symptom types

Plant viruses in the genus Carlavirus include more than 65 members. Plants infected with carlaviruses exhibit various symptoms, including leaf malformation and plant stunting. Cysteine-rich protein (CRP) encoded by carlaviruses has been reported to be a pathogenicity determinant. Carlavirus CRPs contain two motifs in their central part: a nuclear localization signal (NLS) and a zinc finger motif (ZF). In addition to these two conserved motifs, carlavirus CRPs possess highly divergent, N-terminal, 34 amino acid residues with unknown function. In this study, to analyse the role of these distinct domains, we tested six carlavirus CRPs for their RNA silencing suppressor activity, ability to enhance the pathogenicity of a heterologous virus and effects on virus accumulation levels. Although all six tested carlavirus CRPs showed RNA silencing suppressor activity at similar levels, symptoms induced by the Potato virus X (PVX) heterogeneous system exhibited two different patterns: leaf malformation and whole-plant stunting. The expression of each carlavirus CRP enhanced PVX accumulation levels, which were not correlated with symptom patterns. PVX-expressing CRP with mutations in either NLS or ZF did not induce symptoms, suggesting that both motifs play critical roles in symptom expression. Further analysis using chimeric CRPs, in which the N-terminal region was replaced with the corresponding region of another CRP, suggested that the N-terminal region of carlavirus CRPs determined the exhibited symptom types. The up-regulation of a plant gene upp-L, which has been reported in a previous study, was also observed in this study; however, the expression level was not responsible for symptom types.

A furoviral replicase recruits host HSP70 to membranes for viral RNA replication

Many host factors have been identified to be involved in viral infection. However, although furoviruses cause important diseases of cereals worldwide, no host factors have yet been identified that interact with furoviral genes or participate in the viral infection cycle. In this study, both TaHSP70 and NbHSP70 were up-regulated in Chinese wheat mosaic furovirus (CWMV)-infected plants. Their overexpression and inhibition were correlated with the accumulation of viral genomic RNAs, suggesting that the HSP70 genes could be necessary for CWMV infection. The subcellular distributions of TaHSP70 and NbHSP70 were significantly affected by CWMV infection or by infiltration of RNA1 alone. Further assays showed that the viral replicase encoded by CWMV RNA1 interacts with both TaHSP70 and NbHSP70 in vivo and vitro and that its region aa167-333 was responsible for the interaction. Subcellular assays showed that the viral replicase could recruit both TaHSP70 and NbHSP70 from the cytoplasm or nucleus to the granular aggregations or inclusion-like structures on the intracellular membrane system, suggesting that both HSP70s may be recruited into the viral replication complex (VRC) to promote furoviral replication. This is the first host factor identified to be involved in furoviral infection, which extends the list and functional scope of HSP70 chaperones.

Interplays between Soil-Borne Plant Viruses and RNA Silencing-Mediated Antiviral Defense in Roots

Although the majority of plant viruses are transmitted by arthropod vectors and invade the host plants through the aerial parts, there is a considerable number of plant viruses that infect roots via soil-inhabiting vectors such as plasmodiophorids, chytrids, and nematodes. These soil-borne viruses belong to diverse families, and many of them cause serious diseases in major crop plants. Thus, roots are important organs for the life cycle of many viruses. Compared to shoots, roots have a distinct metabolism and particular physiological characteristics due to the differences in development, cell composition, gene expression patterns, and surrounding environmental conditions. RNA silencing is an important innate defense mechanism to combat virus infection in plants, but the specific information on the activities and molecular mechanism of RNA silencing-mediated viral defense in root tissue is still limited. In this review, we summarize and discuss the current knowledge regarding RNA silencing aspects of the interactions between soil-borne viruses and host plants. Overall, research evidence suggests that soil-borne viruses have evolved to adapt to the distinct mechanism of antiviral RNA silencing in roots.

