A cucumber mosaic virus mutant lacking the 2b counter-defence protein gene provides protection against wild-type strains

Strain-specific differences in the interactions of the cucumber mosaic virus 2b protein with the viral 1a and host Argonaute 1 proteins

The cucumber mosaic virus (CMV) 2b protein is a potent counter-defense factor and symptom determinant that inhibits antiviral silencing by titrating short double-stranded RNAs. Expression of the CMV subgroup IA strain Fny-CMV 2b protein in transgenic Arabidopsis thaliana plants disrupts microRNA-mediated cleavage of host mRNAs by binding Argonaute 1 (AGO1), leading to symptom-like phenotypes. This also triggers AGO2-mediated antiviral resistance and resistance to CMV's aphid vectors. However, in authentic viral infections, the Fny-CMV 1a protein modulates 2b-AGO1 interactions, inhibiting induction of AGO2-mediated virus resistance and aphid resistance. Contrastingly, 2b proteins encoded by the subgroup II strain LS-CMV and the recently discovered subgroup IA strain Ho-CMV induce no symptoms. Confocal laser scanning microscopy, bimolecular fluorescence complementation, and co-immunoprecipitation showed that Fny-CMV and Ho-CMV 2b proteins interact with Fny-CMV and LS-CMV 1a proteins, while the CMV-LS 2b protein cannot. However, Fny-CMV, Ho-CMV, and LS-CMV 2b proteins, all interacted with AGO1, but while AGO1-Fny2b complexes occurred in the nucleus and cytoplasm, corresponding AGO1-2b complexes for LS-CMV and Ho-CMV accumulated almost exclusively in nuclei. AGO2 transcript accumulation was used to assess the inhibition of AGO1-mediated mRNA degradation. Fny-CMV 2b induced a fivefold increase in AGO2 accumulation, but LS-CMV and Ho-CMV 2b proteins induced only twofold increases. Thus, these 2b proteins bind AGO1 but are less effective at inhibiting AGO1 activity. We conclude that the intracellular localization of 2b-AGO1 complexes influences the degree to which a 2b protein inhibits microRNA-mediated host mRNA degradation and that cytoplasmic AGO1 has the strongest influence on miRNA-mediated cellular mRNA turnover.

IMPORTANCE

The cucumber mosaic virus (CMV) 2b protein was among the first discovered viral suppressors of RNA silencing. It has additional pro-viral functions through effects on plant defensive signaling pathways mediated by salicylic acid and jasmonic acid, the abscisic acid pathway and virus-induced drought resistance, and on host plant interactions with insect vectors. Many of these effects occur due to interaction with the important host RNA silencing component Argonaute 1 (AGO1). It was thought that only 2b proteins of "severe" CMV strains interacted with AGO1 and inhibited its microRNA-mediated "slicing" of cellular mRNAs and that the lack of interaction with AGO1 explained the moderate symptoms typically seen in plants infected with mild CMV strains. Our work overthrows this paradigm by showing that mild strain CMV 2b proteins can interact with AGO1, but their in vivo localization prevents them from interacting with AGO1 molecules present in the infected cell cytoplasm.

Visual tracking of viral infection dynamics reveals the synergistic interactions between cucumber mosaic virus and broad bean wilt virus 2

Cucumber mosaic virus (CMV) is one of the most prevalent plant viruses in the world, and causes severe damage to various crops. CMV has been studied as a model RNA virus to better understand viral replication, gene functions, evolution, virion structure, and pathogenicity. However, CMV infection and movement dynamics remain unexplored due to the lack of a stable recombinant virus tagged with a reporter gene. In this study, we generated a CMV infectious cDNA construct tagged with a variant of the flavin-binding LOV photoreceptor (iLOV). The iLOV gene was stably maintained in the CMV genome after more than four weeks of three serial passages between plants. Using the iLOV-tagged recombinant CMV, we visualized CMV infection and movement dynamics in living plants in a time course manner. We also examined whether CMV infection dynamics is influenced by co-infection with broad bean wilt virus 2 (BBWV2). Our results revealed that no spatial interference occurred between CMV and BBWV2. Specifically, BBWV2 facilitated the cell-to-cell movement of CMV in the upper young leaves. In addition, the BBWV2 accumulation level increased after co-infection with CMV.

Tomato brown rugose fruit virus: An emerging and rapidly spreading plant RNA virus that threatens tomato production worldwide

Tomato brown rugose fruit virus (ToBRFV) is an emerging and rapidly spreading RNA virus that infects tomato and pepper, with tomato as the primary host. The virus causes severe crop losses and threatens tomato production worldwide. ToBRFV was discovered in greenhouse tomato plants grown in Jordan in spring 2015 and its first outbreak was traced back to 2014 in Israel. To date, the virus has been reported in at least 35 countries across four continents in the world. ToBRFV is transmitted mainly via contaminated seeds and mechanical contact (such as through standard horticultural practices). Given the global nature of the seed production and distribution chain, and ToBRFV's seed transmissibility, the extent of its spread is probably more severe than has been disclosed. ToBRFV can break down genetic resistance to tobamoviruses conferred by R genes Tm-1, Tm-2, and Tm-2² in tomato and L¹ and L² alleles in pepper. Currently, no commercial ToBRFV-resistant tomato cultivars are available. Integrated pest management-based measures such as rotation, eradication of infected plants, disinfection of seeds, and chemical treatment of contaminated greenhouses have achieved very limited success. The generation and application of attenuated variants may be a fast and effective approach to protect greenhouse tomato against ToBRFV. Long-term sustainable control will rely on the development of novel genetic resistance and resistant cultivars, which represents the most effective and environment-friendly strategy for pathogen control.

TAXONOMY

Tomato brown rugose fruit virus belongs to the genus Tobamovirus, in the family Virgaviridae. The genus also includes several economically important viruses such as Tobacco mosaic virus and Tomato mosaic virus.

GENOME AND VIRION

The ToBRFV genome is a single-stranded, positive-sense RNA of approximately 6.4 kb, encoding four open reading frames. The viral genomic RNA is encapsidated into virions that are rod-shaped and about 300 nm long and 18 nm in diameter. Tobamovirus virions are considered extremely stable and can survive in plant debris or on seed surfaces for long periods of time.

DISEASE SYMPTOMS

Leaves, particularly young leaves, of tomato plants infected by ToBRFV exhibit mild to severe mosaic symptoms with dark green bulges, narrowness, and deformation. The peduncles and calyces often become necrotic and fail to produce fruit. Yellow blotches, brown or black spots, and rugose wrinkles appear on tomato fruits. In pepper plants, ToBRFV infection results in puckering and yellow mottling on leaves with stunted growth of young seedlings and small yellow to brown rugose dots and necrotic blotches on fruits.

