Characterization of Synergy Between Cucumber mosaic virus and Potyviruses in Cucurbit Hosts

Association mapping of tan spot and septoria nodorum blotch resistance in cultivated emmer wheat

KEY MESSAGE

A total of 65 SNPs associated with resistance to tan spot and septoria nodorum blotch were identified in a panel of 180 cultivated emmer accessions through association mapping Tan spot and septoria nodorum blotch (SNB) are foliar diseases caused by the respective fungal pathogens Pyrenophora tritici-repentis and Parastagonospora nodorum that affect global wheat production. To find new sources of resistance, we evaluated a panel of 180 cultivated emmer wheat (Triticum turgidum ssp. dicoccum) accessions for reactions to four P. tritici-repentis isolates Pti2, 86-124, 331-9 and DW5, two P. nodorum isolate, Sn4 and Sn2000, and four necrotrophic effectors (NEs) produced by the pathogens. About 8-36% of the accessions exhibited resistance to the four P. tritici-repentis isolates, with five accessions demonstrating resistance to all isolates. For SNB, 64% accessions showed resistance to Sn4, 43% to Sn2000 and 36% to both isolates, with Spain (11% accessions) as the most common origin of resistance. To understand the genetic basis of resistance, association mapping was performed using SNP (single nucleotide polymorphism) markers generated by genotype-by-sequencing and the 9 K SNP Infinium array. A total of 46 SNPs were significantly associated with tan spot and 19 SNPs with SNB resistance or susceptibility. Six trait loci on chromosome arms 1BL, 3BL, 4AL (2), 6BL and 7AL conferred resistance to two or more isolates. Known NE sensitivity genes for disease development were undetected except Snn5 for Sn2000, suggesting novel genetic factors are controlling host-pathogen interaction in cultivated emmer. The emmer accessions with the highest levels of resistance to the six pathogen isolates (e.g., CItr 14133-1, PI 94634-1 and PI 377672) could serve as donors for tan spot and SNB resistance in wheat breeding programs.

The N-Terminal Region of Cucumber Mosaic Virus 2a Protein Is Involved in the Systemic Infection in Brassica juncea

Brassica juncea belongs to the Brassicaceae family and is used as both an oilseed and vegetable crop. As only a few studies have reported on the cucumber mosaic virus (CMV) in B. juncea, we conducted this study to provide a basic understanding of the B. juncea and CMV interactions. B. juncea-infecting CMV (CMV-Co6) and non-infecting CMV (CMV-Rs1) were used. To identify the determinants of systemic infection in B. juncea, we first constructed infectious clones of CMV-Co6 and CMV-Rs1 and used them as pseudo-recombinants. RNA2 of CMV was identified as an important determinant in B. juncea because B. juncea were systemically infected with RNA2-containing pseudo-recombinants; CMV-Co6, R/6/R, and R/6/6 were systemically infected B. juncea. Subsequently, the amino acids of the 2a and 2b proteins were compared, and a chimeric clone was constructed. The chimeric virus R/6Rns/R6cp, containing the C-terminal region of the 2a protein of CMV-Rs1, still infects B. juncea. It is the 2a protein that determines the systemic CMV infection in B. juncea, suggesting that conserved 160G and 214A may play a role in systemic CMV infection in B. juncea.

Inter-virus relationships in mixed infections and virus-drought relationships in plants: a quantitative review

Inter-virus relationships in mixed infections and virus-drought relationships are important in agriculture and natural vegetation. In this quantitative review, we sampled published factorial experiments to probe for relationships against the null hypothesis of additivity. Our sample captured antagonistic, additive and synergistic inter-virus relationships in double infections. Virus-drought relationships in our sample were additive or antagonistic, reinforcing the notion that viruses have neutral or positive effects on droughted plants, or that drought enhances plant tolerance to viruses. Both inter-virus and virus-drought relationships vary with virus species, host plant to the level of cultivar or accession, timing of infection, plant age and trait and growing conditions. The trait-dependence of these relationships has implications for resource allocation in plants. Owing to lagging theories, more experimental research in these fields is bound to return phenomenological outcomes. Theoretical work can advance in two complementary directions. First, the effective theory models the behaviour of the system without specifying all the underlying causes that lead to system state change. Second, mechanistic theory based on a nuanced view of the plant phenotype that explicitly considers downward causation; the influence of the plant phenotype on inter-virus relations and vice versa; the impact of timing, intensity and duration of drought interacting with viruses to modulate the plant phenotype; both the soil (moisture) and atmospheric (vapour pressure deficit) aspects of drought. Theories should scale in time, from short term to full growing season, and in levels of organisation up to the relevant traits: crop yield in agriculture and fitness in nature.

cmv1-Mediated Resistance to CMV in Melon Can Be Overcome by Mixed Infections with Potyviruses

Resistance to cucumber mosaic virus (CMV) strain LS in melon is controlled by the gene cmv1, which restricts phloem entry. In nature, CMV is commonly found in mixed infections, particularly with potyviruses, where a synergistic effect is frequently produced. We have explored the possibility that this synergism could help CMV-LS to overcome cmv1-mediated resistance. We demonstrate that during mixed infection with a potyvirus, CMV-LS is able to overcome cmv1-controlled resistance and develop a systemic infection and that this ability does not depend on an increased accumulation of CMV-LS in mechanically inoculated cotyledons. Likewise, during a mixed infection initiated by aphids, the natural vector of both cucumoviruses and potyviruses that can very efficiently inoculate plants with a low number of virions, CMV-LS also overcomes cmv1-controlled resistance. This indicates that in the presence of a potyvirus, even a very low amount of inoculum, can be sufficient to surpass the resistance and initiate the infection. These results indicate that there is an important risk for this resistance to be broken in nature as a consequence of mixed infections, and therefore, its deployment in elite cultivars would not be enough to ensure a long-lasting resistance.

Molecular Population Genetics of Aspen Mosaic-Associated Virus in Finland and Sweden

Aspen mosaic-associated virus (AsMaV) is a newly identified Emaravirus, in the family Fimoviridae, Bunyavirales, associated with mosaic symptoms in aspen trees (Populus tremula). Aspen trees are widely distributed in Europe and understanding the population structure of AsMaV may aid in the development of better management strategies. The virus genome consists of five negative-sense single-stranded RNA (-ssRNA) molecules. To investigate the genetic diversity and population parameters of AsMaV, different regions of the genome were amplified and analyzed and full-length sequence of the divergent isolates were cloned and sequenced. The results show that RNA3 or nucleoprotein is a good representative for studying genetic diversity in AsMaV. Developed RT-PCR-RFLP was able to identify areas with a higher number of haplotypes and could be applied for screening the large number of samples. In general, AsMaV has a conserved genome and based on the phylogenetic studies, geographical structuring was observed in AsMaV isolates from Sweden and Finland, which could be attributed to founder effects. The genome of AsMaV is under purifying selection but not distributed uniformly on genomic RNAs. Distant AsMaV isolates displayed amino acid sequence variations compared to other isolates, and bioinformatic analysis predicted potential post-translational modification sites in some viral proteins.

