Barley yellow dwarf viruses

Amand P, Rupp J, Pumphrey M, Fritz A, Zhang G, Jordan KW, Bai G. A novel quantitative trait locus for barley yellow dwarf virus resistance and kernel traits on chromosome 2D of a wheat cultivar Jagger

Barley yellow dwarf (BYD) is one of the most serious viral diseases in cereal crops worldwide. Identification of quantitative trait loci (QTLs) underlining wheat resistance to barley yellow dwarf virus (BYDV) is essential for breeding BYDV-tolerant wheat cultivars. In this study, a recombinant inbred line (RIL) population was developed from the cross between Jagger (PI 593688) and a Jagger mutant (JagMut1095). A linkage map of 3106 cM consisting of 21 wheat chromosomes was developed using 1003 unique single nucleotide polymorphisms (SNPs) from the RIL population and was used to identify QTLs for BYDV resistance and yield-related traits, including 1000-kernel weight (TKW), kernel area (KA), kernel width (KW), and kernel length (KL). QByd.hwwg-2DL, a QTL on chromosome arm 2DL for BYDV resistance, was consistently identified in three field experiments and explained 11.6%-44.5% of the phenotypic variation. For yield-related traits, six major and repeatable QTLs were identified on 1AS (QKa.hwwg-1AS), 2DL (QTkw.hwwg-2DL, QKa.hwwg-2DL, QKw.hwwg-2DL, and QKl.hwwg-2DL), and 5AL (QKw.hwwg-5AL). The major QTLs on chromosome 2DL for TKW, KA, KW, and KL were mapped between 621 and 643 Mb, overlapping with QByd.hwwg-2DL with all the favorable alleles from Jagger. This study reports the first native BYDV resistance QTL (QByd.hwwg-2DL) originating from common wheat and tightly linked markers to the QTL for improvement of wheat BYDV resistance in wheat breeding.

The plant growth-promoting rhizobacterium Azospirillum brasilense reduces symptoms and aphid population growth on wheat plants infected with barley yellow dwarf virus

There is increasing interest in the potential of plant growth-promoting rhizobacteria (PGPR) in agriculture to improve plant quality and control pests and diseases. Emerging evidence indicates that some PGPR can influence interactions between plants and their pathogens, while less work has explored whether PGPR may also influence interactions between plants and arthropod vectors. We address this issue in a major agricultural pathosystem involving wheat infection by barley yellow dwarf virus (BYDV), the most economically important aphid-transmitted viral disease of cereal crops. We found that plant association with the PGPR Azospirillum brasilense mitigated both viral effects on plant growth and population growth of the BYDV aphid vector, Rhopalosiphum padi. Although effects varied across A. brasilense strains, PGPR treatments that attenuated virus effects were also associated with reduced induction of salicylic acid in response to infection, suggesting PGPR inoculation may induce systemic resistance against BYDV. These findings suggest that PGPR may have significant capacity for application in the sustainable management of crop growth. However, further investigation of the complex interactions among PGPR, plants, pathogens and their vectors is needed to better understand this potential.

Barley Yellow Dwarf Virus Influences Its Vector's Endosymbionts but Not Its Thermotolerance

The barley yellow dwarf virus (BYDV) of cereals is thought to substantially increase the high-temperature tolerance of its aphid vector, Rhopalosiphum padi, which may enhance its transmission efficiency. This is based on experiments with North American strains of BYDV and R. padi. Here, we independently test these by measuring the temperature tolerance, via Critical Thermal Maximum (CTmax) and knockdown time, of Australian R. padi infected with a local BYDV isolate. We further consider the interaction between BYDV transmission, the primary endosymbiont of R. padi (Buchnera aphidicola), and a transinfected secondary endosymbiont (Rickettsiella viridis) which reduces the thermotolerance of other aphid species. We failed to find an increase in tolerance to high temperatures in BYDV-infected aphids or an impact of Rickettsiella on thermotolerance. However, BYDV interacted with R. padi endosymbionts in unexpected ways, suppressing the density of Buchnera and Rickettsiella. BYDV density was also fourfold higher in Rickettsiella-infected aphids. Our findings indicate that BYDV does not necessarily increase the temperature tolerance of the aphid transmission vector to increase its transmission potential, at least for the genotype combinations tested here. The interactions between BYDV and Rickettsiella suggest new ways in which aphid endosymbionts may influence how BYDV spreads, which needs further testing in a field context.

Global molecular evolution and phylogeographic analysis of barley yellow dwarf virus based on the cp and mp genes

Barley yellow dwarf virus (BYDV) has caused considerable losses in the global production of grain crops such as wheat, barley and maize. We investigated the phylodynamics of the virus by analysing 379 and 485 nucleotide sequences of the genes encoding the coat protein and movement protein, respectively. The maximum clade credibility tree indicated that BYDV-GAV and BYDV-MAV, BYDV-PAV and BYDV-PAS share the same evolutionary lineage, respectively. The diversification of BYDV arises from its adaptability to vector insects and geography. Bayesian phylogenetic analyses showed that the mean substitution rates of the coat and movement proteins of BYDV ranged from 8.327 × 10^- 4 (4.700 × 10^- 4-1.228 × 10^- 3) and 8.671 × 10^- 4 (6.143 × 10^- 4-1.130 × 10^- 3) substitutions/site/year, respectively. The time since the most recent common BYDV ancestor was 1434 (1040-1766) CE (Common Era). The Bayesian skyline plot (BSP) showed that the BYDV population experienced dramatic expansions approximately 8 years into the 21st century, followed by a dramatic decline in less than 15 years. Our phylogeographic analysis showed that the BYDV population originating in the United States was subsequently introduced to Europe, South America, Australia and Asia. The migration pathways of BYDV suggest that the global spread of BYDV is associated with human activities.

The Expanding Menagerie of Prunus-Infecting Luteoviruses

Members of the genus Luteovirus are responsible for economically destructive plant diseases worldwide. Over the past few years, three luteoviruses infecting Prunus trees have been characterized. However, the biological properties, prevalence, and genetic diversity of those viruses have not yet been studied. High-throughput sequencing of samples of various wild, cultivated, and ornamental Prunus species enabled the identification of four novel species in the genus Luteovirus for which we obtained complete or nearly complete genomes. Additionally, we identified another new putative species recovered from Sequence Read Archive data. Furthermore, we conducted a survey on peach-infecting luteoviruses in eight European countries. Analyses of 350 leaf samples collected from germplasm, production orchards, and private gardens showed that peach-associated luteovirus (PaLV), nectarine stem pitting-associated virus (NSPaV), and a novel luteovirus, peach-associated luteovirus 2 (PaLV2), are present in all countries; the most prevalent virus was NSPaV, followed by PaLV. The genetic diversity of these viruses was also analyzed. Moreover, the biological indexing on GF305 peach indicator plants demonstrated that PaLV and PaLV2, like NSPaV, are transmitted by graft at relatively low rates. No clear viral symptoms have been observed in either graft-inoculated GF305 indicators or different peach tree varieties observed in an orchard. The data generated during this study provide a broader overview of the genetic diversity, geographical distribution, and prevalence of peach-infecting luteoviruses and suggest that these viruses are likely asymptomatic in peach under most circumstances.

Important wheat diseases in the US and their management in the 21st century

Wheat is a crop of historical significance, as it marks the turning point of human civilization 10,000 years ago with its domestication. Due to the rapid increase in population, wheat production needs to be increased by 50% by 2050 and this growth will be mainly based on yield increases, as there is strong competition for scarce productive arable land from other sectors. This increasing demand can be further achieved using sustainable approaches including integrated disease pest management, adaption to warmer climates, less use of water resources and increased frequency of abiotic stress tolerances. Out of 200 diseases of wheat, 50 cause economic losses and are widely distributed. Each year, about 20% of wheat is lost due to diseases. Some major wheat diseases are rusts, smut, tan spot, spot blotch, fusarium head blight, common root rot, septoria blotch, powdery mildew, blast, and several viral, nematode, and bacterial diseases. These diseases badly impact the yield and cause mortality of the plants. This review focuses on important diseases of the wheat present in the United States, with comprehensive information of causal organism, economic damage, symptoms and host range, favorable conditions, and disease management strategies. Furthermore, major genetic and breeding efforts to control and manage these diseases are discussed. A detailed description of all the QTLs, genes reported and cloned for these diseases are provided in this review. This study will be of utmost importance to wheat breeding programs throughout the world to breed for resistance under changing environmental conditions.

