Plant virus HC-Pro is a determinant of eriophyid mite transmission

Helper Component-Proteinase of Triticum Mosaic Virus Is a Viral Determinant of Wheat Curl Mite Transmission

Triticum mosaic virus (TriMV; genus Poacevirus; family Potyviridae) is an economically important virus in the Great Plains region of the United States. TriMV is transmitted by the wheat curl mite (Aceria tosichella) Type 2 genotype but not by Type 1. Helper component-proteinase (HC-Pro) is a vector transmission determinant for several potyvirids, but the role of HC-Pro in TriMV transmission is unknown. In this study, we examined the requirement of the HC-Pro cistron of TriMV for wheat curl mite (Type 2) transmission through deletion and point mutations and constructing TriMV chimeras with heterologous HC-Pros from other potyvirids. TriMV with complete deletion of HC-Pro failed to be transmitted by wheat curl mites at detectable levels. Furthermore, TriMV chimeras with heterologous HC-Pros from aphid-transmitted turnip mosaic virus and tobacco etch virus, or wheat curl mite-transmitted wheat streak mosaic virus, failed to be transmitted by wheat curl mites. These data suggest that heterologous HC-Pros did not complement TriMV for wheat curl mite transmission. A decreasing series of progressive nested in-frame deletions at the N-terminal region of HC-Pro comprising amino acids 3 to 125, 3 to 50, 3 to 25, 3 to 15, 3 to 8, and 3 and 4 abolished TriMV transmission by wheat curl mites. Additionally, mutation of conserved His20, Cys49, or Cys52 to Ala in HC-Pro abolished TriMV transmissibility by wheat curl mites. These data suggest that the N-terminal region of HC-Pro is crucial for TriMV transmission by wheat curl mites. Collectively, these data demonstrate that the HC-Pro cistron of TriMV is a viral determinant for wheat curl mite transmission.

Rubisco small subunit (RbCS) is co-opted by potyvirids as the scaffold protein in assembling a complex for viral intercellular movement

Plant viruses must move through plasmodesmata (PD) to complete their life cycles. For viruses in the Potyviridae family (potyvirids), three viral factors (P3N-PIPO, CI, and CP) and few host proteins are known to participate in this event. Nevertheless, not all the proteins engaging in the cell-to-cell movement of potyvirids have been discovered. Here, we found that HCPro2 encoded by areca palm necrotic ring spot virus (ANRSV) assists viral intercellular movement, which could be functionally complemented by its counterpart HCPro from a potyvirus. Affinity purification and mass spectrometry identified several viral factors (including CI and CP) and host proteins that are physically associated with HCPro2. We demonstrated that HCPro2 interacts with both CI and CP in planta in forming PD-localized complexes during viral infection. Further, we screened HCPro2-associating host proteins, and identified a common host protein in Nicotiana benthamiana-Rubisco small subunit (NbRbCS) that mediates the interactions of HCPro2 with CI or CP, and CI with CP. Knockdown of NbRbCS impairs these interactions, and significantly attenuates the intercellular and systemic movement of ANRSV and three other potyvirids (turnip mosaic virus, pepper veinal mottle virus, and telosma mosaic virus). This study indicates that a nucleus-encoded chloroplast-targeted protein is hijacked by potyvirids as the scaffold protein to assemble a complex to facilitate viral movement across cells.

The HC-Pro cistron of Triticum mosaic virus is dispensable for systemic infection in wheat but is required for symptom phenotype and efficient genome amplification

Triticum mosaic virus (TriMV), the type species of the genus Poacevirus in the family Potyviridae, is an economically important wheat curl mite-transmitted wheat-infecting virus in the Great Plains region of the USA. In this study, the functional genomics of helper component-proteinase (HC-Pro) encoded by TriMV was examined using a reverse genetics approach. TriMV with complete deletion of HC-Pro cistron elicited systemic infection in wheat, indicating that HC-Pro cistron is dispensable for TriMV systemic infection. However, TriMV lacking HC-Pro caused delayed systemic infection with mild symptoms that resulted in little or no stunting of plants with a significant reduction in the accumulation of genomic RNA copies and coat protein (CP). Sequential deletion mutagenesis from the 5' end of HC-Pro cistron in the TriMV genome revealed that deletions within amino acids 3 to 25, except for amino acids 3 and 4, elicited mild symptoms with reduced accumulation of genomic RNA and CP. Surprisingly, TriMV with deletion of amino acids 3 to 50 or 3 to 125 in HC-Pro elicited severe symptoms with a substantial increase in genomic RNA copies but a drastic reduction in CP accumulation. Additionally, TriMV with heterologous HC-Pro from other potyvirids produced symptom phenotype and genomic RNA accumulation similar to that of TriMV without HC-Pro, suggesting that HC-Pros of other potyvirids were not effective in complementing TriMV in wheat. Our data indicate that HC-Pro is expendable for replication of TriMV but is required for efficient viral genomic RNA amplification and symptom development. The availability of TriMV with various deletions in the HC-Pro cistron will facilitate the examination of the requirement of HC-Pro for wheat curl mite transmission.