Natural Selection, Intracellular Bottlenecks of Virus Populations, and Viral Superinfection Exclusion

Natural selection acts on cellular organisms by ensuring the genes responsible for an advantageous phenotype consistently reap the phenotypic advantage. This is possible because reproductive cells of these organisms are almost always haploid, separating the beneficial gene from its rival allele at every generation. How natural selection acts on plus-strand RNA viruses is unclear because these viruses frequently load host cells with numerous genome copies and replicate thousands of progeny genomes in each cell. Recent studies suggest that these viruses encode the Bottleneck, Isolate, Amplify, Select (BIAS) mechanism that blocks all but a few viral genome copies from replication, thus creating the environment in which the bottleneck-escaping viral genome copies are isolated from each other, allowing natural selection to reward beneficial mutations and purge lethal errors. This BIAS mechanism also blocks the genomes of highly homologous superinfecting viruses, thus explaining cellular-level superinfection exclusion. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Efficient and high-throughput pseudorecombinant-chimeric Cucumber mosaic virus-based VIGS in maize

Virus-induced gene silencing (VIGS) is a versatile and attractive approach for functional gene characterization in plants. Although several VIGS vectors for maize (Zea mays) have been previously developed, their utilities are limited due to low viral infection efficiency, insert instability, short maintenance of silencing, inadequate inoculation method, or abnormal requirement of growth temperature. Here, we established a Cucumber mosaic virus (CMV)-based VIGS system for efficient maize gene silencing that overcomes many limitations of VIGS currently available for maize. Using two distinct strains, CMV-ZMBJ and CMV-Fny, we generated a pseudorecombinant-chimeric (Pr) CMV. Pr CMV showed high infection efficacy but mild viral symptoms in maize. We then constructed Pr CMV-based vectors for VIGS, dubbed Pr CMV VIGS. Pr CMV VIGS is simply performed by mechanical inoculation of young maize leaves with saps of Pr CMV-infected Nicotiana benthamiana under normal growth conditions. Indeed, suppression of isopentenyl/dimethylallyl diphosphate synthase (ZmIspH) expression by Pr CMV VIGS resulted in non-inoculated leaf bleaching as early as 5 d post-inoculation (dpi) and exhibited constant and efficient systemic silencing over the whole maize growth period up to 105 dpi. Furthermore, utilizing a ligation-independent cloning (LIC) strategy, we developed a modified Pr CMV-LIC VIGS vector, allowing easy gene cloning for high-throughput silencing in maize. Thus, our Pr CMV VIGS system provides a much-improved toolbox to facilitate efficient and long-duration gene silencing for large-scale functional genomics in maize, and our pseudorecombination-chimera combination strategy provides an approach to construct efficient VIGS systems in plants.

A Double Mutation in the Conserved Motifs of the Helper Component Protease of Papaya Leaf Distortion Mosaic Virus for the Generation of a Cross-Protective Attenuated Strain

Potyviral helper component protease (HC-Pro), as a major determinant of symptom expression in susceptible plants, is a likely target candidate in the production of attenuated strains for cross-protection. In this study, single or double mutations of Lys (K) to Glu (E) in the Lys-Ile-Thr-Cys motif and Arg (R) to Ile (I) in the Phe-Arg-Asn-Lys motif of the HC-Pro from the severe papaya leaf distortion mosaic virus strain DF (PLDMV-DF) reduced symptom expression and virus accumulation in infected papaya (Carica papaya) plants. The papaya plants infected with the attenuated double mutant of PLDMV-EI presented as symptomless. PLDMV-EI provided effective protection against PLDMV-DF infection in three papaya cultivars and had no effect on plant growth and development. Our result showed that PLDMV-EI is a promising mild strain for the practical use of cross-protection in the field.

Spatiotemporal Spread of Cacao Swollen Shoot Virus Severe Strain 1A in Mixed Hybrid Cacao Pre-inoculated With Mild Strain N1

The spatiotemporal spread of cocoa swollen shoot virus disease (CSSVD), which is caused by cacao swollen shoot virus (CSSV) severe strain 1A in mixed hybrid cacao pre-inoculated with CSSV mild strain N1 (CSSV-N1), was investigated during a field experiment from 2006 to 2017, at the Cocoa Research Institute of Ghana. The development of disease epidemics has been described by the use of statistical modeling. Protecting all cacao plants with CSSV-N1 reduced the rate of CSSV-1A symptom appearance by 43% (P = 0.05) compared with the nonprotected control and by 33% compared with plots where cacao plants in the outer three or five rows were protected with CSSV-N1. Similarly, creating the protective outer rings three or five rows deep reduced the rate of CSSV-1A symptoms by 14% (P = 0.05) compared with the nonprotected control. CSSV-1A epidemics increased approximately 18% faster (P = 0.05) in transects oriented from the north and east compared with those oriented from the south and west. During the last 2 years of the study, CSSVD spread decreased significantly (P = 0.05) faster in plots where all test cacao plants were inoculated with CSSV-N1 compared with other treatments. The growth of cacao did not differ significantly among the treatments over the 9-year assessment period. Similarly, differences in the cumulative yield among the treatments over the 8-year assessment period were not significant.

The Matrix Protein of a Plant Rhabdovirus Mediates Superinfection Exclusion by Inhibiting Viral Transcription

Superinfection exclusion (SIE) or cross-protection phenomena have been documented for plant viruses for nearly a century and are widespread among taxonomically diverse viruses, but little information is available about SIE of plant negative-strand RNA viruses. Here, we demonstrate that SIE by sonchus yellow net nucleorhabdovirus virus (SYNV) is mediated by the viral matrix (M) protein, a multifunctional protein involved in transcription regulation, virion assembly, and virus budding. We show that fluorescent protein-tagged SYNV variants display mutual exclusion/cross-protection in Nicotiana benthamiana plants. Transient expression of the SYNV M protein, but not other viral proteins, interfered with SYNV local infections. In addition, SYNV M deletion mutants failed to exclude superinfection by wild-type SYNV. An SYNV minireplicon reporter gene expression assay showed that the M protein inhibited viral transcription. However, M protein mutants with weakened nuclear localization signals (NLS) and deficient nuclear interactions with the SYNV nucleocapsid protein were unable to suppress transcription. Moreover, SYNV with M NLS mutations exhibited compromised SIE against wild-type SYNV. From these data, we propose that M protein accumulating in nuclei with primary SYNV infections either coils or prevents uncoiling of nucleocapsids released by the superinfecting SYNV virions and suppresses transcription of superinfecting genomes, thereby preventing superinfection. Our model suggests that the rhabdovirus M protein regulates the transition from replication to virion assembly and renders the infected cells nonpermissive for secondary infections.IMPORTANCE Superinfection exclusion (SIE) is a widespread phenomenon in which an established virus infection prevents reinfection by closely related viruses. Understanding the mechanisms governing SIE will not only advance our basic knowledge of virus infection cycles but may also lead to improved design of antiviral measures. Despite the significance of SIE, our knowledge about viral SIE determinants and their modes of actions remain limited. In this study, we show that sonchus yellow net virus (SYNV) SIE is mediated by the viral matrix (M) protein. During primary infections, accumulation of M protein in infected nuclei results in coiling of genomic nucleocapsids and suppression of viral transcription. Consequently, nucleocapsids released by potential superinfectors are sequestered and are unable to initiate new infections. Our data suggest that SYNV SIE is caused by M protein-mediated transition from replication to virion assembly and that this process prevents secondary infections.

