Rescue of the first alphanucleorhabdovirus entirely from cloned complementary DNA: An efficient vector for systemic expression of foreign genes in maize and insect vectors

Recent reverse genetics technologies have enabled genetic manipulation of plant negative-strand RNA virus (NSR) genomes. Here, we report construction of an infectious clone for the maize-infecting Alphanucleorhabdovirus maydis, the first efficient NSR vector for maize. The full-length infectious clone was established using agrobacterium-mediated delivery of full-length maize mosaic virus (MMV) antigenomic RNA and the viral core proteins (nucleoprotein N, phosphoprotein P, and RNA-directed RNA polymerase L) required for viral transcription and replication into Nicotiana benthamiana. Insertion of intron 2 ST-LS1 into the viral L gene increased stability of the infectious clone in Escherichia coli and Agrobacterium tumefaciens. To monitor virus infection in vivo, a green fluorescent protein (GFP) gene was inserted in between the N and P gene junctions to generate recombinant MMV-GFP. Complementary DNA (cDNA) clones of MMV-wild type (WT) and MMV-GFP replicated in single cells of agroinfiltrated N. benthamiana. Uniform systemic infection and high GFP expression were observed in maize inoculated with extracts of the infiltrated N. benthamiana leaves. Insect vectors supported virus infection when inoculated via feeding on infected maize or microinjection. Both MMV-WT and MMV-GFP were efficiently transmitted to maize by planthopper vectors. The GFP reporter gene was stable in the virus genome and expression remained high over three cycles of transmission in plants and insects. The MMV infectious clone will be a versatile tool for expression of proteins of interest in maize and cross-kingdom studies of virus replication in plant and insect hosts.

Transmission, localization, and infectivity of seedborne maize chlorotic mottle virus

Maize lethal necrosis is a destructive virus disease of maize caused by maize chlorotic mottle virus (MCMV) in combination with a virus in the family Potyviridae. Emergence of MLN is typically associated with the introduction of MCMV or its vectors and understanding its spread through seed is critical for disease management. Previous studies suggest that although MCMV is detected on seed, the seed transmission rate of this virus is low. However, mechanisms influencing its transmission are poorly understood. Elucidating these mechanisms is crucial for informing strategies to prevent spread on contaminated seed. In this study, we evaluated the rate of MCMV seed transmission using seed collected from plants that were artificially inoculated with MCMV isolates from Hawaii and Kenya. Grow-out tests indicated that MCMV transmission through seed was rare, with a rate of 0.004% among the more than 85,000 seed evaluated, despite detection of MCMV at high levels in the seed lots. To understand factors that limit transmission from seed, MCMV distribution in seed tissues was examined using serology and immunolocalization. The virus was present at high levels in maternal tissues, the pericarp and pedicel, but absent from filial endosperm and embryo seed tissues. The ability to transmit MCMV from seed to uninfected plants was tested to evaluate virus viability. Transmission was negatively associated with both seed maturity and moisture content. Transmission of MCMV from infested seed dried to less than 15% moisture was not detected, suggesting proper handling could be important for minimizing spread of MCMV through seed.

Soybean Aphid (Hemiptera: Aphididae) Feeding Behavior is Largely Unchanged by Soybean Mosaic Virus but Significantly Altered by the Beetle-Transmitted Bean Pod Mottle Virus

The soybean aphid (Aphis glycines Matsumura) is an economically important invasive pest of soybean. In addition to damage caused by soybean aphid feeding on the phloem sap, this insect also transmits many plant viruses, including soybean mosaic virus (SMV). Previous work has shown that plant viruses can change plant host phenotypes to alter the behavior of their insect vectors to promote virus spread, known as the vector manipulation hypothesis. In this study, we used electropenetography (EPG) to examine the effects of two plant viruses on soybean aphid feeding behavior: SMV, which is transmitted by many aphid species including the soybean aphid, and bean pod mottle virus (BPMV), which is transmitted by chrysomelid and some coccinellid beetles but not aphids. These two viruses often co-occur in soybean production and can act synergistically. Surprisingly, our results showed little to no effect of SMV on soybean aphid feeding behaviors measured by EPG, but profound differences were observed in aphids feeding on BPMV-infected plants. Aphids took longer to find the vascular bundle of BPMV-infected plants, and once found, spent more time entering and conditioning the phloem than ingesting phloem sap. Interestingly, these observed alterations are similar to those of aphids feeding on insect-resistant soybean plants. The cause of these changes in feeding behavior is not known, and how they impact virus transmission and soybean aphid populations in the field will require further study.

VIGE: virus-induced genome editing for improving abiotic and biotic stress traits in plants

Agricultural production is hampered by disease, pests, and environmental stresses. To minimize yield loss, it is important to develop crop cultivars with resistance or tolerance to their respective biotic and abiotic constraints. Transformation techniques are not optimized for many species and desirable cultivars may not be amenable to genetic transformation, necessitating inferior cultivar usage and time-consuming introgression through backcrossing to the preferred variety. Overcoming these limitations will greatly facilitate the development of disease, insect, and abiotic stress tolerant crops. One such avenue for rapid crop improvement is the development of viral systems to deliver CRISPR/Cas-based genome editing technology to plants to generate targeted beneficial mutations. Viral delivery of genomic editing constructs can theoretically be applied to span the entire host range of the virus utilized, circumventing the challenges associated with traditional transformation and breeding techniques. Here we explore the types of viruses that have been optimized for CRISPR/Cas9 delivery, the phenotypic outcomes achieved in recent studies, and discuss the future potential of this rapidly advancing technology.

Detection of Diverse Maize Chlorotic Mottle Virus Isolates in Maize Seed

Maize chlorotic mottle virus (MCMV) has driven the emergence of maize lethal necrosis worldwide, where it threatens maize production in areas of East Africa, South America, and Asia. It is thought that MCMV transmission through seed may be important for introduction of the virus in new regions. Identification of infested seed lots is critical for preventing the spread of MCMV through seed. Although methods for detecting MCMV in leaf tissue are available, diagnostic methods for its detection in seed lots are lacking. In this study, ELISA, RT-PCR, and RT-qPCR were adapted for detection of MCMV in maize seed. Purified virions of MCMV isolates from Kansas, Mexico, and Kenya were then used to determine the virus detection thresholds for each diagnostic assay. No substantial differences in response were detected among the isolates in any of the three assays. The RT-PCR and a SYBR Green-based RT-qPCR assays were >3,000 times more sensitive than commercial ELISA for MCMV detection. For ELISA using seed extracts, selection of positive and negative controls was critical, most likely because of relatively high backgrounds. Use of seed soak solutions in ELISA detected MCMV with similar sensitivity to seed extracts, produced minimal background, and required substantially less labor. ELISA and RT-PCR were both effective for detecting MCMV in seed lots from Hawaii and Kenya, with ELISA providing a reliable and inexpensive diagnostic assay that could be implemented routinely in seed testing facilities.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

A CRISPR/dCas9 toolkit for functional analysis of maize genes

BACKGROUND

The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 system has become a powerful tool for functional genomics in plants. The RNA-guided nuclease can be used to not only generate precise genomic mutations, but also to manipulate gene expression when present as a deactivated protein (dCas9).

RESULTS

In this study, we describe a vector toolkit for analyzing dCas9-mediated activation (CRISPRa) or inactivation (CRISPRi) of gene expression in maize protoplasts. An improved maize protoplast isolation and transfection method is presented, as well as a description of dCas9 vectors to enhance or repress maize gene expression.

CONCLUSIONS

We anticipate that this maize protoplast toolkit will streamline the analysis of gRNA candidates and facilitate genetic studies of important trait genes in this transformation-recalcitrant plant.

Maize lethal necrosis viruses and other maize viruses in Rwanda

Maize lethal necrosis (MLN) is emergent in East Africa, first reported in 2011 in Kenya, and is devastating to maize production in the region. MLN is caused by coinfection of maize with the emergent maize chlorotic mottle virus (MCMV) and any of several maize-infecting potyviruses endemic in East Africa and worldwide. Here, we examined the distribution of MCMV and sugarcane mosaic virus (SCMV), the major viruses contributing to MLN in Rwanda. These and other viruses in maize across Rwanda were further characterized by deep sequencing. When identified, MCMV had high titres and minimal sequence variability, whereas SCMV showed moderate titres and high sequence variability. Deep sequencing also identified maize streak virus and other maize-associated viruses, including a previously described polerovirus, maize yellow mosaic virus, and barley yellow dwarf virus, diverse maize-associated totiviruses, maize-associated pteridovirus, Zea mays chrysovirus 1, and a maize-associated betaflexivirus. Detection of each virus was confirmed in maize samples by reverse transcription polymerase chain reaction.

