Wheat streak mosaic virus Lacking Helper Component-Proteinase Is Competent to Produce Disease Synergism in Double Infections with Maize chlorotic mottle virus

Maize Lethal Necrosis disease: review of molecular and genetic resistance mechanisms, socio-economic impacts, and mitigation strategies in sub-Saharan Africa

BACKGROUND

Maize lethal necrosis (MLN) disease is a significant constraint for maize producers in sub-Saharan Africa (SSA). The disease decimates the maize crop, in some cases, causing total crop failure with far-reaching impacts on regional food security.

RESULTS

In this review, we analyze the impacts of MLN in Africa, finding that resource-poor farmers and consumers are the most vulnerable populations. We examine the molecular mechanism of MLN virus transmission, role of vectors and host plant resistance identifying a range of potential opportunities for genetic and phytosanitary interventions to control MLN. We discuss the likely exacerbating effects of climate change on the MLN menace and describe a sobering example of negative genetic association between tolerance to heat/drought and susceptibility to viral infection. We also review role of microRNAs in host plant response to MLN causing viruses as well as heat/drought stress that can be carefully engineered to develop resistant varieties using novel molecular techniques.

CONCLUSIONS

With the dual drivers of increased crop loss due to MLN and increased demand of maize for food, the development and deployment of simple and safe technologies, like resistant cultivars developed through accelerated breeding or emerging gene editing technologies, will have substantial positive impact on livelihoods in the region. We have summarized the available genetic resources and identified a few large-effect QTLs that can be further exploited to accelerate conversion of existing farmer-preferred varieties into resistant cultivars.

Differential Synergistic Interactions Among Four Different Wheat-Infecting Viruses

Field-grown wheat (Triticum aestivum L.) plants can be co-infected by multiple viruses, including wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), brome mosaic virus (BMV), and barley stripe mosaic virus (BSMV). These viruses belong to four different genera in three different families and are, hence, genetically divergent. However, the impact of potential co-infections with two, three, or all four of them on the viruses themselves, as well as the wheat host, has yet to be examined. This study examined bi-, tri-, and quadripartite interactions among these viruses in wheat for disease development and accumulation of viral genomic RNAs, in comparison with single virus infections. Co-infection of wheat by BMV and BSMV resulted in BMV-like symptoms with a drastic reduction in BSMV genomic RNA copies and coat protein accumulation, suggesting an antagonism-like effect exerted by BMV toward BSMV. However, co-infection of either BMV or BSMV with WSMV or TriMV led to more severe disease than singly infected wheat, but with a decrease or no significant change in titers of interacting viruses in the presence of BMV or BSMV, respectively. These results were in stark contrast with exacerbated disease phenotype accompanied with enhanced virus titers caused by WSMV and TriMV co-infection. Co-infection of wheat by WSMV, TriMV, and BMV or BSMV resulted in enhanced synergistic disease accompanied by increased accumulation of TriMV and BMV but not WSMV or BSMV. Quadripartite interactions in co-infected wheat by all four viruses resulted in very severe disease synergism, leading to the death of the most infected plants, but paradoxically, a drastic reduction in BSMV titer. Our results indicate that interactions among different viruses infecting the same plant host are more complex than previously thought, do not always entail increases in virus titers, and likely involve multiple mechanisms. These findings lay the foundation for additional mechanistic dissections of synergistic interactions among unrelated plant viruses.

Comparative Analysis of Biological Characteristics among P0 Proteins from Different Brassica Yellows Virus Genotypes

Simple Summary

Polerovirus P0 proteins are multifunctional proteins. Besides their viral suppressor of RNA silencing (VSR) functions, several P0 proteins can induce a cell death phenotype within the infiltrated region of Nicotiana benthamiana or Nicotiana glutinosa. Recently, the Brassica yellows virus (BrYV) genotype A P0 protein (P0^BrA) was identified as a strong viral suppressor of RNAi. In this study, we compared the features of the P0 proteins encoded by different genotypes of BrYV and revealed their difference in inducing cell death in N. benthamiana. Key residues in P0^BrA for inducing cell death were also identified. We also showed that all three BrYV genotypes had synergistic interaction with PEMV 2 in N. benthamiana. This study provides theoretical guidance for controlling the viral disease caused by poleroviruses in the future.

Abstract

Brassica yellows virus (BrYV) is a tentative species of the genus Polerovirus, which has at least three genotypes (A, B, and C) in China. The P0 protein of BrYV-A (P0^BrA) has been identified as a viral suppressor of RNA silencing (VSR), which can also induce cell death in infiltrated Nicotiana benthamiana leaves. In this study, we demonstrated that the cell death induced by P0^BrA was accompanied by the accumulation of reactive oxygen species (ROS) and increased Pathogenesis-related protein genes-1 (PR1) expression. Meanwhile, this cell death phenotype was delayed by salicylic acid (SA) pretreatment. Biological function comparison of the three P0 proteins showed that transiently expressed P0^BrB or P0^BrC induced a significantly delayed and milder cell death response compared with P0^BrA. However, like P0^BrA, they also suppressed local and systemic RNA silencing. Six residues of P0^BrA essential for inducing cell death were identified by comparative analysis and amino acid substitution assay. We also show that all three BrYV genotypes have synergistic interactions with pea enation mosaic virus 2 (PEMV 2) in N. benthamiana. This study provides theoretical guidance for controlling the viral disease caused by poleroviruses in the future.

Maize Yellow Mosaic Virus Interacts with Maize Chlorotic Mottle Virus and Sugarcane Mosaic Virus in Mixed Infections, But Does Not Cause Maize Lethal Necrosis

A maize-infecting polerovirus, variously named maize yellow dwarf virus RMV2 (MYDV RMV2), MYDV-like, and maize yellow mosaic virus (MaYMV), is frequently found in mixed infections in plants also infected with maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV), known to synergistically cause maize lethal necrosis (MLN). MaYMV was discovered in deep sequencing studies precipitated by recent MLN emergence and is prevalent at global locations with MLN, but its role in or contribution to disease was not known. We examined how MaYMV impacted disease development in mixed infections with MCMV, SCMV, and both MCMV and SCMV compared with mock-inoculated plants. Results demonstrated that MaYMV symptoms included stunting as well as leaf reddening in single and mixed infections. MaYMV did not recapitulate MLN synergistic disease in double infections in which either MCMV or SCMV was missing (MaYMV + MCMV or MaYMV + SCMV), but did significantly enhance stunting in mixed infections and suppressed titers of both MCMV and SCMV in double infections. Interestingly, MaYMV strongly suppressed the SCMV-induced titer increase of MCMV in triple infections, but MLN symptoms still occurred with the reduced MCMV titer. These data indicate the potential disease impact of this newly discovered ubiquitous maize virus, alone and in the context of MLN.