6K1, NIa-VPg, NIa-Pro, and CP of Wheat Streak Mosaic Virus Are Collective Determinants of Wheat Streak Mosaic Disease in Wheat

Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) is the causal agent of the most economically important wheat streak mosaic disease of wheat (Triticum aestivum) in the Great Plains region of the United States. WSMV determinants responsible for wheat streak mosaic disease in wheat are unknown. Triticum mosaic virus (TriMV), a wheat-infecting virus, was used as an expression vector for the transient expression of each of the WSMV-encoded cistrons in wheat. WSMV-encoded 6K1, NIa-VPg, NIa-Pro, and CP cistrons in TriMV elicited symptoms specific to different stages of wheat streak mosaic disease without significantly affecting the genomic RNA accumulation. WSMV 6K1 produced early wheat streak mosaic disease-like symptoms of severe chlorotic streaks and patches. NIa-VPg and CP caused severe chlorotic streaks, followed by moderate stunting (only with NIa-VPg) of wheat, mimicking early- and mid-stage symptoms of wheat streak mosaic disease. WSMV NIa-Pro caused mild chlorotic streaks, followed by dark green leaves with severe stunting, representing the late symptoms of wheat streak mosaic disease. Collectively, these data suggest that cumulative effects of WSMV-encoded 6K1, NIa-VPg, NIa-Pro, and CP are responsible for different stages of wheat streak mosaic disease symptoms in wheat. Furthermore, deletion analysis of wheat streak mosaic disease determinants revealed that complete 6K1 and NIa-Pro, amino acids 3 to 60 and 121 to 197 of NIa-VPg, and amino acids 101 to 294 of CP are responsible for wheat streak mosaic disease-like symptoms in wheat. This study suggests that management strategies for wheat streak mosaic disease in wheat should target WSMV determinants of the disease phenotype.

Complete genome sequence of zoysia mosaic virus, a novel member of the genus Poacevirus

Here, we report the detection and characterization of the genome of a novel poacevirus isolated from Zoysia matrella (Merrill) imported into the United States from Japan. The novel virus, tentatively named "zoysia mosaic virus" (ZoMV), is a single-stranded RNA virus with a genome of 9,728 nucleotides (nt) in length, encoding a large putative polyprotein of 3,119 amino acids (aa). The ZoMV genome is closely related to the triticum mosaic virus (TriMV; FJ263671) genome, with 57.18% nt and 51.74% aa sequence identity in the polyprotein region. Moreover, phylogenetic analysis showed that ZoMV is closely related to all other members of the genus Poacevirus. A survey of imported grasses showed that ZoMV was detected only in zoysiagrass. This is the first report of the complete genome sequence of a novel viral pathogen of zoysiagrass of the genus Poacevirus, for which we propose the binomial species name "Poacevirus zoisiae".

A Newly Identified Virus in the Family Potyviridae Encodes Two Leader Cysteine Proteases in Tandem That Evolved Contrasting RNA Silencing Suppression Functions