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.

Diverse Chromosomal Locations of Quantitative Trait Loci for Tolerance to Maize chlorotic mottle virus in Five Maize Populations

The recent rapid emergence of maize lethal necrosis (MLN), caused by coinfection of maize with Maize chlorotic mottle virus (MCMV) and a second virus usually from the family Potyviridae, is causing extensive losses for farmers in East Africa, Southeast Asia, and South America. Although the genetic basis of resistance to potyviruses is well understood in maize, little was known about resistance to MCMV. The responses of five maize inbred lines (KS23-5, KS23-6, N211, DR, and Oh1VI) to inoculation with MCMV, Sugarcane mosaic virus, and MLN were characterized. All five lines developed fewer symptoms than susceptible controls after inoculation with MCMV; however, the virus was detected in systemic leaf tissue from each of the lines similarly to susceptible controls, indicating that the lines were tolerant of MCMV rather than resistant to it. Except for KS23-5, the inbred lines also developed fewer symptoms after inoculation with MLN than susceptible controls. To identify genetic loci associated with MCMV tolerance, large F₂ or recombinant inbred populations were evaluated for their phenotypic responses to MCMV, and the most resistant and susceptible plants were genotyped by sequencing. One to four quantitative trait loci (QTL) were identified in each tolerant population using recombination frequency and positional mapping strategies. In contrast to previous studies of virus resistance in maize, the chromosomal positions and genetic character of the QTL were unique to each population. The results suggest that different, genotype-specific mechanisms are associated with MCMV tolerance in maize. These results will allow for the development of markers for marker-assisted selection of MCMV- and MLN-tolerant maize hybrids for disease control.

Identification of Soybean Host Plant Resistance to Brown Marmorated Stink Bugs in Maturity Group III Plant Introductions

Halyomorpha halys (Stål; Hemiptera: Pentatomidae), brown marmorated stink bug (BMSB), is a polyphagous nonnative insect first found in the United States in 1996. As of 2017, BMSB has been detected in 43 states and is a severe agricultural pest in mid-Atlantic states. On soybean, Glycine max (L.) Merr (Fabales: Fabaceae), damage from BMSB infestation ranges from puncture marks with seed discoloration and deformities to seed and pod abortion. Host plant resistance has been used for managing pest populations and mitigating soybean yield losses caused by neotropical stink bugs (Eushistus heros, Nezara viridula, and Piezodorus guildinii) in Brazil and on the U.S. Gulf Coast. We evaluated maturity group III plant introductions (PIs) for resistance to BMSB damage. In 2014, field cage choice tests of 106 PIs revealed a range of both BMSB damage incidence and severity. In field choice tests, PIs 085665 and 097139 showed the lowest incidence of BMSB damage and seed weight loss due to BMSB, while PIs 243532, 243540, and 567252 had the highest. In whole plant no-choice tests, PIs 085665 and 097139 also had high levels of resistance. However, PI 085665 had a higher incidence of damage but lower seed weight loss than PI 097139, which may suggest bimodal resistance. Moreover, PIs 085665 and 097139 are from Japan and North Korea, respectively, two geographically isolated countries where BMSB is native. Thus, further characterization of host plant resistance to BMSB in each of these lines may elucidate distinct mechanisms that could be synergistic if stacked in breeding lines.

Modeling Virus Coinfection to Inform Management of Maize Lethal Necrosis in Kenya

Maize lethal necrosis (MLN) has emerged as a serious threat to food security in sub-Saharan Africa. MLN is caused by coinfection with two viruses, Maize chlorotic mottle virus and a potyvirus, often Sugarcane mosaic virus. To better understand the dynamics of MLN and to provide insight into disease management, we modeled the spread of the viruses causing MLN within and between growing seasons. The model allows for transmission via vectors, soil, and seed, as well as exogenous sources of infection. Following model parameterization, we predict how management affects disease prevalence and crop performance over multiple seasons. Resource-rich farmers with large holdings can achieve good control by combining clean seed and insect control. However, crop rotation is often required to effect full control. Resource-poor farmers with smaller holdings must rely on rotation and roguing, and achieve more limited control. For both types of farmer, unless management is synchronized over large areas, exogenous sources of infection can thwart control. As well as providing practical guidance, our modeling framework is potentially informative for other cropping systems in which coinfection has devastating effects. Our work also emphasizes how mathematical modeling can inform management of an emerging disease even when epidemiological information remains scanty. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

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.

Feeding Behavior of Soybean Aphid (Hemiptera: Aphididae) Biotype 2 on Resistant and Susceptible Soybean

Host plant resistance to the soybean aphid, Aphis glycines Matsumura, is an effective means of controlling populations of this introduced pest species in the United States. Rag (Resistance to Aphis glycines) genes identified in soybean germplasm have been incorporated into commercial cultivars, but differential responses by soybean aphid biotypes to the Rag genes have made understanding mechanisms underlying resistance associated with Rag genes increasingly important. We compared the behavior of biotype 2 aphids on the resistant soybean line PI243540, which is a source of Rag2, and the susceptible cultivar Wyandot. Scanning electron microscopy revealed that the abaxial surface of leaves from resistant plants had a higher density of both long and glandulartrichomes, which might repel aphids, on veins. Time-lapse animation also suggested a repellent effect of resistant plants on aphids. However, electropenatography (EPG) indicated that the time to first probe did not differ between aphids feeding on the resistant and susceptible lines. EPG also indicated that fewer aphids feeding on resistant plants reached the phloem, and the time before reaching the phloem was much longer relative to susceptible soybean. For aphids that reached the phloem, there was no difference in either number of feedings or their duration in phloem. However, aphids feeding on resistant soybean had fewer prolonged phases of active salivation (E1) and many more pathway activities and non-probing intervals. Together, the feeding behavior of aphids suggested that Rag2 resistance has strong antixenosis effects, in addition to previously reported antibiosis, and was associated with epidermal and mesophyll tissues.

Recommended Reference Genes for Quantitative PCR Analysis in Soybean Have Variable Stabilities during Diverse Biotic Stresses

For real-time reverse transcription-PCR (qRT-PCR) in soybean, reference genes in different tissues, developmental stages, various cultivars, and under stress conditions have been suggested but their usefulness for research on soybean under various biotic stresses occurring in North-Central U.S. is not known. Here, we investigated the expression stabilities of ten previously recommended reference genes (ABCT, CYP, EF1A, FBOX, GPDH, RPL30, TUA4, TUB4, TUA5, and UNK2) in soybean under biotic stress from Bean pod mottle virus (BPMV), powdery mildew (PMD), soybean aphid (SBA), and two-spotted spider mite (TSSM). BPMV, PMD, SBA, and TSSM are amongst the most common pest problems on soybean in North-Central U.S. and other regions. Reference gene stability was determined using three software algorithms (geNorm, NormFinder, BestKeeper) and a web-based tool (RefFinder). Reference genes showed variability in their expression as well as stability across various stressors and the best reference genes were stress-dependent. ABCT and FBOX were found to be the most stable in soybean under both BPMV and SBA stress but these genes had only minimal to moderate stability during PMD and TSSM stress. Expression of TUA4 and CYP was found to be most stable during PMD stress; TUB4 and TUA4 were stable under TSSM stress. Under various biotic stresses on soybean analyzed, GPDH expression was found to be consistently unstable. For all biotic stressors on soybean, we obtained pairwise variation (V2/3) values less than 0.15 which suggested that combined use of the two most stable reference genes would be sufficient for normalization. Further, we demonstrated the utility of normalizing the qRT-PCR data for target genes using the most stable reference genes validated in current study. Following of the recommendations from our current study will enable an accurate and reliable normalization of qRT-PCR data in soybean under biotic stress.

