Plant RNA Viruses Research Articles

Comparison of Nanopore Sequencing Protocols and Real-Time Analysis for Phytopathogen Diagnostics

Next-generation sequencing technologies have aided molecular detection and diagnosis of plant pathogens. Short read sequencing technologies can be expensive, time consuming, and lack portability. The Oxford Nanopore Technologies (ONT) MinION sequencer offers plant diagnosticians an alternative for whole-genome and transcriptome sequencing with the advantages of portability, rapid results and access to sequences generated, and relatively low cost. Although this company offers multiple sequencing products, most previous articles focused on comparing this technology with other sequencing technology (Sanger, Illumina) and not comparing ONT product performance. In this project, we explored the potential of using ONT’s kits without modifications for the sequencing of plant pathogens. We compared the performance of DNA library preparation protocols (1D versus 1D2 versus Rapid) and RNA library preparation protocols (cDNA PCR Sequencing versus Direct RNA) on the MinION sequencer and examined different data analysis approaches for two well-characterized plant pathogens of agricultural concern, Ralstonia solanacearum and plum pox virus. For bacterial DNA sequencing, the Rapid Sequencing kit had the lowest throughput but offered the shortest sample-to-sequence turnaround time. The cDNA PCR Sequencing kit was better suited for plant RNA viruses than the Direct RNA. Whole-genome assemblies using all tested kits provided accurate identification of the target pathogen when pure cultures or infected plant material was used. This technology offers a shorter sample-to-sequence timeline than previous technologies for both DNA and RNA samples. Data analysis can still be time- and resource-consuming, but we found that 1 h of data generation was sufficient for accurate identification of the target pathogens, which can alleviate computing resources because a full 48-h run of sequencing data is not required. With the decreasing costs of flow cells, MinION is now an additional tool to be considered by plant diagnosticians for whole-genome and transcriptome-based diagnostics.

Rapid generation of inoculum of a plant RNA virus using overlap PCR

We have developed an efficient method to rapidly generate infectious inoculum of a plant RNA virus and confirmed its infectivity by mechanical inoculation. The method takes advantage of overlap PCR to bypass the cloning steps, which makes it relatively simple, rapid, and inexpensive compared to the traditional methods. Using this approach, inoculum of a tobamovirus, Turnip vein clearing virus (TVCV), was generated. PCR products specific for the 35S promoter and TVCV genome were used as templates for overlap PCR to form a single product containing the full-length TVCV cDNA under the control of the double 35S promoter, and the entire process took only 8 h. This inoculum was infectious in Nicotiana benthamiana, and its infectivity was ca. 67% compared to 0% and 100% with negative and positive controls, respectively. Thus, this rapid method generates efficient infectious inoculum for a plant RNA virus.

Intercellular movement of plant RNA viruses: Targeting replication complexes to the plasmodesma for both accuracy and efficiency.

Replication and movement are two critical steps in plant virus infection. Recent advances in the understanding of the architecture and subcellular localization of virus-induced inclusions and the interactions between viral replication complex (VRC) and movement proteins (MPs) allow for the dissection of the intrinsic relationship between replication and movement, which has revealed that recruitment of VRCs to the plasmodesma (PD) via direct or indirect MP-VRC interactions is a common strategy used for cell-to-cell movement by most plant RNA viruses. In this review, we summarize the recent advances in the understanding of virus-induced inclusions and their roles in virus replication and cell-to-cell movement, analyze the advantages of such coreplicational movement from a viral point of view and discuss the possible mechanical force by which MPs drive the movement of virions or viral RNAs through the PD. Finally, we highlight the missing pieces of the puzzle of viral movement that are especially worth investigating in the near future.