A New Era for Mild Strain Cross-Protection

Societal and environmental pressures demand high-quality and resilient cropping plants and plant-based foods grown with the use of low or no synthetic chemical inputs. Mild strain cross-protection (MSCP), the pre-immunization of a plant using a mild strain of a virus to protect against subsequent infection by a severe strain of the virus, fits with future-proofing of production systems. New examples of MSCP use have occurred recently. New technologies are converging to support the discovery and mechanism(s) of action of MSCP strains thereby accelerating the popularity of their use.

Rice black-streaked dwarf virus P10 acts as either a synergistic or antagonistic determinant during superinfection with related or unrelated virus

Rice black-streaked dwarf virus (RBSDV), a member of the genus Fijivirus, is a devastating pathogen of crop plants. RBSDV S10 encodes a capsid protein (P10) that is an important component of the double-layered particle. However, little information is available on the roles of RBSDV P10 in viral infection or in interactions with other viruses. Here, we demonstrate that the expression of P10 in plants alleviates the symptoms of both RBSDV and the closely related Southern rice black-streaked dwarf virus (SRBSDV), and reduces the disease incidence, but renders the plants more susceptible to the unrelated Rice stripe virus (RSV). Further experiments suggest that P10-mediated resistance to RBSDV and SRBSDV operates at the protein level, rather than the RNA level, and is not a result of post-transcriptional gene silencing. Transcriptomic data reveal that the expression of P10 in plants significantly suppresses the expression of rice defence-related genes, which may play important roles in resistance to RSV infection. After infection with RBSDV, plants are more resistant to subsequent challenge by SRBSDV, but more susceptible to RSV. Overall, these results indicate that P10 acts as an important effector in virus interactions.

iTRAQ-based analysis of leaf proteome identifies important proteins in secondary metabolite biosynthesis and defence pathways crucial to cross-protection against TMV

Cross-protection is a phenomenon in which infection with a mild virus strain protects host plants against subsequent infection with a closely related severe virus strain. This study showed that a mild strain mutant virus, Tobacco mosaic virus (TMV)-43A could cross protect Nicotiana benthamiana plants against wild-type TMV. Furthermore, we investigated the host responses at the proteome level to identify important host proteins involved in cross-protection. We used the isobaric tags for relative and absolute quantification (iTRAQ) technique to analyze the proteome profiles of TMV, TMV-43A and cross-protected plants at different time-points. Our results showed that TMV-43A can cross-protect N. benthamiana plants from TMV. In cross-protected plants, photosynthetic activities were augmented, as supported by the increased accumulation of 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) and geranylgeranyl diphosphate synthase (GGPS) enzymes, which are crucial for chlorophyll biosynthesis. The increased abundance of ROS scavenging enzymes like thioredoxins and L-ascorbate peroxidase would prevent oxidative damage in cross-protected plants. Interestingly, the abundance of defence-related proteins (14-3-3 and NbSGT1) decreased, along with a reduction in virus accumulation during cross-protection. In conclusion, we have identified several important host proteins that are crucial in cross-protection to counter TMV infection in N. benthamiana plants. BIOLOGICAL SIGNIFICANCE: TMV is the most studied model for host-virus interaction in plants. It can infect wide varieties of plant species, causing significant economic losses. Cross protection is one of the methods to combat virus infection. A few cross-protection mechanisms have been proposed, including replicase/coat protein-mediated resistance, RNA silencing, and exclusion/spatial separation between virus strains. However, knowledge on host responses at the proteome level during cross protection is limited. To address this knowledge gap, we have leveraged on a global proteomics analysis approach to study cross protection. We discovered that TMV-43A (protector) protects N. benthamiana plants from TMV (challenger) infection through multiple host pathways: secondary metabolite biosynthesis, photosynthesis, defence, carbon metabolism, protein translation and processing and amino acid biosynthesis. In the secondary metabolite biosynthesis pathway, enzymes 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) and geranylgeranyl diphosphate synthase (GGPS) play crucial roles in chlorophyll biosynthesis during cross protection. In addition, accumulation of ROS scavenging enzymes was also found in cross-protected plants, providing rescues from excessive oxidative damage. Reduced abundance of plant defence proteins is correlated to reduced virus accumulation in host plants. These findings have increased our knowledge in host responses during cross-protection.

Evaluation of Potato virus X mild mutants for cross protection against severe infection in China

BACKGROUND

Cross protection is a promising alternative to control plant viral diseases. One critical factor limiting the application of cross protection is the availability of attenuated mutants or mild strains. Potato virus X (PVX) infects many crops and induces huge economic losses to agricultural production. However, researches on the variability and mechanism of PVX virulence are scarce.

METHODS

The mutants were obtained by introducing mutations into the RNA dependent RNA polymerase (RdRp) gene of PVX via site-directed mutagenesis. Attenuated mutants were screen according to their symptoms in Nicotiana benthamiana plants. The protection efficacy against severe infection were evaluated with interval of 5, 10 and 15 days.

RESULTS

Among the 40 mutants obtained, four mutants carrying substitutions of either Glu46, Asn863, Asn968 or Glu1001 to Ala in PVX RdRp showed drastically attenuated symptom, accompanying with reduced accumulation levels of coat protein, plus- and minus-sense RNAs. When the interval between protective and challenging inoculations was 15 days, mutant E1001A (with substitution of Glu1001 to Ala in RdRp) provided complete protection against severe infection in both Nicotiana benthamiana and tomato, while E46A (Glu46 mutated to Ala) provided incomplete protection. To reduce the risk of reverse mutation, we constructed mutant dM which carries double mutations of both Glu46 and Glu1001 to Ala in RdRp. The mutant dM could provide effective protection against severe PVX infection.

CONCLUSION

Mutations of Glu46, Asn863, Asn968 or Glu1001 to Ala in PVX RdRp significantly reduced the viral symptoms. Mutants E1001A and E46A could provide effective protection against wild type PVX in both Nicotiana benthamiana and tomato. These results provide theoretical and practical bases for the control of PVX via cross protection.

RNA Interference Mechanisms and Applications in Plant Pathology

The origin of RNA interference (RNAi), the cell sentinel system widely shared among eukaryotes that recognizes RNAs and specifically degrades or prevents their translation in cells, is suggested to predate the last eukaryote common ancestor ( 138 ). Of particular relevance to plant pathology is that in plants, but also in some fungi, insects, and lower eukaryotes, RNAi is a primary and effective antiviral defense, and recent studies have revealed that small RNAs (sRNAs) involved in RNAi play important roles in other plant diseases, including those caused by cellular plant pathogens. Because of this, and because RNAi can be manipulated to interfere with the expression of endogenous genes in an intra- or interspecific manner, RNAi has been used as a tool in studies of gene function but also for plant protection. Here, we review the discovery of RNAi, canonical mechanisms, experimental and translational applications, and new RNA-based technologies of importance to plant pathology.