Shifts in Buchnera aphidicola density in soybean aphids (Aphis glycines) feeding on virus-infected soybean

Vertically transmitted bacterial symbionts are common in arthropods. Aphids undergo an obligate symbiosis with Buchnera aphidicola, which provides essential amino acids to its host and contributes directly to nymph growth and reproduction. We previously found that newly adult Aphis glycines feeding on soybean infected with the beetle-transmitted Bean pod mottle virus (BPMV) had significantly reduced fecundity. We hypothesized that the reduced fecundity was attributable to detrimental impacts of the virus on the aphid microbiome, namely Buchnera. To test this, mRNA sequencing and quantitative real-time PCR were used to assay Buchnera transcript abundance and titre in A. glycines feeding on Soybean mosaic virus-infected, BPMV-infected, and healthy soybean for up to 14 days. Our results indicated that Buchnera density was lower and ultimately suppressed in aphids feeding on virus-infected soybean. While the decreased Buchnera titre may be associated with reduced aphid fecundity, additional mechanisms are probably involved. The present report begins to describe how interactions among insects, plants, and plant pathogens influence endosymbiont population dynamics.

Control of virus diseases in maize

Diseases caused by viruses are found throughout the maize-growing regions of the world and can cause significant losses for producers. In this review, virus diseases of maize and the pathogens that cause them are discussed. Factors leading to the spread of disease and measures for disease control are reviewed, as is our current knowledge of the genetics of virus resistance in this important crop.

Maize Lethal Necrosis (MLN), an Emerging Threat to Maize-Based Food Security in Sub-Saharan Africa

In sub-Saharan Africa, maize is a staple food and key determinant of food security for smallholder farming communities. Pest and disease outbreaks are key constraints to maize productivity. In September 2011, a serious disease outbreak, later diagnosed as maize lethal necrosis (MLN), was reported on maize in Kenya. The disease has since been confirmed in Rwanda and the Democratic Republic of Congo, and similar symptoms have been reported in Tanzania, Uganda, South Sudan, and Ethiopia. In 2012, yield losses of up to 90% resulted in an estimated grain loss of 126,000 metric tons valued at $52 million in Kenya alone. In eastern Africa, MLN was found to result from coinfection of maize with Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV), although MCMV alone appears to cause significant crop losses. We summarize here the results of collaborative research undertaken to understand the biology and epidemiology of MLN in East Africa and to develop disease management strategies, including identification of MLN-tolerant maize germplasm. We discuss recent progress, identify major issues requiring further research, and discuss the possible next steps for effective management of MLN.

Molecular interactions and immune responses between Maize fine streak virus and the leafhopper vector Graminella nigrifrons through differential expression and RNA interference

Graminella nigrifrons is the only known vector for Maize fine streak virus (MFSV). In this study, we used real-time quantitative PCR to compare the expression profiles of transcripts that putatively function in the insect immune response: four peptidoglycan recognition proteins (PGRP-SB1, -SD, -LC and LB), Toll, spaetzle, defensin, Dicer-2 (Dcr-2), Argonaut-2 (Ago-2) and Arsenic resistance protein 2 (Ars-2). Except for PGRP-LB and defensin, transcripts involved in humoral pathways were significantly suppressed in G. nigrifrons fed on MFSV-infected maize. The abundance of three RNA interference (RNAi) pathway transcripts (Dcr-2, Ago-2, Ars-2) was significantly lower in nontransmitting relative to transmitting G. nigrifrons. Injection with double-stranded RNA (dsRNA) encoding segments of the PGRP-LC and Dcr-2 transcripts effectively reduced transcript levels by 90 and 75% over 14 and 22 days, respectively. MFSV acquisition and transmission were not significantly affected by injection of either dsRNA. Knock-down of PGRP-LC resulted in significant mortality (greater than 90%) at 27 days postinjection, and resulted in more abnormal moults relative to those injected with Dcr-2 or control dsRNA. The use of RNAi to silence G. nigrifrons transcripts will facilitate the study of gene function and pathogen transmission, and may provide approaches for developing novel targets of RNAi-based pest control.

Viruses in maize and Johnsongrass in southern Ohio

The two major U.S. maize viruses, Maize dwarf mosaic virus (MDMV) and Maize chlorotic dwarf virus (MCDV), emerged in southern Ohio and surrounding regions in the 1960s and caused significant losses. Planting resistant varieties and changing cultural practices has dramatically reduced virus impact in subsequent decades. Current information on the distribution, diversity, and impact of known and potential U.S. maize disease-causing viruses is lacking. To assess the current reservoir of viruses present at the sites of past disease emergence, we used a combination of serological testing and next-generation RNA sequencing approaches. Here, we report enzyme-linked immunosorbent assay and RNA-Seq data from samples collected over 2 years to assess the presence of viruses in cultivated maize and an important weedy reservoir, Johnsongrass (Sorghum halepense). Results revealed a persistent reservoir of MDMV and two strains of MCDV in Ohio Johnsongrass. We identified sequences of several other grass-infecting viruses and confirmed the presence of Wheat mosaic virus in Ohio maize. Together, these results provide important data for managing virus disease in field corn and sweet corn maize crops, and identifying potential future virus threats.

Genetic insights into Graminella nigrifrons Competence for maize fine streak virus infection and transmission

BACKGROUND

Most plant-infecting rhabdoviruses are transmitted by one or a few closely related insect species. Additionally, intraspecific differences in transmission efficacy often exist among races/biotypes within vector species and among strains within a virus species. The black-faced leafhopper, Graminella nigrifrons, is the only known vector of the persistent propagative rhabdovirus Maize fine streak virus (MFSV). Only a small percentage of leafhoppers are capable of transmitting the virus, although the mechanisms underlying vector competence are not well understood.

METHODOLOGY

RNA-Seq was carried out to explore transcript expression changes and sequence variation in G. nigrifrons and MFSV that may be associated with the ability of the vector to acquire and transmit the virus. RT-qPCR assays were used to validate differential transcript accumulation.

RESULTS/SIGNIFICANCE

Feeding on MFSV-infected maize elicited a considerable transcriptional response in G. nigrifrons, with increased expression of cytoskeleton organization and immunity transcripts in infected leafhoppers. Differences between leafhoppers capable of transmitting MFSV, relative to non-transmitting but infected leafhoppers were more limited, which may reflect difficulties discerning between the two groups and/or the likelihood that the transmitter phenotype results from one or a few genetic differences. The ability of infected leafhoppers to transmit MFSV did not appear associated with virus transcript accumulation in the infected leafhoppers or sequence polymorphisms in the viral genome. However, the non-structural MFSV 3 gene was expressed at unexpectedly high levels in infected leafhoppers, suggesting it plays an active role in the infection of the insect host. The results of this study begin to define the functional roles of specific G. nigrifrons and MFSV genes in the viral transmission process.

Reduction in fecundity and shifts in cellular processes by a native virus on an invasive insect

Pathogens and their vectors have coevolutionary histories that are intricately intertwined with their ecologies, environments, and genetic interactions. The soybean aphid, Aphis glycines, is native to East Asia but has quickly become one of the most important aphid pests in soybean-growing regions of North America. In this study, we used bioassays to examine the effects of feeding on soybean infected with a virus it vectors (Soybean mosaic virus [SMV]) and a virus it does not vector (Bean pod mottle virus [BPMV]) have on A. glycines survival and fecundity. The genetic underpinnings of the observed changes in fitness phenotype were explored using RNA-Seq. Aphids fed on SMV-infected soybean had transcriptome and fitness profiles that were similar to that of aphids fed on healthy control plants. Strikingly, a significant reduction in fecundity was seen in aphids fed on BPMV-infected soybean, concurrent with a large and persistent downregulation of A. glycines transcripts involved in regular cellular activities. Although molecular signatures suggested a small regulatory RNA pathway defense response was repressed in aphids feeding on infected plants, BPMV did not appear to be replicating in the vector. These results suggest that incompatibilities with BPMV or the effects of BPMV infection on soybean caused A. glycines to allot available energy resources to survival rather than reproduction and other core cellular processes. Ultimately, the detrimental impacts to A. glycines may reflect the short tritrophic evolutionary histories between the insect, plant, and virus.