Crystal structure of tomato spotted wilt virus GN reveals a dimer complex formation and evolutionary link to animal-infecting viruses

Tospoviridae is a family of enveloped RNA plant viruses that infect many field crops, inflicting a heavy global economic burden. These tripartite, single-stranded, negative-sense RNA viruses are transmitted from plant to plant by thrips as the insect vector. The medium (M) segment of the viral genome encodes two envelope glycoproteins, GN and GC, which together form the envelope spikes. GC is considered the virus fusogen, while the accompanying GN protein serves as an attachment protein that binds to a yet unknown receptor, mediating the virus acquisition by the thrips carrier. Here we present the crystal structure of glycoprotein N (GN) from the tomato spotted wilt virus (TSWV), a representative member of the Tospoviridae family. The structure suggests that GN is organized as dimers on TSWV's outer shell. Our structural data also suggest that this dimerization is required for maintaining GN structural integrity. Although the structure of the TSWV GN is different from other bunyavirus GN proteins, they all share similar domain connectivity that resembles glycoproteins from unrelated animal-infecting viruses, suggesting a common ancestor for these accompanying proteins.

Long Noncoding RNAs in Plant Viroids and Viruses: A Review.

Infectious long-noncoding (lnc) RNAs related to plants can be of both viral and non-viral origin. Viroids are infectious plant lncRNAs that are not related to viruses and carry the circular, single-stranded, non-coding RNAs that replicate with host enzymatic activities via a rolling circle mechanism. Viroids interact with host processes in complex ways, emerging as one of the most productive tools for studying the functions of lncRNAs. Defective (D) RNAs, another category of lnc RNAs, are found in a variety of plant RNA viruses, most of which are noncoding. These are derived from and are replicated by the helper virus. D RNA-virus interactions evolve into mutually beneficial combinations, enhancing virus fitness via competitive advantages of moderated symptoms. Yet the satellite RNAs are single-stranded and include either large linear protein-coding ss RNAs, small linear ss RNAs, or small circular ss RNAs (virusoids). The satellite RNAs lack sequence homology to the helper virus, but unlike viroids need a helper virus to replicate and encapsidate. They can attenuate symptoms via RNA silencing and enhancement of host defense, but some can be lethal as RNA silencing suppressor antagonists. Moreover, selected viruses produce lncRNAs by incomplete degradation of genomic RNAs. They do not replicate but may impact viral infection, gene regulation, and cellular functions. Finally, the host plant lncRNAs can also contribute during plant-virus interactions, inducing plant defense and the regulation of gene expression, often in conjunction with micro and/or circRNAs.

Capsicum annum Hsp26.5 promotes defense responses against RNA viruses via ATAF2 but is hijacked as a chaperone for tobamovirus movement protein.

The expression of Capsicum annuum HEAT SHOCK PROTEIN 26.5 (CaHsp26.5) was triggered by the inoculation of Tobacco mosaic virus pathotype P0 (TMV-P0) but its function in the defense response of plants is unknown. We used gene silencing and overexpression approaches to investigate the effect of CaHsp26.5 expression on different plant RNA viruses. Moreover, we performed protein-protein and protein-RNA interaction assays to study the mechanism of CaHsp26.5 function. CaHsp26.5 binding to a short poly-cytosine motif in the 3'-untranslated region of the genome of some viruses triggers the expression of several defense-related genes such as PATHOGENESIS-RELATED GENE 1 with the help of a transcription factor, NAC DOMAIN-CONTAINING PROTEIN 81 (ATAF2). Thus, an elevated CaHsp26.5 level was accompanied by increased plant resistance against plant viruses such as Cucumber mosaic virus strain Korea. However, the movement proteins of Pepper mild mottle virus pathotype P1,2,3 and TMV-P0 were shown to be able to interact with CaHsp26.5 to maintain the integrity of their proteins. Our work shows CaHsp26.5 as a positive player in the plant defense response against several plant RNA viruses. However, some tobamoviruses can hijack CaHsp26.5's chaperone activity for their own benefit.