Alfalfa dwarf cytorhabdovirus P protein is a local and systemic RNA silencing supressor which inhibits programmed RISC activity and prevents transitive amplification of RNA silencing

Plants employ RNA silencing as an innate defense mechanism against viruses. As a counter-defense, plant viruses have evolved to express RNA silencing suppressor proteins (RSS), which target one or more steps of the silencing pathway. In this study, we show that the phosphoprotein (P) encoded by the negative-sense RNA virus alfalfa dwarf virus (ADV), a species of the genus Cytorhabdovirus, family Rhabdoviridae, is a suppressor of RNA silencing. ADV P has a relatively weak local RSS activity, and does not prevent siRNA accumulation. On the other hand, ADV P strongly suppresses systemic RNA silencing, but does not interfere with the short-distance spread of silencing, which is consistent with its lack of inhibition of siRNA accumulation. The mechanism of suppression appears to involve ADV P binding to RNA-induced silencing complex proteins AGO1 and AGO4 as shown in protein-protein interaction assays when ectopically expressed. In planta, we demonstrate that ADV P likely functions by inhibiting miRNA-guided AGO1 cleavage and prevents transitive amplification by repressing the production of secondary siRNAs. As recently described for lettuce necrotic yellows cytorhabdovirus P, but in contrast to other viral RSS known to disrupt AGO activity, ADV P sequence does not contain any recognizable GW/WG or F-box motifs, which suggests that cytorhabdovirus P proteins may use alternative motifs to bind to AGO proteins.

Functional identification of two minor capsid proteins from Chinese wheat mosaic virus using its infectious full-length cDNA clones

Full-length cDNA clones of Chinese wheat mosaic virus (CWMV) RNA1 and RNA2 were produced from single reverse transcription PCR reactions and transcripts were shown to be infectious in both wheat and Nicotiana benthamiana. An efficient and reliable agro-infiltration method was then developed for reverse genetic assays in N. benthamiana. Inoculation of infectious cDNA clones resulted in obvious chlorotic symptoms, and CWMV viral genomic RNAs, capsid protein (CP)-related proteins, and typical rod-shaped particles were detectable on the inoculated and upper leaves, similar to those of WT virus. The optimal temperature for virus multiplication was 12 °C, but the optimum for systematic infection in plants was 17 °C. Mutant clones that abolished the N- or C-terminal extensions of the major CP did not inhibit systemic infection or the formation of rod-shaped particles but sometimes modified the symptoms in inoculated plants. These results suggest that the two minor CP-related proteins of CWMV are dispensable for viral infection, replication, systemic movement and virion assembly in plants.

Identification and molecular characterization of a novel monopartite geminivirus associated with mulberry mosaic dwarf disease

High-throughput sequencing of small RNAs allowed the identification of a novel DNA virus in a Chinese mulberry tree affected by a disease showing mosaic and dwarfing symptoms. Rolling-circle amplification and PCR with specific primers, followed by sequencing of eleven independent full-length clones, showed that this virus has a monopartite circular DNA genome (∼ 2.95 kb) containing ORFs in both polarity strands, as reported previously for geminiviruses. A field survey showed the close association of the virus with diseased mulberries, so we tentatively named the virus mulberry mosaic dwarf-associated virus (MMDaV). The MMDaV genome codes for five and two putative proteins in the virion-sense and in the complementary-sense strands, respectively. Although three MMDaV virion-sense putative proteins did not share sequence homology with any protein in the databases, functional domains [coiled-coil and transmembrane (TM) domains] were identified in two of them. In addition, the protein containing a TM domain was encoded by an ORF located in a similar genomic position in MMDaV and in several other geminiviruses. As reported for members of the genera Mastrevirus and Becurtovirus, MMDaV replication-associated proteins are expressed through the alternative splicing of an intron, which was shown to be functional in vivo. A similar intron was found in the genome of citrus chlorotic dwarf-associated virus (CCDaV), a divergent geminivirus found recently in citrus. On the basis of pairwise comparisons and phylogenetic analyses, CCDaV and MMDaV appear to be closely related to each other, thus supporting their inclusion in a putative novel genus in the family Geminiviridae.