Potyviridae is the largest family of plant-infecting RNA viruses and includes many agriculturally and economically important viral pathogens. The viruses in the family, known as potyvirids, possess single-stranded, positive-sense RNA genomes with polyprotein processing as a gene expression strategy. The N-terminal regions of potyvirid polyproteins vary greatly in sequence. Previously, we identified a novel virus species within the family, Areca palm necrotic spindle-spot virus (ANSSV), which was predicted to encode two cysteine proteases, HCPro1 and HCPro2, in tandem at the N-terminal region. Here, we present evidence showing self-cleavage activity of these two proteins and define their cis-cleavage sites. We demonstrate that HCPro2 is a viral suppressor of RNA silencing (VSR), and both the variable N-terminal and conserved C-terminal (protease domain) moieties have antisilencing activity. Intriguingly, the N-terminal region of HCPro1 also has RNA silencing suppression activity, which is, however, suppressed by its C-terminal protease domain, leading to the functional divergence of HCPro1 and HCPro2 in RNA silencing suppression. Moreover, the deletion of HCPro1 or HCPro2 in a newly created infectious clone abolishes viral infection, and the deletion mutants cannot be rescued by addition of corresponding counterparts of a potyvirus. Altogether, these data suggest that the two closely related leader proteases of ANSSV have evolved differential and essential functions to concertedly maintain viral viability.IMPORTANCE The Potyviridae represent the largest group of known plant RNA viruses and account for more than half of the viral crop damage worldwide. The leader proteases of viruses within the family vary greatly in size and arrangement and play key roles during the infection. Here, we experimentally demonstrate the presence of a distinct pattern of leader proteases, HCPro1 and HCPro2 in tandem, in a newly identified member within the family. Moreover, HCPro1 and HCPro2, which are closely related and typically characterized with a short size, have evolved contrasting RNA silencing suppression activity and seem to function in a coordinated manner to maintain viral infectivity. Altogether, the new knowledge fills a missing piece in the evolutionary relationship history of potyvirids and improves our understanding of the diversification of potyvirid genomes.

Aphid Transmission of Potyvirus: The Largest Plant-Infecting RNA Virus Genus

Potyviruses are the largest group of plant infecting RNA viruses that cause significant losses in a wide range of crops across the globe. The majority of viruses in the genus Potyvirus are transmitted by aphids in a non-persistent, non-circulative manner and have been extensively studied vis-à-vis their structure, taxonomy, evolution, diagnosis, transmission, and molecular interactions with hosts. This comprehensive review exclusively discusses potyviruses and their transmission by aphid vectors, specifically in the light of several virus, aphid and plant factors, and how their interplay influences potyviral binding in aphids, aphid behavior and fitness, host plant biochemistry, virus epidemics, and transmission bottlenecks. We present the heatmap of the global distribution of potyvirus species, variation in the potyviral coat protein gene, and top aphid vectors of potyviruses. Lastly, we examine how the fundamental understanding of these multi-partite interactions through multi-omics approaches is already contributing to, and can have future implications for, devising effective and sustainable management strategies against aphid-transmitted potyviruses to global agriculture.

The Potyviruses: An Evolutionary Synthesis Is Emerging

In this review, encouraged by the dictum of Theodosius Dobzhansky that "Nothing in biology makes sense except in the light of evolution", we outline the likely evolutionary pathways that have resulted in the observed similarities and differences of the extant molecules, biology, distribution, etc. of the potyvirids and, especially, its largest genus, the potyviruses. The potyvirids are a family of plant-infecting RNA-genome viruses. They had a single polyphyletic origin, and all share at least three of their genes (i.e., the helicase region of their CI protein, the RdRp region of their NIb protein and their coat protein) with other viruses which are otherwise unrelated. Potyvirids fall into 11 genera of which the potyviruses, the largest, include more than 150 distinct viruses found worldwide. The first potyvirus probably originated 15,000-30,000 years ago, in a Eurasian grass host, by acquiring crucial changes to its coat protein and HC-Pro protein, which enabled it to be transmitted by migrating host-seeking aphids. All potyviruses are aphid-borne and, in nature, infect discreet sets of monocotyledonous or eudicotyledonous angiosperms. All potyvirus genomes are under negative selection; the HC-Pro, CP, Nia, and NIb genes are most strongly selected, and the PIPO gene least, but there are overriding virus specific differences; for example, all turnip mosaic virus genes are more strongly conserved than those of potato virus Y. Estimates of dN/dS (ω) indicate whether potyvirus populations have been evolving as one or more subpopulations and could be used to help define species boundaries. Recombinants are common in many potyvirus populations (20%-64% in five examined), but recombination seems to be an uncommon speciation mechanism as, of 149 distinct potyviruses, only two were clear recombinants. Human activities, especially trade and farming, have fostered and spread both potyviruses and their aphid vectors throughout the world, especially over the past five centuries. The world distribution of potyviruses, especially those found on islands, indicates that potyviruses may be more frequently or effectively transmitted by seed than experimental tests suggest. Only two meta-genomic potyviruses have been recorded from animal samples, and both are probably contaminants.