F-actin dynamics in midgut cells enables virus persistence in vector insects

Hemipteran insects that transmit plant viruses in a persistent circulative manner acquire, retain and transmit viruses for their entire life. The mechanism enabling this persistence has remained unclear for many years. Here, we determined how wheat dwarf virus (WDV) persists in its leafhopper vector Psammotettix alienus. We found that WDV caused the up-regulation of actin-depolymerizing factor (ADF) at the mRNA and protein levels in the midgut cells of leafhoppers after experiencing a WDV acquisition access period (AAP) of 6, 12 or 24 h. Experimental inhibition of F-actin depolymerization by jasplakinolide and dsRNA injection led to lower virus accumulation levels and transmission efficiencies, suggesting that depolymerization of F-actin regulated by ADF is essential for WDV invasion of midgut cells. Exogenous viral capsid protein (CP) inhibited ADF depolymerization of actin filaments in vitro and in Spodoptera frugiperda 9 (Sf9) cells because the CP competed with actin to bind ADF and then blocked actin filament disassembly. Interestingly, virions colocalized with ADF after a 24-h AAP, just as actin polymerization occurred, indicating that the binding of CP with ADF affects the ability of ADF to depolymerize F-actin, inhibiting WDV entry. Similarly, the luteovirus barley yellow dwarf virus also induced F-actin depolymerization and then polymerization in the gut cells of its vector Schizaphis graminum. Thus, F-actin dynamics are altered by nonpropagative viruses in midgut cells to enable virus persistence in vector insects.

Ecology of Yellow Dwarf Viruses in Crops and Grasslands: Interactions in the Context of Climate Change

Our understanding of the ecological interactions between plant viruses, their insect vectors, and their host plants has increased rapidly over the past decade. The suite of viruses known collectively as the yellow dwarf viruses infect an extensive range of cultivated and noncultivated grasses worldwide and is one of the best-studied plant virus systems. The yellow dwarf viruses are ubiquitous in cereal crops, where they can significantly limit yields, and there is growing recognition that they are also ubiquitous in grassland ecosystems, where they can influence community dynamics. Here, we discuss recent research that has explored (a) the extent and impact of yellow dwarf viruses in a diversity of plant communities, (b) the role of vector behavior in virus transmission, and (c) the prospects for impacts of climate change-including rising temperatures, drought, and elevated CO₂-on the epidemiology of yellow dwarf viruses.

Barley GRIK1-SnRK1 kinases subvert a viral virulence protein to upregulate antiviral RNAi and inhibit infection

Viruses often usurp host machineries for their amplification, but it remains unclear if hosts may subvert virus proteins to regulate viral proliferation. Here, we show that the 17K protein, an important virulence factor conserved in barley yellow dwarf viruses (BYDVs) and related poleroviruses, is phosphorylated by host GRIK1‐SnRK1 kinases, with the phosphorylated 17K (P17K) capable of enhancing the abundance of virus‐derived small interfering RNAs (vsiRNAs) and thus antiviral RNAi. Furthermore, P17K interacts with barley small RNA‐degrading nuclease 1 (HvSDN1) and impedes HvSDN1‐catalyzed vsiRNA degradation. Additionally, P17K weakens the HvSDN1‐HvAGO1 interaction, thus hindering HvSDN1 from accessing and degrading HvAGO1‐carried vsiRNAs. Importantly, transgenic expression of 17K phosphomimetics (17K^5D), or genome editing of SDN1, generates stable resistance to BYDV through elevating vsiRNA abundance. These data validate a novel mechanism that enhances antiviral RNAi through host subversion of a viral virulence protein to inhibit SDN1‐catalyzed vsiRNA degradation and suggest new ways for engineering BYDV‐resistant crops.

Applied phenomics and genomics for improving barley yellow dwarf resistance in winter wheat

Barley yellow dwarf is one of the major viral diseases of cereals. Phenotyping barley yellow dwarf in wheat is extremely challenging due to similarities to other biotic and abiotic stresses. Breeding for resistance is additionally challenging as the wheat primary germplasm pool lacks genetic resistance, with most of the few resistance genes named to date originating from a wild relative species. The objectives of this study were to (1) evaluate the use of high-throughput phenotyping to improve barley yellow dwarf assessment; (2) identify genomic regions associated with barley yellow dwarf resistance; and (3) evaluate the ability of genomic selection models to predict barley yellow dwarf resistance. Up to 107 wheat lines were phenotyped during each of 5 field seasons under both insecticide treated and untreated plots. Across all seasons, barley yellow dwarf severity was lower within the insecticide treatment along with increased plant height and grain yield compared with untreated entries. Only 9.2% of the lines were positive for the presence of the translocated segment carrying the resistance gene Bdv2. Despite the low frequency, this region was identified through association mapping. Furthermore, we mapped a potentially novel genomic region for barley yellow dwarf resistance on chromosome 5AS. Given the variable heritability of the trait (0.211–0.806), we obtained a predictive ability for barley yellow dwarf severity ranging between 0.06 and 0.26. Including the presence or absence of Bdv2 as a covariate in the genomic selection models had a large effect for predicting barley yellow dwarf but almost no effect for other observed traits. This study was the first attempt to characterize barley yellow dwarf using field-high-throughput phenotyping and apply genomic selection to predict disease severity. These methods have the potential to improve barley yellow dwarf characterization, additionally identifying new sources of resistance will be crucial for delivering barley yellow dwarf resistant germplasm.

Metagenomics Analysis of the Wheat Virome Identifies Novel Plant and Fungal-Associated Viral Sequences

Wheat viruses including wheat streak mosaic virus, Triticum mosaic virus, and barley yellow dwarf virus cost substantial losses in crop yields every year. Although there have been extensive studies conducted on these known wheat viruses, currently, there is limited knowledge about all components of the wheat (Triticum aestivum L.) virome. Here, we determined the composition of the wheat virome through total RNA deep sequencing of field-collected leaf samples. Sequences were de novo assembled after removing the host reads, and BLASTx searches were conducted. In addition to the documented wheat viruses, novel plant and fungal-associated viral sequences were identified. We obtained the full genome sequence of the first umbra-like associated RNA virus tentatively named wheat umbra-like virus in cereals. Moreover, a novel bi-segmented putative virus tentatively named wheat-associated vipovirus sharing low but significant similarity with both plant and fungal-associated viruses was identified. Additionally, a new putative fungal-associated tobamo-like virus and novel putative Mitovirus were discovered in wheat samples. The discovery and characterization of novel viral sequences associated with wheat is important to determine if these putative viruses may pose a threat to the wheat industry or have the potential to be used as new biological control agents for wheat pathogens either as wild-type or recombinant viruses.

Host plant resistance in wheat to barley yellow dwarf viruses and their aphid vectors: a review

Cereal aphids are vectors of at least 11 species of Barley Yellow Dwarf Viruses (BYDV) in wheat that alone and/or in combination can cause between 5%-80% grain yield losses. They establish complex virus-vector interactions, with variations in specificity and transmission efficiency that need to be considered for control purposes. In general, these viruses and vectors have a global distribution, however, BYDV-PAV is the most prevalent and abundant virus species worldwide, likely due to its vectoring efficiency and the wide distribution of its primary vector Rhopalosiphum padi. Host plant resistance (HPR) is an environmentally friendly, efficient and cost-effective tool to reduce crop losses to biotic stressors such as aphids and viruses. Finding resistance sources is paramount to breed for HPR. Currently, most of the resistance identified for aphids and BYDV derives from wheat related and wild relative species. However, breeding for HPR to BYDV and its vectors has additional challenges besides the source identification, for example, the lack of selection tools for certain aphid species, which likely prevents the development of elite wheat germplasm carrying resistance to these constraints. Nonetheless, modern technologies such as high-throughput phenotyping, genomic and advanced statistical tools can contribute to make HPR to aphids and BYDV more efficient. In the present review we describe the main sources of resistance, discuss the challenges and opportunities for incorporating the resistance in wheat breeding programs and present a workflow to breed for BYDV and its vectors in wheat.