Impact of viral silencing suppressors on plant viral synergism: a global agro-economic concern

Plant viruses are known for their devastating impact on global agriculture. These intracellular biotrophic pathogens can infect a wide variety of plant hosts all over the world. The synergistic association of plant viruses makes the situation more alarming. It usually promotes the replication, movement, and transmission of either or both the coexisting synergistic viral partners. Although plants elicit a robust antiviral immune reaction, including gene silencing, to limit these infamous invaders, viruses counter it by encoding viral suppressors of RNA silencing (VSRs). Growing evidence also suggests that VSRs play a driving role in mediating the plant viral synergism. This review briefly discusses the evil impacts of mixed infections, especially synergism, and then comprehensively describes the emerging roles of VSRs in mediating the synergistic association of plant viruses. KEY POINTS: • Synergistic associations of plant viruses have devastating impacts on global agriculture. • Viral suppressors of RNA silencing (VSRs) play key roles in driving plant viral synergism.

Recombinase Polymerase Amplification Assay for Simultaneous Detection of Maize Chlorotic Mottle Virus and Sugarcane Mosaic Virus in Maize

Maize chlorotic mottle virus (MCMV) can cause maize lethal necrosis (MLN) when coinfected with potyvirids, such as sugarcane mosaic virus (SCMV), maize dwarf mosaic virus, or wheat streak mosaic virus. MLN is often caused by coinfection of MCMV and SCMV, which has been reported in China and several countries of Africa. In this study, a recombinase polymerase amplification (RPA) assay was established for simultaneous detection of MCMV and SCMV in maize. The RPA assay can be completed within 30 min at 38 °C. The primers for the RPA assay were specific since no crossreaction was detected with other selected viruses that infected maize in China. The detection limit of the RPA method was 10² copies μL^-1, which was about 10-fold more sensitive than that of the conventional PCR method. Moreover, the RPA assay can be successfully applied to detect maize samples collected in the field. These results demonstrated that the established RPA assay is a rapid and efficient method to conduct simultaneous detection of MCMV and SCMV, which provides an alternative technology for MLN diagnosis.

Synergistic Effects of a Tomato chlorosis virus and Tomato yellow leaf curl virus Mixed Infection on Host Tomato Plants and the Whitefly Vector

In China, Tomato chlorosis virus (ToCV) and Tomato yellow leaf curl virus (TYLCV) are widely present in tomato plants. The epidemiology of these viruses is intimately associated with their vector, the whitefly (Bemisia tabaci MED). However, how a ToCV+TYLCV mixed infection affects viral acquisition by their vector remains unknown. In this study, we examined the growth parameters of tomato seedlings, including disease symptoms and the heights and weights of non-infected, singly infected and mixed infected tomato plants. Additionally, the spatio-temporal dynamics of the viruses in tomato plants, and the viral acquisition and transmission by B. tabaci MED, were determined. The results demonstrated that: (i) ToCV+TYLCV mixed infections induced tomato disease synergism, resulting in a high disease severity index and decreased stem heights and weights; (ii) as the disease progressed, TYLCV accumulated more in upper leaves of TYLCV-infected tomato plants than in lower leaves, whereas ToCV accumulated less in upper leaves of ToCV-infected tomato plants than in lower leaves; (iii) viral accumulation in ToCV+TYLCV mixed infected plants was greater than in singly infected plants; and (iv) B. tabaci MED appeared to have a greater TYLCV, but a lower ToCV, acquisition rate from mixed infected plants compared with singly infected plants. However, mixed infections did not affect transmission by whiteflies. Thus, ToCV+TYLCV mixed infections may induce synergistic disease effects in tomato plants.

Occurrence, genetic diversity, and recombination of maize lethal necrosis disease-causing viruses in Kenya

Maize is the most important food crop in Kenya accounting for more than 51 % of all staples grown in the country. Out of Kenya's 5.3 million ha total crops area, more than 2.1 million ha is occupied by maize which translates to 40 % of all crops area. However, with the emergence of maize lethal necrosis (MLN) disease in 2011, the average yields plummeted to all-time lows with severely affected counties recording 90-100% yield loss in 2013 and 2014. The disease is mainly caused by Maize chlorotic mottle virus (MCMV) in combination with Sugarcane mosaic virus (SCMV) or other potyviruses. In this study, a country-wide survey was carried out to assess the MLN causing viruses in Kenya, their distribution, genetic diversity, and recombination. The causative viruses of MLN were determined by RT-PCR using virus-specific primers and DAS-ELISA. Next-generation sequencing (NGS) data was generated, viral sequences identified, genetic diversity of MLN viruses was determined, and recombination was evaluated. MCMV and SCMV were detected in all the maize growing regions at varying levels of incidence, and severity while MaYMV, a polerovirus was detected in some samples through NGS. However, there were some samples in this study where only MCMV was detected with severe MLN symptoms. SCMV Sequences were highly diverse while MCMV sequences exhibited low variability. Potential recombination events were detected only in SCMV explaining the elevated level of diversity and associated risk of this virus in Kenya and the eastern Africa region.

Transgenic Wheat Harboring an RNAi Element Confers Dual Resistance Against Synergistically Interacting Wheat Streak Mosaic Virus and Triticum Mosaic Virus

Wheat streak mosaic virus (WSMV) and triticum mosaic virus (TriMV) are economically important viruses of wheat (Triticum aestivum L.), causing significant yield losses in the Great Plains region of the United States. These two viruses are transmitted by wheat curl mites, which often leads to mixed infections with synergistic interaction in grower fields that exacerbates yield losses. Development of dual-resistant wheat lines would provide effective control of these two viruses. In this study, a genetic resistance strategy employing an RNA interference (RNAi) approach was implemented by assembling a hairpin element composed of a 202-bp (404-bp in total) stem sequence of the NIb (replicase) gene from each of WSMV and TriMV in tandem and of an intron sequence in the loop. The derived RNAi element was cloned into a binary vector and was used to transform spring wheat genotype CB037. Phenotyping of T₁ lineages across eight independent transgenic events for resistance revealed that i) two of the transgenic events provided resistance to WSMV and TriMV, ii) four events provided resistance to either WSMV or TriMV, and iii) no resistance was found in two other events. T₂ populations derived from the two events classified as dual-resistant were subsequently monitored for stability of the resistance phenotype through the T₄ generation. The resistance phenotype in these events was temperature-dependent, with a complete dual resistance at temperatures ≥25°C and an increasingly susceptible response at temperatures below 25°C. Northern blot hybridization of total RNA from transgenic wheat revealed that virus-specific small RNAs (vsRNAs) accumulated progressively with an increase in temperature, with no detectable levels of vsRNA accumulation at 20°C. Thus, the resistance phenotype of wheat harboring an RNAi element was correlated with accumulation of vsRNAs, and the generation of vsRNAs can be used as a molecular marker for the prediction of resistant phenotypes of transgenic plants at a specific temperature.