The Biological Impact of the Hypervariable N-Terminal Region of Potyviral Genomes

Potyviridae is the largest family of plant-infecting RNA viruses, encompassing over 30% of known plant viruses. The family is closely related to animal picornaviruses such as enteroviruses and belongs to the picorna-like supergroup. Like all other picorna-like viruses, potyvirids employ polyprotein processing as a gene expression strategy and have single-stranded, positive-sense RNA genomes, most of which are monopartite with a long open reading frame. The potyvirid polyproteins are highly conserved in the central and carboxy-terminal regions. In contrast, the N-terminal region is hypervariable and contains position-specific mutations resulting from transcriptional slippage during viral replication, leading to translational frameshift to produce additional viral proteins essential for viral infection. Some potyvirids even lack one of the N-terminal proteins P1 or helper component-protease and have a genus-specific or species-specific protein instead. This review summarizes current knowledge about the conserved and divergent features of potyvirid genomes and biological relevance and discusses future research directions.

Wheat streak mosaic virus alters the transcriptome of its vector, wheat curl mite (Aceria tosichella Keifer), to enhance mite development and population expansion

Wheat streak mosaic virus (WSMV; genus Tritimovirus; family Potyviridae) is an economically important wheat virus that is transmitted by the wheat curl mite (WCM; Aceria tosichella Keifer) in a persistent manner. Virus-vector coevolution may potentially influence vector gene expression to prolong viral association and thus increase virus transmission efficiency and spread. To understand the transcriptomic responses of WCM to WSMV, RNA sequencing was performed to assemble and analyse transcriptomes of WSMV viruliferous and aviruliferous mites. Among 7291 de novo-assembled unigenes, 1020 were differentially expressed between viruliferous and aviruliferous WCMs using edgeR at a false discovery rate ≤0.05. Differentially expressed unigenes were enriched for 108 gene ontology terms, with the majority of the unigenes showing downregulation in viruliferous mites in comparison to only a few unigenes that were upregulated. Protein family and metabolic pathway enrichment analyses revealed that most downregulated unigenes encoded enzymes and proteins linked to stress response, immunity and development. Mechanistically, these predicted changes in mite physiology induced by viral association could be suggestive of pathways needed for promoting virus-vector interactions. Overall, our data suggest that transcriptional changes in viruliferous mites facilitate prolonged viral association and alter WCM development to expedite population expansion, both of which could enhance viral transmission.

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

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

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

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

Genetics and mechanisms underlying transmission of Wheat streak mosaic virus by the wheat curl mite

Wheat streak mosaic virus (WSMV, genus Tritimovirus; family Potyviridae) is the most economically important virus of wheat in the Great Plains region of the USA. WSMV is transmitted by the eriophyid wheat curl mite (WCM), Aceria tosichella Keifer. In contrast to Hemipteran-borne plant viruses, the mode and mechanism of eriophyid mite transmission of viruses have remained poorly understood, mostly due to difficulty of working with these ∼200 μm long microscopic creatures. Among eriophyid-transmitted plant viruses, relatively extensive work has been performed on population genetics of WCMs, WSMV determinants involved in WCM transmission, and localization of WSMV virions and inclusion bodies in WCMs. The main focus of this review is to appraise readers on WCM, WSMV encoded proteins required for WCM transmission and further details and questions on the mode of WSMV transmission by WCMs, and potential advances in management strategies for WCMs and WSMV with increased understanding of transmission.

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.

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

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

Wheat streak mosaic virus Coat Protein Deletion Mutants Elicit More Severe Symptoms Than Wild-Type Virus in Multiple Cereal Hosts

Previously, we reported that coat protein (CP) of Wheat streak mosaic virus (WSMV) (genus Tritimovirus, family Potyviridae) tolerates deletion of amino acids 36 to 84 for efficient systemic infection of wheat. In this study, we demonstrated that WSMV mutants with deletion of CP amino acids 58 to 84 but not of 36 to 57 induced severe chlorotic streaks and spots, followed by acute chlorosis in wheat, maize, barley, and rye compared with mild to moderate chlorotic streaks and mosaic symptoms by wild-type virus. Deletion of CP amino acids 58 to 84 from the WSMV genome accelerated cell-to-cell movement, with increased accumulation of genomic RNAs and CP, compared with the wild-type virus. Microscopic examination of wheat tissues infected by green fluorescent protein-tagged mutants revealed that infection by mutants lacking CP amino acids 58 to 84 caused degradation of chloroplasts, resulting in acute macroscopic chlorosis. The profile of CP-specific proteins was altered in wheat infected by mutants causing acute chlorosis, compared with mutants eliciting wild-type symptoms. All deletion mutants accumulated CP-specific major protein similarly to that in wild-type virus; however, mutants that elicit acute chlorosis failed to accumulate a 31-kDa minor protein compared with wild-type virus or mutants lacking amino acids 36 to 57. Taken together, these data suggest that deletion of CP amino acids 58 to 84 from the WSMV genome enhanced accumulation of CP and genomic RNA, altered CP-specific protein profiles, and caused severe symptom phenotypes in multiple cereal hosts.