Genetic analysis of resistance to six virus diseases in a multiple virus-resistant maize inbred line

Novel and previously known resistance loci for six phylogenetically diverse viruses were tightly clustered on chromosomes 2, 3, 6 and 10 in the multiply virus-resistant maize inbred line, Oh1VI. Virus diseases in maize can cause severe yield reductions that threaten crop production and food supplies in some regions of the world. Genetic resistance to different viruses has been characterized in maize populations in diverse environments using different screening techniques, and resistance loci have been mapped to all maize chromosomes. The maize inbred line, Oh1VI, is resistant to at least ten viruses, including viruses in five different families. To determine the genes and inheritance mechanisms responsible for the multiple virus resistance in this line, F1 hybrids, F2 progeny and a recombinant inbred line (RIL) population derived from a cross of Oh1VI and the virus-susceptible inbred line Oh28 were evaluated. Progeny were screened for their responses to Maize dwarf mosaic virus, Sugarcane mosaic virus, Wheat streak mosaic virus, Maize chlorotic dwarf virus, Maize fine streak virus, and Maize mosaic virus. Depending on the virus, dominant, recessive, or additive gene effects were responsible for the resistance observed in F1 plants. One to three gene models explained the observed segregation of resistance in the F2 generation for all six viruses. Composite interval mapping in the RIL population identified 17 resistance QTLs associated with the six viruses. Of these, 15 were clustered in specific regions of chr. 2, 3, 6, and 10. It is unknown whether these QTL clusters contain single or multiple virus resistance genes, but the coupling phase linkage of genes conferring resistance to multiple virus diseases in this population could facilitate breeding efforts to develop multi-virus resistant crops.

The bean pod mottle virus RNA2-encoded 58-kilodalton protein P58 is required in cis for RNA2 accumulation

Bean pod mottle virus (BPMV) is a bipartite, positive-sense (+) RNA plant virus in the Secoviridae family. Its RNA1 encodes proteins required for genome replication, whereas RNA2 primarily encodes proteins needed for virion assembly and cell-to-cell movement. However, the function of a 58-kDa protein (P58) encoded by RNA2 has not been resolved. P58 and the movement protein (MP) of BPMV are two largely identical proteins differing only at their N termini, with P58 extending MP upstream by 102 amino acid residues. In this report, we unveil a unique role for P58. We show that BPMV RNA2 accumulation in infected cells was abolished when the start codon of P58 was eliminated. The role of P58 does not require the region shared by MP, as RNA2 accumulation in individual cells remained robust even when most of the MP coding sequence was removed. Importantly, the function of P58 required the P58 protein, rather than its coding RNA, as compensatory mutants could be isolated that restored RNA2 accumulation by acquiring new start codons upstream of the original one. Most strikingly, loss of P58 function could not be complemented by P58 provided in trans, suggesting that P58 functions in cis to selectively promote the accumulation of RNA2 copies that encode a functional P58 protein. Finally, we found that all RNA1-encoded proteins are cis-acting relative to RNA1. Together, our results suggest that P58 probably functions by recruiting the RNA1-encoded polyprotein to RNA2 to enable RNA2 reproduction.

IMPORTANCE

Bean pod mottle virus (BPMV) is one of the most important pathogens of the crop plant soybean, yet its replication mechanism is not well understood, hindering the development of knowledge-based control measures. The current study examined the replication strategy of BPMV RNA2, one of the two genomic RNA segments of this virus, and established an essential role for P58, one of the RNA2-encoded proteins, in the process of RNA2 replication. Our study demonstrates for the first time that P58 functions preferentially with the very RNA from which it is translated, thus greatly advancing our understanding of the replication mechanisms of this and related viruses. Furthermore, this study is important because it provides a potential target for BPMV-specific control, and hence could help to mitigate soybean production losses caused by this virus.

Virus-independent and common transcriptome responses of leafhopper vectors feeding on maize infected with semi-persistently and persistent propagatively transmitted viruses

BACKGROUND

Insects are the most important epidemiological factors for plant virus disease spread, with >75% of viruses being dependent on insects for transmission to new hosts. The black-faced leafhopper (Graminella nigrifrons Forbes) transmits two viruses that use different strategies for transmission: Maize chlorotic dwarf virus (MCDV) which is semi-persistently transmitted and Maize fine streak virus (MFSV) which is persistently and propagatively transmitted. To date, little is known regarding the molecular and cellular mechanisms in insects that regulate the process and efficiency of transmission, or how these mechanisms differ based on virus transmission strategy.

RESULTS

RNA-Seq was used to examine transcript changes in leafhoppers after feeding on MCDV-infected, MFSV-infected and healthy maize for 4 h and 7 d. After sequencing cDNA libraries constructed from whole individuals using Illumina next generation sequencing, the Rnnotator pipeline in Galaxy was used to reassemble the G. nigrifrons transcriptome. Using differential expression analyses, we identified significant changes in transcript abundance in G. nigrifrons. In particular, transcripts implicated in the innate immune response and energy production were more highly expressed in insects fed on virus-infected maize. Leafhoppers fed on MFSV-infected maize also showed an induction of transcripts involved in hemocoel and cell-membrane linked immune responses within four hours of feeding. Patterns of transcript expression were validated for a subset of transcripts by quantitative real-time reverse transcription polymerase chain reaction using RNA samples collected from insects fed on healthy or virus-infected maize for between a 4 h and seven week period.

CONCLUSIONS

We expected, and found, changes in transcript expression in G. nigrifrons feeding of maize infected with a virus (MFSV) that also infects the leafhopper, including induction of immune responses in the hemocoel and at the cell membrane. The significant induction of the innate immune system in G. nigrifrons fed on a foregut-borne virus (MCDV) that does not infect leafhoppers was less expected. The changes in transcript accumulation that occur independent of the mode of pathogen transmission could be key for identifying insect factors that disrupt vector-mediated plant virus transmission.

Identification of Resistance to Maize rayado fino virus in Maize Inbred Lines

Maize rayado fino virus (MRFV) causes one of the most important virus diseases of maize in America. Severe yield losses, ranging from 10 to 50% in landraces to nearly 100% in contemporary cultivars, have been reported. Resistance has been reported in maize populations, but few resistant inbred lines have been identified. Maize inbred lines representing the range of diversity in the cultivated types and selected lines known to be resistant to other viruses were evaluated to identify novel sources of resistance to MRFV. The virus was transmitted to maize seedlings using the vector Dalbulus maidis, and disease incidence and severity were evaluated beginning 7 days postinoculation. Most of the 36 lines tested were susceptible to MRFV, with mean disease incidence ranging from 21 to 96%, and severity from 1.0 to 4.3 (using a 0 to 5 severity scale). A few genotypes, including CML333 and Ki11, showed intermediate levels of resistance, with 14 and 10% incidence, respectively. Novel sources of resistance, with incidence of less than 5% and severity ratings of 0.4 or less, included the inbred lines Oh1VI, CML287, and Cuba. In Oh1VI, resistance appeared to be dominant, and segregation of resistance in F₂ plants was consistent with one or two resistance genes. The discovery of novel sources of resistance in maize inbred lines will facilitate the identification of virus resistance genes and their incorporation into breeding programs.

Wheat mosaic virus (WMoV), the Causal Agent of High Plains Disease, is Present in Ohio Wheat Fields

High Plains disease was first described in wheat (Triticum aestivum) in Nebraska, Idaho, Texas, and other High Plains states in 1993 to 1994 (1). The causal agent is a negative sense RNA virus in the genus Emaravirus with at least three genome segments, which is transmitted by the wheat curl mite (Aceria tosichella Keifer) (2). This virus is variously referred to as High Plains virus (HPV), Maize red stripe virus (MRSV/MRStV), or Wheat mosaic virus (WMoV) in the literature. We adopt the name WMoV based on the latest recommendation (3). The presence of WMoV in Ohio was revealed through a comprehensive survey conducted in early spring 2012. Specifically, wheat plants exhibiting virus-like symptoms including chlorosis, reddening, stunting, spotting, or striping were collected from 27 wheat fields in 14 counties throughout Ohio, between March 20 and April 15, 2012. Total RNA was extracted from individual leaf samples, then pooled prior to ribosomal RNA removal and high throughput RNA-sequencing (RNA-Seq) using the Illumina HiSeq2000 platform (University of Illinois Biotechnology Center, Champaign-Urbana, IL). The resulting sequences were assembled and analyzed using CLC Genomics Workbench 5.5 software (CLC Bio, Cambridge, MA). One 983-nt contig was 99% identical to the nucleocapsid protein (NP)-coding RNA segment of WMoV (GenBank Accession DQ324466). We used reverse transcription (RT)-PCR to determine the distribution of WMoV in individual samples using WMoV-specific primers: WMoV NPf1 (TGCTATGTCATTGTTCAGGTGGTC), and WMoV NPr1 (TTAGGCAGTCCTTGATTGTGCTG). WMoV was identified in one sample each from Miami, Auglaize, and Paulding Counties, which are all in western Ohio. The WMoV-positive plants were chlorotic, with varying degrees of stunting and leaf striping. The presence of WMoV in the three samples was confirmed using protein A sandwich (PAS)-ELISA with WMoV-specific antiserum. Vascular puncture inoculation (VPI) (4) was used to inoculate germinating maize seed (cv. Spirit) with the extracts from the WMoV-positive samples. WMoV was detected in two of 378 surviving inoculated plants by RT-PCR and PAS-ELISA. These two WMoV-positive maize plants developed flecking mosaic symptoms on upper uninoculated leaves, consistent with reported WMoV symptoms. The WMoV-positive sample from Auglaize County also contained Wheat streak mosaic virus (WSMV), and 60 of the 120 surviving plants inoculated with this sample were positive for WSMV. This result suggests that, even with VPI, mechanical transmission of WMoV remains a great challenge. To our knowledge, this is the first report of WMoV in Ohio, and demonstrates that WMoV is more widespread than previously thought, reaching at least the eastern edge of the Midwest wheat production region. The expanding distribution of this emerging virus is significant because of its potential to cause additional yield losses in wheat. References: (1) S. G. Jensen et al. Plant Dis. 80:1387, 1996. (2) N. Mielke-Ehred and H.-P. Muhlbach. Viruses 4:1515, 2012. (3) J. M. Skare et al. Virology 347:343, 2006. (4) R. Louie et al. J. Virol. Methods 135:214, 2006.