Recombinase polymerase amplification assay for rapid detection of maize chlorotic mottle virus in maize

Maize chlorotic mottle virus (MCMV), an important quarantine virus, causes lethal necrosis in maize when coinfected with a potyvirid, which is seriously threatening the production of maize worldwide. In this study, recombinase polymerase amplification (RPA), a novel isothermal DNA amplification and detection technique, was developed to detect MCMV in maize crops. A pair of specific primers was designed based on the conserved sequences of the MCMV coat protein region. The RT-RPA assay was carried out as an isothermal reaction at 38 °C that was complete within 30 min, and no cross-reactivity was detected with other viruses infecting maize in China. The limit of detection of the RT-RPA assay was tenfold lower than that of ordinary RT-PCR. Moreover, this method was successfully applied to test field-collected samples. The newly developed RT-RPA assay offers a reliable, sensitive and efficient method for rapid detection of MCMV in maize in equipment-limited diagnostic laboratories and on-site facilities.

Maize lethal necrosis and the molecular basis of variability in concentrations of the causal viruses in co-infected maize plant

Maize lethal necrosis (MLN) disease is new to Africa. First report was in Kenya in 2012, since then the disease has rapidly spread to most parts of eastern and central Africa region including Tanzania, Burundi, DRC Congo, Rwanda, Uganda, Ethiopia and similar symptoms were observed in South Sudan. Elsewhere, the disease was caused by infection of Maize Chlorotic Mottle Virus (MCMV) in combination with any of the potyviruses namely; maize dwarf mosaic virus (MDMV), sugarcane mosaic virus (SCMV) and tritimovirus wheat streak mosaic virus (WSMV). In Africa, the disease occurs due to combined infections of maize by MCMV and SCMV, leading to severe yield losses. Efforts to address the disease spread have been ongoing. Serological techniques including enzyme-linked immuno-sorbent assay (ELISA), polymerase chain reaction (PCR), genome-wide association (GWAS) mapping and next generation sequencing have been effectively used to detect and characterize MLN causative pathogens. Various management strategies have been adapted to control MLN including use of resistant varieties, phytosanitary measures and better cultural practices. This review looks at the current knowledge on MLN causative viruses, genetic architecture and molecular basis underlying their synergistic interactions. Lastly, some research gaps towards MLN management will be identified. The information gathered may be useful for developing strategies towards future MLN management and maize improvement in Africa.

Asymmetry in Synergistic Interaction Between Wheat streak mosaic virus and Triticum mosaic virus in Wheat

Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV), distinct members in the family Potyviridae, are economically important wheat-infecting viruses in the Great Plains region. Previously, we reported that coinfection of wheat by WSMV and TriMV caused disease synergism with increased concentration of both viruses. The mechanisms of synergistic interaction between WSMV and TriMV and the effects of prior infection of wheat by either of these "synergistically interacting partner" (SIP) viruses on the establishment of local and systemic infection by the other SIP virus are not known. In this study, using fluorescent protein-tagged viruses, we found that prior infection of wheat by WSMV or TriMV negatively affected the onset and size of local foci elicited by subsequent SIP virus infection compared with those in buffer-inoculated wheat. These data revealed that prior infection of wheat by an SIP virus has no measurable advantage for another SIP virus on the initiation of infection and cell-to-cell movement. In TriMV-infected wheat, WSMV exhibited accelerated long-distance movement and increased accumulation of genomic RNAs compared with those in buffer-inoculated wheat, indicating that TriMV-encoded proteins complemented WSMV for efficient systemic infection. In contrast, TriMV displayed delayed systemic infection in WSMV-infected wheat, with fewer genomic RNA copies in early stages of infection compared with those in buffer-inoculated wheat. However, during late stages of infection, TriMV accumulation in WSMV-infected wheat increased rapidly with accelerated long-distance movement compared with those in buffer-inoculated wheat. Taken together, these data suggest that interactions between synergistically interacting WSMV and TriMV are asymmetrical; thus, successful establishment of synergistic interaction between unrelated viruses will depend on the order of infection of plants by SIP viruses.

Wheat streak mosaic virus: a century old virus with rising importance worldwide

Wheat streak mosaic virus (WSMV) causes wheat streak mosaic, a disease of cereals and grasses that threatens wheat production worldwide. It is a monopartite, positive-sense, single-stranded RNA virus and the type member of the genus Tritimovirus in the family Potyviridae. The only known vector is the wheat curl mite (WCM, Aceria tosichella), recently identified as a species complex of biotypes differing in virus transmission. Low rates of seed transmission have been reported. Infected plants are stunted and have a yellow mosaic of parallel discontinuous streaks on the leaves. In the autumn, WCMs move from WSMV-infected volunteer wheat and other grass hosts to newly emerged wheat and transmit the virus which survives the winter within the plant, and the mites survive as eggs, larvae, nymphs or adults in the crown and leaf sheaths. In the spring/summer, the mites move from the maturing wheat crop to volunteer wheat and other grass hosts and transmit WSMV, and onto newly emerged wheat in the fall to which they transmit the virus, completing the disease cycle. WSMV detection is by enzyme-linked immunosorbent assay (ELISA), reverse transcription-polymerase chain reaction (RT-PCR) or quantitative RT-PCR (RT-qPCR). Three types of WSMV are recognized: A (Mexico), B (Europe, Russia, Asia) and D (USA, Argentina, Brazil, Australia, Turkey, Canada). Resistance genes Wsm1, Wsm2 and Wsm3 have been identified. The most effective, Wsm2, has been introduced into several wheat cultivars. Mitigation of losses caused by WSMV will require enhanced knowledge of the biology of WCM biotypes and WSMV, new or improved virus detection techniques, the development of resistance through traditional and molecular breeding, and the adaptation of cultural management tactics to account for climate change.