Plant insects and mites uptake double-stranded RNA upon its exogenous application on tomato leaves

Exogenously applied double-stranded RNA (dsRNA) molecules onto tomato leaves, moved rapidly from local to systemic leaves and were uptaken by agricultural pests namely aphids, whiteflies and mites. Four small interfering RNAs, deriving from the applied dsRNA, were molecularly detected in plants, aphids and mites but not in whiteflies. Double-stranded RNA (dsRNA) acts as the elicitor molecule of the RNA silencing (RNA interference, RNAi), the endogenous and evolutionary conserved surveillance system present in all eukaryotes. DsRNAs and their subsequent degradation products, namely the small interfering RNAs (siRNAs), act in a sequence-specific manner to control gene expression. Exogenous application of dsRNAs onto plants elicits resistance against plant viruses. In the present work, exogenously applied dsRNA molecules, derived from Zucchini yellow mosaic virus (ZYMV) HC-Pro region, onto tomato plants were detected in aphids (Myzus persicae), whiteflies (Trialeurodes vaporariorum) and mites (Tetranychus urticae) that were fed on treated as well as systemic tomato leaves. Furthermore, four siRNAs, deriving from the dsRNA applied, were detected in tomato and the agricultural pests fed on treated tomato plants. More specifically, dsRNA was detected in agricultural pests at 3 and 10 dpt (days post treatment) in dsRNA-treated leaves and at 14 dpt in systemic leaves. In addition, using stem-loop RT-PCR, siRNAs were detected in agricultural pests at 3 and 10 dpt in aphids and mites. Surprisingly, in whiteflies carrying the applied dsRNA, siRNAs were not molecularly detected. Our results showed that, upon exogenous application of dsRNAs molecules, these moved rapidly within tomato and were uptaken by agricultural pests fed on treated tomato. As a result, this non-transgenic method has the potential to control important crop pests via RNA silencing of vital genes of the respective pests.

Wheat streak mosaic virus coat protein is a host-specific long-distance transport determinant in oat

Viral determinants involved in systemic infection of hosts by monocot-infecting plant viruses are poorly understood. Wheat streak mosaic virus (WSMV, genus Tritimovirus, family Potyviridae) exclusively infects monocotyledonous crops such as wheat, oat, barley, maize, triticale, and rye. Previously, we reported that WSMV CP amino acids 36-84 are expendable for systemic infection of wheat, maize, barley and rye. In this study, the role of coat protein (CP) in systemic infection of oat by WSMV was examined by using a series of viable deletion mutants. WSMV bearing deletions within or encompassing all of amino acids 36-57 efficiently infected oat, indicating that these amino acids are dispensable for systemic infection of oat. However, WSMV mutants lacking CP amino acids 58-84 or 85-100 failed to systemically infect oat. Furthermore, green fluorescent protein-tagged WSMV mutants lacking CP amino acids 58-100 elicited local foci in oat but failed to enter the vasculature. These data suggest that CP amino acids 58-100 are required for systemic infection of oat by WSMV by specifically facilitating virus long-distance transport in oat.