Surveillance of Landraces' Seed Health in South Italy and New Evidence on Crop Diseases

During the last three years, more than 300 landraces belonging to different plant species have been the main focus of an Italian valorization research project (AgroBiodiversità Campana, ABC) aiming at analyzing, recovering, preserving, and collecting local biodiversity. In this context, phytosanitary investigation plays a key role in identifying potential threats to the preservation of healthy seeds in gene banks and the successful cultivation of landraces. The surveillance carried out in this study, in addition to highlighting the expected presence of common species-specific pathogens such as Ascochyta pisi in peas, Ascochyta fabae in broad beans, and Macrophomina phaseolina, Xanthomonas axonopodis pv. phaseoli, and Xanthomonas fuscans subsp. fuscans in beans, pointed to the presence of novel microorganisms never detected before in the seeds of some hosts (Apiospora arundinis in common beans or Sclerotinia sclerotiorum and Stemphylium vesicarium in broad beans). These novel seedborne pathogens were fully characterized by (i) studying their morphology, (ii) identifying them by molecular methods, and (iii) studying their impact on adult crop plants. For the first time, this study provides key information about three novel seedborne pathogens that can be used to correctly diagnose their presence in seed lots, helping prevent the outbreaks of new diseases in the field.

High-Throughput Sequencing Discloses the Cucumber Mosaic Virus (CMV) Diversity in Slovakia and Reveals New Hosts of CMV from the Papaveraceae Family

Cucumber mosaic virus (CMV; Cucumovirus, Bromoviridae) is an omnipresent virus characterized by a large host range and high genetic variability. Using high-throughput sequencing, we have characterized near complete genomes of 14 Slovak CMV variants from different plant hosts. Of these, three variants originated from the Papaveraceae species (oilseed poppy, common poppy and great celandine), previously poorly described as CMV natural hosts. Based on a BLAST search and phylogenetic analysis, the Slovak CMV isolates can be divided into two genetically different Groups, Ia and II, respectively. The SL50V variant, characterized by a divergent RNA2 sequence, potentially represents a reassortant variant. In four samples (T101, SL50V, CP2, MVU2-21), the presence of satellite CMV RNA was identified along with CMV. Although mechanically transmitted to experimental cucumber plants, the role of satellite RNA in the symptomatology observed could not be established due to a complex infection of original hosts with different viruses.

The Virome of ‘Lamon Bean’: Application of MinION Sequencing to Investigate the Virus Population Associated with Symptomatic Beans in the Lamon Area, Italy

‘Lamon bean’ is a protected geographical indication (PGI) for a product of four varieties of bean (Phaseolus vulgaris L.) grown in a specific area of production, which is located in the Belluno district, Veneto region (N.E. of Italy). In the last decade, the ‘Lamon bean’ has been threatened by severe virus epidemics that have compromised its profitability. In this work, the full virome of seven bean samples showing different foliar symptoms was obtained by MinION sequencing. Evidence that emerged from sequencing was validated through RT-PCR and ELISA in a large number of plants, including different ecotypes of Lamon bean and wild herbaceous hosts that may represent a virus reservoir in the field. Results revealed the presence of bean common mosaic virus (BCMV), cucumber mosaic virus (CMV), peanut stunt virus (PSV), and bean yellow mosaic virus (BYMV), which often occurred as mixed infections. Moreover, both CMV and PSV were reported in association with strain-specific satellite RNAs (satRNAs). In conclusion, this work sheds light on the cause of the severe diseases affecting the ‘Lamon bean’ by exploitation of MinION sequencing.

Characterization of Pyrenophora tritici-repentis in Tunisia and Comparison with a Global Pathogen Population

Pyrenophora tritici-repentis causes tan spot, an important foliar disease of wheat. A collection of P. tritici-repentis isolates from Tunisia, located in one of the main secondary centers of diversification of durum wheat, was tested for phenotypic race classification based on virulence on a host differential set and for the presence of the necrotrophic effector (NE) genes ToxA, ToxB, and toxb by PCR analysis. While races 2, 4, 5, 6, 7, and 8 were identified according to their virulence phenotypes, PCR testing indicated the presence of "atypical" isolates that induced necrosis on the wheat differential 'Glenlea,' but lacked the expected ToxA gene, suggesting the involvement of other NEs in the P. tritici-repentis/wheat interaction. Genetic diversity and the P. tritici-repentis population structure were explored further by examining 59 Tunisian isolates and 35 isolates from Algeria, Azerbaijan, Canada, Iran, and Syria using 24 simple sequence repeat markers. Average genetic diversity, overall gene flow, and percentage polymorphic loci were estimated as 0.58, 2.09, and 87%, respectively. Analysis of molecular variance showed that 81% of the genetic variance occurred within populations and 19% occurred between populations. Cluster analysis by the unweighted pair group method indicated that ToxB^- isolates grouped together and were distantly related to ToxB⁺ isolates. Based on Nei's analysis, the global collection clustered into two distinct groups according to their region of origin. The results suggest that geographic origin and the host specificity imposed by different NEs can lead to differentiation among P. tritici-repentis populations.

MIXED VIRAL INFECTIONS IN VEGETABLES IN UKRAINE

Aim. Screening of vegetable crops for mixed viral infections caused by 8 viruses, and evaluation of relative share for different combinations of pathogens using serological methods for plant virus diagnostics. Methods. Double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) was performed for detection of viral antigens. Results. For this study, the samples from plants with virus-like symptoms were collected from the agroecosystems in 4 regions of Ukraine during three vegetative seasons in 2019-2021. Plant samples were tested for presence of the following viruses: cucumber mosaic virus (CMV), watermelon mosaiс virus 2 (WMV2), zucchini yellow mosaic virus (ZYMV), tomato mosaic virus (ToMV), tobacco rattle virus (TRV), potato virus Y (PVY), potato virus X (PVX), and tomato spotted wilt virus (TSWV). Among the screened plants with virus-like symptoms, three viruses were the most common: CMV, WMV2 and ZYMV, with the incidence rate of 32%, 33% and 29%, respectively. Apart from monoinfected plants, several patterns of mixed infections were shown typically induced by two and less commonly by three viruses (CMV+WMV2+ZYMV). From these patterns of mixed infections, five groups of pathogens were registered. Conclusions. In this work, we have analyzed cucurbit and solanaceous vegetable crops with virus-like symptoms for viral infections caused by 8 pathogens, and established virus combinations inducing mixed infections in the field. Five groups of pathogens were demonstrated as such combinations: CMV+ToMV, CMV+PVY, CMV+WMV2, CMV+ZYMV, and WMV2+ZYMV, with CMV+ToMV being the most common for tested plants. Also, there was one group of pathogens formed by three viruses CMV+WMV2+ZYMV. CMV has been shown present in every group of mixed viral infections in plants from both families, Cucurbitaceae and Solanaceae. The incidence rate for these combinations differed from 4 to 8%. The presented results are important in the context of ecology and epidemiology of viral diseases of vegetables.