Identification and functional analyses of host factors interacting with the 17-kDa protein of Barley yellow dwarf virus-GAV

Barley yellow dwarf viruses (BYDVs) cause significant economic losses on barley, wheat, and oats worldwide. 17-kDa protein (17K) of BYDVs plays a key role in viral infection in plants, whereas the underlying regulation mechanism of 17K in virus infection remains elusive. In this study, we determined that 17K of BYDV-GAV, the most common species found in China in recent years, was involved in viral pathogenicity. To identify the host factors interacting with 17K, the full length coding sequence of 17K was cloned into pGBKT7 to generate the bait plasmid pGBKT7-17K. 114 positive clones were identified as possible host factors to interact with 17K through screening a tobacco cDNA library. Gene ontology enrichment analysis showed that they were classified into 35 functional groups, involving three main categories including biological processes (BP), cellular components (CC), and molecular functions (MF). Kyoto Encyclopedia of Genes and Genome (KEGG) analysis indicated the acquired genes were assigned to 49 KEGG pathways. The majority of these genes were involved in glyoxylate and dicarboxylate metabolism, carbon fixation in photosynthetic organisms, and glycolysis/gluconeogenesis. The interactions between 17K and the 27 proteins with well-documented annotations were verified by conducting yeast two-hybrid assays and 12 of the 27 proteins were verified to interact with 17K. To explore the putative function of the 12 proteins in BYDV-GAV infection, the subcellular localization and expression alterations in the presence of BYDV-GAV were monitored. The results showed that, under the condition of BYDV-GAV infection, RuBisCo, POR, and PPD5 were significantly up-regulated, whereas AEP and CAT1 were significantly down-regulated. Our findings provide insights into the 17K-mediated BYDV-GAV infection process.

Pliant pathogens: Estimating viral spread when confronted with new vector, host, and environmental conditions

Pathogen spread rates are determined, in part, by the performance of pathogens under altered environmental conditions and their ability to persist while switching among hosts and vectors.To determine the effects of new conditions (host, vector, and nutrient) on pathogen spread rate, we introduced a vector-borne viral plant pathogen, Barley Yellow Dwarf Virus PAV (BYDV-PAV) into hosts, vectors, and host nutrient supplies that it had not encountered for thousands of viral generations. We quantified pathogen prevalence over the course of two serial inoculations under the new conditions. Using individual-level transmission rates from this experiment, we parameterized a dynamical model of disease spread and projected spread across host populations through a growing season.A change in nutrient conditions (increased supply of phosphorus) reduced viral transmission whereas shifting to a new vector or host species had no effect on infection prevalence. However, the reduction in the new nutrient environment was only temporary; infection prevalence recovered after the second inoculation. Synthesis. These results highlight how robust the pathogen, BYDV-PAV, is to changes in its biotic and abiotic environment. Our study also highlights the need to quantify longitudinal infection information beyond snapshot assessments to project disease risk for pathogens in new environments.

Global Plant Virus Disease Pandemics and Epidemics

The world's staple food crops, and other food crops that optimize human nutrition, suffer from global virus disease pandemics and epidemics that greatly diminish their yields and/or produce quality. This situation is becoming increasingly serious because of the human population's growing food requirements and increasing difficulties in managing virus diseases effectively arising from global warming. This review provides historical and recent information about virus disease pandemics and major epidemics that originated within different world regions, spread to other continents, and now have very wide distributions. Because they threaten food security, all are cause for considerable concern for humanity. The pandemic disease examples described are six (maize lethal necrosis, rice tungro, sweet potato virus, banana bunchy top, citrus tristeza, plum pox). The major epidemic disease examples described are seven (wheat yellow dwarf, wheat streak mosaic, potato tuber necrotic ringspot, faba bean necrotic yellows, pepino mosaic, tomato brown rugose fruit, and cucumber green mottle mosaic). Most examples involve long-distance virus dispersal, albeit inadvertent, by international trade in seed or planting material. With every example, the factors responsible for its development, geographical distribution and global importance are explained. Finally, an overall explanation is given of how to manage global virus disease pandemics and epidemics effectively.

Enhancing Capsid Proteins Capacity in Plant Virus-Vector Interactions and Virus Transmission

Vector transmission of plant viruses is basically of two types that depend on the virus helper component proteins or the capsid proteins. A number of plant viruses belonging to disparate groups have developed unusual capsid proteins providing for interactions with the vector. Thus, cauliflower mosaic virus, a plant pararetrovirus, employs a virion associated p3 protein, the major capsid protein, and a helper component for the semi-persistent transmission by aphids. Benyviruses encode a capsid protein readthrough domain (CP-RTD) located at one end of the rod-like helical particle, which serves for the virus transmission by soil fungal zoospores. Likewise, the CP-RTD, being a minor component of the luteovirus icosahedral virions, provides for persistent, circulative aphid transmission. Closteroviruses encode several CPs and virion-associated proteins that form the filamentous helical particles and mediate transmission by aphid, whitefly, or mealybug vectors. The variable strategies of transmission and evolutionary ‘inventions’ of the unusual capsid proteins of plant RNA viruses are discussed.

Barley yellow dwarf virus-GAV-derived vsiRNAs are involved in the production of wheat leaf yellowing symptoms by targeting chlorophyll synthase

BACKGROUND

Wheat yellow dwarf virus disease is infected by barley yellow dwarf virus (BYDV), which causes leaf yellowing and dwarfing symptoms in wheat, thereby posing a serious threat to China's food production. The infection of plant viruses can produce large numbers of vsiRNAs, which can target host transcripts and cause symptom development. However, few studies have been conducted to explore the role played by vsiRNAs in the interaction between BYDV-GAV and host wheat plants.

METHODS

In this study, small RNA sequencing was conducted to profile vsiRNAs in BYDV-GAV-infected wheat plants. The putative targets of vsiRNAs were predicted by the bioinformatics software psRNATarget. RT-qPCR and VIGS were employed to identify the function of selected target transcripts. To confirm the interaction between vsiRNA and the target, 5' RACE was performed to analyze the specific cleavage sites.

RESULTS

From the sequencing data, we obtained a total of 11,384 detected vsiRNAs. The length distribution of these vsiRNAs was mostly 21 and 22 nt, and an A/U bias was observed at the 5' terminus. We also observed that the production region of vsiRNAs had no strand polarity. The vsiRNAs were predicted to target 23,719 wheat transcripts. GO and KEGG enrichment analysis demonstrated that these targets were mostly involved in cell components, catalytic activity and plant-pathogen interactions. The results of RT-qPCR analysis showed that most chloroplast-related genes were downregulated in BYDV-GAV-infected wheat plants. Silencing of a chlorophyll synthase gene caused leaf yellowing that was similar to the symptoms exhibited by BYDV-GAV-inoculated wheat plants. A vsiRNA from an overlapping region of BYDV-GAV MP and CP was observed to target chlorophyll synthase for gene silencing. Next, 5' RACE validated that vsiRNA8856 could cleave the chlorophyll synthase transcript in a sequence-specific manner.

CONCLUSIONS

This report is the first to demonstrate that BYDV-GAV-derived vsiRNAs can target wheat transcripts for symptom development, and the results of this study help to elucidate the molecular mechanisms underlying leaf yellowing after viral infection.

Simultaneous Increase in CO2 and Temperature Alters Wheat Growth and Aphid Performance Differently Depending on Virus Infection

Climate change impacts crop production, pest and disease pressure, yield stability, and, therefore, food security. In order to understand how climate and atmospheric change factors affect trophic interactions in agriculture, we evaluated the combined effect of elevated carbon dioxide (CO2) and temperature on the interactions among wheat (Triticum aestivum L.), Barley yellow dwarf virus species PAV (BYDV-PAV) and its vector, the bird cherry-oat aphid (Rhopalosiphum padi L.). Plant traits and aphid biological parameters were examined under two climate and atmospheric scenarios, current (ambient CO2 and temperature = 400 ppm and 20 °C), and future predicted (elevated CO2 and temperature = 800 ppm and 22 °C), on non-infected and BYDV-PAV-infected plants. Our results show that combined elevated CO2 and temperature increased plant growth, biomass, and carbon to nitrogen (C:N) ratio, which in turn significantly decreased aphid fecundity and development time. However, virus infection reduced chlorophyll content, biomass, wheat growth and C:N ratio, significantly increased R. padi fecundity and development time. Regardless of virus infection, aphid growth rates remained unchanged under simulated future conditions. Therefore, as R. padi is currently a principal pest in temperate cereal crops worldwide, mainly due to its role as a plant virus vector, it will likely continue to have significant economic importance. Furthermore, an earlier and more distinct virus symptomatology was highlighted under the future predicted scenario, with consequences on virus transmission, disease epidemiology and, thus, wheat yield and quality. These research findings emphasize the complexity of plant-vector-virus interactions expected under future climate and their implications for plant disease and pest incidence in food crops.