A critical domain of Sweet potato chlorotic fleck virus nucleotide-binding protein (NaBp) for RNA silencing suppression, nuclear localization and viral pathogenesis

RNA silencing is an important mechanism of antiviral defence in plants. To counteract this resistance mechanism, many viruses have evolved RNA silencing suppressors. In this study, we analysed five proteins encoded by Sweet potato chlorotic fleck virus (SPCFV) for their abilities to suppress RNA silencing using a green fluorescent protein (GFP)-based transient expression assay in Nicotiana benthamiana line 16c plants. Our results showed that a putative nucleotide-binding protein (NaBp), but not other proteins encoded by the virus, could efficiently suppress local and systemic RNA silencing induced by either sense or double-stranded RNA (dsRNA) molecules. Deletion mutation analysis of NaBp demonstrated that the basic motif (an arginine-rich region) was critical for its RNA silencing suppression activity. Using confocal laser scanning microscopy imaging of transfected protoplasts expressing NaBp fused to GFP, we showed that NaBp accumulated predominantly in the nucleus. Mutational analysis of NaBp demonstrated that the basic motif represented part of the nuclear localization signal. In addition, we demonstrated that the basic motif in NaBp was a pathogenicity determinant in the Potato virus X (PVX) heterogeneous system. Overall, our results demonstrate that the basic motif of SPCFV NaBp plays a critical role in RNA silencing suppression, nuclear localization and viral pathogenesis.

The P2 of Wheat yellow mosaic virus rearranges the endoplasmic reticulum and recruits other viral proteins into replication-associated inclusion bodies

Viruses commonly modify host endomembranes to facilitate biological processes in the viral life cycle. Infection by viruses belonging to the genus Bymovirus (family Potyviridae) has long been known to induce the formation of large membranous inclusion bodies in host cells, but their assembly and biological roles are still unclear. Immunoelectron microscopy of cells infected with the bymovirus Wheat yellow mosaic virus (WYMV) showed that P1, P2 and P3 are the major viral protein constituents of the membranous inclusions, whereas NIa-Pro (nuclear inclusion-a protease) and VPg (viral protein genome-linked) are probable minor components. P1, P2 and P3 associated with the endoplasmic reticulum (ER), but only P2 was able to rearrange ER and form large aggregate structures. Bioinformatic analyses and chemical experiments showed that P2 is an integral membrane protein and depends on the active secretory pathway to form aggregates of ER membranes. In planta and in vitro assays demonstrated that P2 interacts with P1, P3, NIa-Pro or VPg and recruits these proteins into the aggregates. In vivo RNA labelling using WYMV-infected wheat protoplasts showed that the synthesis of viral RNAs occurs in the P2-associated inclusions. Our results suggest that P2 plays a major role in the formation of membranous compartments that house the genomic replication of WYMV.

Nucleo-cytoplasmic shuttling of VPg encoded by Wheat yellow mosaic virus requires association with the coat protein

VPg (virus protein, genome-linked) is a multifunctional protein that plays important roles in viral multiplication in the cytoplasm. However, a number of VPgs encoded by plant viruses target the nucleus and this appears to be biologically significant. These VPgs may therefore be translocated between nuclear and cytoplasmic compartments during virus infection, but such nucleo-cytoplasmic transport has not been demonstrated. We report that VPg encoded by Wheat yellow mosaic virus (WYMV, genus Bymovirus, family Potyviridae) accumulated in both the nucleus and cytoplasm of infected cells, but localized exclusively in the nucleus when expressed alone in plants. Computational analyses predicted the presence of a nuclear localization signal (NLS) and a nuclear export signal (NES) in WYMV VPg. Mutational analyses showed that both the N-terminal and the NLS domains of VPg contribute to the efficiency of nuclear targeting. In vitro and in planta assays indicated that VPg interacts with WYMV coat protein (CP) and proteinase 1 (P1) proteins. Observation of VPg fused to a fluorescent protein and subcellular fractionation experiments showed that VPg was translocated to the cytoplasm when co-expressed with CP, but not with P1. Mutations in the NES domain or treatment with leptomycin B prevented VPg translocation to the cytoplasm when co-expressed with CP. Our results suggest that association with CP facilitates the nuclear export of VPg during WYMV infection.