Co-existence of chlorosis inducing strain of Cucumber mosaic virus with tospoviruses on hot pepper (Capsicum annuum) in India

Cucumo- and tospoviruses are the most destructive viruses infecting hot pepper (chilli). A diagnostic survey was conducted to assess the prevalence of cucumo and tospoviruses in chilli growing tracts of Tamil Nadu. Infected plants showing mosaic with chlorotic and necrotic rings, veinal necrosis, mosaic mottling, leaf filiformity and malformation were collected. Molecular indexing carried out through reverse transcription polymerase chain reaction (RT-PCR) with coat protein gene specific primer of Cucumber mosaic virus (CMV) and tospovirus degenerate primer corresponding to the L segment (RdRp). Ostensibly, amplifications were observed for both CMV and tospoviruses as sole as well for mixed infections. The sequence analysis indicated that the Capsicum chlorosis virus (CaCV) and Groundnut bud necrosis virus (GBNV) to be involved with CMV in causing combined infections. The co-infection of CMV with CaCV was detected in 10.41% of the symptomatic plant samples and combined infection of CMV with GBNV was recorded in around 6.25% of the symptomatic plants surveyed. The amino acid substitution of Ser¹²⁹ over conserved Pro¹²⁹ in coat protein of CMV implies that CMV strain involved in mixed infection as chlorosis inducing strain. Further, the electron microscopy of symptomatic plant samples explicated the presence of isometric particles of CMV and quasi spherical particles of tospoviruses. This is the first molecular evidence for the natural co-existence of chlorosis inducing CMV strain with CaCV and GBNV on hot pepper in India.

Illuminating an Ecological Blackbox: Using High Throughput Sequencing to Characterize the Plant Virome Across Scales

The ecology of plant viruses began to be explored at the end of the 19th century. Since then, major advances have revealed mechanisms of virus-host-vector interactions in various environments. These advances have been accelerated by new technlogies for virus detection and characterization, most recently including high throughput sequencing (HTS). HTS allows investigators, for the first time, to characterize all or nearly all viruses in a sample without a priori information about which viruses might be present. This powerful approach has spurred new investigation of the viral metagenome (virome). The rich virome datasets accumulated illuminate important ecological phenomena such as virus spread among host reservoirs (wild and domestic), effects of ecosystem simplification caused by human activities (and agriculture) on the biodiversity and the emergence of new viruses in crops. To be effective, however, HTS-based virome studies must successfully navigate challenges and pitfalls at each procedural step, from plant sampling to library preparation and bioinformatic analyses. This review summarizes major advances in plant virus ecology associated with technological developments, and then presents important considerations and best practices for HTS use in virome studies.

Detection of Plant Viruses and Disease Management: Relevance of Genetic Diversity and Evolution

Plant viruses cause considerable economic losses and are a threat for sustainable agriculture. The frequent emergence of new viral diseases is mainly due to international trade, climate change, and the ability of viruses for rapid evolution. Disease control is based on two strategies: i) immunization (genetic resistance obtained by plant breeding, plant transformation, cross-protection, or others), and ii) prophylaxis to restrain virus dispersion (using quarantine, certification, removal of infected plants, control of natural vectors, or other procedures). Disease management relies strongly on a fast and accurate identification of the causal agent. For known viruses, diagnosis consists in assigning a virus infecting a plant sample to a group of viruses sharing common characteristics, which is usually referred to as species. However, the specificity of diagnosis can also reach higher taxonomic levels, as genus or family, or lower levels, as strain or variant. Diagnostic procedures must be optimized for accuracy by detecting the maximum number of members within the group (sensitivity as the true positive rate) and distinguishing them from outgroup viruses (specificity as the true negative rate). This requires information on the genetic relationships within-group and with members of other groups. The influence of the genetic diversity of virus populations in diagnosis and disease management is well documented, but information on how to integrate the genetic diversity in the detection methods is still scarce. Here we review the techniques used for plant virus diagnosis and disease control, including characteristics such as accuracy, detection level, multiplexing, quantification, portability, and designability. The effect of genetic diversity and evolution of plant viruses in the design and performance of some detection and disease control techniques are also discussed. High-throughput or next-generation sequencing provides broad-spectrum and accurate identification of viruses enabling multiplex detection, quantification, and the discovery of new viruses. Likely, this technique will be the future standard in diagnostics as its cost will be dropping and becoming more affordable.

When Viruses Play Team Sports: Mixed Infections in Plants

The pathological importance of mixed viral infections in plants might be underestimated except for a few well-characterized synergistic combinations in certain crops. Considering that the host ranges of many viruses often overlap and that most plant species can be infected by several unrelated viruses, it is not surprising to find more than one virus simultaneously in the same plant. Furthermore, dispersal of the majority of plant viruses relies on efficient transmission mechanisms mediated by vector organisms, mainly but not exclusively insects, which can contribute to the occurrence of multiple infections in the same plant. Recent work using different experimental approaches has shown that mixed viral infections can be remarkably frequent, up to the point that they could be considered the rule more than the exception. The purpose of this review is to describe the impact of multiple infections not only on the participating viruses themselves but also on their vectors and on the common host. From this standpoint, mixed infections arise as complex events that involve several cross-interacting players, and they consequently require a more general perspective than the analysis of single-virus/single-host approaches for a full understanding of their relevance.

Development of a molecular assay for the detection of Cucumber mosaic virus and the discrimination of its subgroups I and II

A nucleic acid based test for the detection of the economically important plant virus Cucumber mosaic virus (CMV) based on the Luminex xTAG technology was developed. This technology has the advantage of allowing the simultaneous detection of various targets. Applying this method, we prove the presence of CMV in general and differentiate between its two subgroups I and II for which significant differences concerning severity of symptoms and virulence have been reported. For the development of the test procedure the coat protein gene sequences of 29 CMV isolates were cloned, sequenced and classified into subgroups. Sequences from GenBank were used to design primers. Additionally, a subgroup specific ELISA was conducted for comparison. This work is part of a project which aims to develop a test for the simultaneous detection of various plant pathogens (viral, bacterial and fungal) in plant material.

Synergies and antagonisms in virus interactions

Metagenomic surveys and data from next generation sequencing revealed that mixed infections among plant viruses are probably a rule rather than an exception in natural pathosystems. The documented cases may range from synergism to antagonism, which may depend from the spatiotemporal order of arrival of the viruses on the host and upon the host itself. In synergistic interactions, the measurable differences in replication, phenotypic and cytopathological changes, cellular tropism, within host movement, and transmission rate of one of the two viruses or both are increased. Conversely, a decrease in replication, or inhibition of one or more of the above functions by one virus against the other, leads to an antagonistic interaction. Viruses may interact directly and by transcomplementation of defective functions or indirectly, through responses mediated by the host like the defense mechanism based on RNA silencing. Outcomes of these interactions can be applied to the risk assessment of transgenic crops expressing viral proteins, or cross-protected crops for the identification of potential hazards. Prior to experimental evidence, mathematical models may help in forecasting challenges deriving from the great variety of pathways of synergistic and antagonistic interactions. Actually, it seems that such predictions do not receive sufficient credit in the framework of agriculture.

Satellite RNAs and Satellite Viruses

Satellite RNAs and satellite viruses are extraviral components that can affect either the pathogenicity, the accumulation, or both of their associated viruses while themselves being dependent on the associated viruses as helper viruses for their infection. Most of these satellite RNAs are noncoding RNAs, and in many cases, have been shown to alter the interaction of their helper viruses with their hosts. In only a few cases have the functions of these satellite RNAs in such interactions been studied in detail. In particular, work on the satellite RNAs of Cucumber mosaic virus and Turnip crinkle virus have provided novel insights into RNAs functioning as noncoding RNAs. These effects are described and potential roles for satellite RNAs in the processes involved in symptom intensification or attenuation are discussed. In most cases, models describing these roles involve some aspect of RNA silencing or its suppression, either directly or indirectly involving the particular satellite RNA.