Potyviral Gene-Silencing Suppressor HCPro Interacts with Salicylic Acid (SA)-Binding Protein 3 to Weaken SA-Mediated Defense Responses

The viral infection process is a battle between host defense response and pathogen antagonizing action. Several studies have established a tight link between the viral RNA silencing suppressor (RSS) and the repression of salicylic acid (SA)-mediated defense responses, nonetheless host factors directly linking an RSS and the SA pathway remains unidentified. From yeast two-hybrid analysis, we identified an interaction between the potyviral RSS helper-component proteinase (HCPro) and SA-binding protein SABP3. Co-localization and bimolecular fluorescence complementation analyses validated the direct in vivo interaction between Turnip mosaic virus (TuMV) HCPro and the Arabidopsis homologue of SABP3, AtCA1. Additionally, transient expression of TuMV HCPro demonstrated its ability to act as a negative regulator of AtCA1. When the plants of the AtCA1 knockout mutant line were inoculated with TuMV, our results indicated that AtCA1 is essential to restrict viral spreading and accumulation, induce SA accumulation, and trigger the SA pathway. Unexpectedly, the AtCA1 overexpression line also displayed a similar phenotype, suggesting that the constitutive expression of AtCA1 antagonizes the SA pathway. Taken together, our results depict AtCA1 as an essential regulator of SA defense responses. Moreover, the interaction of potyviral HCPro with this regulator compromises the SA pathway to weaken host defense responses and facilitate viral infection.

Discovery of novel inhibitors of RNA silencing suppressor P19 based on virtual screening

The combined virtual and experimental screening method is a efficient strategy to discover inhibitors of RNA silencing suppressor.

A New Mechanistic Model for Viral Cross Protection and Superinfection Exclusion

Plants pre-infected with a mild variant of a virus frequently become protected against more severe variants of the same virus through the cross protection phenomenon first discovered in 1929. Despite its widespread use in managing important plant virus diseases, the mechanism of cross protection remains poorly understood. Recent investigations in our labs, by analyzing the whole-plant dynamics of a turnip crinkle virus (TCV) population, coupled with cell biological interrogation of individual TCV variants, revealed possible novel mechanisms for cross protection and the closely related process of superinfection exclusion (SIE). Our new mechanistic model postulates that, for RNA viruses like TCV, SIE manifests a viral function that denies progeny viruses the chance of re-replicating their genomes in the cells of their "parents," and it collaterally targets highly homologous superinfecting viruses that are indistinguishable from progeny viruses. We further propose that SIE may be evolutionarily selected to maintain an optimal error frequency in progeny genomes. Although primarily based on observations made with TCV, this new model could be broadly applicable to other viruses as it provides a molecular basis for maintaining virus genome fidelity in the face of the error-prone nature of virus replication process.

Prospects for engineering and improvement of cross-protective virus strains

Mild strain cross-protection is currently an important method for the production of high quality plant products; despite challenge from severe virus isolates the initial protecting strain precludes symptom development. The mechanism of cross-protection is not yet resolved as RNA silencing does not sufficiently explain the phenomenon. Six requirements have been put forward to ensure long-lasting protection. We propose two additional requirements for effective and durable mild strain cross-protection; mild strains based on knowledge of the mechanism and consideration of impacts to consumers. Future research on predicting phenotype from genotype and understanding virus-plant and virus-vector interactions will enable improvement of cross-protective strains. Shared international databases of whole ecosystem interactions across a wide range of virus patho- and symbiotic-systems will form the basis for making step-change advances towards our collective ability to engineer and improve mild strain cross-protection.

Cucumber mosaic virus and its 2b protein alter emission of host volatile organic compounds but not aphid vector settling in tobacco

BACKGROUND

Aphids, including the generalist herbivore Myzus persicae, transmit cucumber mosaic virus (CMV). CMV (strain Fny) infection affects M. persicae feeding behavior and performance on tobacco (Nicotiana tabacum), Arabidopsis thaliana and cucurbits in varying ways. In Arabidopsis and cucurbits, CMV decreases host quality and inhibits prolonged feeding by aphids, which may enhance virus transmission rates. CMV-infected cucurbits also emit deceptive, aphid-attracting volatiles, which may favor virus acquisition. In contrast, aphids on CMV-infected tobacco (cv. Xanthi) exhibit increased survival and reproduction. This may not increase transmission but might increase virus and vector persistence within plant communities. The CMV 2b counter-defense protein diminishes resistance to aphid infestation in CMV-infected tobacco plants. We hypothesised that in tobacco CMV and its 2b protein might also alter the emission of volatile organic compounds that would influence aphid behavior.

RESULTS

Analysis of headspace volatiles emitted from tobacco plants showed that CMV infection both increased the total quantity and altered the blend produced. Furthermore, experiments with a CMV 2b gene deletion mutant (CMV∆2b) showed that the 2b counter-defense protein influences volatile emission. Free choice bioassays were conducted where wingless M. persicae could choose to settle on infected or mock-inoculated plants under a normal day/night regime or in continual darkness. Settling was recorded at 15 min, 1 h and 24 h post-release. Statistical analysis indicated that aphids showed no marked preference to settle on mock-inoculated versus infected plants, except for a marginally greater settlement of aphids on mock-inoculated over CMV-infected plants under normal illumination.

CONCLUSIONS

CMV infection of tobacco plants induced quantitative and qualitative changes in host volatile emission and these changes depended in part on the activity of the 2b counter-defense protein. However, CMV-induced alterations in tobacco plant volatile emission did not have marked effects on the settling of aphids on infected versus mock-inoculated plants even though CMV-infected plants are higher quality hosts for M. persicae.

The interaction between Turnip crinkle virus p38 and Cucumber mosaic virus 2b and its critical domains

Cross protection is a common phenomenon among closely related strain viruses in co-infected plants. However, unrelated viruses, Turnip crinkle virus (TCV) and Cucumber mosaic virus (CMV), also show an antagonistic effect in co-infected Arabidopsis plants. In many cases, viral suppressors of RNA silencing (VSRs) have important roles in the interactions between viruses in mixed infections. CMV 2b and TCV p38 are multifunctional proteins and both of them are well characterized VSRs and have important roles in operation synergistic interactions with other viruses. Here, we demonstrated antagonistic effects of TCV toward CMV and showed that RNA silencing-mediated resistance protein, RCY1 and TCV-interacting protein (TIP) of Arabidopsis plants did not affect this antagonism effect. We further showed that TCV p38 and CMV 2b could interact with each other in vivo but not in vitro. Further mutational analysis showed that C-terminal of 2b and middle domains of p38 had more important roles in the interaction between the two viruses.

Random Plant Viral Variants Attain Temporal Advantages During Systemic Infections and in Turn Resist other Variants of the Same Virus

Infection of plants with viruses containing multiple variants frequently leads to dominance by a few random variants in the systemically infected leaves (SLs), for which a plausible explanation is lacking. We show here that SL dominance by a given viral variant is adequately explained by its fortuitous lead in systemic spread, coupled with its resistance to superinfection by other variants. We analyzed the fate of a multi-variant turnip crinkle virus (TCV) population in Arabidopsis and N. benthamiana plants. Both wild-type and RNA silencing-defective plants displayed a similar pattern of random dominance by a few variant genotypes, thus discounting a prominent role for RNA silencing. When introduced to plants sequentially as two subpopulations, a twelve-hour head-start was sufficient for the first set to dominate. Finally, SLs of TCV-infected plants became highly resistant to secondary invasions of another TCV variant. We propose that random distribution of variant foci on inoculated leaves allows different variants to lead systemic movement in different plants. The leading variants then colonize large areas of SLs, and resist the superinfection of lagging variants in the same areas. In conclusion, superinfection resistance is the primary driver of random enrichment of viral variants in systemically infected plants.