Maize Lethal Necrosis: An Emerging, Synergistic Viral Disease

Maize lethal necrosis (MLN) is a disease of maize caused by coinfection of maize with maize chlorotic mottle virus (MCMV) and one of several viruses from the Potyviridae, such as sugarcane mosaic virus, maize dwarf mosaic virus, Johnsongrass mosaic virus or wheat streak mosaic virus. The coinfecting viruses act synergistically to result in frequent plant death or severely reduce or negligible yield. Over the past eight years, MLN has emerged in sub-Saharan East Africa, Southeast Asia, and South America, with large impacts on smallholder farmers. Factors associated with MLN emergence include multiple maize crops per year, the presence of maize thrips ( Frankliniella williamsi), and highly susceptible maize crops. Soil and seed transmission of MCMV may also play significant roles in development and perpetuation of MLN epidemics. Containment and control of MLN will likely require a multipronged approach, and more research is needed to identify and develop the best measures.

Metagenomic analysis of viruses associated with maize lethal necrosis in Kenya

BACKGROUND

Maize lethal necrosis is caused by a synergistic co-infection of Maize chlorotic mottle virus (MCMV) and a specific member of the Potyviridae, such as Sugarcane mosaic virus (SCMV), Wheat streak mosaic virus (WSMV) or Johnson grass mosaic virus (JGMV). Typical maize lethal necrosis symptoms include severe yellowing and leaf drying from the edges. In Kenya, we detected plants showing typical and atypical symptoms. Both groups of plants often tested negative for SCMV by ELISA.

METHODS

We used next-generation sequencing to identify viruses associated to maize lethal necrosis in Kenya through a metagenomics analysis. Symptomatic and asymptomatic leaf samples were collected from maize and sorghum representing sixteen counties.

RESULTS

Complete and partial genomes were assembled for MCMV, SCMV, Maize streak virus (MSV) and Maize yellow dwarf virus-RMV (MYDV-RMV). These four viruses (MCMV, SCMV, MSV and MYDV-RMV) were found together in 30 of 68 samples. A geographic analysis showed that these viruses are widely distributed in Kenya. Phylogenetic analyses of nucleotide sequences showed that MCMV, MYDV-RMV and MSV are similar to isolates from East Africa and other parts of the world. Single nucleotide polymorphism, nucleotide and polyprotein sequence alignments identified three genetically distinct groups of SCMV in Kenya. Variation mapped to sequences at the border of NIb and the coat protein. Partial genome sequences were obtained for other four potyviruses and one polerovirus.

CONCLUSION

Our results uncover the complexity of the maize lethal necrosis epidemic in Kenya. MCMV, SCMV, MSV and MYDV-RMV are widely distributed and infect both maize and sorghum. SCMV population in Kenya is diverse and consists of numerous strains that are genetically different to isolates from other parts of the world. Several potyviruses, and possibly poleroviruses, are also involved.

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

RNA viruses have very compact genomes and so provide a unique opportunity to study how evolution works to optimize the use of very limited genomic information. A widespread viral strategy to solve this issue concerning the coding space relies on the expression of proteins with multiple functions. Members of the family Potyviridae, the most abundant group of RNA viruses in plants, offer several attractive examples of viral factors which play roles in diverse infection-related pathways. The Helper Component Proteinase (HCPro) is an essential and well-characterized multitasking protein for which at least three independent functions have been described: (i) viral plant-to-plant transmission; (ii) polyprotein maturation; and (iii) RNA silencing suppression. Moreover, multitudes of host factors have been found to interact with HCPro. Intriguingly, most of these partners have not been ascribed to any of the HCPro roles during the infectious cycle, supporting the idea that this protein might play even more roles than those already established. In this comprehensive review, we attempt to summarize our current knowledge about HCPro and its already attributed and putative novel roles, and to discuss the similarities and differences regarding this factor in members of this important viral family.

Johnsongrass mosaic virus Contributes to Maize Lethal Necrosis in East Africa

Maize lethal necrosis (MLN), a severe virus disease of maize, has emerged in East Africa in recent years with devastating effects on production and food security where maize is a staple subsistence crop. In extensive surveys of MLN-symptomatic plants in East Africa, sequences of Johnsongrass mosaic virus (JGMV) were identified in Uganda, Kenya, Rwanda, and Tanzania. The East African JGMV is distinct from previously reported isolates and infects maize, sorghum, and Johnsongrass but not wheat or oat. This isolate causes MLN in coinfection with Maize chlorotic mottle virus (MCMV), as reported for other potyviruses, and was present in MLN-symptomatic plants in which the major East African potyvirus, Sugarcane mosaic virus (SCMV), was not detected. Virus titers were compared in single and coinfections by quantitative reverse transcription-polymerase chain reaction. MCMV titer increased in coinfected plants whereas SCMV, Maize dwarf mosaic virus, and JGMV titers were unchanged compared with single infections at 11 days postinoculation. Together, these results demonstrate the presence of an East African JGMV that contributes to MLN in the region.

One-step reverse transcription loop-mediated isothermal amplification for the detection of Maize chlorotic mottle virus in maize

Maize chlorotic mottle virus (MCMV) is spreading in many regions worldwide, causing maize lethal necrosis when co-infected with a potyvirid. In this study, one-step reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed to detect MCMV in maize. A set of four specific primers was designed based on the conserved coat protein gene sequences of MCMV. The RT-LAMP could be completed within 60min under isothermal condition at 63°C. The sensitivity test showed that the RT-LAMP was about 10-fold more sensitive than RT-PCR and no cross-reactivity was detected with other viral pathogens infecting maize in China. Moreover, the results of RT-LAMP could be visually inspected by SYBR Green I staining in a closed-tube, facilitating high-throughput application of MCMV detection. This method was further verified by testing field-collected samples. These results suggested that the developed MCMV RT-LAMP technique is a rapid, efficient and sensitive method which could be used as a routine screen for MCMV infection.