Incidence and Distribution of Insect-Transmitted Cereal Viruses in Wheat in China from 2007 to 2019

Diseases caused by insect-transmitted viruses are the predominant constraint to wheat production worldwide. However, detailed knowledge of virus incidence and dynamics in China in recent years is very limited. Here, major wheat-growing regions of China were surveyed over 10 years for insect-transmitted viruses, and 2,143 samples were collected (in 2007 to 2015) and analyzed by molecular hybridization or multiplex reverse-transcription PCR for barley yellow dwarf viruses (BYDVs: BYDV-GAV, -GPV, and -PAV) and wheat dwarf virus (WDV). In a 4-year survey (2016 to 2019), the incidence of eight insect-transmitted viruses (BYDVs, WDV, wheat yellow striate virus [WYSV], barley yellow striate mosaic virus [BYSMV], northern cereal mosaic virus [NCMV], and rice black-streaked dwarf virus [RBSDV]) was investigated, and BYDVs and WDV were widely distributed across China. BYDV-GAV (29.0% of the tested sample) was the most abundant, followed by BYDV-PAV (23.2%) from 2007 to 2015. From 2016 to 2019, however, BYDV-PAV had become the predominant species (39.5% positive of 952 samples tested), while the incidence of BYDV-GAV (13.4%) had declined. During the entire survey, the incidence of BYDV-GPV was very low in some locations in northwestern and northern China, and all eight viruses caused only local epidemics, not large-scale outbreaks throughout China. Two new cereal-infecting rhabdoviruses, leafhopper-transmitted WYSV and planthopper-transmitted BYSMV, were also found in China in recent years.

A distinct class of plant and animal viral proteins that disrupt mitosis by directly interrupting the mitotic entry switch Wee1-Cdc25-Cdk1

Many animal viral proteins, e.g., Vpr of HIV-1, disrupt host mitosis by directly interrupting the mitotic entry switch Wee1-Cdc25-Cdk1. However, it is unknown whether plant viruses may use this mechanism in their pathogenesis. Here, we report that the 17K protein, encoded by barley yellow dwarf viruses and related poleroviruses, delays G2/M transition and disrupts mitosis in both host (barley) and nonhost (fission yeast, Arabidopsis thaliana, and tobacco) cells through interrupting the function of Wee1-Cdc25-CDKA/Cdc2 via direct protein-protein interactions and alteration of CDKA/Cdc2 phosphorylation. When ectopically expressed, 17K disrupts the mitosis of cultured human cells, and HIV-1 Vpr inhibits plant cell growth. Furthermore, 17K and Vpr share similar secondary structural feature and common amino acid residues required for interacting with plant CDKA. Thus, our work reveals a distinct class of mitosis regulators that are conserved between plant and animal viruses and play active roles in viral pathogenesis.

Silicon-induced changes in plant volatiles reduce attractiveness of wheat to the bird cherry-oat aphid Rhopalosiphum padi and attract the parasitoid Lysiphlebus testaceipes

Silicon (Si) supplementation is well-known for enhancing plant resistance to insect pests, however, only recently studies revealed that Si accumulation in the plant not only confers a mechanical barrier to insect feeding, but also primes jasmonic acid-dependent defenses. Here, we examined whether Si supplementation alters wheat volatile emissions that influence the bird cherry-oat aphid (Rhopalosiphum padi) olfactory preference and the aphid parasitoid Lysiphlebus testaceipes. Even though Si accumulation in wheat did not impact aphid performance, we found that R. padi preferred constitutive volatiles from–Si wheat over those emitted by +Si wheat plants. In Y-tube olfactometer bioassays, the parasitoid was attracted to volatiles from +Si uninfested wheat, but not to those from–Si uninfested wheat. +Si and–Si aphid-infested plants released equally attractive blends to the aphid parasitoid; however, wasps were unable to distinguish +Si uninfested plant odors from those of aphid-infested treatments. GC-MS analyses revealed that +Si uninfested wheat plants emitted increased amounts of a single compound, geranyl acetone, compared to -Si uninfested wheat, but similar to those emitted by aphid-infested treatments. By contrast, Si supplementation in wheat did not alter composition of aphid-induced plant volatiles. Our results show that changes in wheat volatile blend induced by Si accumulation mediate the non-preference behavior of the bird cherry-oat aphid and the attraction of its parasitoid L. testaceipes. Conversely to the literature, Si supplementation by itself seems to work as an elicitor of induced defenses in wheat, and not as a priming agent.

Enhanced heat tolerance of viral-infected aphids leads to niche expansion and reduced interspecific competition

Vector-borne pathogens are known to alter the phenotypes of their primary hosts and vectors, with implications for disease transmission as well as ecology. Here we show that a plant virus, barley yellow dwarf virus, increases the surface temperature of infected host plants (by an average of 2 °C), while also significantly enhancing the thermal tolerance of its aphid vector Rhopalosiphum padi (by 8 °C). This enhanced thermal tolerance, which was associated with differential upregulation of three heat-shock protein genes, allowed aphids to occupy higher and warmer regions of infected host plants when displaced from cooler regions by competition with a larger aphid species, R. maidis. Infection thereby led to an expansion of the fundamental niche of the vector. These findings show that virus effects on the thermal biology of hosts and vectors can influence their interactions with one another and with other, non-vector organisms.

Eukaryotic initiation factor (eIF) 3 mediates Barley Yellow Dwarf Viral mRNA 3'-5' UTR interactions and 40S ribosomal subunit binding to facilitate cap-independent translation

Barley Yellow Dwarf Virus (BYDV) is a positive strand RNA virus that lacks the canonical 5′ 7-methylguanosine cap and a 3′ poly-A tail. Instead, BYDV utilizes a cruciform cap independent translation element (CITE) in its 3′UTR RNA (BYDV-like CITE or BTE) that binds eukaryotic translation initiation factor (eIF) 4F and recruits 40S ribosomal subunits in the presence of active helicase factors (eIF4A, eIF4B, eIF4F and ATP). A long-range, 5-nucleotide, base-pairing kissing loop interaction between the 3′BTE and a 5′UTR stem-loop is necessary for translation to initiate. The 40S–eIF complex does not bind to the BYDV 5′UTR, suggesting the involvement of additional factors. We identified eIF3 as a component of the 3′BTE recruited complex using affinity-tagged 3′BTE RNA pull-down assays. Fluorescence anisotropy binding and gel shift assays showed that the 3′BTE and 5′UTR RNAs can simultaneously and non-competitively bind eIF3 in the presence of active helicase factors forming a single, macromolecular complex. Further, quantitative studies showed eIF3 increased recruitment of the 40S subunit by more than 25-fold. We propose a new role for eIF3, where eIF3 bridges BYDV’s UTRs, stabilizes the long-range 5′-3′ interaction, and facilitates recruitment of the 40S–eIF complex to the 5′UTR, leading to translation initiation.

Non-Invasive Characterization of Single-, Double- and Triple-Viral Diseases of Wheat With a Hand-Held Raman Spectrometer

Plant diseases can reduce crop yield by up to 100%. Therefore, timely and confirmatory diagnosis of plant diseases is strongly desired. Typical pathogen assaying methods include polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). These approaches are quite useful but are also time-consuming and destructive to the sample. Raman spectroscopy (RS) is a modern analytical technique that enables non-invasive plant disease detection. In this study, we report on Raman-based detection of wheat diseases caused by wheat streak mosaic virus (WSMV) and barley yellow dwarf virus (BYDV). Our results show that RS can be used to differentiate between healthy wheat and wheat infected by these two viruses. We also show that RS can be used to identify whether wheat is infected by these individual viruses or by a combination of WSMV and BYDV, as well as WSMV, BYDV, and Triticum mosaic virus (TriMV). We found that wheat spectra showed non-linear spectroscopic responses to coinfection by different viruses. These results suggest that RS can be used to probe pathogen-specific changes in plant metabolism. The portable nature of this approach opens the possibility of RS directly in the field for confirmatory diagnostics of viral diseases.