Molecular insights into the function of the viral RNA silencing suppressor HCPro

Potyviral helper component proteinase (HCPro) is a well-characterized suppressor of antiviral RNA silencing, but its mechanism of action is not yet fully understood. In this study, we used affinity purification coupled with mass spectrometry to identify binding partners of HCPro in potyvirus-infected plant cells. This approach led to identification of various HCPro interactors, including two key enzymes of the methionine cycle, S-adenosyl-L-methionine synthase and S-adenosyl-L-homocysteine hydrolase. This finding, together with the results of enzymatic activity and gene knockdown experiments, suggests a mechanism in which HCPro complexes containing viral and host proteins act to suppress antiviral RNA silencing through local disruption of the methionine cycle. Another group of HCPro interactors identified in this study comprised ribosomal proteins. Immunoaffinity purification of ribosomes demonstrated that HCPro is associated with ribosomes in virus-infected cells. Furthermore, we show that HCPro and ARGONAUTE1 (AGO1), the core component of the RNA-induced silencing complex (RISC), interact with each other and are both associated with ribosomes in planta. These results, together with the fact that AGO1 association with ribosomes is a hallmark of RISC-mediated translational repression, suggest a second mechanism of HCPro action, whereby ribosome-associated multiprotein complexes containing HCPro relieve viral RNA translational repression through interaction with AGO1.

Additive interactions of unrelated viruses in mixed infections of cowpea (Vigna unguiculata L. Walp)

This study was carried out to determine the effects of single infections and co-infections of three unrelated viruses on three cowpea cultivars (one commercial cowpea cultivar "White" and 2 IITA lines; IT81D-985 and TVu 76). The plants were inoculated with Cowpea aphid-borne mosaic virus (CABMV), genus Potyvirus, Cowpea mottle virus (CMeV), genus Carmovirus and Southern bean mosaic virus (SBMV), genus Sobemovirus singly and in mixture (double and triple) at 10, 20, and 30 days after planting (DAP). The treated plants were assessed for susceptibility to the viruses, growth, and yield. In all cases of infection, early inoculation resulted in higher disease severity compared with late infection. The virus treated cowpea plants were relatively shorter than buffer inoculated control plants except the IT81D-985 plants that were taller and produced more foliage. Single infections by CABMV, CMeV, and SBMV led to a complete loss of seeds in the three cowpea cultivars at 10 DAP; only cultivar White produced some seeds at 30 DAP. Double and triple virus infections led to a total loss of seeds in all three cowpea cultivars. None of the virus infected IITA lines produced any seeds except IT81D-985 plants co-infected with CABMV and SBMV at 30 DAP with a reduction of 80%. Overall, the commercial cultivar "White" was the least susceptible to the virus treatments and produced the most yield (flowers, pods, and seeds). CABMV was the most aggressive of these viruses and early single inoculations with this virus resulted in the premature death of some of the seedlings. The presence of the Potyvirus, CABMV in the double virus infections did not appear to increase disease severity or yield loss. There was no strong evidence for synergistic interactions between the viruses in the double virus mixtures.

Synergisms between microbial pathogens in plant disease complexes: a growing trend

Plant diseases are often thought to be caused by one species or even by a specific strain. Microbes in nature, however, mostly occur as part of complex communities and this has been noted since the time of van Leeuwenhoek. Interestingly, most laboratory studies focus on single microbial strains grown in pure culture; we were therefore unaware of possible interspecies and/or inter-kingdom interactions of pathogenic microbes in the wild. In human and animal infections, it is now being recognized that many diseases are the result of multispecies synergistic interactions. This increases the complexity of the disease and has to be taken into consideration in the development of more effective control measures. On the other hand, there are only a few reports of synergistic pathogen-pathogen interactions in plant diseases and the mechanisms of interactions are currently unknown. Here we review some of these reports of synergism between different plant pathogens and their possible implications in crop health. Finally, we briefly highlight the recent technological advances in diagnostics as these are beginning to provide important insights into the microbial communities associated with complex plant diseases. These examples of synergistic interactions of plant pathogens that lead to disease complexes might prove to be more common than expected and understanding the underlying mechanisms might have important implications in plant disease epidemiology and management.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

Detection of Multiple Potato Viruses in the Field Suggests Synergistic Interactions among Potato Viruses in Pakistan

Viral diseases have been a major limiting factor threating sustainable potato (Solanum tuberosum L.) production in Pakistan. Surveys were conducted to serologically quantify the incidence of RNA viruses infecting potato; Potato virus X (PVX), Potato virus Y (PVY), Potato virus S (PVS), Potato virus A (PVA), Potato virus M (PVM) and Potato leaf roll virus (PLRV) in two major potato cultivars (Desiree and Cardinal). The results suggest the prevalence of multiple viruses in all surveyed areas with PVY, PVS and PVX dominantly widespread with infection levels of up to 50% in some regions. Co-infections were detected with the highest incidence (15.5%) for PVX and PVS. Additionally the data showed a positive correlation between co-infecting viruses with significant increase in absorbance value (virus titre) for at least one of the virus in an infected plant and suggested a synergistic interaction. To test this hypothesis, glasshouse grown potato plants were challenged with multiple viruses and analyzed for systemic infections and symptomology studies. The results obtained conclude that multiple viral infections dramatically increase disease epidemics as compared to single infection and an effective resistance strategy in targeting multiple RNA viruses is required to save potato crop.

Molecular genetic analysis of cucumber mosaic virus populations infecting pepper suggests unique patterns of evolution in Korea

Studying genetic structure and diversity of viruses is important to understand the evolutionary mechanisms that generate and maintain variations in viral populations. Cucumber mosaic virus (CMV) is endemic in most pepper fields in Korea. Currently, no effective methods for control of CMV are available due to many environmental and biological factors such as the extensive evolutionary capacity of CMV. Thus, analyzing the genetic structure of CMV populations may facilitate the development of strategies for the control of CMV. In this study, 252 pepper (Capsicum annuum) samples showing virus symptoms were collected by field surveys performed throughout Korea in 2007. Reverse-transcription polymerase chain reaction analyses revealed that, in total, 165 collected samples were infected with CMV. Forty-five CMV isolates were randomly selected within each regional subpopulation and analyzed by full-genome sequencing. Analyses of genetic diversity showed that the 2b gene of CMV is under weaker purifying selection than the other genes. Based on the phylogenetic analysis of RNA1, the CMV isolates from pepper were divided into three clusters in subgroup I. Our full-genome sequence-based molecular analyses of the CMV Korean population suggest that the subpopulations of CMV have been geographically localized in pepper fields in Korea.

Effect of Raspberry bushy dwarf virus, Raspberry leaf mottle virus, and Raspberry latent virus on Plant Growth and Fruit Crumbliness in 'Meeker' Red Raspberry

Raspberry crumbly fruit in red raspberry (Rubus idaeus), widespread in the Pacific Northwest of the United States and British Columbia, Canada, is most commonly caused by a virus infection. Raspberry bushy dwarf virus (RBDV) has long been attributed as the causal agent of the disease. Recently, the identification of two additional viruses, Raspberry leaf mottle virus (RLMV) and Raspberry latent virus (RpLV), in northern Washington and British Columbia, suggested the existence of a possible new virus complex responsible for the increased severity of the disease. Virus testing of crumbly fruited plants from five fields in northern Washington revealed the presence of RLMV and RpLV, in addition to RBDV. Plants with less severe crumbly fruit symptoms had a much lower incidence of RLMV or RpLV. Field trials using replicated plots of 'Meeker' plants containing single and mixed infections of RBDV, RLMV, or RpLV, along with a virus-free control, were developed to determine the role of RLMV and RpLV in crumbly fruit. Field evaluations during establishment and two fruiting years revealed that plants infected with the three viruses or the combinations RBDV+RLMV and RBDV+RpLV had the greatest reduction in cane growth, or fruit firmness and fruit weight, respectively. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) showed that the titer of RBDV was increased ~400-fold when it occurred in mixed infections with RLMV compared to RBDV in single infections. In addition, a virus survey revealed that RLMV and RpLV are present at high incidence in northern Washington; whereas the incidence in southern Washington and Oregon, where crumbly fruit is not as serious a problem, was considerably lower.