Synergistic infection of two viruses MCMV and SCMV increases the accumulations of both MCMV and MCMV-derived siRNAs in maize

The co-infection of Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) can cause maize lethal necrosis. However, the mechanism underlying the synergistic interaction between these two viruses remains elusive. In this study, we found that the co-infection of MCMV and SCMV increased the accumulation of MCMV. Moreover, the profiles of virus-derived siRNAs (vsiRNAs) from MCMV and SCMV in single- and co-infected maize plants were obtained by high-throughput sequencing. Our data showed that synergistic infection of MCMV and SCMV increased remarkably the accumulation of vsiRNAs from MCMV, which were mainly 22 and 21 nucleotides in length. The single-nucleotide resolution maps of vsiRNAs revealed that vsiRNAs were almost continuously but heterogeneously distributed throughout MCMV and SCMV genomic RNAs, respectively. Moreover, we predicted and annotated dozens of host transcript genes targeted by vsiRNAs. Our results also showed that maize DCLs and several AGOs RNAs were differentially accumulated in maize plants with different treatments (mock, single or double inoculations), which were associated with the accumulation of vsiRNAs. Our findings suggested possible roles of vsiRNAs in the synergistic interaction of MCMV and SCMV in maize plants.

Evolutionary and Epidemiological Implications of Multiple Infection in Plants

Recent methodological advances have uncovered tremendous microbial diversity cohabiting in the same host plant, and many of these microbes cause disease. In this review we highlight how the presence of other pathogen species, or other pathogen genotypes, within a plant can affect key components of host-pathogen interactions: (i) within-plant virulence and pathogen accumulation, through direct and host-mediated mechanisms; (ii) evolutionary trajectories of pathogen populations, through virulence evolution, generation of novel genetic combinations, and maintenance of genetic diversity; and (iii) disease dynamics, with multiple infection likely to render epidemics more devastating. The major future challenges are to couple a community ecology approach with a molecular investigation of the mechanisms operating under coinfection and to evaluate the evolution and effectiveness of resistance within a coinfection framework.

Insight into the three-dimensional structure of maize chlorotic mottle virus revealed by Cryo-EM single particle analysis

Maize chlorotic mottle virus (MCMV) is the only member of the Machlomovirus genus in the family Tombusviridae. Here, we obtained the Cryo-EM structure of MCMV by single particle analysis with most local resolution at approximately 4 Å. The Cα backbone was built based on residues with bulky side chains. The resolved C-terminus of the capsid protein subunit and obvious openings at the 2-fold axis demonstrated the compactness of the asymmetric unit, which indicates an important role in the stability of MCMV. The Asp116 residue from each subunit around the 5-fold and 3-fold axes contributed to the negative charges in the centers of the pentamers and hexamers, which might serve as a solid barrier against the leakage of genomic RNA. Finally, the loops most exposed on the surface were analyzed and are proposed to be potential functional sites related to MCMV transmission.

Impact of Wheat streak mosaic virus and Triticum mosaic virus Coinfection of Wheat on Transmission Rates by Wheat Curl Mites

Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) are transmitted by the wheat curl mite (WCM, Aceria tosichella), and coinfections of wheat by these viruses are common in the field. Previous work has shown that mite genotypes vary in their ability to transmit TriMV. However, the degree to which coinfection of wheat modifies WCM vector competence has not been studied. The objective was to determine whether mite genotypes differed in virus transmission ability when feeding on wheat coinfected by WSMV and TriMV. First, WCM genotype type 2 was used to determine virus transmission rates from mock-, WSMV-, TriMV-, and coinfected wheat plants. Transmission rates were determined by using single-mite transfers from replicated source plants. Coinfection reduced WSMV transmission by type 2 WCM from 50 to 35.6%; however, coinfection increased TriMV transmission from 43.3 to 56.8%. Mite survival on single-mite transfer test plants indicates that the reduction in WSMV transmission may result from poor mite survival when TriMV is present. In a second study, two separate colonies of WCM genotype type 1 were tested to assess the impact of coinfection on transmission. Type 1 mites did not transmit TriMV from coinfected plants but the two colonies varied in transmission rates for WSMV (20.9 to 36.5%). Even though these changes in mite transmission rates are moderate, they help explain the high relative incidence of TriMV-positive plants that are coinfected with WSMV in field observations. These findings begin to demonstrate the complicated interactions found in this mite-virus complex.

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.

Dynamics of small RNA profiles of virus and host origin in wheat cultivars synergistically infected by Wheat streak mosaic virus and Triticum mosaic virus: virus infection caused a drastic shift in the endogenous small RNA profile

Co-infection of wheat (Triticum aestivum L.) by Wheat streak mosaic virus (WSMV, a Tritimovirus) and Triticum mosaic virus (TriMV, a Poacevirus) of the family Potyviridae causes synergistic interaction. In this study, the effects of the synergistic interaction between WSMV and TriMV on endogenous and virus-derived small interfering RNAs (vsiRNAs) were examined in susceptible ('Arapahoe') and temperature-sensitive resistant ('Mace') wheat cultivars at 18°C and 27°C. Single and double infections in wheat caused a shift in the profile of endogenous small RNAs from 24 nt being the most predominant in healthy plants to 21 nt in infected wheat. Massive amounts of 21 and 22 nt vsiRNAs accumulated in singly and doubly infected Arapahoe at both temperatures and in Mace at 27°C but not 18°C. The plus- and minus-sense vsiRNAs were distributed throughout the genomic RNAs in Arapahoe at both temperature regimens and in Mace at 27°C, although some regions served as hot-spots, spawning an excessive number of vsiRNAs. The vsiRNA peaks were conserved among cultivars, suggesting that the Dicer-like enzymes in susceptible and resistant cultivars similarly accessed the genomic RNAs of WSMV or TriMV. Accumulation of large amounts of vsiRNAs in doubly infected plants suggests that the silencing suppressor proteins encoded by TriMV and WSMV do not prevent the formation of vsiRNAs; thus, the synergistic effect observed is independent from RNA-silencing mediated vsiRNA biogenesis. The high-resolution map of endogenous and vsiRNAs from WSMV- and/or TriMV-infected wheat cultivars may form a foundation for understanding the virus-host interactions, the effect of synergistic interactions on host defense, and virus resistance mechanisms in wheat.