Generation of an Attenuated, Cross-Protective Pepino mosaic virus Variant Through Alignment-Guided Mutagenesis of the Viral Capsid Protein

Mild variants of many viruses are able to protect infected plants from subsequent invasion by more severe variants of the same viruses through a process known as cross-protection. In the past, the cross-protective viral variants were commonly derived from mild field isolates that were sometimes genetically heterogeneous, providing variable levels of cross-protection. Here, we report a novel approach to rapidly generate cross-protective variants of the tomato-infecting Pepino mosaic virus (PepMV) independently of the availability of mild field isolates. Our approach sought to attenuate PepMV by mutating less conserved amino acid residues of the abundantly produced capsid protein (CP). These less-conserved amino acid residues were identified through multiple alignments of CPs of six potexviruses including PepMV, and were altered systematically to yield six PepMV mutants. These mutants were subsequently inoculated onto the model plant Nicotiana benthamiana, as well as tomato, to evaluate their accumulation levels, symptom severities, and cross-protection potentials. The mutant KD, in which the threonine (T) and alanine (A) residues at CP positions 66 and 67 were replaced with lysine (K) and aspartic acid (D), respectively, were found to accumulate to low levels in infected plants, cause very mild symptoms, and effectively protect both N. benthamiana and tomato against secondary infections by wild-type PepMV. These data suggest that our approach represents a simple, fast, and reliable way of generating attenuated viral variants capable of cross-protection.

Crop immunity against viruses: outcomes and future challenges

Viruses cause epidemics on all major cultures of agronomic importance, representing a serious threat to global food security. As strict intracellular pathogens, they cannot be controlled chemically and prophylactic measures consist mainly in the destruction of infected plants and excessive pesticide applications to limit the population of vector organisms. A powerful alternative frequently employed in agriculture relies on the use of crop genetic resistances, approach that depends on mechanisms governing plant-virus interactions. Hence, knowledge related to the molecular bases of viral infections and crop resistances is key to face viral attacks in fields. Over the past 80 years, great advances have been made on our understanding of plant immunity against viruses. Although most of the known natural resistance genes have long been dominant R genes (encoding NBS-LRR proteins), a vast number of crop recessive resistance genes were cloned in the last decade, emphasizing another evolutive strategy to block viruses. In addition, the discovery of RNA interference pathways highlighted a very efficient antiviral system targeting the infectious agent at the nucleic acid level. Insidiously, plant viruses evolve and often acquire the ability to overcome the resistances employed by breeders. The development of efficient and durable resistances able to withstand the extreme genetic plasticity of viruses therefore represents a major challenge for the coming years. This review aims at describing some of the most devastating diseases caused by viruses on crops and summarizes current knowledge about plant-virus interactions, focusing on resistance mechanisms that prevent or limit viral infection in plants. In addition, I will discuss the current outcomes of the actions employed to control viral diseases in fields and the future investigations that need to be undertaken to develop sustainable broad-spectrum crop resistances against viruses.

Genetic analyses of the FRNK motif function of Turnip mosaic virus uncover multiple and potentially interactive pathways of cross-protection

Cross-protection triggered by a mild strain of virus acts as a prophylaxis to prevent subsequent infections by related viruses in plants; however, the underling mechanisms are not fully understood. Through mutagenesis, we isolated a mutant strain of Turnip mosaic virus (TuMV), named Tu-GK, that contains an Arg182Lys substitution in helper component-proteinase (HC-Pro(K)) that confers complete cross-protection against infection by a severe strain of TuMV in Nicotiana benthamiana, Arabidopsis thaliana Col-0, and the Arabidopsis dcl2-4/dcl4-1 double mutant defective in DICER-like ribonuclease (DCL)2/DCL4-mediated silencing. Our analyses showed that HC-Pro(K) loses the ability to interfere with microRNA pathways, although it retains a partial capability for RNA silencing suppression triggered by DCL. We further showed that Tu-GK infection triggers strong salicylic acid (SA)-dependent and SA-independent innate immunity responses. Our data suggest that DCL2/4-dependent and -independent RNA silencing pathways are involved, and may crosstalk with basal innate immunity pathways, in host defense and in cross-protection.

Pseudorecombination between Two Distinct Strains of Cucumber mosaic virus Results in Enhancement of Symptom Severity

Recently, a Cucumber mosaic virus (CMV) strain, named as CMV-209, was isolated from Glycine soja. In this study, symptom expression of CMV-209 was analyzed in detail in Nicotiana benthamiana by comparing with that of CMV-Fny, which is a representative strain of CMV. Using infectious cDNA clones of CMV strains 209 and Fny, symptom expression of various pseudorecombinants between these two strains were examined in the early and late infection stages. In the early infection stage, the pseudorecombinants containing Fny-RNA2 induced stunting and leaf distortion on the newly emerged leaves whereas the pseudorecombinants containing 209-RNA2 caused no obvious symptoms. In the late infection stage, the pseudorecombinants containing 209-RNA1 and Fny-RNA2 induced severe leaf distortion and stunting, while CMV-209 induced mild symptom and CMV-Fny caused typical mosaic, general stunting, and leaf distortion symptoms, indicating that RNA 2 encodes a symptom determinant(s) of CMV, which is capable of enhancing symptoms. Furthermore, our results support the possibility that natural recombination between compatible viruses can result in emergence of novel viruses causing severe damages in crop fields.

Nuclear-cytoplasmic partitioning of cucumber mosaic virus protein 2b determines the balance between its roles as a virulence determinant and an RNA-silencing suppressor

The Cucumber Mosaic Virus (CMV) 2b protein is an RNA-silencing suppressor that plays roles in CMV accumulation and virulence. The 2b proteins of subgroup IA CMV strains partition between the nucleus and cytoplasm, but the biological significance of this is uncertain. We fused an additional nuclear localization signal (NLS) to the 2b protein of subgroup IA strain Fny-CMV to create 2b-NLS and tested its effects on subcellular distribution, silencing, and virulence. The additional NLS enhanced 2b protein nuclear and nucleolar accumulation, but nuclear and nucleolar enrichment correlated with markedly diminished silencing suppressor activity in patch assays and abolished 2b protein-mediated disruption of microRNA activity in transgenic Arabidopsis. Nucleus/nucleolus-localized 2b protein possesses at least some ability to inhibit antiviral silencing, but this was not sufficient to prevent recovery from disease in younger, developing leaves in Arabidopsis. However, enhanced nuclear and nucleolar accumulation of 2b increased virulence and accelerated symptom appearance in older leaves. Experiments with Arabidopsis lines carrying mutant Dicer-like alleles demonstrated that compromised suppressor activity explained the diminished ability of 2b-NLS to enhance virus accumulation. Remarkably, the increased virulence that 2b-NLS engendered was unrelated to effects on microRNA- or short interfering RNA-regulated host functions. Thus, although nucleus- and nucleolus-localized 2b protein is less efficient at silencing suppression than cytoplasm-localized 2b, it enhances CMV virulence. We propose that partitioning of the 2b protein between the cytoplasmic and nuclear/nucleolar compartments allows CMV to regulate the balance between virus accumulation and damage to the host, presumably to maximize the benefit for the virus.