Identification of New QTL Contributing to Barley Yellow Dwarf Virus-PAV (BYDV-PAV) Resistance in Wheat

Barley yellow dwarf (BYD) is a major virus disease which dramatically reduces wheat yield. Introducing BYD resistance genes into commercial varieties has been proven to be effective in reducing damage caused by barley yellow dwarf virus (BYDV). However, only one major resistance gene is readily deployable for breeding; Bdv2 derived from Thinopyrum intermedium is deployed as a chromosomal translocation. In this study, a double haploid (DH) population was developed from a cross between XuBYDV (introduced from China showing very good resistance to BYD) and H-120 (a BYD-sensitive Chinese accession), and was used to identify QTL for BYD resistance. The population was genotyped using an Infinium iSelect bead chip array targeting 90K gene-based SNPs. The disease resistance of DH lines inoculated with BYDV was assessed at the heading stage. The infections were assessed by tissue blot immunoassay (TBIA). Three new QTL were identified on chromosomes 5A, 6A, and 7A for both symptom and TBIA, with all three resistance alleles being inherited from XuBYDV. Some DH lines with the resistance alleles from all three QTL showed high level resistance to BYD. These new QTL will be useful in breeding programs for pyramiding BYD resistance genes.

Catch Me If You Can! RNA Silencing-Based Improvement of Antiviral Plant Immunity

Viruses are obligate parasites which cause a range of severe plant diseases that affect farm productivity around the world, resulting in immense annual losses of yield. Therefore, control of viral pathogens continues to be an agronomic and scientific challenge requiring innovative and ground-breaking strategies to meet the demands of a growing world population. Over the last decade, RNA silencing has been employed to develop plants with an improved resistance to biotic stresses based on their function to provide protection from invasion by foreign nucleic acids, such as viruses. This natural phenomenon can be exploited to control agronomically relevant plant diseases. Recent evidence argues that this biotechnological method, called host-induced gene silencing, is effective against sucking insects, nematodes, and pathogenic fungi, as well as bacteria and viruses on their plant hosts. Here, we review recent studies which reveal the enormous potential that RNA-silencing strategies hold for providing an environmentally friendly mechanism to protect crop plants from viral diseases.

Characterization of an Ohio Isolate of Brome Mosaic Virus and Its Impact on the Development and Yield of Soft Red Winter Wheat

Brome mosaic virus (BMV) is generally thought to be of little economic importance to crops; consequently, there is little information about its impact on wheat production under field conditions. After repeated detection of BMV in Ohio wheat fields at incidences up to 25%, the virus was isolated, sequenced, characterized, and tested for its impact on soft red winter wheat (SRWW). The Ohio isolate of brome mosaic virus (BMV-OH) was found to be >99% identical to a BMV-Fescue isolate (accession no. DQ530423-25) and capable of systemically infecting multiple monocot and dicot species, including cowpea and soybean, in experimental inoculations. BMV-OH was used in field experiments during the 2016 and 2017 growing seasons to quantify its effect on SRWW grain yield and development when inoculated at Feekes 1, 5, 8, and 10 in two to four cultivars. Cultivar and timing of inoculation had statistically significant (P < 0.05) main and interaction effects on grain yield, wheat growth, and multiple components of yield. Compared with noninoculated controls, BMV-OH reduced grain yield by up to 61% when inoculated at Feekes 1 and by as much as 25, 36, and 31% for inoculations at Feekes 5, 8, and 10, respectively. The magnitude of the yield reduction varied among cultivars and was associated with reductions in grain size and weight or plant population. These findings suggest that BMV could impact wheat productivity in Ohio and will serve as the basis for more large-scale investigations of the effects of this virus in commercial fields.

Prevalence and Incidence of Yellow Dwarf Viruses Across a Climatic Gradient: A Four-Year Field Study in Southeastern Australia

Yellow dwarf viruses (YDVs) form a complex of economically important pathogens that affect cereal production worldwide, reducing yield and quality. The prevalence and incidence of YDVs including barley yellow dwarf viruses (BYDV-PAV and BYDV-MAV) and cereal yellow dwarf virus (CYDV-RPV) in cereal fields in Victoria, Australia were measured. As temperature decreases and rainfall increases from north to south in Victoria, fields in three geographical regions were evaluated to determine potential differences in virus prevalence and incidence across the weather gradient. Cereal samples randomly collected from each field during spring for four consecutive years (2014-2017) were tested for BYDV-PAV, BYDV-MAV, and CYDV-RPV using tissue blot immunoassay. BYDV-PAV was the most prevalent YDV species overall and had the highest overall mean incidence. Higher temperature and lower rainfall were associated with reduced prevalence and incidence of YDVs as the northern region, which is hotter and drier, had a 17-fold decrease in virus incidence compared with the cooler and wetter regions. Considerable year-to-year variation in virus prevalence and incidence was observed. This study improves our understanding of virus epidemiology, which will aid the development of more targeted control measures and predictive models. It also highlights the need to monitor for YDVs and their vectors over multiple years to assess the level of risk and to make more informed and appropriate disease management decisions.

Spatiotemporal Distribution and Environmental Drivers of Barley yellow dwarf virus and Vector Abundance in Kansas

Several aphid species transmit barley yellow dwarf, a globally destructive disease caused by viruses that infect cereal grain crops. Data from >400 samples collected across Kansas wheat fields in 2014 and 2015 were used to develop spatiotemporal models predicting the extent to which landcover, temperature and precipitation affect spring aphid vector abundance and presence of individuals carrying Barley yellow dwarf virus (BYDV). The distribution of Rhopalosiphum padi abundance was not correlated with climate or landcover, but Sitobion avenae abundance was positively correlated with fall temperature and negatively correlated to spring temperature and precipitation. The abundance of Schizaphis graminum was negatively correlated with fall precipitation and winter temperature. The incidence of viruliferous (+BYDV) R. padi was positively correlated with fall precipitation but negatively correlated with winter precipitation. In contrast, the probability of +BYDV S. avenae was unaffected by precipitation but was positively correlated with fall temperatures and distance to forest or shrubland. R. padi and S. avenae were more prevalent at eastern sample sites where ground cover is more grassland than cropland, suggesting that grassland may provide over-summering sites for vectors and pose a risk as potential BYDV reservoirs. Nevertheless, land cover patterns were not strongly associated with differences in abundance or the probability that viruliferous aphids were present.

Barley RNA viromes in six different geographical regions in Korea

Barley is a kind of cereal grass belonging to the family Poaceae. To examine viruses infecting winter barley in Korea, we carried out a comprehensive study of barley RNA viromes using next-generation sequencing (NGS). A total of 110 barley leaf samples from 17 geographical locations were collected. NGS followed by extensive bioinformatics analyses revealed six different barley viromes: Barley yellow mosaic virus (BaYMV), Barley mild mosaic virus (BaMMV), Barley yellow dwarf virus (BYDV), Hordeum vulgare endornavirus (HvEV), and Barley virus G (BVG). BaYMV and HvEV were identified in all libraries, while other viruses were identified in some specific library. Based on the number of virus-associated reads, BaYMV was a dominant virus infecting winter barley in Korea causing yellow disease symptoms. We obtained nearly complete genomes of six BaYMV isolates and two BaMMV isolates. Phylogenetic analyses indicate that BaYMV and BaMMV were largely grouped based on geographical regions such as Asia and Europe. Single nucleotide polymorphisms analyses suggested that most BaYMV and BaMMV showed strong genetic variations; however, BaYMV isolate Jeonju and BaMMV isolate Gunsan exhibited a few and no SNPs, respectively, suggesting low level of genetic variation. Taken together, this is the first study of barley RNA viromes in Korea.

Global Ramifications of Dust and Sandstorm Microbiota

Dust and sandstorm events inject substantial quantities of foreign microorganisms into global ecosystems, with the ability to impact distant environments. The majority of these microorganisms originate from deserts and drylands where the soil is laden with highly stress-resistant microbes capable of thriving under extreme environmental conditions, and a substantial portion of them survive long journeys through the atmosphere. This large-scale transmission of highly resilient alien microbial contaminants raises concerns with regards to the invasion of sensitive and/or pristine sink environments, and to human health-concerns exacerbated by increases in the rate of desertification. Further increases in the transport of dust-associated microbiota could extend the spread of foreign microbes to new ecosystems, increase their load in present sink environments, disrupt ecosystem balance, and potentially introduce new pathogens. Our present understanding of these microorganisms, their phylogenic affiliations and functional significance, is insufficient to determine their impact. The purpose of this review is to provide an overview of available data regarding dust and sandstorm microbiota and their potential ramifications on human and ecosystem health. We conclude by discussing current gaps in dust and sandstorm microbiota research, and the need for collaborative studies involving high-resolution meta-omic approaches in conjunction with extensive ecological time-series studies to advance the field towards an improved and sufficient understanding of these invisible atmospheric travelers and their global ramifications.