P3N-PIPO of Clover yellow vein virus exacerbates symptoms in pea infected with white clover mosaic virus and is implicated in viral synergism

Mixed infection of pea (Pisum sativum) with Clover yellow vein virus (ClYVV) and White clover mosaic virus (WClMV) led to more severe disease symptoms (a phenomenon called viral synergism). Similar to the mixed ClYVV/WClMV infection, a WClMV-based vector encoding P3N-PIPO of ClYVV exacerbated the disease symptoms. Infection with the WClMV vector encoding ClYVV HC-Pro (a suppressor of RNA silencing involved in potyviral synergisms), also resulted in more severe symptoms, although to a lesser extent than infection with the vector encoding P3N-PIPO. Viral genomic RNA accumulated soon after inoculation (at 2 and 4 days) at higher levels in leaves inoculated with WClMV encoding HC-Pro but at lower levels in leaves inoculated with WClMV encoding P3N-PIPO than in peas infected with WClMV encoding GFP. Our results suggest that ClYVV P3N-PIPO is involved in the synergism between ClYVV and WClMV during pea infection through an unknown mechanism different from suppression of RNA silencing.

The role of virulence in in vivo superinfection fitness of the vertebrate RNA virus infectious hematopoietic necrosis virus

We have developed a novel in vivo superinfection fitness assay to examine superinfection dynamics and the role of virulence in superinfection fitness. This assay involves controlled, sequential infections of a natural vertebrate host, Oncorhynchus mykiss (rainbow trout), with variants of a coevolved viral pathogen, infectious hematopoietic necrosis virus (IHNV). Intervals between infections ranged from 12 h to 7 days, and both frequency of superinfection and viral replication levels were examined. Using virus genotype pairs of equal and unequal virulence, we observed that superinfection generally occurred with decreasing frequency as the interval between exposures to each genotype increased. For both the equal-virulence and unequal-virulence genotype pairs, the frequency of superinfection in most cases was the same regardless of which genotype was used in the primary exposure. The ability to replicate in the context of superinfection also did not differ between the genotypes of equal or unequal virulence tested here. For both genotype pairs, the mean viral load of the secondary virus was significantly reduced in superinfection while primary virus replication was unaffected. Our results demonstrate, for the two genotype pairs examined, that superinfection restriction does occur for IHNV and that higher virulence did not correlate with a significant difference in superinfection fitness. To our knowledge, this is the first assay to examine the role of virulence of an RNA virus in determining superinfection fitness dynamics within a natural vertebrate host.

Outcomes of co-infection by two potyviruses: implications for the evolution of manipulative strategies

Recent studies have documented effects of plant viruses on host plants that appear to enhance transmission by insect vectors. But, almost no empirical work has explored the implications of such apparent manipulation for interactions among co-infecting pathogens. We examined single and mixed infections of two potyviruses, watermelon mosaic virus (WMV) and zucchini yellow mosaic virus (ZYMV), that frequently co-occur in cucurbitaceae populations and share the same aphid vectors. We found that ZYMV isolates replicated at similar rates in single and mixed infections, whereas WMV strains accumulated to significantly lower levels in the presence of ZYMV. Furthermore, ZYMV induced changes in leaf colour and volatile emissions that enhanced aphid (Aphis gossypii) recruitment to infected plants. By contrast, WMV did not elicit strong effects on plant-aphid interactions. Nevertheless, WMV was still readily transmitted from mixed infections, despite fairing poorly in in-plant competition. These findings suggest that pathogen effects on host-vector interactions may well influence competition among co-infecting pathogens. For example, if non-manipulative pathogens benefit from the increased vector traffic elicited by manipulative competitors, their costs of competition may be mitigated to some extent. Conversely, the benefits of manipulation may be limited by free-rider effects in systems where there is strong competition among pathogens for host resources and/or access to vectors.

Complementation between two tospoviruses facilitates the systemic movement of a plant virus silencing suppressor in an otherwise restrictive host

BACKGROUND

New viruses pathogenic to plants continue to emerge due to mutation, recombination, or reassortment among genomic segments among individual viruses. Tospoviruses cause significant economic damage to a wide range of crops in many parts of the world. The genetic or molecular basis of the continued emergence of new tospoviruses and new hosts is not well understood though it is generally accepted that reassortment and/or genetic complementation among the three genomic segments of individual viruses could be contributing to this variability since plants infected with more than one tospovirus are not uncommon in nature.

METHODOLOGY/PRINCIPAL FINDINGS

Two distinct and economically important tospoviruses, Iris yellow spot virus (IYSV) and Tomato spotted wilt virus (TSWV), were investigated for inter-virus interactions at the molecular level in dually-infected plants. Datura (Datura stramonium) is a permissive host for TSWV, while it restricts the movement of IYSV to inoculated leaves. In plants infected with both viruses, however, TSWV facilitated the selective movement of the viral gene silencing suppressor (NSs) gene of IYSV to the younger, uninoculated leaves. The small RNA expression profiles of IYSV and TSWV in single- and dually-infected datura plants showed that systemic leaves of dually-infected plants had reduced levels of TSWV N gene-specific small interfering RNAs (siRNAs). No TSWV NSs-specific siRNAs were detected either in the inoculated or systemic leaves of dually-infected datura plants indicating a more efficient suppression of host silencing machinery in the presence of NSs from both viruses as compared to the presence of only TSWV NSs.

CONCLUSION/SIGNIFICANCE

Our study identifies a new role for the viral gene silencing suppressor in potentially modulating the biology and host range of viruses and underscores the importance of virally-coded suppressors of gene silencing in virus infection of plants. This is the first experimental evidence of functional complementation between two distinct tospoviruses in the Bunyaviridae family.

Yellowing disease in zucchini squash produced by mixed infections of Cucurbit yellow stunting disorder virus and Cucumber vein yellowing virus

Zucchini squash is host to Cucurbit yellow stunting disorder virus (CYSDV), a member of the genus Crinivirus, and Cucumber vein yellowing virus (CVYV), a member of the genus Ipomovirus, both transmitted by the whitefly Bemisia tabaci. Field observations suggest the appearance of new symptoms observed on leaves of zucchini squash crops when both viruses were present. When infected during controlled experiments with CYSDV only, zucchini plants showed no obvious symptoms and the virus titer decreased between 15 and 45 days postinoculation (dpi), after which it was no longer detected. CVYV caused inconspicuous symptoms restricted to vein clearing on some of the apical leaves and the virus accumulated progressively between 15 and 60 dpi. Similar accumulations of virus followed single inoculations with the potyvirus Zucchini yellow mosaic virus (ZYMV) and plants showed severe stunting, leaf deformation, and mosaic yellowing. However, in mixed infections with CYSDV and CVYV, intermediate leaves showed chlorotic mottling which evolved later to rolling, brittleness, and complete yellowing of the leaf lamina, with exception of the veins. No consistent alteration of CVYV accumulation was detected but the amounts of CYSDV increased ≈100-fold and remained detectable at 60 dpi. Such synergistic effects on the titer of the crinivirus and symptom expression were not observed when co-infected with ZYMV.