The asparagine residue in the FRNK box of potyviral helper-component protease is critical for its small RNA binding and subcellular localization

The multifunctional potyviral helper-component protease (HcPro) contains variable regions with some functionally conserved domains, such as the FRNK box. Natural variants occur at the FRNK box, a conserved central domain, known for its role in RNA binding and RNAi suppression activities, although no dominant natural variants for the N(182) residue are known to occur. Here, a mutant at HcPro(N182L) was developed to investigate its role in natural populations. Using in vitro studies, we found an increase in the small RNA (sRNA) binding potential of HcPro(N182L) without affecting its protein-protein interaction properties, suggesting that the presence of N(182) is critical to maintain threshold levels of sRNAs, but does not interfere in the self-interaction of HcPro. Furthermore, we found that expression of HcPro(N182L) in Nicotiana benthamiana affected plant growth. Transient expression of HcPro(N182L) induced reporter gene expression in 16c GFP transgenic plants more than HcPro did, suggesting that replacement of asparagine in the FRNK box favours RNA silencing suppression. HcPro was found to be distributed in the nucleus and cytoplasm, whereas HcPro(N182L) was observed only in cytoplasmic inclusion bodies in N. benthamiana leaves, when fused to a GFP tag and expressed by agro-infiltration, suggesting mutation favours oligomerization of HcPro. These findings suggest that amino acid N(182) of the conserved FRNK box may regulate RNA silencing mechanisms, and is required for maintenance of the subcellular localization of the protein for its multi-functionality. Hence, the N(182) residue of the FRNK box seems to be indispensable for potyvirus infection during evolution.

Monoclonal antibody-based serological methods for maize chlorotic mottle virus detection in China

Maize chlorotic mottle virus (MCMV) infects maize plants and causes significant losses in corn production worldwide. In this study, purified MCMV particles were used as the immunogen to produce monoclonal antibodies (MAbs) and polyclonal antibodies (PAbs). Four murine MAbs (4B8, 8C11, 6F4, and 9G1) against MCMV were obtained through the hybridoma technology. The triple antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA), dot-immunobinding assay (DIBA), and immunocapture reverse transcription-polymerase chain reaction (IC-RT-PCR) using the MAb 4B8 were then developed for sensitive, specific, and rapid detection of MCMV in fields. MCMV could be detected in infected leaf crude extracts at dilutions of 1:327680, 1:64000, and 1:3276800 (w/v, g/ml) by TAS-ELISA, DIBA, and IC-RT-PCR, respectively. One hundred and sixty-one maize field samples showing virus-like symptoms and sixty-nine symptomless maize field samples from ten different provinces of China were collected and screened for the presence of MCMV using the established serological methods. A phylogenetic tree was constructed based on the full length CP genes and Chinese MCMV isolates formed one branch with Thailand isolates. The detection results demonstrated that MCMV is one of most prevalent viruses infecting maize in the Yunnan and Sichuan provinces of China.

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.

Rationale for developing new virus vectors to analyze gene function in grasses through virus-induced gene silencing

The exploding availability of genome and EST-based sequences from grasses requires a technology that allows rapid functional analysis of the multitude of genes that these resources provide. There are several techniques available to determine a gene's function. For gene knockdown studies, silencing through RNAi is a powerful tool. Gene silencing can be accomplished through stable transformation or transient expression of a fragment of a target gene sequence. Stable transformation in rice, maize, and a few other species, although routine, remains a relatively low-throughput process. Transformation in other grass species is difficult and labor-intensive. Therefore, transient gene silencing methods including Agrobacterium-mediated and virus-induced gene silencing (VIGS) have great potential for researchers studying gene function in grasses. VIGS in grasses already has been used to determine the function of genes during pathogen challenge and plant development. It also can be used in moderate-throughput reverse genetics screens to determine gene function. However, the number of viruses modified to serve as silencing vectors in grasses is limited, and the silencing phenotype induced by these vectors is not optimal: the phenotype being transient and with moderate penetration throughout the tissue. Here, we review the most recent information available for VIGS in grasses and summarize the strengths and weaknesses in current virus-grass host systems. We describe ways to improve current virus vectors and the potential of other grass-infecting viruses for VIGS studies. This work is necessary because VIGS for the foreseeable future remains a higher throughput and more rapid system to evaluate gene function than stable transformation.

Relative Susceptibility Among Alternative Host Species Prevalent in the Great Plains to Wheat streak mosaic virus

Wild grasses, crops, and grassy weeds are known to host Wheat streak mosaic virus (WSMV) and its vector, the wheat curl mite (WCM). Their relative importance as a source of WSMV was evaluated. A survey of small-grain fields throughout Montana was conducted between 2008 and 2009. Cheatgrass was the most prevalent grassy weed and the most frequent viral host, with 6% infection by WSMV in 2008 (n = 125) and 15% in 2009 (n = 358). By mechanically inoculating plants with WSMV in the greenhouse, the highest susceptibility was found in rye brome (52.1%), jointed goatgrass (80.9%), and wild oat (53.9%. Quackgrass, not previously reported as a host, was susceptible to WSMV (12.7%). Mite transmission efficiency from susceptible grass species was lower than from wheat, and grass species must be a host for both WSMV and the WCM to serve as a virus source. WCM transmission was more efficient than mechanical transmission. Overall, results indicate that grass species can serve as a viral reservoir, regional variation in a weed species' susceptibility to WSMV cannot explain geographic variation in epidemic intensity, and crop species and closely related weeds (e.g., jointed goatgrass) remain the best reservoirs for both WSMV and the WCM.

Effects of Single and Double Infections of Winter Wheat by Triticum mosaic virus and Wheat streak mosaic virus on Yield Determinants

Triticum mosaic virus (TriMV) is a recently discovered virus infecting wheat (Triticum aestivum) in the Great Plains region of the United States. It is transmitted by wheat curl mites (Aceria tosichella) which also transmit Wheat streak mosaic virus (WSMV) and Wheat mosaic virus. In a greenhouse study, winter wheat 'Millennium' (WSMV susceptible) and 'Mace' (WSMV resistant) were mechanically inoculated with TriMV, WSMV, TriMV+WSMV, or sterile water at the two-leaf growth stage. At 28 days after inoculation, final chlorophyll meter (soil plant analysis development [SPAD]) readings, area under the SPAD progress curve (AUSPC), the number of tillers per plant, shoot and root weight, and total nitrogen and carbon content were determined. In Millennium, all measured variables were significantly reduced by single or double virus infections, with the greatest reductions occurring in the double-infection treatment. In Mace, only final SPAD readings, AUSPC, and total nitrogen were significantly reduced by single or double virus infections. There was a significant (P ≤ 0.05), positive linear relationship between SPAD readings and shoot weight in Millennium but not in Mace. The relationship between total nitrogen and shoot weight was positive, linear, and significant in both cultivars. The results from this study indicate that Mace, a WSMV-resistant cultivar, is also resistant to TriMV, and double infection of winter wheat by TriMV and WSMV exacerbates symptom expression and loss of biomass in susceptible cultivars.