A plant DNA virus replicates in the salivary glands of its insect vector via recruitment of host DNA synthesis machinery

Whereas most of the arthropod-borne animal viruses replicate in their vectors, this is less common for plant viruses. So far, only some plant RNA viruses have been demonstrated to replicate in insect vectors and plant hosts. How plant viruses evolved to replicate in the animal kingdom remains largely unknown. Geminiviruses comprise a large family of plant-infecting, single-stranded DNA viruses that cause serious crop losses worldwide. Here, we report evidence and insight into the replication of the geminivirus tomato yellow leaf curl virus (TYLCV) in the whitefly (Bemisia tabaci) vector and that replication is mainly in the salivary glands. We found that TYLCV induces DNA synthesis machinery, proliferating cell nuclear antigen (PCNA) and DNA polymerase δ (Polδ), to establish a replication-competent environment in whiteflies. TYLCV replication-associated protein (Rep) interacts with whitefly PCNA, which recruits DNA Polδ for virus replication. In contrast, another geminivirus, papaya leaf curl China virus (PaLCuCNV), does not replicate in the whitefly vector. PaLCuCNV does not induce DNA-synthesis machinery, and the Rep does not interact with whitefly PCNA. Our findings reveal important mechanisms by which a plant DNA virus replicates across the kingdom barrier in an insect and may help to explain the global spread of this devastating pathogen.

Newly emerged resistance-breaking variants of cucumber mosaic virus represent ongoing host-interactive evolution of an RNA virus

Understanding the evolutionary history of a virus and the mechanisms influencing the direction of its evolution is essential for the development of more durable strategies to control the virus in crop fields. While the deployment of host resistance in crops is the most efficient means to control various viruses, host resistance itself can act as strong selective pressure and thus play a critical role in the evolution of virus virulence. Cucumber mosaic virus (CMV), a plant RNA virus with high evolutionary capacity, has caused endemic disease in various crops worldwide, including pepper (Capsicum annuum L.), because of frequent emergence of resistance-breaking variants. In this study, we examined the molecular and evolutionary characteristics of recently emerged, resistance-breaking CMV variants infecting pepper. Our population genetics analysis revealed that the high divergence capacity of CMV RNA1 might have played an essential role in the host-interactive evolution of CMV and in shaping the CMV population structure in pepper. We also demonstrated that nonsynonymous mutations in RNA1 encoding the 1a protein enabled CMV to overcome the deployed resistance in pepper. Our findings suggest that resistance-driven selective pressures on RNA1 might have contributed in shaping the unique evolutionary pattern of CMV in pepper. Therefore, deployment of a single resistance gene may reduce resistance durability against CMV and more integrated approaches are warranted for successful control of CMV in pepper.

A plant RNA virus activates selective autophagy in a UPR-dependent manner to promote virus infection.

Autophagy is an evolutionarily conserved pathway in eukaryotes that delivers unwanted cytoplasmic materials to the lysosome/vacuole for degradation/recycling. Stimulated autophagy emerges as an integral part of plant immunity against intracellular pathogens. In this study, we used turnip mosaic virus (TuMV) as a model to investigate the involvement of autophagy in plant RNA virus infection. The small integral membrane protein 6K2 of TuMV, known as a marker of the virus replication site and an elicitor of the unfolded protein response (UPR), upregulates the selective autophagy receptor gene NBR1 in a UPR-dependent manner. NBR1 interacts with TuMV NIb, the RNA-dependent RNA polymerase of the virus replication complex (VRC), and the autophagy cargo receptor/adaptor protein ATG8f. The NIb/NBR1/ATG8f interaction complexes colocalise with the 6K2-stained VRC. Overexpression of NBR1 or ATG8f enhances TuMV replication, and deficiency of NBR1 or ATG8f inhibits virus infection. In addition, ATG8f interacts with the tonoplast-specific protein TIP1 and the NBR1/ATG8f-containing VRC is enclosed by the TIP1-labelled tonoplast. In TuMV-infected cells, numerous membrane-bound viral particles are evident in the vacuole. Altogether these results suggest that TuMV activates and manipulates UPR-dependent NBR1-ATG8f autophagy to target the VRC to the tonoplast to promote viral replication and virion accumulation.