Biological and molecular characterization of a U.S. isolate of Hosta virus X

Hosta virus X (HVX) is rapidly becoming a serious pathogen of commercially important hosta plants worldwide. We report here biological and molecular characterization of a U.S. isolate of HVX, HVX-37. HVX-37 infectivity was tested in 21 hosta cultivars over three growth seasons, and three types of responses were defined based upon the ability of the virus to cause local and/or systemic infections. Four cultivars resistant to systemic HVX infection were identified. The full-length sequence of the HVX-37 genome was determined, the first complete sequence of a U.S. HVX isolate. Comparison with the previously sequenced HVX-Korea (Kr) genome revealed a high level of sequence similarity, as well as some differences. Notably, a 105-nucleotide long, near-perfect direct repeat in the Kr isolate is absent in HVX-37. The accuracy of the HVX-37 genome sequence was confirmed by infectivity of in vitro transcripts synthesized from a full-length HVX-37 cDNA on Nicotiana benthamiana and hosta plants.

First Report of Maize chlorotic mottle virus and Maize Lethal Necrosis in Kenya

In September 2011, a high incidence of a new maize (Zea mays L.) disease was reported at lower elevations (1,900 m asl) in the Longisa division of Bomet County, Southern Rift Valley, Kenya. The disease later spread to the Narok South and North and Naivasha Districts. By March 2012, the disease was reported at up to 2,100 m asl. Diseased plants had symptoms characteristic of virus diseases: a chlorotic mottle on leaves, developing from the base of young whorl leaves upward to the leaf tips; mild to severe leaf mottling; and necrosis developing from leaf margins to the mid-rib. Necrosis of young leaves led to a "dead heart" symptom, and plant death. Severely affected plants had small cobs with little or no grain set. Plants frequently died before tasseling. All maize varieties grown in the affected areas had similar symptoms. In these regions, maize is grown continuously throughout the year, with the main planting season starting in November. Maize streak virus was present, but incidence was low (data not shown). Infected plants were distributed throughout affected fields, with heavier infection along field edges. High thrips (Frankliniella williamsi Hood) populations were present in sampled fields, but populations of other potential disease vectors, such as aphids and leafhoppers, were low. Because of the high thrips populations and foliar symptoms, symptomatic plants were tested for the presence of Maize chlorotic mottle virus (MCMV) (3) using tissue blot immunoassay (TBIA) (1). Of 17 symptomatic leaf samples from each Bomet and Naivasha, nine from Bomet and all 17 from Naivasha were positive for MCMV. However, the observed symptoms were more severe than commonly associated with MCMV, suggesting the presence of maize lethal necrosis (MLN), a disease that results from maize infection with both MCMV and a potyvirus (4). Therefore, samples were tested for the presence of Sugarcane mosaic virus (SCMV), which is present in Kenya (2). Twenty-seven samples were positive for SCMV by TBIA, and 23 of 34 samples were infected with both viruses. Virus identities were verified with reverse-transcription (RT)-PCR (Access RT-PCR, Promega) and MCMV or SCMV-specific primers. MCMV primers (2681F: 5'-ATGAGAGCAGTTGGGGAATGCG and 3226R: 5'-CGAATCTACACACACACACTCCAGC) amplified the expected 550-bp product from three leaf samples. Amplicon sequences were identical, and had 95 to 98% identity with MCMV sequences in GenBank. SCMV primers (8679F: 5'-GCAATGTCGAAGAAAATGCG) and 9595R: 5'-GTCTCTCACCAAGAGACTCGCAGC) amplified the expected 900-bp product from four leaf samples. Amplicon sequences had 96 to 98% identity, and were 88 to 96% identical with SCMV sequences in GenBank. To our knowledge, this is the first report of MCMV and of maize coinfection with MCMV and SCMV associated with MLN in Kenya and Africa. MLN is a serious threat to farmers in the affected areas, who are experiencing extensive to complete crop loss. References: (1) P. G. S. Chang et al. J. Virol. Meth. 171:345, 2011. (2) Delgadillo Sanchez et al. Rev. Mex. Fitopat. 5:21, 1987. (3) Jiang et al., Crop Prot. 11:248, 1992. (4) R. Louie, Plant Dis. 64:944, 1980.

The bacterium Pantoea stewartii uses two different type III secretion systems to colonize its plant host and insect vector

Plant- and animal-pathogenic bacteria utilize phylogenetically distinct type III secretion systems (T3SS) that produce needle-like injectisomes or pili for the delivery of effector proteins into host cells. Pantoea stewartii subsp. stewartii (herein referred to as P. stewartii), the causative agent of Stewart's bacterial wilt and leaf blight of maize, carries phylogenetically distinct T3SSs. In addition to an Hrc-Hrp T3SS, known to be essential for maize pathogenesis, P. stewartii has a second T3SS (Pantoea secretion island 2 [PSI-2]) that is required for persistence in its flea beetle vector, Chaetocnema pulicaria (Melsh). PSI-2 belongs to the Inv-Mxi-Spa T3SS family, typically found in animal pathogens. Mutagenesis of the PSI-2 psaN gene, which encodes an ATPase essential for secretion of T3SS effectors by the injectisome, greatly reduces both the persistence of P. stewartii in flea beetle guts and the beetle's ability to transmit P. stewartii to maize. Ectopic expression of the psaN gene complements these phenotypes. In addition, the PSI-2 psaN gene is not required for P. stewartii pathogenesis of maize and is transcriptionally upregulated in insects compared to maize tissues. Thus, the Hrp and PSI-2 T3SSs play different roles in the life cycle of P. stewartii as it alternates between its insect vector and plant host.

Characterization and Distribution of a Potyvirus Associated with Passion Fruit Woodiness Disease in Uganda

This article describes the incidence and etiology of a viral disease of passion fruit in Uganda. Symptoms, including those characteristic of passion fruit woodiness disease (PWD), were observed on 32% of plants in producing areas. Electron microscopic observations of infected tissues revealed flexuous filaments of ca. 780 nm. Enzymelinked immunosorbent assays indicated a serological relationship with Cowpea aphid-borne mosaic virus (CABMV) and Passion fruit ringspot virus (PFRSV). In host range studies, only species in the families Solanaceae and Chenopodiaceae were susceptible, and neither Vigna unguiculata nor Phaseolus vulgaris became infected. Coat protein (CP) gene sequences of eight isolates exhibited features typical of potyviruses and were highly similar (88 to 100% identity). However, the sequences had limited sequence identity with CP genes of two of the three potyviruses reported to cause PWD: East Asian Passiflora virus and Passion fruit woodiness virus (PWV). Deduced amino acid sequences for the CP of isolates from Uganda had highest identity with Bean common mosaic necrosis virus (BCMNV) (72 to 79%, with evolutionary divergence values between 0.17 and 0.19) and CABMV (73 to 76%, with divergence values between 0.21 and 0.25). Based on these results and in accordance with International Committee for Taxonomy of Viruses criteria for species demarcation in the family Potyviridae, we conclude that a previously unreported virus causes viral diseases on passion fruit in Uganda. The name "Ugandan Passiflora virus" is proposed for this virus.