Facilitative and antagonistic interactions between plant viruses in mixed infections

Mixed infections of plant viruses are common in nature, and a number of important virus diseases of plants are the outcomes of interactions between causative agents. Multiple infections lead to a variety of intrahost virus-virus interactions, many of which may result in the generation of variants showing novel genetic features, and thus change the genetic structure of the viral population. Hence, virus-virus interactions in plants may be of crucial significance for the understanding of viral pathogenesis and evolution, and consequently for the development of efficient and stable control strategies. The interactions between plant viruses in mixed infections are generally categorized as synergistic or antagonistic. Moreover, mixtures of synergistic and antagonistic interactions, creating usually unpredictable biological and epidemiological consequences, are likely to occur in plants. The mechanisms of some of these are still unknown. This review aims to bring together the current knowledge on the most commonly occurring facilitative and antagonistic interactions between related or unrelated viruses infecting the same host plant. The best characterized implications of these interactions for virus-vector-host relationships are included. The terms 'synergism' and 'helper dependence' for facilitative virus-virus interactions, and 'cross-protection' and 'mutual exclusion' for antagonistic interactions, are applied in this article.

Tritimovirus P1 functions as a suppressor of RNA silencing and an enhancer of disease symptoms

Wheat streak mosaic virus (WSMV) is an eriophyid mite-transmitted virus of the genus Tritimovirus, family Potyviridae. Complete deletion of helper component-proteinase (HC-Pro) has no effect on WSMV virulence or disease synergism, suggesting that a different viral protein suppresses RNA silencing. RNA silencing suppression assays using Nicotiana benthamiana 16C plants expressing GFP were conducted with each WSMV protein; only P1 suppressed RNA silencing. Accumulation of GFP siRNAs was markedly reduced in leaves infiltrated with WSMV P1 at both 3 and 6 days post infiltration relative to WSMV HC-Pro and the empty vector control. On the other hand, helper component-proteinase (HC-Pro) of two species in the mite-transmitted genus Rymovirus, family Potyviridae was demonstrated to be a suppressor of RNA silencing. Symptom enhancement assays were conducted by inoculating Potato virus X (PVX) onto transgenic N. benthamiana. Symptoms produced by PVX were more severe on transgenic plants expressing WSMV P1 or potyvirus HC-Pro compared to transgenic plants expressing GFP or WSMV HC-Pro.

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.

Real-time RT-PCR for detection of Raspberry bushy dwarf virus, Raspberry leaf mottle virus and characterizing synergistic interactions in mixed infections

Two TaqMan-based real-time One-Step RT-PCR assays were developed for the rapid and efficient detection of Raspberry bushy dwarf virus (RBDV) and Raspberry leaf mottle virus (RLMV), two of the most common raspberry viruses in North America and Europe. The primers and probes were designed from conserved fragments of the polymerase region of each virus and were effective for the detection of different isolates tested in this study. The RBDV assay amplified a 94bp amplicon and was able to detect as few as 30 viral copies. Whereas the RLMV assay amplified a 180bp amplicon and detected as few as 300 viral copies from plant and aphid RNA extracts. Both assays were significantly more sensitive than their corresponding conventional RT-PCR methods. The sensitivity of the RLMV assay was also tested on single aphids after a fixed acquisition access period (AAP). In addition, the assays revealed a novel synergistic interaction between the two viruses, where the concentration of RBDV was enhanced ∼400-fold when it occurred in combination with RLMV compared to its concentration in single infections. The significance of this finding and the importance of the development of real-time RT-PCR assays for the detection of RBDV and RLMV are discussed.

The N-terminal region of wheat streak mosaic virus coat protein is a host- and strain-specific long-distance transport factor

Understanding the genetics underlying host range differences among plant virus strains can provide valuable insights into viral gene functions and virus-host interactions. In this study, we examined viral determinants and mechanisms of differential infection of Zea mays inbred line SDp2 by Wheat streak mosaic virus (WSMV) isolates. WSMV isolates Sidney 81 (WSMV-S81) and Type (WSMV-T) share 98.7% polyprotein sequence identity but differentially infect SDp2: WSMV-S81 induces a systemic infection, but WSMV-T does not. Coinoculation and sequential inoculation of SDp2 with WSMV-T and/or WSMV-S81 did not affect systemic infection by WSMV-S81, suggesting that WSMV-T does not induce a restrictive defense response but that virus-encoded proteins may be involved in differential infection of SDp2. The viral determinant responsible for strain-specific host range was mapped to the N terminus of coat protein (CP) by systematic exchanges of WSMV-S81 sequences with those of WSMV-T and by reciprocal exchanges of CP or CP codons 1 to 74. Green fluorescent protein (GFP)-tagged WSMV-S81 with CP or CP residues 1 to 74 from WSMV-T produced similar numbers of infection foci and genomic RNAs and formed virions in inoculated leaves as those produced with WSMV-S81, indicating that failure to infect SDp2 systemically is not due to defects in replication, cell-to-cell movement, or virion assembly. However, these GFP-tagged hybrids showed profound defects in long-distance transport of virus through the phloem. Furthermore, we found that four of the five differing amino acids in the N terminus of CP between the WSMV-S81 and WSMV-T isolates were collectively involved in systemic infection of SDp2. Taken together, these results demonstrate that the N-terminal region of tritimoviral CP functions in host- and strain-specific long-distance movement.

Real-time TaqMan RT-PCR for detection of maize chlorotic mottle virus in maize seeds

Maize chlorotic mottle virus (MCMV) causes corn lethal necrosis disease, and can be transmitted through infected maize seeds. It remains a challenge to detect this virus in the seeds to prevent its introduction and infection. For this purpose, a real-time TaqMan RT-PCR procedure for efficient detection of MCMV was developed. The sensitivity of the method was 4 fg of total RNA or 25 copies of RNA transcripts, which was approximately ten-fold higher than conventional RT-PCR gel electrophoresis method. The successful detection of MCMV in maize seeds suggested the feasibility of this procedure for routine testing.