The Coat Protein and NIa Protease of Two Potyviridae Family Members Independently Confer Superinfection Exclusion

Superinfection exclusion (SIE) is an antagonistic virus-virus interaction whereby initial infection by one virus prevents subsequent infection by closely related viruses. Although SIE has been described in diverse viruses infecting plants, humans, and animals, its mechanisms, including involvement of specific viral determinants, are just beginning to be elucidated. In this study, SIE determinants encoded by two economically important wheat viruses, Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) and Triticum mosaic virus (TriMV; genus Poacevirus, family Potyviridae), were identified in gain-of-function experiments that used heterologous viruses to express individual virus-encoded proteins in wheat. Wheat plants infected with TriMV expressing WSMV P1, HC-Pro, P3, 6K1, CI, 6K2, NIa-VPg, or NIb cistrons permitted efficient superinfection by WSMV expressing green fluorescent protein (WSMV-GFP). In contrast, wheat infected with TriMV expressing WSMV NIa-Pro or coat protein (CP) substantially excluded superinfection by WSMV-GFP, suggesting that both of these cistrons are SIE effectors encoded by WSMV. Importantly, SIE is due to functional WSMV NIa-Pro or CP rather than their encoding RNAs, as altering the coded protein products by minimally changing RNA sequences led to abolishment of SIE. Deletion mutagenesis further revealed that elicitation of SIE by NIa-Pro requires the entire protein while CP requires only a 200-amino-acid (aa) middle fragment (aa 101 to 300) of the 349 aa. Strikingly, reciprocal experiments with WSMV-mediated expression of TriMV proteins showed that TriMV CP, and TriMV NIa-Pro to a lesser extent, likewise excluded superinfection by TriMV-GFP. Collectively, these data demonstrate that WSMV- and TriMV-encoded CP and NIa-Pro proteins are effectors of SIE and that these two proteins trigger SIE independently of each other.

IMPORTANCE

Superinfection exclusion (SIE) is an antagonistic virus-virus interaction that prevents secondary invasions by identical or closely related viruses in the same host cells. Although known to occur in diverse viruses, SIE remains an enigma in terms of key molecular determinants and action mechanisms. In this study, we found that Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) encode two independently functioning cistrons that serve as effectors of SIE at the protein but not the RNA level. The coat protein and NIa-Pro encoded by these two viruses, when expressed from a heterologous virus, exerted SIE to the cognate viruses. The identification of virus-encoded effectors of SIE and their transgenic expression could potentially facilitate the development of virus-resistant crop plants. Additionally, functional conservation of SIE in diverse virus groups suggests that a better understanding of the underlying mechanisms of SIE could facilitate the development of novel antiviral therapies against viral diseases.

Temperature-Dependent Wsm1 and Wsm2 Gene-Specific Blockage of Viral Long-Distance Transport Provides Resistance to Wheat streak mosaic virus and Triticum mosaic virus in Wheat

Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) are economically important viral pathogens of wheat. Wheat cvs. Mace, carrying the Wsm1 gene, is resistant to WSMV and TriMV, and Snowmass, with Wsm2, is resistant to WSMV. Viral resistance in both cultivars is temperature sensitive and is effective at 18°C or below but not at higher temperatures. The underlying mechanisms of viral resistance of Wsm1 and Wsm2, nonallelic single dominant genes, are not known. In this study, we found that fluorescent protein-tagged WSMV and TriMV elicited foci that were approximately similar in number and size at 18 and 24°C, on inoculated leaves of resistant and susceptible wheat cultivars. These data suggest that resistant wheat cultivars at 18°C facilitated efficient cell-to-cell movement. Additionally, WSMV and TriMV efficiently replicated in inoculated leaves of resistant wheat cultivars at 18°C but failed to establish systemic infection, suggesting that Wsm1- and Wsm2-mediated resistance debilitated viral long-distance transport. Furthermore, we found that neither virus was able to enter the leaf sheaths of inoculated leaves or crowns of resistant wheat cultivars at 18°C but both were able to do so at 24°C. Thus, wheat cvs. Mace and Snowmass provide resistance at the long-distance movement stage by specifically blocking virus entry into the vasculature. Taken together, these data suggest that both Wsm1 and Wsm2 genes similarly confer virus resistance by temperature-dependent impairment of viral long-distance movement.

Differential Transmission of Two Isolates of Wheat streak mosaic virus by Five Wheat Curl Mite Populations