Responses of maize (Zea mays L.) near isogenic lines carrying Wsm1, Wsm2, and Wsm3 to three viruses in the Potyviridae

Genes on chromosomes six (Wsm1), three (Wsm2) and ten (Wsm3) in the maize (Zea mays L.) inbred line Pa405 control resistance to Wheat streak mosaic virus (WSMV), and the same or closely linked genes control resistance to Maize dwarf mosaic virus (MDMV) and Sugarcane mosaic virus (SCMV). Near isogenic lines (NIL) carrying one or two of the genes were developed by introgressing regions of the respective chromosomes into the susceptible line Oh28 and tested for their responses to WSMV, MDMV, and SCMV in the field and greenhouse. F(1) progeny from NIL × Oh28 were also tested. Wsm1, or closely linked genes, provided resistance to all three viruses, as determined by symptom incidence and severity. Wsm2 and Wsm3 provided resistance to WSMV. Wsm2 and/or Wsm3 provided no resistance to MDMV, but significantly increased resistance in plants with one Wsm1 allele. NIL carrying Wsm1, Wsm2, or Wsm3 had similar SCMV resistance in the field, but NIL with Wsm2 and Wsm3 were not resistant in the greenhouse. Addition of Wsm2 to Wsm1 increased SCMV resistance in the field. For all viruses, symptom incidence was higher in the greenhouse than in the field, and relative disease severity was higher in the greenhouse for WSMV and MDMV. An Italian MDMV isolate and the Ohio SCMV infected the Wsm1 NIL, while the Ohio MDMV and Seehausen SCMV isolates did not. Our results indicate that the three genes, or closely linked loci, provide virus resistance. Resistance conferred by the three genes is influenced by interactions among the genes, the virus species, the virus isolate, and the environment.

Responses of maize (Zea mays L.) near isogenic lines carrying Wsm1, Wsm2, and Wsm3 to three viruses in the Potyviridae

Genes on chromosomes six (Wsm1), three (Wsm2) and ten (Wsm3) in the maize (Zea mays L.) inbred line Pa405 control resistance to Wheat streak mosaic virus (WSMV), and the same or closely linked genes control resistance to Maize dwarf mosaic virus (MDMV) and Sugarcane mosaic virus (SCMV). Near isogenic lines (NIL) carrying one or two of the genes were developed by introgressing regions of the respective chromosomes into the susceptible line Oh28 and tested for their responses to WSMV, MDMV, and SCMV in the field and greenhouse. F(1) progeny from NIL × Oh28 were also tested. Wsm1, or closely linked genes, provided resistance to all three viruses, as determined by symptom incidence and severity. Wsm2 and Wsm3 provided resistance to WSMV. Wsm2 and/or Wsm3 provided no resistance to MDMV, but significantly increased resistance in plants with one Wsm1 allele. NIL carrying Wsm1, Wsm2, or Wsm3 had similar SCMV resistance in the field, but NIL with Wsm2 and Wsm3 were not resistant in the greenhouse. Addition of Wsm2 to Wsm1 increased SCMV resistance in the field. For all viruses, symptom incidence was higher in the greenhouse than in the field, and relative disease severity was higher in the greenhouse for WSMV and MDMV. An Italian MDMV isolate and the Ohio SCMV infected the Wsm1 NIL, while the Ohio MDMV and Seehausen SCMV isolates did not. Our results indicate that the three genes, or closely linked loci, provide virus resistance. Resistance conferred by the three genes is influenced by interactions among the genes, the virus species, the virus isolate, and the environment.

Plant host range and leafhopper transmission of maize fine streak virus

Maize fine streak virus (MFSV), an emerging Rhabdovirus sp. in the genus Nucleorhabdovirus, is persistently transmitted by the black-faced leafhopper, Graminella nigrifrons (Forbes). MFSV was transmitted to maize, wheat, oat, rye, barley, foxtail, annual ryegrass, and quackgrass by G. nigrifrons. Parameters affecting efficiency of MFSV acquisition (infection) and transmission (inoculation) to maize were evaluated using single-leafhopper inoculations and enzyme-linked immunosorbent assay. MFSV was detected in ≈20% of leafhoppers that fed on infected plants but <10% of insects transmitted the virus. Nymphs became infected earlier and supported higher viral titers than adults but developmental stage at aquisition did not affect the rate of MFSV transmission. Viral titer and transmission also increased with longer post-first access to diseased periods (PADPs) (the sum of the intervals from the beginning of the acquisition access period to the end of the inoculation access period). Length of the acquisition access period was more important for virus accumulation in adults, whereas length of the interval between acquisition access and inoculation access was more important in nymphs. A threshold viral titer was needed for transmission but no transmission occurred, irrespective of titer, with a PADP of <4 weeks. MFSV was first detected by immunofluorescence confocal laser scanning microscopy at 2-week PADPs in midgut cells, hemocytes, and neural tissues; 3-week PADPs in tracheal cells; and 4-week PADPs in salivary glands, coinciding with the time of transmission to plants.

Bean pod mottle virus Spread in Insect-Feeding-Resistant Soybean

Bean pod mottle virus (BPMV) infection reduces yield and seed quality in soybean. To test the hypothesis that virus incidence and movement within plots would be reduced in soybean with resistance to feeding by the virus' bean leaf beetle (Cerotoma trifurcata) vector, BPMV spread was evaluated in five soybean genotypes at two inoculum levels over 2 years at two locations in Ohio. Soybean genotypes included two insect-feeding-susceptible genotypes (Williams 82 and Resnik), two insect-feeding-resistant, semidwarf genotypes (HC95-15 and HC95-24), and an insect-feeding-susceptible, semidwarf genotype (Troll). BPMV incidence was assessed in individual plants at growth stages R5/R6 and R7/R8 using enzyme-linked immunosorbent assay. Beetle feeding was visually assessed in 2004. Data for infection of individual plants were analyzed using a generalized linear mixed model, with a binomial distribution and logit-link. Within plots, BPMV incidence was highest in Resnik and Williams 82 and significantly lower in Troll. Incidence in HC95-15 was not significantly different than in Williams 82 and Resnik but incidence in HC95-24 was lower than in Resnik. BPMV incidence was also significantly (P < 0.05) affected by year, location, inoculum level and sampling date, with increasing incidence over time and higher incidence at the higher inoculum level. Beetle feeding damage was affected by the interaction of location-genotype. Significant spatial aggregation of infected plants was found for most plots but aggregation was independent of host genotype and inoculum level. Although the results indicate that BPMV infection varied by genotype, they do not support the hypothesis that insect-feeding resistance is sufficient to reduce the incidence and spread of BPMV.

Stolbur phytoplasma transmission to maize by Reptalus panzeri and the disease cycle of maize redness in Serbia

Maize redness (MR), induced by stolbur phytoplasma ('Candidatus Phytoplasma solani', subgroup 16SrXII-A), is characterized by midrib, leaf, and stalk reddening and abnormal ear development. MR has been reported from Serbia, Romania, and Bulgaria for 50 years, and recent epiphytotics reduced yields by 40 to 90% in South Banat District, Serbia. Potential vectors including leafhoppers and planthoppers in the order Hemiptera, suborder Auchenorrhyncha, were surveyed in MR-affected and low-MR-incidence fields, and 33 different species were identified. Only Reptalus panzeri populations displayed characteristics of a major MR vector. More R. panzeri individuals were present in MR-affected versus low-MR fields, higher populations were observed in maize plots than in field border areas, and peak population levels preceded the appearance of MR in late July. Stolbur phytoplasma was detected in 17% of R. panzeri adults using nested polymerase chain reaction but not in any other insects tested. Higher populations of R. panzeri nymphs were found on maize, Johnsongrass (Sorghum halepense), and wheat (Triticum aestivum) roots. Stolbur phytoplasma was detected in roots of these three plant species, as well as in R. panzeri L(3) and L(5) nymphs. When stolbur phytoplasma-infected R. panzeri L(3) nymphs were introduced into insect-free mesh cages containing healthy maize and wheat plants, 89 and 7%, respectively, became infected. These results suggest that the MR disease cycle in South Banat involves mid-July transmission of stolbur phytoplasma to maize by infected adult R. panzeri. The adult R. panzeri lay eggs on infected maize roots, and nymphs living on these roots acquire the phytoplasma from infected maize. The nymphs overwinter on the roots of wheat planted into maize fields in the autumn, allowing emergence of phytoplasma-infected vectors the following July.