IMPORTANCE In this work, the main finding is that nucleus/nucleolus-localized 2b protein is strongly associated with CMV virulence, which is independent of its effect on small RNA pathways. Moreover, this work supports the contention that the silencing suppressor activity of CMV 2b protein is predominantly exerted by that portion of the 2b protein residing in the cytoplasm. Thus, we propose that partitioning of the 2b protein between the cytoplasmic and nuclear/nucleolar compartments allows CMV to regulate the balance between virus accumulation and damage to the host, presumably to maximize the benefit for the virus.

Profiling of genes related to cross protection and competition for NbTOM1 by HLSV and TMV

Cross protection is the phenomenon through which a mild strain virus suppresses symptoms induced by a closely related severe strain virus in infected plants. Hibiscus latent Singapore virus (HLSV) and Tobacco mosaic virus (TMV) are species within the genus tobamovirus. HLSV can protect Nicotianabenthamiana against TMV-U1 strain, resulting in mild symptoms instead of severe systemic necrosis. The mechanism of cross protection between HLSV and TMV is unknown. In the past, some researchers suggest that the protecting virus strain might occupy virus-specific replication sites within a cell leaving no room for the challenge virus. Quantitative real-time RT-PCR was performed to detect viral RNA levels during cross protection. HLSV accumulation increased in cross protected plants compared with that of single HLSV infected plants, while TMV decreased in cross protected plants. This suggests that there is a competition for host factors between HLSV and TMV for replication. To investigate the mechanism under the cross protection between HLSV and TMV, microarray analysis was conducted to examine the transcriptional levels of global host genes during cross protection, using Tobacco Gene Expression Microarray, 4 x 44 k slides. The transcriptional level of some host genes corresponded to accumulation level of TMV. Some host genes were up-regulated only by HLSV. Tobamovirus multiplication gene 1 (TOM1), essential for tobamovirus multiplication, was involved in competition for replication by HLSV and TMV during cross protection. Both HLSV and TMV accumulation decreased when NbTOM1 was silenced. A large quantity of HLSV resulted in decreased TMV accumulation in HLSV+TMV (100:1) co-infection. These results indicate that host genes involved in the plant defense response and virus multiplication are up-regulated by challenge virus TMV but not by protecting virus HLSV during cross protection.

Viral and cellular factors involved in Phloem transport of plant viruses

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

The VP3 factor from viruses of Birnaviridae family suppresses RNA silencing by binding both long and small RNA duplexes

RNA silencing is directly involved in antiviral defense in a wide variety of eukaryotic organisms, including plants, fungi, invertebrates, and presumably vertebrate animals. The study of RNA silencing-mediated antiviral defences in vertebrates is hampered by the overlap with other antiviral mechanisms; thus, heterologous systems are often used to study the interplay between RNA silencing and vertebrate-infecting viruses. In this report we show that the VP3 protein of the avian birnavirus Infectious bursal disease virus (IBDV) displays, in addition to its capacity to bind long double-stranded RNA, the ability to interact with double-stranded small RNA molecules. We also demonstrate that IBDV VP3 prevents the silencing mediated degradation of a reporter mRNA, and that this silencing suppression activity depends on its RNA binding ability. Furthermore, we find that the anti-silencing activity of IBDV VP3 is shared with the homologous proteins expressed by both insect- and fish-infecting birnaviruses. Finally, we show that IBDV VP3 can functionally replace the well-characterized HCPro silencing suppressor of Plum pox virus, a potyvirus that is unable to infect plants in the absence of an active silencing suppressor. Altogether, our results support the idea that VP3 protects the viral genome from host sentinels, including those of the RNA silencing machinery.

Helicase domain encoded by Cucumber mosaic virus RNA1 determines systemic infection of Cmr1 in pepper

The Cmr1 gene in peppers confers resistance to Cucumber mosaic virus isolate-P0 (CMV-P0). Cmr1 restricts the systemic spread of CMV strain-Fny (CMV-Fny), whereas this gene cannot block the spread of CMV isolate-P1 (CMV-P1) to the upper leaves, resulting in systemic infection. To identify the virulence determinant of CMV-P1, six reassortant viruses and six chimeric viruses derived from CMV-Fny and CMV-P1 cDNA clones were used. Our results demonstrate that the C-terminus of the helicase domain encoded by CMV-P1 RNA1 determines susceptibility to systemic infection, and that the helicase domain contains six different amino acid substitutions between CMV-Fny and CMV-P1(.) To identify the key amino acids of the helicase domain determining systemic infection with CMV-P1, we then constructed amino acid substitution mutants. Of the mutants tested, amino acid residues at positions 865, 896, 957, and 980 in the 1a protein sequence of CMV-P1 affected the systemic infection. Virus localization studies with GFP-tagged CMV clones and in situ localization of virus RNA revealed that these four amino acid residues together form the movement determinant for CMV-P1 movement from the epidermal cell layer to mesophyll cell layers. Quantitative real-time PCR revealed that CMV-P1 and a chimeric virus with four amino acid residues of CMV-P1 accumulated more genomic RNA in inoculated leaves than did CMV-Fny, indicating that those four amino acids are also involved in virus replication. These results demonstrate that the C-terminal region of the helicase domain is responsible for systemic infection by controlling virus replication and cell-to-cell movement. Whereas four amino acids are responsible for acquiring virulence in CMV-Fny, six amino acid (positions at 865, 896, 901, 957, 980 and 993) substitutions in CMV-P1 were required for complete loss of virulence in 'Bukang'.

The Cucumber vein yellowing virus silencing suppressor P1b can functionally replace HCPro in Plum pox virus infection in a host-specific manner

Plant viruses of the genera Potyvirus and Ipomovirus (Potyviridae family) use unrelated RNA silencing suppressors (RSS) to counteract antiviral RNA silencing responses. HCPro is the RSS of Potyvirus spp., and its activity is enhanced by the upstream P1 protein. Distinctively, the ipomovirus Cucumber vein yellowing virus (CVYV) lacks HCPro but contains two P1 copies in tandem (P1aP1b), the second of which functions as RSS. Using chimeras based on the potyvirus Plum pox virus (PPV), we found that P1b can functionally replace HCPro in potyviral infections of Nicotiana plants. Interestingly, P1a, the CVYV protein homologous to potyviral P1, disrupted the silencing suppression activity of P1b and reduced the infection efficiency of PPV in Nicotiana benthamiana. Testing the influence of RSS in host specificity, we found that a P1b-expressing chimera poorly infected PPV's natural host, Prunus persica. Conversely, P1b conferred on PPV chimeras the ability to replicate locally in cucumber, CVYV's natural host. The deleterious effect of P1a on PPV infection is host dependent, because the P1aP1b-expressing PPV chimera accumulated in cucumber to higher levels than PPV expressing P1b alone. These results demonstrate that a potyvirus can use different RSS, and that particular RSS and upstream P1-like proteins contribute to defining the virus host range.