Distinct modes of manipulation of rice auxin response factor OsARF17 by different plant RNA viruses for infection

Plant auxin response factor (ARF) transcription factors are an important class of key transcriptional modulators in auxin signaling. Despite the well-studied roles of ARF transcription factors in plant growth and development, it is largely unknown whether, and how, ARF transcription factors may be involved in plant resistance to pathogens. We show here that two fijiviruses (double-stranded RNA viruses) utilize their proteins to disturb the dimerization of OsARF17 and repress its transcriptional activation ability, while a tenuivirus (negative-sense single-stranded RNA virus) directly interferes with the DNA binding activity of OsARF17. These interactions impair OsARF17-mediated antiviral defense. OsARF17 also confers resistance to a cytorhabdovirus and was directly targeted by one of the viral proteins. Thus, OsARF17 is the common target of several very different viruses. This suggests that OsARF17 plays a crucial role in plant defense against different types of plant viruses, and that these viruses use independently evolved viral proteins to target this key component of auxin signaling and facilitate infection.

Eukaryotic Initiation Factor 3 Interactions with 5′ and 3′ RNA structures of the Barley Yellow Dwarf Virus Facilitate a Non‐Canonical Translation Mechanism

Barley Yellow Dwarf Virus (BYDV) is a positive strand RNA plant virus that translates without using a 5′ 7‐methylguanosine cap or a 3′ poly‐adenosine tail that are required for the canonical translation mechanism used by most mRNA. BYDV’s non‐canonical translation relies on RNA structures in the 5′ and 3′ untranslated regions (UTRs) that recruit eukaryotic initiation factors (eIFs) and ribosomes. Using a combination of biophysical, biochemical and chemical probing techniques, we have measured direct binding of eukaryotic initiation factors (eIFs) to BYVD 5′ and 3′ UTRs and probed binding sites. Our data show that one of the initiation factors, eIF3, can bind tightly, specifically, and simultaneously to structures in both 5′ and 3′ UTRs. This suggests that eIF3’s mechanistic role includes stabilization of the long‐distance RNA‐RNA interaction and the transfer of 3′‐bound factors/complexes to the 5′ UTR. Further work with RNA chemical probing has identified regions of the BYDV UTRs that interact with eIF3. This data highlights commonalities between BYDV structures and structures found in other viruses (e.g. Hepatitis C virus) and cellular mRNAs (e.g. c_JUN mRNA).Support or Funding InformationNSF MCB‐1902054 to Dixie J. Goss, NSF IGERT‐0965983 to Hunter College

The Multifunctional Long-Distance Movement Protein of Pea Enation Mosaic Virus 2 Protects Viral and Host Transcripts from Nonsense-Mediated Decay.

The nonsense-mediated decay (NMD) pathway presents a challenge for RNA viruses with termination codons that precede extended 3' untranslated regions (UTRs). The umbravirus Pea enation mosaic virus 2 (PEMV2) is a nonsegmented, positive-sense RNA virus with an unusually long 3' UTR that is susceptible to NMD. To establish a systemic infection, the PEMV2 long-distance movement protein p26 was previously shown to both stabilize viral RNAs and bind them for transport through the plant's vascular system. The current study demonstrated that p26 protects both viral and nonviral messenger RNAs from NMD. Although p26 localizes to both the cytoplasm and nucleolus, p26 exerts its anti-NMD effects exclusively in the cytoplasm independently of long-distance movement. Using a transcriptome-wide approach in the model plant Nicotiana benthamiana, p26 protected a subset of cellular NMD target transcripts, particularly those containing long, structured, GC-rich 3' UTRs. Furthermore, transcriptome sequencing (RNA-seq) revealed that the NMD pathway is highly dysfunctional during PEMV2 infection, with 1,820 (48%) of NMD targets increasing in abundance. Widespread changes in the host transcriptome are common during plant RNA virus infections, and these results suggest that, in at least some instances, virus-mediated NMD inhibition may be a major contributing factor.IMPORTANCE Nonsense-mediated decay (NMD) represents an RNA regulatory pathway that degrades both natural and faulty messenger RNAs with long 3' untranslated regions. NMD targets diverse families of RNA viruses, requiring that viruses counteract the NMD pathway for successful amplification in host cells. A protein required for long-distance movement of Pea enation mosaic virus 2 (PEMV2) is shown to also protect both viral and host mRNAs from NMD. RNA-seq analyses of the Nicotiana benthamiana transcriptome revealed that PEMV2 infection significantly impairs the host NMD pathway. RNA viruses routinely induce large-scale changes in host gene expression, and, like PEMV2, may use NMD inhibition to alter the host transcriptome in an effort to increase virus amplification.