Molecular and Ultrastructural Mechanisms Underlying Yellow Dwarf Symptom Formation in Wheat after Infection of Barley Yellow Dwarf Virus

Wheat (Tritium aestivum L.) production is essential for global food security. Infection of barley yellow dwarf virus-GAV (BYDV-GAV) results in wheat showing leaf yellowing and plant dwarfism symptom. To explore the molecular and ultrastructural mechanisms underlying yellow dwarf symptom formation in BYDV-GAV-infected wheat, we investigated the chloroplast ultrastructure via transmission electron microscopy (TEM), examined the contents of the virus, H₂O₂, and chlorophyll in Zhong8601, and studied the comparative transcriptome through microarray analyses in the susceptible wheat line Zhong8601 after virus infection. TEM images indicated that chloroplasts in BYDV-GAV-infected Zhong8601 leaf cells were fragmentized. Where thylakoids were not well developed, starch granules and plastoglobules were rare. Compared with mock-inoculated Zhong8601, chlorophyll content was markedly reduced, but the virus and H₂O₂ contents were significantly higher in BYDV-GAV-infected Zhong8601. The transcriptomic analyses revealed that chlorophyll biosynthesis and chloroplast related transcripts, encoding chlorophyll a/b binding protein, glucose-6-phosphate/phosphate translocator 2, and glutamyl-tRNA reductase 1, were down-regulated in BYDV-GAV-infected Zhong8601. Some phytohormone signaling-related transcripts, including abscisic acid (ABA) signaling factors (phospholipase D alpha 1 and calcineurin B-like protein 9) and nine ethylene response factors, were up-regulated. Additionally, reactive oxygen species (ROS)-related genes were transcriptionally regulated in BYDV-GAV infected Zhong8601, including three up-regulated transcripts encoding germin-like proteins (promoting ROS accumulation) and four down-regulated transcripts encoding peroxides (scavenging ROS). These results clearly suggest that the yellow dwarf symptom formation is mainly attributed to reduced chlorophyll content and fragmentized chloroplasts caused by down-regulation of the chlorophyll and chloroplast biosynthesis related genes, ROS excessive accumulation, and precisely transcriptional regulation of the above-mentioned ABA and ethylene signaling- and ROS-related genes in susceptible wheat infected by BYDV-GAV.

Overexpression of the aphid-induced serine protease inhibitor CI2c gene in barley affects the generalist green peach aphid, not the specialist bird cherry-oat aphid

Aphids are serious pests in crop plants. In an effort to identify plant genes controlling resistance against aphids, we have here studied a protease inhibitor, CI2c in barley (Hordeum vulgare L.). The CI2c gene was earlier shown to be upregulated by herbivory of the bird cherry-oat aphid (Rhopalosiphum padi L.) in barley genotypes with moderate resistance against this aphid, but not in susceptible lines. We hypothesized that CI2c contributes to the resistance. To test this idea, cDNA encoding CI2c was overexpressed in barley and bioassays were carried out with R. padi. For comparison, tests were carried out with the green peach aphid (Myzus persicae Sulzer), for which barley is a poor host. The performance of R. padi was not different on the CI2c-overexpressing lines in comparison to controls in test monitoring behavior and fecundity. M. persicae preference was affected as shown in the choice test, this species moved away from control plants, but remained on the CI2c-overexpressing lines. R. padi-induced responses related to defense were repressed in the overexpressing lines as compared to in control plants or the moderately resistant genotypes. A putative susceptibility gene, coding for a β-1,3-glucanase was more strongly induced by aphids in one of the CI2c-overexpressing lines. The results indicate that the CI2c inhibitor in overexpressing lines affects aphid-induced responses by suppressing defense. This is of little consequence to the specialist R.padi, but causes lower non-host resistance towards the generalist M. persicae in barley.

Identification and Characterization of Wheat Yellow Striate Virus, a Novel Leafhopper-Transmitted Nucleorhabdovirus Infecting Wheat

A new wheat viral disease was found in China. Bullet-shaped viral particles within the nucleus of the infected wheat leave cells, which possessed 180-210 nm length and 35-40 nm width, were observed under transmission electron microscopy. A putative wheat-infecting rhabdovirus vectored by the leafhopper Psammotettix alienus was identified and tentatively named wheat yellow striate virus (WYSV). The full-length nucleotide sequence of WYSV was determined using transcriptome sequencing and RACE analysis of both wheat samples and leafhoppers P. alienus. The negative-sense RNA genome of WYSV contains 14,486 nucleotides (nt) and seven open reading frames (ORFs) encode deduced proteins in the order N-P-P3-M-P6-G-L on the antisense strand. In addition, WYSV genome has a 76-nt 3' leader RNA and a 258-nt 5' trailer, and the ORFs are separated by conserved intergenic sequences. The entire genome sequence shares 58.1 and 57.7% nucleotide sequence identity with two strains of rice yellow stunt virus (RYSV-A and RYSV-B) genomes, respectively. The highest amino acid sequence identity was 63.8% between the L proteins of the WYSV and RYSV-B, but the lowest was 29.5% between the P6 proteins of these viruses. Phylogenetic analysis firmly established WYSV as a new member of the genus Nucleorhabdovirus. Collectively, this study provided evidence that WYSV is likely the first nucleorhabdovirus described infecting wheat via leafhopper P. alienus transmission.

Overexpression and Down-Regulation of Barley Lipoxygenase LOX2.2 Affects Jasmonate-Regulated Genes and Aphid Fecundity

Aphids are pests on many crops and depend on plant phloem sap as their food source. In an attempt to find factors improving plant resistance against aphids, we studied the effects of overexpression and down-regulation of the lipoxygenase gene LOX2.2 in barley (Hordeum vulgare L.) on the performance of two aphid species. A specialist, bird cherry-oat aphid (Rhopalosiphum padi L.) and a generalist, green peach aphid (Myzus persicae Sulzer) were studied. LOX2.2 overexpressing lines showed up-regulation of some other jasmonic acid (JA)-regulated genes, and antisense lines showed down-regulation of such genes. Overexpression or suppression of LOX2.2 did not affect aphid settling or the life span on the plants, but in short term fecundity tests, overexpressing plants supported lower aphid numbers and antisense plants higher aphid numbers. The amounts and composition of released volatile organic compounds did not differ between control and LOX2.2 overexpressing lines. Up-regulation of genes was similar for both aphid species. The results suggest that LOX2.2 plays a role in the activation of JA-mediated responses and indicates the involvement of LOX2.2 in basic defense responses.

Coinfection Timing Drives Host Population Dynamics through Changes in Virulence

Infections of one host by multiple parasites are common, and several studies have found that the order of parasite invasion can affect both within-host competition and disease severity. However, it is unclear to what extent coinfection timing might be important to consider when modeling parasite impacts on host populations. Using a model system of two viruses infecting barley, we found that simultaneous infections of the two viruses were significantly more damaging to hosts than sequential coinfections. While priority effects were evident in within-host concentrations of sequential coinfections, priority did not influence any parameters (such as virulence or transmission rate) that affect host population dynamics. We built a susceptible-infected model to examine whether the observed difference in coinfection virulence could impact host population dynamics under a range of scenarios. We found that coinfection timing can have an important but context-dependent effect on projected host population dynamics. Studies that examine only simultaneous coinfections could inflate disease impact predictions.

Environmental Nutrient Supply Directly Alters Plant Traits but Indirectly Determines Virus Growth Rate

Ecological stoichiometry and resource competition theory both predict that nutrient rates and ratios can alter infectious disease dynamics. Pathogens such as viruses hijack nutrient rich host metabolites to complete multiple steps of their epidemiological cycle. As the synthesis of these molecules requires nitrogen (N) and phosphorus (P), environmental supply rates, and ratios of N and P to hosts can directly limit disease dynamics. Environmental nutrient supplies also may alter virus epidemiology indirectly by changing host phenotype or the dynamics of coinfecting pathogens. We tested whether host nutrient supplies and coinfection control pathogen growth within hosts and transmission to new hosts, either directly or through modifications of plant tissue chemistry (i.e., content and stoichiometric ratios of nutrients), host phenotypic traits, or among-pathogen interactions. We examined two widespread plant viruses (BYDV-PAV and CYDV-RPV) in cultivated oats (Avena sativa) grown along a range of N and of P supply rates. N and P supply rates altered plant tissue chemistry and phenotypic traits; however, environmental nutrient supplies and plant tissue content and ratios of nutrients did not directly alter virus titer. Infection with CYDV-RPV altered plant traits and resulted in thicker plant leaves (i.e., higher leaf mass per area) and there was a positive correlation between CYDV-RPV titer and leaf mass per area. CYDV-RPV titer was reduced by the presence of a competitor, BYDV-PAV, and higher CYDV-RPV titer led to more severe chlorotic symptoms. In our experimental conditions, virus transmission was unaffected by nutrient supply rates, co-infection, plant stoichiometry, or plant traits, although nutrient supply rates have been shown to increase infection and coinfection rates. This work provides a robust test of the role of plant nutrient content and ratios in the dynamics of globally important pathogens and reveals a more complex relationship between within-host virus growth and alterations of plant traits. A deeper understanding of the differential effects of environmental nutrient supplies on virus epidemiology and ecology is particularly relevant given the rapid increase of nutrients flowing into Earth's ecosystems as a result of human activities.