Wheat streak mosaic virus (WSMV), type member of the genus Tritimovirus in the family Potyviridae, is an economically important virus causing annual average yield losses of approximately 2 to 3% in winter wheat across the Great Plains. The wheat curl mite (WCM), Aceria tosichella, transmits WSMV along with two other viruses found throughout the Great Plains of the United States. Two common genotypes of WSMV (Sidney 81 and Type) in the United States share 97.6% nucleotide sequence identity but their transmission relationships with the WCM are unknown. The objective of this study was to determine transmission of these two isolates of WSMV by five WCM populations ('Nebraska', 'Montana', 'South Dakota', 'Type 1', and 'Type 2'). Nonviruliferous mites from each population were reared on wheat source plants mechanically inoculated with either Sidney 81 or Type WSMV isolates. For each source plant, individual mites were transferred to 10 separate test plants and virus transmission was determined by a double-antibody sandwich enzyme-linked immunosorbent assay. Source plants were replicated nine times for each treatment (90 individual mite transfers). Results indicate that three mite populations transmitted Sidney 81 at higher rates compared with Type. Two mite populations (Nebraska and Type 2) transmitted Sidney 81 and Type at higher rates compared with the other three populations. Results from this study demonstrate that interactions between virus isolates and mite populations influence the epidemiology of WSMV.

Ipomovirus--an atypical genus in the family Potyviridae transmitted by whiteflies

Ipomoviruses (genus Ipomovirus) are whitefly-transmitted viruses assigned to the family Potyviridae. They are characterised by filamentous flexible particles and a positive-sense single-stranded RNA (+ssRNA) genome. The viral genome is translated into a polyprotein precursor, which is processed into mature proteins and a short overlapping open reading frame. The genus Ipomovirus contains four accepted species and one unapproved species, and two other tentative members have recently been characterised. Ipomoviruses cause serious economic losses in many important crops, including cassava, sweet potato, cucurbits, tomato and aubergine. These viruses are transmitted by whiteflies in a non-circulative, semi-persistent manner, the virions being retained on the external surface of the vectors' mouthparts for a few days or weeks. Comparison of the available complete genome sequences of different ipomoviruses revealed differences in their genome organisation and a considerable variation in their proteins and conserved motifs that may reflect functional differences. This review summarises the current knowledge of the members within the genus Ipomovirus, focusing on genome organisation, taxonomic classification and the mechanism by which they are transmitted.

The P2 of Wheat yellow mosaic virus rearranges the endoplasmic reticulum and recruits other viral proteins into replication-associated inclusion bodies

Viruses commonly modify host endomembranes to facilitate biological processes in the viral life cycle. Infection by viruses belonging to the genus Bymovirus (family Potyviridae) has long been known to induce the formation of large membranous inclusion bodies in host cells, but their assembly and biological roles are still unclear. Immunoelectron microscopy of cells infected with the bymovirus Wheat yellow mosaic virus (WYMV) showed that P1, P2 and P3 are the major viral protein constituents of the membranous inclusions, whereas NIa-Pro (nuclear inclusion-a protease) and VPg (viral protein genome-linked) are probable minor components. P1, P2 and P3 associated with the endoplasmic reticulum (ER), but only P2 was able to rearrange ER and form large aggregate structures. Bioinformatic analyses and chemical experiments showed that P2 is an integral membrane protein and depends on the active secretory pathway to form aggregates of ER membranes. In planta and in vitro assays demonstrated that P2 interacts with P1, P3, NIa-Pro or VPg and recruits these proteins into the aggregates. In vivo RNA labelling using WYMV-infected wheat protoplasts showed that the synthesis of viral RNAs occurs in the P2-associated inclusions. Our results suggest that P2 plays a major role in the formation of membranous compartments that house the genomic replication of WYMV.

The C-terminus of Wheat streak mosaic virus coat protein is involved in differential infection of wheat and maize through host-specific long-distance transport

Viral determinants and mechanisms involved in extension of host range of monocot-infecting viruses are poorly understood. Viral coat proteins (CP) serve many functions in almost every aspect of the virus life cycle. The role of the C-terminal region of Wheat streak mosaic virus (WSMV) CP in virus biology was examined by mutating six negatively charged aspartic acid residues at positions 216, 289, 290, 326, 333, and 334. All of these amino acid residues are dispensable for virion assembly, and aspartic acid residues at positions 216, 333, and 334 are expendable for normal infection of wheat and maize. However, mutants D289N, D289A, D290A, DD289/290NA, and D326A exhibited slow cell-to-cell movement in wheat, which resulted in delayed onset of systemic infection, followed by a rapid recovery of genomic RNA accumulation and symptom development. Mutants D289N, D289A, and D326A inefficiently infected maize, eliciting milder symptoms, while D290A and DD289/290NA failed to infect systemically, suggesting that the C-terminus of CP is involved in differential infection of wheat and maize. Mutation of aspartic acid residues at amino acid positions 289, 290, and 326 severely debilitated virus ingress into the vascular system of maize but not wheat, suggesting that these amino acids facilitate expansion of WSMV host range through host-specific long-distance transport.