Eukaryotic Translation Initiation Factors Shape RNA Viruses Resistance in Plants

Viruses are representative of a global threat to agricultural production. Genetic resistance is the preferred strategy for the control of viral infection and against loss of crop yield. Viral protein synthesis requires host cellular factors for translating their viral RNAs, and for regulating their replication and cell to cell systemic movement. Therefore, the viruses are dependent on cellular translation factors. Mutations in the gene encoding eIF4E and eIF4G or their isoforms, eIFiso4E, eIFiso4G and eIF2Bβ have been mapped as a source of plant potyvirus while other genus of plant virus recessive resistance genes in many species are originated from these loci. Some of other plant translation factors, such as eIF3, eIF4A-like helicases, eEF1A and eEF1B, which are required in interacting with viral RNAs and regulating various aspects of the infection cycle, have also been identified. Here, we summarized the mechanisms utilized by RNA viruses of eukaryotic plants and the essential roles of eIFs in virus infection. Moreover, we discussed the potential of eIFs as a target gene in the development of genetic resistance to viruses for crop improvement. This review highlighted newly revealed examples of abnormal translational strategies and provided insights into natural host resistance mechanisms that have been linked to 3′ cap-independent translational enhancer activity.

The NIa-Protease Protein Encoded by the Pepper Mottle Virus Is a Pathogenicity Determinant and Releases DNA Methylation of Nicotiana benthamiana.

It is well documented that the canonical function of NIa-protease (NIa-Pro) of the potyviruses is responsible for cleaving the viral polyprotein into functional proteins. Although NIa-Pro is vital for the infection cycle of potyviruses, the function of NIa-Pro in the interaction of the potyvirus host is not clear. In this study, NIa-Pro is ectopically expressed from a potato virus X (PVX) vector and infiltrates Nicotiana benthamiana wild type and 16-TGS. The pathogenicity and inhibition of host transcriptional gene silencing (TGS) are characterized. Ectopic expression of NIa-Pro from a PVX vector resulted in severe mosaic symptoms followed by a hypersensitive-like response in N. benthamiana. Furthermore, PepMoV NIa-Pro was able to reverse established TGS of a green fluorescent protein transgene by reducing methylation of promoter sequences in N. benthamiana and possessed the capacity to interfere with the global methylation of N. benthamiana. Taken together, the results of this study likely suggest that PepMoV NIa-Pro is a pathogenicity determinant and a potent suppressor of host TGS and suggest that NIa-Pro may employ novel mechanisms to suppress host antiviral defenses. To the best of our knowledge, this is the first report of a plant RNA virus modulating host TGS in a novel manner by interfering with the establishment of the methylation step of the plant DNA methylation pathway.

Plant Virus Genome Is Shaped by Specific Dinucleotide Restrictions That Influence Viral Infection.