Infection dynamics in viral spread and interference under the synergism between Cucumber mosaic virus and Turnip mosaic virus

Mixed infection of Cucumber mosaic virus (CMV) and Turnip mosaic virus (TuMV) induced more severe symptoms on Nicotiana benthamiana than single infection. To dissect the relationships between spatial infection patterns and the 2b protein (2b) of CMV in single or mixed infections, the CMV vectors expressing enhanced green fluorescent or Discosoma sp. red fluorescent proteins (EGFP [EG] or DsRed2 [Ds], respectively were constructed from the same wild-type CMV-Y and used for inoculation onto N. benthamiana. CMV2-A1 vector (C2-A1 [A1]) has a functional 2b while CMV-H1 vector (C2-H1 [H1]) is 2b deficient. As we expected from the 2b function as an RNA silencing suppressor (RSS), in a single infection, A1Ds retained a high level of accumulation at initial infection sites and showed extensive fluorescence in upper, noninoculated leaves, whereas H1Ds disappeared rapidly at initial infection sites and could not spread efficiently in upper, noninoculated leaf tissues. In various mixed infections, we found two phenomena providing novel insights into the relationships among RSS, viral synergism, and interference. First, H1Ds could not spread efficiently from vasculature into nonvascular tissues with or without TuMV, suggesting that RNA silencing was not involved in CMV unloading from vasculature. These results indicated that 2b could promote CMV to unload from vasculature into nonvascular tissues, and that this 2b function might be independent of its RSS activity. Second, we detected spatial interference (local interference) between A1Ds and A1EG in mixed infection with TuMV, between A1Ds (or H1Ds) and TuMV, and between H1Ds and H1EG. This observation suggested that local interference between two viruses was established even in the synergism between CMV and TuMV and, again, RNA silencing did not seem to contribute greatly to this phenomenon.

Cucumber mosaic virus

Cucumber mosaic virus (CMV) is an important virus because of its agricultural impact in the Mediterranean Basin and worldwide, and also as a model for understanding plant-virus interactions. This review focuses on those areas where most progress has been made over the past decade in our understanding of CMV. Clearly, a deep understanding of the role of the recently described CMV 2b gene in suppression of host RNA silencing and viral virulence is the most important discovery. These findings have had an impact well beyond the virus itself, as the 2b gene is an important tool in the studies of eukaryotic gene regulation. Protein 2b was shown to be involved in most of the steps of the virus cycle and to interfere with several basal host defenses. Progress has also been made concerning the mechanisms of virus replication and movement. However, only a few host proteins that interact with viral proteins have been identified, making this an area of research where major efforts are still needed. Another area where major advances have been made is CMV population genetics, where contrasting results were obtained. On the one hand, CMV was shown to be prone to recombination and to show high genetic diversity based on sequence data of different isolates. On the other hand, populations did not exhibit high genetic variability either within plants, or even in a field and the nearby wild plants. The situation was partially clarified with the finding that severe bottlenecks occur during both virus movement within a plant and transmission between plants. Finally, novel studies were undertaken to elucidate mechanisms leading to selection in virus population, according to the host or its environment, opening a new research area in plant-virus coevolution.

Cucumber mosaic virus and its 2b RNA silencing suppressor modify plant-aphid interactions in tobacco

The cucumber mosaic virus (CMV) 2b protein not only inhibits anti-viral RNA silencing but also quenches transcriptional responses of plant genes to jasmonic acid, a key signalling molecule in defence against insects. This suggested that it might affect interactions between infected plants and aphids, insects that transmit CMV. We found that infection of tobacco with a 2b gene deletion mutant (CMVΔ2b) induced strong resistance to aphids (Myzus persicae) while CMV infection fostered aphid survival. Using electrical penetration graph methodology we found that higher proportions of aphids showed sustained phloem ingestion on CMV-infected plants than on CMVΔ2b-infected or mock-inoculated plants although this did not increase the rate of growth of individual aphids. This indicates that while CMV infection or certain viral gene products might elicit aphid resistance, the 2b protein normally counteracts this during a wild-type CMV infection. Our findings suggest that the 2b protein could indirectly affect aphid-mediated virus transmission.

The 2b silencing suppressor of a mild strain of Cucumber mosaic virus alone is sufficient for synergistic interaction with Tobacco mosaic virus and induction of severe leaf malformation in 2b-transgenic tobacco plants

Tobacco plants infected simultaneously by Tobacco mosaic virus (TMV) and Cucumber mosaic virus (CMV) are known to produce a specific synergistic disease in which the emerging leaves are filiformic. Similar developmental malformations are also caused to a lesser extent by the severe strains (e.g., Fny) of CMV alone, but mild strains (e.g., Kin) cause them only in mixed infection with TMV. We show here that transgenic tobacco plants expressing 2b protein of CMV-Kin produce filiformic symptoms when infected with TMV, indicating that only 2b protein is needed from CMV-Kin for this synergistic relationship. On the other hand, transgenic plants that express either the wild-type TMV genome or a modified TMV genome with its coat protein deleted or movement protein (MP) inactivated also develop filiformic or at least distinctly narrow leaves, while plants expressing the MP alone do not develop any malformations when infected with CMV-Kin. These results show that either TMV helicase/replicase protein or active TMV replication are required for this synergistic effect. The effect appears to be related to an efficient depletion of silencing machinery, caused jointly by both viral silencing suppressors, i.e., CMV 2b protein and the TMV 126-kDa replicase subunit.

Cucumber mosaic virus 2b-Deficient Mutant Causes Limited, Asymptomatic Infection of Bell Pepper

Cucumber mosaic virus Fast New York strain (CMV-Fny) containing a mutated 2b protein (CMV-FnyΔ2b) was evaluated for the ability to infect 'Calwonder' bell pepper (Capsicum annuum) plants in comparative tests with the parent virus, CMV-Fny. Plants inoculated with CMV-FnyΔ2b did not develop local or systemic symptoms of infection, whereas CMV-Fny-infected plants developed systemic chlorosis by 7 days post inoculation (dpi), followed by mosaic and leaf deformation. Virus accumulation, determined by enzyme-linked immunosorbent assay (ELISA), revealed that CMV-FnyΔ2b accumulated in inoculated Calwonder leaves and inconsistently infected some noninoculated leaves at a low titer but was not detected in the youngest, noninoculated leaves. Immuno-tissue blot tests did not detect CMV-FnyΔ2b in the stems of infected plants, whereas CMV-Fny accumulated throughout the length of the stems of inoculated plants. In two experiments, protoplasts were isolated from Calwonder leaves, inoculated with viral RNAs of CMV-Fny or CMV-FnyΔ2b, and tested by ELISA for infection. In both experiments, less CMV-FnyΔ2b than CMV-Fny accumulated in protoplasts. These results suggest that the CMV 2b protein is needed for systemic infection of Calwonder pepper plants and for accumulation of the virus in inoculated protoplasts.