Cellular and molecular aspects of rhabdovirus interactions with insect and plant hosts

The rhabdoviruses form a large family (Rhabdoviridae) whose host ranges include humans, other vertebrates, invertebrates, and plants. There are at least 90 plant-infecting rhabdoviruses, several of which are economically important pathogens of various crops. All definitive plant-infecting and many vertebrate-infecting rhabdoviruses are persistently transmitted by insect vectors, and a few putative plant rhabdoviruses are transmitted by mites. Plant rhabdoviruses replicate in their plant and arthropod hosts, and transmission by vectors is highly specific, with each virus species transmitted by one or a few related insect species, mainly aphids, leafhoppers, or planthoppers. Here, we provide an overview of plant rhabdovirus interactions with their insect hosts and of how these interactions compare with those of vertebrate-infecting viruses and with the Sigma rhabdovirus that infects Drosophila flies. We focus on cellular and molecular aspects of vector/host specificity, transmission barriers, and virus receptors in the vectors. In addition, we briefly discuss recent advances in understanding rhabdovirus-plant interactions.

Insect vector interactions with persistently transmitted viruses

The majority of described plant viruses are transmitted by insects of the Hemipteroid assemblage that includes aphids, whiteflies, leafhoppers, planthoppers, and thrips. In this review we highlight progress made in research on vector interactions of the more than 200 plant viruses that are transmitted by hemipteroid insects beginning a few hours or days after acquisition and for up to the life of the insect, i.e., in a persistent-circulative or persistent-propagative mode. These plant viruses move through the insect vector, from the gut lumen into the hemolymph or other tissues and finally into the salivary glands, from which these viruses are introduced back into the plant host during insect feeding. The movement and/or replication of the viruses in the insect vectors require specific interactions between virus and vector components. Recent investigations have resulted in a better understanding of the replication sites and tissue tropism of several plant viruses that propagate in insect vectors. Furthermore, virus and insect proteins involved in overcoming transmission barriers in the vector have been identified for some virus-vector combinations.

Response of Soybean Cultivars to Bean pod mottle virus Infection

Bean pod mottle virus (BPMV) has become increasingly common in soybean throughout the north-central region of the United States. Yield loss assessments on southern soybean germplasm have reported reductions ranging from 3 to 52%. Currently, no soybean cultivars have been identified with resistance to BPMV. The objective of this study was to determine the impact of BPMV infection on soybean cultivars representing a broad range of northern soybean germ-plasm by comparing inoculated and noninoculated soybean plants in paired row studies. In all, 30 and 24 cultivars were evaluated in Nebraska (NE) in which soybean plants were inoculated at the V3 to V4 growth stage. Eleven cultivars from public and breeding lines were inoculated at the VC and R5 to R6 growth stages in Ohio (OH). Disease severity, yield, and percent seed coat mottling were assessed at both locations, whereas protein and oil content also were assessed at NE. Yield and percent seed coat mottling was significantly reduced following inoculation at the VC (OH) and V3 to V4 (NE) growth stages. In addition, seed oil and protein composition were impacted in 1 of the 2 years of the study. This study demonstrates that substantial yield losses can occur in soybean due to BPMV infection. In addition, protein and oil may be affected depending on the environment during the production season.

Infectious cDNA transcripts of Maize necrotic streak virus: infectivity and translational characteristics

Maize necrotic streak virus (MNeSV) is a unique member of the family Tombusviridae that is not infectious by leaf rub inoculation and has a coat protein lacking the protruding domain of aureusviruses, carmoviruses, and tombusviruses (Louie et al., Plant Dis. 84, 1133-1139, 2000). Completion of the MNeSV sequence indicated a genome of 4094 nt. RNA blot and primer extension analysis identified subgenomic RNAs of 1607 and 781 nt. RNA and protein sequence comparisons and RNA secondary structure predictions support the classification of MNeSV as the first monocot-infecting tombusvirus, the smallest tombusvirus yet reported. Uncapped transcripts from cDNAs were infectious in maize (Zea mays L.) protoplasts and plants. Translation of genomic and subgenomic RNA transcripts in wheat germ extracts indicated that MNeSV has a 3' cap-independent translational enhancer (3'CITE) located within the 3' 156 nt. The sequence, predicted structure, and the ability to function in vitro differentiate the MNeSV 3'CITE from that of Tomato bushy stunt virus.

Shotgun sequencing of the negative-sense RNA genome of the rhabdovirus Maize mosaic virus

The maize-infecting nucleorhabdovirus, Maize mosaic virus (MMV), was sequenced to near completion using the random shotgun approach. Sequences of 102 clones from a cDNA library constructed from randomly-primed viral RNA were compiled into a 12,133 nucleotide (nt) contig containing six open reading frames. The contig consisted of 97 sequences averaging 660 bp in length. The average sequence coverage was six-fold, and 93% of the contig had sequence reads covering both strands. The remaining sequence was derived from single (5%) or multiple (2%) reads on the same strand. Three of the six ORFs showed significant similarities to the deduced protein sequences of the nucleocapsid, glycoprotein and polymerase sequences of other rhabdoviruses. The predicted gene order of the MMV genome was 3'-N-P-3-M-G-L-5'. Shotgun sequencing of the MMV genome took approximately 127 h and cost 0.38 dollars per nt (including labor), whereas the primer walking approach for sequencing the 13,782-nt MFSV genome [Tsai, C.-W., Redinbaugh, M.G., Willie, K.J., Reed, S., Goodin, M., Hogenhout, S. A., 2005. Complete genome sequence and in planta subcellular localization of maize fine streak virus proteins. J. Virol. 79, 5304-5314] took about 217 h and cost 0.50 dollars per nt. Thus, the shotgun approach gave good depth of coverage for the viral genome sequence while being significantly faster and less expensive than the primer walking method. This technique will facilitate the sequencing of multiple rhabdovirus genomes.

Plant rhabdoviruses

This chapter provides an overview of plant rhabdovirus structure and taxonomy, genome structure, protein function, and insect and plant infection. It is focused on recent research and unique aspects of rhabdovirus biology. Plant rhabdoviruses are transmitted by aphid, leafhopper or planthopper vectors, and the viruses replicate in both their insect and plant hosts. The two plant rhabdovirus genera, Nucleorhabdovirus and Cytorhabdovirus, can be distinguished on the basis of their intracellular site of morphogenesis in plant cells. All plant rhabdoviruses carry analogs of the five core genes: the nucleocapsid (N), phosphoprotein (P), matrix (M), glycoprotein (G) and large or polymerase (L). However, compared to vesiculoviruses that are composed of the five core genes, all plant rhabdoviruses encode more than these five genes, at least one of which is inserted between the P and M genes in the rhabdoviral genome. Interestingly, while these extra genes are not similar among plant rhabdoviruses, two encode proteins with similarity to the 30K superfamily of plant virus movement proteins. Analysis of nucleorhabdoviral protein sequences revealed nuclear localization signals for the N, P, M and L proteins, consistent with virus replication and morphogenesis of these viruses in the nucleus. Plant and insect factors that limit virus infection and transmission are discussed.

Complete genome sequence and in planta subcellular localization of maize fine streak virus proteins

The genome of the nucleorhabdovirus maize fine streak virus (MFSV) consists of 13,782 nucleotides of nonsegmented, negative-sense, single-stranded RNA. The antigenomic strand consisted of seven open reading frames (ORFs), and transcripts of all ORFs were detected in infected plants. ORF1, ORF6, and ORF7 had significant similarities to the nucleocapsid protein (N), glycoprotein (G), and polymerase (L) genes of other rhabdoviruses, respectively, whereas the ORF2, ORF3, ORF4, and ORF5 proteins had no significant similarities. The N (ORF1), ORF4, and ORF5 proteins localized to nuclei, consistent with the presence of nuclear localization signals (NLSs) in these proteins. ORF5 likely encodes the matrix protein (M), based on its size, the position of its NLS, and the localization of fluorescent protein fusions to the nucleus. ORF2 probably encodes the phosphoprotein (P) because, like the P protein of Sonchus yellow net virus (SYNV), it was spread throughout the cell when expressed alone but was relocalized to a subnuclear locus when coexpressed with the MFSV N protein. Unexpectedly, coexpression of the MFSV N and P proteins, but not the orthologous proteins of SYNV, resulted in accumulations of both proteins in the nucleolus. The N and P protein relocalization was specific to cognate proteins of each virus. The subcellular localizations of the MFSV ORF3 and ORF4 proteins were distinct from that of the SYNV sc4 protein, suggesting different functions. To our knowledge, this is the first comparative study of the cellular localizations of plant rhabdoviral proteins. This study indicated that plant rhabdoviruses are diverse in genome sequence and viral protein interactions.