Identification of novel genomic regions associated with resistance to Pyrenophora tritici-repentis races 1 and 5 in spring wheat landraces using association analysis

Tan spot, caused by Pyrenophora tritici-repentis, is a major foliar disease of wheat worldwide. Host plant resistance is the best strategy to manage this disease. Traditionally, bi-parental mapping populations have been used to identify and map quantitative trait loci (QTL) affecting tan spot resistance in wheat. The association mapping (AM) could be an alternative approach to identify QTL based on linkage disequilibrium (LD) within a diverse germplasm set. In this study, we assessed resistance to P. tritici-repentis races 1 and 5 in 567 spring wheat landraces from the USDA-ARS National Small Grains Collection (NSGC). Using 832 diversity array technology (DArT) markers, QTL for resistance to P. tritici-repentis races 1 and 5 were identified. A linear model with principal components suggests that at least seven and three DArT markers were significantly associated with resistance to P. tritici-repentis races 1 and 5, respectively. The DArT markers associated with resistance to race 1 were detected on chromosomes 1D, 2A, 2B, 2D, 4A, 5B, and 7D and explained 1.3-3.1% of the phenotypic variance, while markers associated with resistance to race 5 were distributed on 2D, 6A and 7D, and explained 2.2-5.9% of the phenotypic variance. Some of the genomic regions identified in this study correspond to previously identified loci responsible for resistance to P. tritici-repentis, offering validation for our AM approach. Other regions identified were novel and could possess genes useful for resistance breeding. Some DArT markers associated with resistance to race 1 also were localized in the same regions of wheat chromosomes where QTL for resistance to yellow rust, leaf rust and powdery mildew, have been mapped previously. This study demonstrates that AM can be a useful approach to identify and map novel genomic regions involved in resistance to P. tritici-repentis.

Analysis of the autoproteolytic activity of the recombinant helper component proteinase from zucchini yellow mosaic virus

The multifunctional helper component proteinase (HC-Pro) of potyviruses contains an autoproteolytic function that, together with the protein 1 (P1) and NIa proteinase, processes the polyprotein into mature proteins. In this study, we analysed the autoproteolytic active domain of zucchini yellow mosaic virus (ZYMV) HC-Pro. Several Escherichia coli-expressed MBP:HC-Pro:GFP mutants containing deletions or point mutations at either the N- or C-terminus of the HC-Pro protein were examined. Our results showed that amino acids essential for the proteolytic activity of ZYMV HC-Pro are distinct from those of the tobacco etch virus HC-Pro, although the amino acid sequences in the proteolytic active domain are conserved among potyviruses.

Tissue blot immunoassay and direct RT-PCR of cucumoviruses and potyviruses from the same NitroPure nitrocellulose membrane

A method is described for using Nitropure nitrocellulose (NPN) membranes as an effective solid matrix for retrieval of template RNA of three potyviruses, Tobacco etch virus, Soybean mosaic virus and Turnip mosaic virus, and two cucumoviruses, Cucumber mosaic virus and Peanut stunt virus. These NPN membranes were also used for tissue blot immunosorbent assays (TBIAs) to identify and detect plant viruses. For RNA detection, discs from dried membranes blotted with infected tissue were minimally cleaned with Triton X-100 and placed directly into reverse transcription (RT) reactions to initiate cDNA synthesis. Aliquots of cDNA plus primers specific for coat protein produced PCR amplicons of expected sizes for each of the viruses. Intensity of PCR-amplified bands from cDNA transcribed from both non-processed and TBIA-processed NPN membranes was comparable to those using FTA Card protocols. Direct sequencing of PCR products yielded high quality runs enabling identification to species. NPN membranes retained immunologically detectable virus particles, as well as intact template viral RNA, for more than a year at room temperature. The quantity of amplification product decreased after several months of storage, but could be increased by increasing the number of PCR cycles.

Characterization of the interactions between Cucumber mosaic virus and Potato virus Y in mixed infections in tomato

Mixed infection with the SON41 strain of Potato virus Y (PVY-SON41) in tomato increased accumulation of RNAs of strains Fny and LS of Cucumber mosaic virus (CMV-Fny and CMV-LS, respectively) and enhanced disease symptoms. By contrast, replication of PVY-SON41 was downregulated by CMV-Fny and this was due to the CMV-Fny 2b protein. The CMV-FnyΔ2b mutant was unable to systemically invade the tomato plant because its movement was blocked at the bundle sheath of the phloem. The function needed for invading the phloem was complemented by PVY-SON41 in plants grown at 22°C whereas this complementation was not necessary in plants grown at 15°C. Mutations in the 2b protein coding sequence of CMV-Fny as well as inhibition of translation of the 2a/2b overlapping region of the 2a protein lessened both the accumulation of viral RNAs and the severity of symptoms. Both of these functions were complemented by PVY-SON41. Infection of CMV-Fny supporting replication of the Tfn-satellite RNA reduced the accumulation of CMV RNA and suppressed symptom expression also in plants mixed-infected with PVY-SON41. The interaction between CMV and PVY-SON41 in tomato exhibited different features from that documented in other hosts. The results of this work are relevant from an ecological and epidemiological perspective due to the frequency of natural mixed infection of CMV and PVY in tomato.

Arabidopsis thaliana as a model for the study of plant-virus co-evolution

Understanding plant-virus coevolution requires wild systems in which there is no human manipulation of either host or virus. To develop such a system, we analysed virus infection in six wild populations of Arabidopsis thaliana in Central Spain. The incidence of five virus species with different life-styles was monitored during four years, and this was analysed in relation to the demography of the host populations. Total virus incidence reached 70 per cent, which suggests a role of virus infection in the population structure and dynamics of the host, under the assumption of a host fitness cost caused by the infection. Maximum incidence occurred at early growth stages, and co-infection with different viruses was frequent, two factors often resulting in increased virulence. Experimental infections under controlled conditions with two isolates of the most prevalent viruses, cauliflower mosaic virus and cucumber mosaic virus, showed that there is genetic variation for virus accumulation, although this depended on the interaction between host and virus genotypes. Comparison of Q(ST)-based genetic differentiations between both host populations with F(ST) genetic differentiation based on putatively neutral markers suggests different selection dynamics for resistance against different virus species or genotypes. Together, these results are compatible with a hypothesis of plant-virus coevolution.