Detection of Cereal yellow dwarf virus using small interfering RNAs and enhanced infection rate with Cocksfoot streak virus in wild cocksfoot grass (Dactylis glomerata)

Small RNA sequences were obtained from leaf extracts of wild Dactylis glomerata (cocksfoot grass) using deep sequencing (454 Life Sciences, Roche Diagnostics), and were screened against virus sequences in GenBank using a local BLASTn search program (BioEdit). Putative small interfering (si)RNAs complementary in sequence to Cereal yellow dwarf virus (CYDV, genus Luteovirus) genomes were identified. Primer sequences were made against the "high scoring" siRNA sequences and RT-PCR was used to amplify a 438 bp CYDV fragment in total RNA extracts from D. glomerata leaves. Sequencing of the RT-PCR product confirmed the occurrence of a previously undescribed CYDV population with phylogenetic affinity to CYDV-RPS. In D. glomerata the CYDV infection rates were 42.3% (n=78) in 2008 and 50.0% (n=48) in 2009. Specific RT-PCR tests also showed that this D. glomerata population harboured Cocksfoot streak virus (CSV, genus Potyvirus). Dual infections by these viruses were observed in 20.5-22.9% of all plants tested in 2008-2009. Interestingly, infections of either CYDV or CSV enhanced the occurrence of the other virus in individual grasses. Opportunities are discussed for using siRNA sequencing approaches in virus survey and other ecology studies under field conditions.

Wheat cultivar-specific disease synergism and alteration of virus accumulation during co-infection with Wheat streak mosaic virus and Triticum mosaic virus

Triticum mosaic virus (TriMV), the type member of the newly proposed Poacevirus genus, and Wheat streak mosaic virus (WSMV), the type member of Tritimovirus genus of the family Potyviridae, infect wheat naturally in the Great Plains and are transmitted by wheat curl mites. In this study, we examined the ability of these viruses to infect selected cereal hosts, and found several differential hosts between TriMV and WSMV. Additionally, we examined the interaction between WSMV and TriMV in three wheat cultivars at two temperature regimens (19 and 20 to 26 degrees C), and quantified the virus concentration in single and double infections by real-time reverse-transcription polymerase chain reaction. Double infections in wheat cvs. Arapahoe and Tomahawk at both temperature regimens induced disease synergism with severe leaf deformation, bleaching, and stunting, with a 2.2- to 7.4-fold increase in accumulation of both viruses over single infections at 14 days postinoculation (dpi). However, at 28 dpi, in double infections at 20 to 26 degrees C, TriMV concentration was increased by 1.4- to 1.8-fold in Arapahoe and Tomahawk but WSMV concentration was decreased to 0.5-fold. WSMV or TriMV replicated poorly in Mace at 19 degrees C with no synergistic interaction whereas both viruses accumulated at moderate levels at 20 to 26 degrees C and induced mild to moderate disease synergism in doubly infected Mace compared with Arapahoe and Tomahawk. Co-infections in Mace at 20 to 26 degrees C caused increased TriMV accumulation at 14 and 28 dpi by 2.6- and 1.4-fold and WSMV accumulated at 0.5- and 1.6-fold over single infections, respectively. Our data suggest that WSMV and TriMV induced cultivar-specific disease synergism in Arapahoe, Tomahawk, and Mace, and these findings could have several implications for management of wheat viruses in the Great Plains.

Cassava brown streak virus (Potyviridae) encodes a putative Maf/HAM1 pyrophosphatase implicated in reduction of mutations and a P1 proteinase that suppresses RNA silencing but contains no HC-Pro

The complete positive-sense single-stranded RNA genome of Cassava brown streak virus (CBSV; genus Ipomovirus; Potyviridae) was found to consist of 9,069 nucleotides and predicted to produce a polyprotein of 2,902 amino acids. It was lacking helper-component proteinase but contained a single P1 serine proteinase that strongly suppressed RNA silencing. Besides the exceptional structure of the 5'-proximal part of the genome, CBSV also contained a Maf/HAM1-like sequence (678 nucleotides, 226 amino acids) recombined between the replicase and coat protein domains in the 3'-proximal part of the genome, which is highly conserved in Potyviridae. HAM1 was flanked by consensus proteolytic cleavage sites for ipomovirus NIaPro cysteine proteinase. Homology of CBSV HAM1 with cellular Maf/HAM1 pyrophosphatases suggests that it may intercept noncanonical nucleoside triphosphates to reduce mutagenesis of viral RNA.

N-terminal of Papaya ringspot virus type-W (PRSV-W) helper component proteinase (HC-Pro) is essential for PRSV systemic infection in zucchini

The Papaya ringspot virus (PRSV) is one of the limiting factors affecting papaya and cucurbits production worldwide. PRSV belongs to the potyvirus genus which consists of 30% of known plant viruses. Two serological closely related strains, namely type-P and -W, have been reported. PRSV type-P infects both papaya and cucurbits, while type-W infects only cucurbits. The genome of PRSV Thailand isolate consists of a (+) RNA molecule of 10323 nucleotides, which is first translated into a single polypeptide and further cleaved by three viral encoded proteases into ten gene products. Helper-component proteinase (HC-Pro), which is encoded by the 2nd cistron of the potyviral genome, has been implicated in aphid transmission, viral movement, viral replication and suppression of host viral defense system. Studies of the Tobacco etch virus (TEV), Lettuce mosaic virus (LMV), Onion yellow dwarf virus (OYDV) and Wheat streak mosaic virus (WSMV) indicate that the N-terminal of HC-Pro is dispensable for systemic infection in their respective hosts. However, deletion analysis of the Tobacco vein mottling virus (TVMV) indicates otherwise. In this study, we examined whether HC-Pro is essential for PRSV systemic infection in cucurbits and the role of its N-terminal in systemic infection. Our results indicated that HC-Pro is indispensable for PRSV infection in zucchini. Deletion analysis of PRSV HC-Pro showed that deletion of as few as 54 amino acids at the N-terminal of HC-Pro completely abolished the infectivity of the corresponding cDNA clone. Therefore, it is proposed that the N-terminal of HC-Pro is involved in systemic infection of PRSV, in addition to its conserved function in aphid transmission.

Complete nucleotide sequence of a maize chlorotic mottle virus isolate from Nebraska

The complete genome of a maize chlorotic mottle virus isolate from Nebraska (MCMV-NE) was cloned and sequenced. The MCMV-NE genome consists of 4,436 nucleotides and shares 99.5 nucleotide sequence identity with an MCMV isolate from Kansas (MCMV-KS). Of 22 polymorphic sites, most resulted from transition with a clear bias for U to C and C to U substitutions. The MCMV-NE genome was assembled into a single plasmid insert and used as a template to transcribe RNA in vitro. As RNA transcribed from the cloned MCMV-NE genome was infectious to maize plants, sequence differences between MCMV-NE and MCMV-KS are most likely neutral with respect to pathogenicity and virulence.