Identification of quantitative trait loci controlling resistance to maize chlorotic dwarf virus

Ineffective screening methods and low levels of disease resistance have hampered genetic analysis of maize (Zea mays L.) resistance to disease caused by maize chlorotic dwarf virus (MCDV). Progeny from a cross between the highly resistant maize inbred line Oh1VI and the susceptible inbred line Va35 were evaluated for MCDV symptoms after multiple virus inoculations, using the viral vector Graminella nigrifrons. Symptom severity scores from three rating dates were used to calculate area under the disease progress curve (AUDPC) scores for vein banding, leaf twist and tear, and whorl chlorosis. AUDPC scores for the F(2) population indicated that MCDV resistance was quantitatively inherited. Genotypic and phenotypic analyses of 314 F(2) individuals were compared using composite interval mapping (CIM) and analysis of variance. CIM identified two major quantitative trait loci (QTL) on chromosomes 3 and 10 and two minor QTL on chromosomes 4 and 6. Resistance was additive, with alleles from Oh1VI at the loci on chromosomes 3 and 10 contributing equally to resistance.

Accumulation of maize chlorotic dwarf virus proteins in its plant host and leafhopper vector

The genome of Maize chlorotic dwarf virus (MCDV; genus Waikavirus; family Sequiviridae) consists of a monopartite positive-sense RNA genome encoding a single large polyprotein. Antibodies were produced to His-fusions of three undefined regions of the MCDV polyprotein: the N-terminus of the polyprotein (R78), a region between coat proteins (CPs) and the nucleotide-binding site (NBS) (R37), and a region between the NBS and a 3C-like protease (R69). The R78 antibodies react with proteins of 50 kDa (P50), 35 kDa (P35), and 25 kDa (P25) in virus preparations, and with P35 in plant extracts. In extracts of the leafhopper vector Graminella nigrifrons fed on MCDV-infected plants, the R78 antibodies reacted with P25 but not with P50 and P35. The R69 antibodies bound proteins of approximately 36 kDa (P36), 30 kDa (P30), and 26 kDa (P26) in virus preparations, and P36 and P26 in plant extracts. Antibodies to R37 reacted with a 26-kDa protein in purified virus preparations, but not in plant extracts. Neither the R69 nor the R37 antibodies bound any proteins in G. nigrifrons. Thus, in addition to the three CPs, cysteine protease and RNA-dependent RNA polymerase, the MCDV polyprotein is apparently post-transitionally cleaved into P50, P35, P25, P36, P30, and P26.

Effect of environmental conditions and leafhopper gender on Maize chlorotic dwarf virus transmission by Graminella nigrifrons (Homoptera: Cicadellidae)

To determine the most economical and efficient means to maintain cultures of Maize chlorotic dwarf virus (MCDV) and to screen for host plant resistance to MCDV, we evaluated the effects of temperature, light intensity, daylength, atmospheric pressure, and leafhopper gender on the frequency of transmission of MCDV by Grarminella nigrifrons Forbes (Homoptera: Cicadellidae). Female leafhoppers transmitted at higher frequencies than males under most conditions. In temperature studies, transmission rates for both male and female leafhoppers progressively increased as temperatures rose from 20 to 30 degrees C. At high light intensities, both males and females transmitted at greater frequencies than they did at low. Similarly, longer day lengths were correlated with higher transmission rates for both sexes. No significant differences in transmission rates were observed in response to differences in atmospheric pressure. The results also showed that transmission rates under most conditions are high enough to overcome potential ambiguities caused by inoculated susceptible plants that do not become infected (disease escapes) when screening for resistance.

Plant and animal rhabdovirus host range: a bug's view

Rhabdoviruses affect human health, terrestrial and aquatic livestock and crops. Most rhabdoviruses are transmitted by insects to their vertebrate or plant hosts. For insect transmission to occur, rhabdoviruses must negotiate barriers to acquisition, replication, movement, escape and inoculation. A better understanding of the molecular interactions of rhabdoviruses with insects will clarify the complexities of rhabdovirus infection processes and epidemiology. A unique opportunity for studying how insects become hosts and vectors of rhabdoviruses is provided by five maize-infecting rhabdoviruses that are differentially transmitted by one or more related species of two divergent homopteran families.

Transmission of Maize streak virus by vascular puncture inoculation with unit-length genomic DNA

The infectivity of cloned unit-length genomes of Maize streak virus (MSV) was tested using vascular puncture inoculation (VPI). VPI of kernels with plasmid DNA (pUC19) carrying a tandem repeat of the MSV genome produced 33+/-8% infection. Similar plasmids carrying the unit-length MSV genome were not infectious. If the MSV genome was released from the plasmid prior to VPI, 16+/-4% of plants became infected. Ligation of the free linear MSV genome did not increase infectivity. The three infective inocula produced symptoms of similar severity in maize. Bioassay of systemically infected leaves indicated the virus was equally infectious regardless of inoculum. In Southern blots of bioassay plants, no differences in MSV genome restriction endonuclease sites were observed. Thus, inoculation with the free linear or circularized MSV unit-length genome produced infections similar to those with plasmids carrying tandemly repeated genomes. The infectivity of free linear MSV unit-length genomes will facilitate molecular analysis of MSV, because cloning steps are minimized.

Maize fine streak virus, a New Leafhopper-Transmitted Rhabdovirus

ABSTRACT A previously uncharacterized virus was isolated from fall-planted sweet corn (Zea mays L., Syngenta GSS 0966) leaves showing fine chlorotic streaks. Symptomatic plants were negative in enzyme-linked immunosorbent assay against many maize viruses, but reacted weakly with antisera to Sorghum stunt mosaic virus suggesting a distant relationship between the viruses. The virus was readily transmitted by vascular puncture inoculation (VPI), but not by leaf-rub inoculation. Symptoms on maize included dwarfing and fine chlorotic streaks along intermediate and small veins that developed 12 to 17 days post-VPI. The isolated virus was bacilliform (231 +/- 5 nm long and 71 +/- 2 nm wide), with a knobby surface, and obvious helical structure typical of rhabdovirus morphology. Nucleorhabdovirus virions were observed by transmission electron microscopy of infected maize leaf tissue sections. Proteins unique to infected plants were observed in extracts of infected leaves, and the isolated virion contained three proteins with molecular masses 82 +/- 2, 50 +/- 3, and 32 +/- 2 kDa. Preliminary sequence analysis indicated the virus had similarity to members of the family Rhabdoviridae. The virus was transmitted by Graminella nigrifrons under persistent conditions. The data indicate the virus, provisionally designated Maize fine streak virus, is a new species in the genus Nucleorhabdovirus.

Transmission of viral RNA and DNA to maize kernels by vascular puncture inoculation

Vascular puncture inoculation (VPI) is an effective technique for transmission of maize viruses without using arthropods or other biological vectors. It involves using a jeweler's engraving tool to push minuten pins through a droplet of virus inoculum toward the major vascular bundle in the scutellum of germinating kernels. Here, VPI is shown to be useful for introducing RNA and DNA viral genomes into maize. Maize dwarf mosaic potyvirus (MDMV) virions, MDMV genomic RNA, foxtail mosaic potexvirus (FoMV) genomic RNA and maize streak geminivirus (MSV) DNA were introduced into kernels by VPI, and infection rates determined. At high concentrations, both MDMV virion and genomic RNA preparations produced 100% infection of susceptible maize. However, MDMV genomic RNA was transmitted with about 100-fold lower efficiency than virions. FoMV genomic RNA and MSV DNA were transmitted at lower efficiency than the MDMV RNA, and the highest transmission rates were about 50%. Ribonuclease A pretreatment prevented genomic MDMV and FoMV RNA transmission, but not MDMV virion transmission indicating the viral RNA was the infectious entity. Proteinase K (ProK) pretreatment reduced transmission of MDMV RNA suggesting that integrity of the viral genomic protein bound covalently to the viral RNA may be important for efficient transmission.