Differential effects of mild and severe Cucumber mosaic virus strains in the perturbation of MicroRNA-regulated gene expression in tomato map to the 3' sequence of RNA 2

Viral infections interfere with the microRNA (miRNA)-mediated regulation of gene expression, determining developmental defects. In tomato leaves, the accumulation levels of six miRNA species and their target transcripts corresponding to transcription factors with roles in plant development and leaf morphogenesis and two genes involved in the short RNA processing, DCL1 and AGO1, were significantly enhanced upon infection with the severe strain Cucumber mosaic virus (CMV)-Fny, while that of AGO4 was reduced. In plants harboring the infection of the mild strain CMV-LS, the effects on miRNA pathway were reduced, although AGO1, DCL1, and NAC1 also were shown to overaccumulate during infections exhibiting a mild phenotype. The use of the recombinant strain CMV-Fny(LS2b), in which the 3'-terminal region of CMV-Fny RNA 2, including the 2b coding sequence, was replaced with the corresponding region of CMV-LS RNA 2, provided evidence that the exchanged region was implicated in the perturbation of miRNA metabolism. In tomato plants infected with CMV-Fny supporting the ameliorative satellite (sat)RNA variant Tfn-satRNA, the symptomless phenotype correlated, with the exception of NAC1 upregulation, with the absence of effects on mitochondrial RNA and miRNA expression. Some of the aspects of miRNA pathway perturbation described were peculiar to CMV-tomato interactions and involved in the etiology of the disease phenotype elicited in this host.

Application of game theory to the interaction between plant viruses during mixed infections

Natural mixed infections of plant viruses are frequent, often leading to unpredictable variations in symptoms, infectivity, accumulation and/or vector transmissibility. Cauliflower mosaic caulimovirus (CaMV) has often been found in mixed infections with turnip mosaic potyvirus (TuMV) in plants of the genus Brassica. This study addressed the effect of mixed infection on infectivity, pathogenicity and accumulation of CaMV and TuMV in Arabidopsis thaliana plants inoculated mechanically with cDNA infectious clones. In singly infected plants, TuMV accumulation was approximately 8-fold higher than that of CaMV. In co-infected plants, there was 77 % more TuMV accumulation compared with single infections, whilst the accumulation of CaMV was 56 % lower. This outcome describes a biological game in which TuMV always plays the winner strategy, leading to the competitive exclusion of CaMV. However, the infectivity of each virus was not affected by the presence of the other, and no symptom synergism was observed.

Systemic gene delivery into soybean by simple rub-inoculation with plasmid DNA of a Soybean mosaic virus-based vector

Plant virus-based vectors provide attractive and valuable tools for conventional transgenic technology and gene function studies in plants. In the present study, we established the infectivity of intact plasmid DNA of Soybean mosaic virus (SMV) cDNA upon simple rub-inoculation of soybean leaves by utilizing viral transcription and processing signals to produce infectious in vivo transcripts. Furthermore, we engineered this SMV cDNA clone as a gene delivery vector for systemic expression of foreign proteins in soybean. Using this SMV-based vector, several genes with different biological activities were successfully expressed and stably maintained following serial plant passage in soybean. Thus, DNA-mediated gene delivery using this SMV-based vector provides a rapid and cost-effective approach for the overproduction of valuable proteins and for the evaluation of new traits in soybean after simple rub-inoculation onto leaves.

A simple technique for separation of Cowpea chlorotic mottle virus from Cucumber mosaic virus in natural mixed infections

A simple technique was developed to separate Cowpea chlorotic mottle virus (CCMV) from Cucumber mosaic virus (CMV) in natural mixed infections. Sap from cowpea leaves infected naturally with a mixture of CCMV and CMV was inoculated mechanically on the first tri-foliolate leaf of cowpea seedlings. Both inoculated and non-inoculated upper leaves were sampled 3 or 8 days post-inoculation and tested by reverse transcription polymerase chain reaction (RT-PCR) using primers specific to CCMV and CMV. RT-PCR analysis showed the presence of only CCMV in the inoculated leaf and both viruses in the non-inoculated systemically infected upper leaves. Total RNA from the inoculated leaves positive to CCMV only was further confirmed upon re-inoculation to cowpea seedlings. Typical CCMV symptoms were produced within 1 week and RT-PCR analysis showed only the presence of CCMV in both inoculated and non-inoculated systemically infected upper leaves. Systemically infected upper leaves of the same plants were used for CCMV purification. RT-PCR analysis of the purified virion and RNA extracted from the virion further confirmed the absence of CMV contamination. To our knowledge, this is the first report of a method separating CCMV directly from mixed infections with CMV in cowpea.

Identification of novel tan spot resistance loci beyond the known host-selective toxin insensitivity genes in wheat

Tan spot, caused by Pyrenophora tritici-repentis, is a destructive foliar disease of wheat causing significant yield reduction in major wheat growing areas throughout the world. The objective of this study was to identify quantitative trait loci (QTL) conferring resistance to tan spot in the synthetic hexaploid wheat (SHW) line TA4152-60. A doubled haploid (DH) mapping population derived from TA4152-60 x ND495 was inoculated with conidia produced by isolates of each of four virulent races of P. tritici-repentis found in North America. QTL analysis revealed a total of five genomic regions significantly associated with tan spot resistance, all of which were contributed by the SHW line. Among them, two novel QTLs located on chromosome arms 2AS and 5BL conferred resistance to all isolates tested. Another novel QTL on chromosome arm 5AL conferred resistance to isolates of races 1, 2 and 5, and a QTL specific to a race 3 isolate was detected on chromosome arm 4AL. None of these QTLs corresponded to known host selective toxin (HST) insensitivity loci, but a second QTL on chromosome arm 5BL conferred resistance to the Ptr ToxA producing isolates of races 1 and 2 and corresponded to the Tsn1 (Ptr ToxA sensitivity) locus. This indicates that the wheat-P. tritici-repentis pathosystem is much more complex than previously thought and that selecting for toxin insensitivity alone will not necessarily lead to tan spot resistance. The markers associated with the QTLs identified in this work will be useful for deploying the SHW line as a tan spot resistance source in wheat breeding.

Transcomplementation and synergism in plants: implications for viral transgenes?

In plants, viral synergisms occur when one virus enhances infection by a distinct or unrelated virus. Such synergisms may be unidirectional or mutualistic but, in either case, synergism implies that protein(s) from one virus can enhance infection by another. A mechanistically related phenomenon is transcomplementation, in which a viral protein, usually expressed from a transgene, enhances or supports the infection of a virus from a distinct species. To gain an insight into the characteristics and limitations of these helper functions of individual viral genes, and to assess their effects on the plant-pathogen relationship, reports of successful synergism and transcomplementation were compiled from the peer-reviewed literature and combined with data from successful viral gene exchange experiments. Results from these experiments were tabulated to highlight the phylogenetic relationship between the helper and dependent viruses and, where possible, to identify the protein responsible for the altered infection process. The analysis of more than 150 publications, each containing one or more reports of successful exchanges, transcomplementation or synergism, revealed the following: (i) diverse viral traits can be enhanced by synergism and transcomplementation; these include the expansion of host range, acquisition of mechanical transmission, enhanced specific infectivity, enhanced cell-to-cell and long-distance movement, elevated or novel vector transmission, elevated viral titre and enhanced seed transmission; (ii) transcomplementation and synergism are mediated by many viral proteins, including inhibitors of gene silencing, replicases, coat proteins and movement proteins; (iii) although more frequent between closely related viruses, transcomplementation and synergism can occur between viruses that are phylogenetically highly divergent. As indicators of the interoperability of viral genes, these results are of general interest, but they can also be applied to the risk assessment of transgenic crops expressing viral proteins. In particular, they can contribute to the identification of potential hazards, and can be used to identify data gaps and limitations in predicting the likelihood of transgene-mediated transcomplementation.