Risk-managed production of bioactive recombinant proteins using a novel plant virus vector with a helper plant to complement viral systemic movement

A plant viral vector has the potential to efficiently produce recombinant proteins at a low cost in a short period. Although recombinant proteins can be also produced by transgenic plants, a plant viral vector, if available, may be more convenient when urgent scale-up in production is needed. However, it is difficult to use a viral vector in open fields because of the risk of escape to the environment. In this study, we constructed a novel viral vector system using a movement-defective Cucumber mosaic virus (CMV) vector, which is theoretically localized in the inoculated cells but infects systemically only with the aid of the transgenic helper plant that complements viral movement, diminishing the risk of viral proliferation. Interestingly, the helper plant systemically infected with the vector gave strong cross-protection against challenge inoculation with wild-type CMVs. Using CMV strains belonging to two discrete CMV groups (subgroups I and II), we also improved the system to prevent recombination between the vector and the transgene transcript in the helper plant. We here demonstrate the expression of an anti-dioxin single chain variable fragment (DxscFv) and interleukin-1 receptor antagonist (IL1-Ra) in Nicotiana benthamiana by this viral vector confinement system, which is applicable for many useful high-quality recombinant proteins.

Disruption of two defensive signaling pathways by a viral RNA silencing suppressor

The Cucumber mosaic virus (CMV) 2b counter-defense protein disrupts plant antiviral mechanisms mediated by RNA silencing and salicylic acid (SA). We used microarrays to investigate defensive gene expression in 2b-transgenic Arabidopsis thaliana plants. Surprisingly, 2b inhibited expression of few SA-regulated genes and, in some instances, enhanced the effect of SA on certain genes. Strikingly, the 2b protein inhibited changes in the expression of 90% of genes regulated by jasmonic acid (JA). Consistent with this, infection of plants with CMV, but not the 2b gene-deletion mutant CMVDelta2b, strongly inhibited JA-inducible gene expression. JA levels were unaffected by infection with either CMV or CMVDelta2b. Although the CMV-Arabidopsis interaction is a compatible one, SA accumulation, usually considered to be an indicator of plant resistance, was increased in CMV-infected plants but not in CMVDelta2b-infected plants. Thus, the 2b protein inhibits JA signaling at a step downstream of JA biosynthesis but it primes induction of SA biosynthesis by another CMV gene product or by the process of infection itself. Like many plant viruses, CMV is aphid transmitted. JA is important in plant defense against insects. This raises the possibility that disruption of JA-mediated gene expression by the 2b protein may influence CMV transmission by aphids.

Cross-protection: a century of mystery

Cross-protection is a phenomenon in which infection of a plant with a mild virus or viroid strain protects it from disease resulting from a subsequent encounter with a severe strain of the same virus or viroid. In this chapter, we review the history of cross-protection with regard to the development of ideas concerning its likely mechanisms, including RNA silencing and exclusion, and its influence on the early development of genetically engineered virus resistance. We also examine examples of the practical use of cross-protection in averting crop losses due to viruses, as well as the use of satellite RNAs to ameliorate the impact of virus-induced diseases. We also discuss the potential of cross-protection to contribute in future to the maintenance of crop health in the face of emerging virus diseases and related threats to agricultural production.

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.

The role of the Cucumber mosaic virus 2b protein in viral movement and symptom induction

The Cucumber mosaic virus (CMV) 2b protein is a counter-defense factor and symptom determinant. Conserved domains in the 2b protein sequence were mutated in the 2b gene of strain Fny-CMV. The effects of these mutations were assessed by infection of Nicotiana tabacum, N. benthamiana, and Arabidopsis thaliana (ecotype Col-0) with mutant viruses and by expression of mutant 2b transgenes in A. thaliana. We confirmed that two nuclear localization signals were required for symptom induction and found that the N-terminal domain was essential for symptom induction. The C-terminal domain and two serine residues within a putative phosphorylation domain modulated symptom severity. Further infection studies were conducted using Fny-CMVdelta2b, a mutant that cannot express the 2b protein and that induces no symptoms in N. tabacum, N. benthamiana, or A. thaliana ecotype Col-0. Surprisingly, in plants of A. thaliana ecotype C24, Fny-CMVdelta2b induced severe symptoms similar to those induced by the wild-type virus. However, C24 plants infected with the mutant virus recovered from disease while those infected with the wild-type virus did not. Expression of 2b transgenes from either Fny-CMV or from LS-CMV (a mild strain) in Col-0 plants enhanced systemic movement of Fny-CMVdelta2b and permitted symptom induction by Fny-CMVdelta2b. Taken together, the results indicate that the 2b protein itself is an important symptom determinant in certain hosts. However, they also suggest that the protein may somehow synergize symptom induction by other CMV-encoded factors.

Cross-Protection as Control Strategy Against Grapevine fanleaf virus in Naturally Infected Vineyards

The efficacy of cross-protection at mitigating the impact of Grapevine fanleaf virus (GFLV) on grapevines (Vitis vinifera) was assessed in two naturally infected vineyard sites. Test vines consisted of scions grafted onto rootstocks that were healthy or infected by mild protective strains GFLV-GHu or Arabis mosaic virus (ArMV)-Ta. Challenge GFLV infection via the nematode Xiphinema index was monitored over nine consecutive years in control and ArMV-Ta cross-protected vines by double-antibody sandwich-enzyme-linked immunosorbent assay using GFLV-specific antibodies, and in GFLV-GHu cross-protected vines by characterizing the coat protein gene of superinfecting isolates by immunocapture-reverse transcription-polymerase chain reaction-restriction fragment length polymorphism. Results were consistent with a significantly reduced challenge infection rate in cross-protected vines compared with control vines, more so in those protected with GFLV-GHu (19 versus 90%) than with ArMV-Ta (40 versus 65% in field A and 63 versus 90% in field B). However, the two mild strains significantly reduced fruit yield by 9% (ArMV-Ta) and 17% (GFLV-GHu) over 8 years and had a limited effect on fruit quality. Therefore, in spite of a great potential at reducing the incidence of challenge field isolates, cross-protection with natural mild protective strains GFLV-GHu and ArMV-Ta is not attractive to control GFLV because the negative impact on yield is a limiting factor for its deployment.

Direct and indirect roles of viral suppressors of RNA silencing in pathogenesis

Plant and animal viruses overcome host antiviral silencing by encoding diverse viral suppressors of RNA silencing (VSRs). Prior to the identification and characterization of their silencing suppression activities mostly in transgene silencing assays, plant VSRs were known to enhance virus accumulation in the inoculated protoplasts, promote cell-to-cell virus movement in the inoculated leaves, facilitate the phloem-dependent long-distance virus spread, and/or intensify disease symptoms in systemically infected tissues. Here we discuss how the various silencing suppression activities of VSRs may facilitate these distinct steps during plant infection and why VSRs may not play a direct role in eliciting disease symptoms by general impairments of host endogenous small RNA pathways. We also highlight many of the key questions still to be addressed on the role of viral suppression of antiviral silencing in plant infection.