Identification, Characterization and Full-Length Sequence Analysis of a Novel Polerovirus Associated with Wheat Leaf Yellowing Disease

To identify the pathogens responsible for leaf yellowing symptoms on wheat samples collected from Jinan, China, we tested for the presence of three known barley/wheat yellow dwarf viruses (BYDV-GAV, -PAV, WYDV-GPV) (most likely pathogens) using RT-PCR. A sample that tested negative for the three viruses was selected for small RNA sequencing. Twenty-five million sequences were generated, among which 5% were of viral origin. A novel polerovirus was discovered and temporarily named wheat leaf yellowing-associated virus (WLYaV). The full genome of WLYaV corresponds to 5,772 nucleotides (nt), with six AUG-initiated open reading frames, one non-AUG-initiated open reading frame, and three untranslated regions, showing typical features of the family Luteoviridae. Sequence comparison and phylogenetic analyses suggested that WLYaV had the closest relationship with sugarcane yellow leaf virus (ScYLV), but the identities of full genomic nucleotides and deduced amino acid sequence of coat protein (CP) were 64.9 and 86.2%, respectively, below the species demarcation thresholds (90%) in the family Luteoviridae. Furthermore, agroinoculation of Nicotiana benthamiana leaves with a cDNA clone of WLYaV caused yellowing symptoms on the plant. Our study adds a new polerovirus that is associated with wheat leaf yellowing disease, which would help to identify and control pathogens of wheat.

Effect of Plant Essential Oils against Rophalosiphum padi on Wheat and Barley

The aim of the present study was to evaluate the effect of six EOs against Rhopalosiphum padi L. (Homoptera: Aphididae). This aphid is one of the most important cereal pests worldwide. EOs used in this study were from cumin ( Cuminum cyminum L.; Apiaceae), hyssop ( Hyssopus officinalis L.; Lamiaceae), costmary ( Tanacetum balsamita L. syn. Chrysanthemum balsamita L. Baill; Asteraceae), lovage ( Levisticum officinale W.D.J. Koch; Apiaceae), lavander ( Lavandula angustifolia Mill; Lamiaceae) and thyme ( Thymus vulgaris L.; Lamiaceae). Cultivar ORCF-10, a soft white winter wheat ( Triticum aestivum L.) cultivar and barley ( Hordeum vulgare L.) cv. ‘Alba’ were selected for the bioassays. Each EO was diluted with acetone (w/v) at different concentrations ranging from 0.1 % to 1.0 %. Number of R. padi landing on treated or untreated leaves was counted 24 h after the initial aphid transfer. In general, the repellency effect increased as the concentration of EOs increased regardless of treatments with the exception of lovage. Regardless of the substrate and treatment, lower numbers of aphids were observed on treated leaves at higher concentrations compared to the untreated control stressing the repellency effect. This study demonstrated that the EOs of cumin, hyssop, costmary, lavender, and thyme may have potential for aphids control.

Resistance of Select Winter Wheat (Triticum aestivum) Cultivars to Rhopalosiphum padi (Hemiptera: Aphididae)

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a global pest of wheat and vectors some of the most damaging strains of barley yellow dwarf virus (BYDV). In years of heavy R. padi infestation, R. padi and BYDV together reduce wheat yields by 30-40% in Kansas and other states of the U.S. Great Plains wheat production area. Cultivation of wheat cultivars resistant to R. padi can greatly reduce production costs and mitigate R. padi-BYDV yield losses, and increase producer profits. This study identified cultivars of hard red and soft white winter wheat with R. padi resistance that suppress R. padi populations or tolerate the effects of R. padi feeding damage. 'Pioneer (S) 25R40,' 'MFA (S) 2248,' 'Pioneer (S) 25R77,' and 'Limagrain LCS Mint' significantly reduced R. padi populations. MFA (S) 2248, Pioneer (S) 25R40, and 'Limagrain LS Wizard' exhibited tolerance expressed as significantly greater aboveground biomass. These findings are significant in that they have identified wheat cultivars currently available to producers, enabling the immediate improvement of tactics to manage R. padi and BYDV in heavily infested areas. Secondarily, these results identify cultivars that are good candidates for use in breeding and genetic analyses of arthropod resistance genes in wheat.

Crop-associated virus reduces the rooting depth of non-crop perennial native grass more than non-crop-associated virus with known viral suppressor of RNA silencing (VSR)

As agricultural acreage expanded and came to dominate landscapes across the world, viruses gained opportunities to move between crop and wild native plants. In the Midwestern USA, virus exchange currently occurs between widespread annual Poaceae crops and remnant native perennial prairie grasses now under consideration as bioenergy feedstocks. In this region, the common aphid species Rhopalosiphum padi L. (the bird cherry-oat aphid) transmits several virus species in the family Luteoviridae, including Barley yellow dwarf virus (BYDV-PAV, genus Luteovirus) and Cereal yellow dwarf virus (CYDV-RPV and -RPS, genus Polerovirus). The yellow dwarf virus (YDV) species in these two genera share genetic similarities in their 3'-ends, but diverge in the 5'-regions. Most notably, CYDVs encode a P0 viral suppressor of RNA silencing (VSR) absent in BYDV-PAV. Because BYDV-PAV has been reported more frequently in annual cereals and CYDVs in perennial non-crop grasses, we examine the hypothesis that the viruses' genetic differences reflect different affinities for crop and non-crop hosts. Specifically, we ask (i) whether CYDVs might persist within and affect a native non-crop grass more strongly than BYDV-PAV, on the grounds that the polerovirus VSR could better moderate the defenses of a well-defended perennial, and (ii) whether the opposite pattern of effects might occur in a less defended annual crop. Because previous work found that the VSR of CYDV-RPS possessed greater silencing suppressor efficiency than that of CYDV-RPV, we further explored (iii) whether a novel grass-associated CYDV-RPS isolate would influence a native non-crop grass more strongly than a comparable CYDV-RPV isolate. In growth chamber studies, we found support for this hypothesis: only grass-associated CYDV-RPS stunted the shoots and crowns of Panicum virgatum L. (switchgrass), a perennial native North American prairie grass, whereas crop-associated BYDV-PAV (and coinfection with BYDV-PAV and CYDV-RPS) most stunted annual Avena sativa L. (oats). These findings suggest that some of the diversity in grass-infecting Luteoviridae reflects viral capacity to modulate defenses in different host types. Intriguingly, while all virus treatments also reduced root production in both host species, only crop-associated BYDV-PAV (or co-infection) reduced rooting depths. Such root effects may increase host susceptibility to drought, and indicate that BYDV-PAV pathogenicity is determined by something other than a P0 VSR. These findings contribute to growing evidence that pathogenic crop-associated viruses may harm native species as well as crops. Critical next questions include the extent to which crop-associated selection pressures drive viral pathogenesis.

Virus incidence in wheat increases under elevated CO2: A 4-year study of yellow dwarf viruses from a free air carbon dioxide facility

The complexities behind the mechanisms associated with virus-host-vector interactions of vector-transmitted viruses, and their consequences for disease development need to be understood to reduce virus spread and disease severity. Climate has a substantial effect on viruses, vectors, host plants and their interactions. Increased atmospheric carbon dioxide (CO₂) is predicted to impact the interactions between them. This study, conducted under ambient and elevated CO₂ (550μmolmol^-1), in the Australian Grains Free Air Carbon Enrichment facility reports on natural yellow dwarf virus incidence on wheat (including Barley/Cereal yellow dwarf viruses (B/CYDV)). A range of wheat cultivars was tested using tissue blot immunoassay to determine the incidence of four yellow dwarf virus species from 2013 to 2016. In 2013, 2014 and 2016, virus incidence was high, reaching upwards of 50%, while in 2015 it was relatively low, with a maximum incidence of 3%. Across all years and most cultivars, BYDV-PAV was the most prevalent virus species. In the years with high virus incidence, a majority plots with the elevated levels of CO₂ (eCO₂) were associated with increased levels of virus relative to the plots with ambient CO₂. In 2013, 2014 and 2016 the recorded mean percent virus incidence was higher under elevated CO₂ when compared to ambient CO₂ by 33%, 14% and 34%, respectively. The mechanism behind increased yellow dwarf virus incidence under elevated CO₂ is not well understood. Potential factors involved in the higher virus incidence under elevated CO₂ conditions are discussed.