Molecular evidence and sequence analysis of a natural reassortant between Cucumber mosaic virus subgroup IA and II strains

Cucumber mosaic virus (CMV) is a tripartite RNA virus and has been divided into three subgroups, named IA, IB, and II. Some studies have found a few natural reassortants between CMV subgroups, although reassortment between CMV subgroups is infrequent. In our present work, a CMV reassortant, named CMV-Tsh, was obtained from a tomato plant. The complete sequence of CMV-Tsh genomic RNAs has been determined and analyzed. The results of sequence comparisons and phylogenetic analyses revealed that CMV-Tsh RNAs 1 and 3 are derived from one or two CMV subgroup II strain(s), while RNA2 is derived from a CMV subgroup IA strain. A PCR and restriction enzyme analysis-based method was developed to analyze the possibility of mixed infection by CMV strains of different subgroup in the CMV-Tsh-infected tomato plant. The results of the restriction enzyme analysis proved that CMV-Tsh is the unique strain in the tomato plant. Taken together, CMV-Tsh is a natural reassortant having CMV subgroup IA RNA2 and subgroup II RNAs 1 and 3.

Cowpea viruses: effect of single and mixed infections on symptomatology and virus concentration

Natural multiple viral infections of cultivated cowpeas have been reported in Nigeria. In this study, three Nigerian commercial cowpea cultivars ("Olo 11", "Oloyin" and "White") and two lines from the IITA (IT86D- 719 and TVU 76) were mechanically inoculated with Cowpea aphid-borne mosaic virus (CABMV), Bean southern mosaic virus (SBMV) and Cowpea mottle virus (CMeV) singly, as well as in all possible combinations at 10, 20 and 30 days after planting (DAP). Samples of leaves or stems were collected at 10, 20 and 30 days after inoculation (DAI) and analyzed for relative virus concentration by Enzyme-Linked Immunosrbent Assay. All the cultivars and lines {CVS/L} were susceptible to the viruses but the commercial CVS showed more severe symptoms and had relatively higher viral concentration. In single virus infections, CABMV which induced the most severe symptoms had absorbance values (at 405 nm) of 0.11 to 0.46 while SBMV and CMeV which induced moderate symptoms had virus titre of 0.74 to 1.99 and 0.11 to 0.90 respectively. Plants inoculated 10 DAP had significantly higher virus concentration than those inoculated 30 DAP. In mixed infections involving CABMV (10 DAP) apical necrosis and death were observed in commercial cultivars "Olo 11" and "White". Enhancement of CMeV titers were observed in plants infected with CMeV + CABMV. Multiple viral infections of cowpeas may result in complete yield loss, hence, the availability of seeds of cultivars with a high level of multiple virus resistance is recommended as a means of control.

The conserved FRNK box in HC-Pro, a plant viral suppressor of gene silencing, is required for small RNA binding and mediates symptom development

The helper component-proteinase (HC-Pro) protein of potyviruses is a suppressor of gene silencing and has been shown to elicit plant developmental-defect-like symptoms. In Zucchini yellow mosaic virus (ZYMV), a mutation in the highly conserved FR180NK box of HC-Pro to FI180NK causes attenuation of these symptoms. At 5 days postinoculation and before symptoms appear, virus accumulation, HC-Pro protein levels, and viral short interfering RNA (siRNA) levels are similar for the severe (FRNK) and attenuated (FINK) strains. At this stage, ZYMV(FRNK) caused greater accumulation of most microRNAs (miRNAs), and especially of their complementary miRNA "passenger" strands (miRNA*s), in systemically infected leaves than the attenuated ZYMV(FINK) did. HC-Pro(FRNK) specifically bound artificial siRNA and miRNA/miRNA* duplexes with a much higher affinity than the mutated HC-Pro(FINK). Further analysis of the mutant and wild-type HC-Pro proteins revealed that suppressor activity of the ZYMV HC(FINK) mutant was not diminished. However, the FINK mutation caused a loss of HC-Pro suppressor function in other potyviruses. Replacement of the second positively charged amino acid in the ZYMV FRNK box to result in FRNA also caused symptom attenuation and reduced small RNA duplex-binding affinity without loss of suppressor activity. Our data suggest that the highly conserved FRNK box in the HC-Pro of potyviruses is a probable point of contact with siRNA and miRNA duplexes. The interaction of the FRNK box with populations of miRNAs directly influences their accumulation levels and regulatory functions, resulting in symptom development.

Synergistic Interaction of Sweet potato chlorotic stunt virus (Crinivirus) with Carla-, Cucumo-, Ipomo-, and Potyviruses Infecting Sweet Potato

Co-infection of Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus) with Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) results in sweet potato virus disease (SPVD), a synergistic disease that is widely distributed in the sweet potato (Ipomoea batatas) growing regions of the world. Since both SPCSV and SPFMV are common and often detected as part of multiple co-infections of severely diseased plants, the occurrence of synergistic interactions with other viruses was investigated. Data from this study show that SPCSV, but not SPFMV, can cause synergistic diseases in sweet potato with all viruses tested, including members of the genus Potyvirus (Sweet potato latent virus, Sweet potato mild speckling virus), Ipomovirus (Sweet potato mild mottle virus), Cucumovirus (Cucumber mosaic virus), and putative members of the genus Carlavirus (Sweet potato chlorotic fleck virus and C-6 virus). The synergism was expressed as an increase in the severity of symptoms, virus accumulation, viral movement in plants, and as an effect on yield of storage roots. The presence of a third different virus in plants affected with SPVD increased the severity of symptoms even further compared with SPVD alone. There was a positive correlation between increase in virus accumulation and symptom expression in double and triple SPCSV-associated co-infections. The epidemiological implications of the results are discussed.

The Tsn1-ToxA interaction in the wheat-Stagonospora nodorum pathosystem parallels that of the wheat-tan spot system

The wheat tan spot fungus (Pyrenophora tritici-repentis) produces a well-characterized host-selective toxin (HST) known as Ptr ToxA, which induces necrosis in genotypes that harbor the Tsn1 gene on chromosome 5B. In previous work, we showed that the Stagonospora nodorum isolate Sn2000 produces at least 2 HSTs (SnTox1 and SnToxA). Sensitivity to SnTox1 is governed by the Snn1 gene on chromosome 1B in wheat. SnToxA is encoded by a gene with a high degree of similarity to the Ptr ToxA gene. Here, we evaluate toxin sensitivity and resistance to S. nodorum blotch (SNB) caused by Sn2000 in a recombinant inbred population that does not segregate for Snn1. Sensitivity to the Sn2000 toxin preparation cosegregated with sensitivity to Ptr ToxA at the Tsn1 locus. Tsn1-disrupted mutants were insensitive to both Ptr ToxA and SnToxA, suggesting that the 2 toxins are functionally similar, because they recognize the same locus in the host to induce necrosis. The locus harboring the tsn1 allele underlies a major quantitative trait locus (QTL) for resistance to SNB caused by Sn2000, and explains 62% of the phenotypic variation, indicating that the toxin is an important virulence factor for this fungus. The Tsn1 locus and several minor QTLs together explained 77% of the phenotypic variation. Therefore, the Tsn1-ToxA interaction in the wheat-S. nodorum pathosystem parallels that of the wheat-tan spot system, and the wheat Tsn1 gene serves as a major determinant for susceptibility to both SNB and tan spot.