The presence of CpG and UpA dinucleotides is restricted in the genomes of animal RNA viruses to avoid specific host defenses. We wondered whether a similar phenomenon exists in nonanimal RNA viruses. Here, we show that these two dinucleotides, especially UpA, are underrepresented in the family Potyviridae, the most important group of plant RNA viruses. Using plum pox virus (PPV; Potyviridae family) as a model, we show that an increase in UpA frequency strongly diminishes virus accumulation. Remarkably, unlike previous observations in animal viruses, PPV variants harboring CpG-rich fragments display just faint (or no) attenuation. The anticorrelation between UpA frequency and viral fitness additionally demonstrates the relevance of this particular dinucleotide: UpA-high mutants are attenuated in a dose-dependent manner, whereas a UpA-low variant displays better fitness than its parental control. Using high-throughput sequencing, we also show that UpA-rich PPV variants are genetically stable, without apparent changes in sequence that revert and/or compensate for the dinucleotide modification despite its attenuation. In addition, we also demonstrate here that the PPV restriction of UpA-rich variants works independently of the classical RNA silencing pathway. Finally, we show that the anticorrelation between UpA frequency and RNA accumulation applies to mRNA-like fragments produced by the host RNA polymerase II. Together, our results inform us about a dinucleotide-based system in plant cells that controls diverse RNAs, including RNA viruses.IMPORTANCE Dinucleotides (combinations of two consecutive nucleotides) are not randomly present in RNA viruses; in fact, the presence of CpG and UpA is significantly repressed in their genomes. Although the meaning of this phenomenon remains obscure, recent studies with animal-infecting viruses have revealed that their low CpG/UpA frequency prevents virus restriction via a host antiviral system that recognizes, and promotes the degradation of, CpG/UpA-rich RNAs. Whether similar systems act in organisms from other life kingdoms has been unknown. To fill this gap in our knowledge, we built several synthetic variants of a plant RNA virus with deoptimized dinucleotide frequencies and analyzed their viral fitness and genome adaptation. In brief, our results inform us for the first time about an effective dinucleotide-based system that acts in plants against viruses. Remarkably, this viral restriction in plants is reminiscent of, but not identical to, the equivalent antiviral response in animals.

Facilitative and synergistic interactions between fungal and plant viruses

Plants and fungi are closely associated through parasitic or symbiotic relationships in which bidirectional exchanges of cellular contents occur. Recently, a plant virus was shown to be transmitted from a plant to a fungus, but it is unknown whether fungal viruses can also cross host barriers and spread to plants. In this study, we investigated the infectivity of Cryphonectria hypovirus 1 (CHV1, family Hypoviridae), a capsidless, positive-sense (+), single-stranded RNA (ssRNA) fungal virus in a model plant, Nicotiana tabacum CHV1 replicated in mechanically inoculated leaves but did not spread systemically, but coinoculation with an unrelated plant (+)ssRNA virus, tobacco mosaic virus (TMV, family Virgaviridae), or other plant RNA viruses, enabled CHV1 to systemically infect the plant. Likewise, CHV1 systemically infected transgenic plants expressing the TMV movement protein, and coinfection with TMV further enhanced CHV1 accumulation in these plants. Conversely, CHV1 infection increased TMV accumulation when TMV was introduced into a plant pathogenic fungus, Fusarium graminearum In the in planta F. graminearum inoculation experiment, we demonstrated that TMV infection of either the plant or the fungus enabled the horizontal transfer of CHV1 from the fungus to the plant, whereas CHV1 infection enhanced fungal acquisition of TMV. Our results demonstrate two-way facilitative interactions between the plant and fungal viruses that promote cross-kingdom virus infections and suggest the presence of plant-fungal-mediated routes for dissemination of fungal and plant viruses in nature.

Identification of eukaryotic translation initiation factors and the temperature-dependent nature of Turnip mosaic virus epidemics in allopolyploid Brassica juncea.