Eukaryotic translation initiation factor 4G (eIF4G) coordinates interactions with eIF4A, eIF4B, and eIF4E in binding and translation of the barley yellow dwarf virus 3' cap-independent translation element (BTE)

Barley yellow dwarf virus RNA, lacking a 5' cap and a 3' poly(A) tail, contains a cap-independent translation element (BTE) in the 3'-untranslated region that interacts with host translation initiation factor eIF4G. To determine how eIF4G recruits the mRNA, three eIF4G deletion mutants were constructed: (i) eIF4G601-1196, containing amino acids 601-1196, including the putative BTE-binding region, and binding domains for eIF4E, eIF4A, and eIF4B; (ii) eIF4G601-1488, which contains an additional C-terminal eIF4A-binding domain; and (iii) eIF4G742-1196, which lacks the eIF4E-binding site. eIF4G601-1196 binds BTE tightly and supports efficient translation. The helicase complex, consisting of eIF4A, eIF4B, and ATP, stimulated BTE binding with eIF4G601-1196 but not eIF4G601-1488, suggesting that the eIF4A binding domains may serve a regulatory role, with the C-terminal binding site having negative effects. eIF4E binding to eIF4G601-1196 induced a conformational change, significantly increasing the binding affinity to BTE. A comparison of the binding of eIF4G deletion mutants with BTEs containing mutations showed a general correlation between binding affinity and ability to facilitate translation. In summary, these results reveal a new role for the helicase complex in 3' cap-independent translation element-mediated translation and show that the functional core domain of eIF4G plus an adjacent probable RNA-binding domain mediate translation initiation.

Localization of Barley yellow dwarf virus Movement Protein Modulating Programmed Cell Death in Nicotiana benthamiana

Barley yellow dwarf virus (BYDV) belongs to Luteovirus and is limited only at phloem related tissues. An open reading frame (ORF) 4 of BYDV codes for the movement protein (MP) of BYDV gating plasmodesmata (PD) to facilitate virus movement. Like other Luteoviruses, ORF 4 of BYDV is embedded in the ORF3 but expressed from the different reading frame in leaky scanning manner. Although MP is a very important protein for systemic infection of BYDV, there was a little information. In this study, MP was characterized in terms of subcellular localization and programmed cell death (PCD). Gene of MP or its mutant (ΔMP) was expressed by Agroinfiltration method. MP was clearly localized at the nucleus and the PD, but ΔMP which was deleted distal N-terminus of MP showed no localization to PD exhibited the different target with original MP. In addition to PD localization, MP appeared associated with small granules in cytoplasm whereas ΔMP did not. MP associated with PD and small granules induced PCD, but ΔMP showed no association with PD and small granules did not exhibit PCD. Based on this study, the distal N-terminal region within MP is seemingly responsible for the localization of PD and the induction small granules and PCD induction. These results suggest that subcellular localization of BYDV MP may modulate the PCD in Nicotiana benthamiana.

The complete genomic sequence of a tentative new polerovirus identified in barley in South Korea

The complete nucleotide sequence of a new barley polerovirus, tentatively named barley virus G (BVG), which was isolated in Gimje, South Korea, has been determined using an RNA sequencing technique combined with polymerase chain reaction methods. The viral genomic RNA of BVG is 5,620 nucleotides long and contains six typical open reading frames commonly observed in other poleroviruses. Sequence comparisons revealed that BVG is most closely related to maize yellow dwarf virus-RMV, with the highest amino acid identities being less than 90 % for all of the corresponding proteins. These results suggested that BVG is a member of a new species in the genus Polerovirus.

Occurrence of Viruses and Associated Grain Yields of Paired Symptomatic and Nonsymptomatic Tillers in Kansas Winter Wheat Fields

Vector-borne virus diseases of wheat are recurrent in nature and pose significant threats to crop production worldwide. In the spring of 2011 and 2012, a state-wide sampling survey of multiple commercial field sites and university-managed Kansas Agricultural Experiment Station variety performance trial locations spanning all nine crop-reporting regions of the state was conducted to determine the occurrence of Barley yellow dwarf virus-PAV (BYDV-PAV), Cereal yellow dwarf virus-RPV, Wheat streak mosaic virus (WSMV), High plains virus, Soilborne wheat mosaic virus, and Wheat spindle streak mosaic virus using enzyme-linked immunosorbent assays (ELISA). As a means of directly coupling tiller infection status with tiller grain yield, multiple pairs of symptomatic and nonsymptomatic plants were selected and individual tillers were tagged for virus species and grain yield determination at the variety performance trial locations. BYDV-PAV and WSMV were the two most prevalent species across the state, often co-occurring within location. Of those BYDV-PAV- or WSMV-positive tillers, 22% and 19%, respectively, were nonsymptomatic, a finding that underscores the importance of sampling criteria to more accurately assess virus occurrence in winter wheat fields. Symptomatic tillers that tested positive for BYDV-PAV produced significantly lower grain yields compared with ELISA-negative tillers in both seasons, as did WSMV-positive tillers in 2012. Nonsymptomatic tillers that tested positive for either of the two viruses in 2011 produced significantly lower grain yields than tillers from nonsymptomatic, ELISA-negative plants, an indication that these tillers were physiologically compromised in the absence of virus-associated symptoms. Overall, the virus survey and tagged paired-tiller sampling strategy revealed effects of virus infection on grain yield of individual tillers of plants grown under field conditions and may provide a complementary approach toward future estimates of the impact of virus incidence on crop health in Kansas.

Brachypodium distachyon is a suitable host plant for study of Barley yellow dwarf virus

Barley yellow dwarf viruses (BYDVs) belong to the family Luteoviridae and cause disease in cereals. Because of the large and complex genome of cereal plants, it is difficult to study host-virus interactions. In order to establish a model host system for the studies on BYDVs, we examined the susceptibility of a monocot model plant, Brachypodium distachyon, to BYDV-GAV infection. Fourteen days after BYDV-GAV inoculation by aphid transmission, B. distachyon plants (inbred line Bd21-3) showed conspicuous disease symptoms such as leaf reddening, dwarfness and root stunting. Virus accumulation was detected in both shoots and roots using reverse transcription PCR and triple antibody sandwich ELISA. Compared with infected wheat plants, B. distachyon plants developed more severe disease symptoms and accumulated a higher level of BYDV-GAV. Under transmission electron microscope, we observed that virus particles accumulated in companion cells and BYDV-GAV infection was associated with the deformation of chloroplasts in the infected leaves of B. distachyon plants. Our results suggest that B. distachyon is a suitable and promising experimental model plant for the host-BYDV-GAV pathosystem and possibly for other BYDVs.

Methodological Guidelines for Accurate Detection of Viruses in Wild Plant Species

Ecological understanding of disease risk, emergence, and dynamics and of the efficacy of control strategies relies heavily on efficient tools for microorganism identification and characterization. Misdetection, such as the misclassification of infected hosts as healthy, can strongly bias estimates of disease prevalence and lead to inaccurate conclusions. In natural plant ecosystems, interest in assessing microbial dynamics is increasing exponentially, but guidelines for detection of microorganisms in wild plants remain limited, particularly so for plant viruses. To address this gap, we explored issues and solutions associated with virus detection by serological and molecular methods in noncrop plant species as applied to the globally important Barley yellow dwarf virus PAV (Luteoviridae), which infects wild native plants as well as crops. With enzyme-linked immunosorbent assays (ELISA), we demonstrate how virus detection in a perennial wild plant species may be much greater in stems than in leaves, although leaves are most commonly sampled, and may also vary among tillers within an individual, thereby highlighting the importance of designing effective sampling strategies. With reverse transcription-PCR (RT-PCR), we demonstrate how inhibitors in tissues of perennial wild hosts can suppress virus detection but can be overcome with methods and products that improve isolation and amplification of nucleic acids. These examples demonstrate the paramount importance of testing and validating survey designs and virus detection methods for noncrop plant communities to ensure accurate ecological surveys and reliable assumptions about virus dynamics in wild hosts.