Root-specific role for Nicotiana benthamiana RDR6 in the inhibition of Chinese wheat mosaic virus accumulation at higher temperatures

Some viruses only infect plants at cool temperatures but the molecular mechanism underlying this low-temperature dependence remains unclear. Chinese wheat mosaic virus (CWMV, genus Furovirus) was able to infect wheat and Nicotiana benthamiana plants at 16 but not at 24°C. When CWMV-infected plants were transferred to 24°C for 2 weeks, the newly emerged leaves and roots became virus free. Co-infection with Potato virus Y rescued CWMV accumulation in N. benthamiana plants after a temperature shift to 24°C. In transgenic N. benthamiana plants silenced for the N. benthamiana RNA-dependent RNA polymerase 6 (NbRDR6), CWMV was able to accumulate in roots but not in leaves after a temperature shift to 24°C. Deep sequencing of small RNAs showed that, at 16°C, abundant CWMV small interfering (si)RNAs accumulated in infected N. benthamiana plants. Silencing of NbRDR6 increased the abundance of CWMV siRNAs and the generation of siRNAs from hotspots in the CWMV genome. In contrast, when shifted to 24°C for 1 week, CWMV siRNAs were markedly fewer in roots of NbRDR6-silenced than in roots of wild-type plants but were similar in the leaves of those plants. Our results demonstrate the root-specific role of NbRDR6 in the inhibition of CWMV accumulation and biogenesis of CWMV siRNAs at higher temperatures.

Orchid fleck virus structural proteins N and P form intranuclear viroplasm-like structures in the absence of viral infection

Orchid fleck virus (OFV) has a unique two-segmented negative-sense RNA genome that resembles that of plant nucleorhabdoviruses. In infected plant cells, OFV and nucleorhabdoviruses induce an intranuclear electron-lucent viroplasm that is believed to be the site for virus replication. In this study, we investigated the molecular mechanism by which OFV viroplasms are produced in vivo. Among OFV-encoded proteins, the nucleocapsid protein (N) and the putative phosphoprotein (P) were present in nuclear fractions of OFV-infected Nicotiana benthamiana plants. Transient coexpression of N and P, in the absence of virus infection, was shown to be sufficient for formation of an intranuclear viroplasm-like structure in plant cells. When expressed independently as a fluorescent protein fusion product in uninfected plant cells, N protein accumulated throughout the cell, while P protein accumulated in the nucleus. However, the N protein, when coexpressed with P, was recruited to a subnuclear region to induce a large viroplasm-like focus. Deletion and substitution mutagenesis demonstrated that the P protein contains a nuclear localization signal (NLS). Artificial nuclear targeting of the N-protein mutant was insufficient for formation of viroplasm-like structures in the absence of P. A bimolecular fluorescence complementation assay confirmed interactions between the N and P proteins within subnuclear viroplasm-like foci and interactions of two of the N. benthamiana importin-α homologues with the P protein but not with the N protein. Taken together, our results suggest that viroplasm formation by OFV requires nuclear accumulation of both the N and P proteins, which is mediated by P-NLS, unlike nucleorhabdovirus viroplasm utilizing the NLS on protein N.

Non-structural protein P6 encoded by rice black-streaked dwarf virus is recruited to viral inclusion bodies by binding to the viroplasm matrix protein P9-1

Like other members of the family Reoviridae, rice black-streaked dwarf virus (RBSDV, genus Fijivirus) is thought to replicate and assemble within cytoplasmic viral inclusion bodies, commonly called viroplasms. RBSDV P9-1 is the key protein for the formation of viroplasms, but little is known about the other proteins of the viroplasm or the molecular interactions amongst its components. RBSDV non-structural proteins were screened for their association with P9-1 using a co-immunoprecipitation assay. Only P6 was found to directly interact with P9-1, an interaction that was confirmed by bimolecular fluorescence complementation assay in Spodoptera frugiperda (Sf9) cells. Immunoelectron microscopy showed that P6 and P9-1 co-localized in electron-dense inclusion bodies, indicating that P6 is a constituent of the viroplasm. In addition, non-structural protein P5 also localized to viroplasms and interacted with P6. In Sf9 cells, P6 was diffusely distributed throughout the cytoplasm when expressed alone, but localized to inclusions when co-expressed with P9-1, suggesting that P6 is recruited to viral inclusion bodies by binding to P9-1. P5 localized to the inclusions formed by P9-1 when co-expressed with P6 but did not when P6 was absent, suggesting that P5 is recruited to viroplasms by binding to P6. This study provides a model by which viral non-structural proteins are recruited to RBSDV viroplasms.