Eukaryotic translation initiation factors (eIFs) are essential protein complexes involved in the translation of mRNA into proteins. These initiation factors are generally used as targets in the control of plant RNA virus infections. In the present study, we identified a total 190 eIFs, clustered phylogenetically into 40 distinct subfamilies in the allopolyploid Brassica juncea. Extensive evolutionary duplications of the eIFs in B. juncea suggest their increased genetic diversity and wide adaptability. The induction of expressions in some of the eIFs after inoculation against Turnip mosaic virus (TuMV) provided candidate targets to be used in the control of viral infections. In addition, the expression profiles of eIFs under different temperatures suggested that the TuMV epidemic was temperature dependent. The eIFs expressions suggested that the systemic viral infections were more acute in plants grown between 20°C and 28°C. In addition, our results revealed that new subgroups of eIFs, eIF2β, eIF2α, eIF2Bβ, EF1A, and PABP could be represented as targets for antiviral strategies in B. juncea. In summary, our findings would be helpful in studying the complex mechanisms of eIF-mediated, temperature-dependent RNA virus control in B. juncea.

Host Range Evolution of Potyviruses: A Global Phylogenetic Analysis

Virus host range, i.e., the number and diversity of host species of viruses, is an important determinant of disease emergence and of the efficiency of disease control strategies. However, for plant viruses, little is known about the genetic or ecological factors involved in the evolution of host range. Using available genome sequences and host range data, we performed a phylogenetic analysis of host range evolution in the genus Potyvirus, a large group of plant RNA viruses that has undergone a radiative evolution circa 7000 years ago, contemporaneously with agriculture intensification in mid Holocene. Maximum likelihood inference based on a set of 59 potyviruses and 38 plant species showed frequent host range changes during potyvirus evolution, with 4.6 changes per plant species on average, including 3.1 host gains and 1.5 host loss. These changes were quite recent, 74% of them being inferred on the terminal branches of the potyvirus tree. The most striking result was the high frequency of correlated host gains occurring repeatedly in different branches of the potyvirus tree, which raises the question of the dependence of the molecular and/or ecological mechanisms involved in adaptation to different plant species.

First Report of Cucurbit Chlorotic Yellows Virus Infecting Cucumber and Zucchini in Algeria

This work was supported by the Spanish Ministerio de Ciencia, Innovacion y Universidades, cofunded with FEDER funds (project nos. AGL2017-85563-C2-1-R and RTA2017-00061-0O3-03 [INIA]) and the programa para grupos de investigacion de excelencia from the Conselleria d'Educacio, Investigacio, Cultura i Esport, (Generalitat Valenciana) (Prometeo Program 2017/078). A. Kheireddine thanks the Erasmus+ Programme of the European Union for her mobility project (KA107 2018-20). C. Saez is a recipient of a predoctoral fellowship from Generalitat Valenciana, cofunded by the Operational Program of the European Social Fund (FSECV 2014-2020) (grant no. ACIF/2016/188). Plant Dis. 104: 1264, 2020; published online as https://doi.org/10.1094/PDIS-10-19-2091PDN.Accepted for publication 19 December 2019.

The RNA-Dependent RNA Polymerase NIb of Potyviruses Plays Multifunctional, Contrasting Roles during Viral Infection.

Potyviruses represent the largest group of known plant RNA viruses and include many agriculturally important viruses, such as Plum pox virus, Soybean mosaic virus, Turnip mosaic virus, and Potato virus Y. Potyviruses adopt polyprotein processing as their genome expression strategy. Among the 11 known viral proteins, the nuclear inclusion protein b (NIb) is the RNA-dependent RNA polymerase responsible for viral genome replication. Beyond its principal role as an RNA replicase, NIb has been shown to play key roles in diverse virus–host interactions. NIb recruits several host proteins into the viral replication complexes (VRCs), which are essential for the formation of functional VRCs for virus multiplication, and interacts with the sumoylation pathway proteins to suppress NPR1-mediated immunity response. On the other hand, NIb serves as a target of selective autophagy as well as an elicitor of effector-triggered immunity, resulting in attenuated virus infection. These contrasting roles of NIb provide an excellent example of the complex co-evolutionary arms race between plant hosts and potyviruses. This review highlights the current knowledge about the multifunctional roles of NIb in potyvirus infection, and discusses future research directions.