Wheat streak mosaic virus infects systemically despite extensive coat protein deletions: identification of virion assembly and cell-to-cell movement determinants

Viral coat proteins function in virion assembly and virus biology in a tightly coordinated manner with a role for virtually every amino acid. In this study, we demonstrated that the coat protein (CP) of Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) is unusually tolerant of extensive deletions, with continued virion assembly and/or systemic infection found after extensive deletions are made. A series of deletion and point mutations was created in the CP cistron of wild-type and/or green fluorescent protein-tagged WSMV, and the effects of these mutations on cell-to-cell and systemic transport and virion assembly of WSMV were examined. Mutants with overlapping deletions comprising N-terminal amino acids 6 to 27, 36 to 84, 85 to 100, 48 to 100, and 36 to 100 or the C-terminal 14 or 17 amino acids systemically infected wheat with different efficiencies. However, mutation of conserved amino acids in the core domain, which may be involved in a salt bridge, abolished virion assembly and cell-to-cell movement. N-terminal amino acids 6 to 27 and 85 to 100 are required for efficient virion assembly and cell-to-cell movement, while the C-terminal 65 amino acids are dispensable for virion assembly but are required for cell-to-cell movement, suggesting that the C terminus of CP functions as a dedicated cell-to-cell movement determinant. In contrast, amino acids 36 to 84 are expendable, with their deletion causing no obvious effects on systemic infection or virion assembly. In total, 152 amino acids (amino acids 6 to 27 and 36 to 100 and the 65 amino acids at the C-terminal end) of 349 amino acids of CP are dispensable for systemic infection and/or virion assembly, which is rare for multifunctional viral CPs.

Genetic variation of wheat streak mosaic virus in the United States Pacific Northwest

Wheat streak mosaic virus (WSMV), the cause of wheat streak mosaic, is a widespread and damaging pathogen of wheat. WSMV is not a chronic problem of annual wheat in the United States Pacific Northwest but could negatively affect the establishment of perennial wheat, which is being developed as an alternative to annual wheat to prevent soil erosion. Fifty local isolates of WSMV were collected from 2008 to 2010 near Lewiston, ID, Pullman, WA, and the United States Department of Agriculture Central Ferry Research Station, near Pomeroy, WA to determine the amount of genetic variation present in the region. The coat protein gene from each isolate was sequenced and the data subjected to four different methods of phylogenetic analyses. Two well-supported clades of WSMV were identified. Isolates in clade I share sequence similarity with isolates from Central Europe; this is the first report of isolates from Central Europe being reported in the United States. Isolates in clade II are similar to isolates originating from Australia, Argentina, and the American Pacific Northwest. Nine isolates showed evidence of recombination and the same two well-supported clades were observed when recombinant isolates were omitted from the analysis. More polymorphic sites, parsimony informative sites, and increased diversity were observed in clade II than clade I, suggesting more recent establishment of the virus in the latter. The observed diversity within both clades could make breeding for durable disease resistance in perennial wheat difficult if there is a differential response of WSMV resistance genes to isolates from different clades.

Status and prospects of plant virus control through interference with vector transmission

Most plant viruses rely on vector organisms for their plant-to-plant spread. Although there are many different natural vectors, few plant virus-vector systems have been well studied. This review describes our current understanding of virus transmission by aphids, thrips, whiteflies, leafhoppers, planthoppers, treehoppers, mites, nematodes, and zoosporic endoparasites. Strategies for control of vectors by host resistance, chemicals, and integrated pest management are reviewed. Many gaps in the knowledge of the transmission mechanisms and a lack of available host resistance to vectors are evident. Advances in genome sequencing and molecular technologies will help to address these problems and will allow innovative control methods through interference with vector transmission. Improved knowledge of factors affecting pest and disease spread in different ecosystems for predictive modeling is also needed. Innovative control measures are urgently required because of the increased risks from vector-borne infections that arise from environmental change.

Sharka: the past, the present and the future

Members the Potyviridae family belong to a group of plant viruses that are causing devastating plant diseases with a significant impact on agronomy and economics. Plum pox virus (PPV), as a causative agent of sharka disease, is widely discussed. The understanding of the molecular biology of potyviruses including PPV and the function of individual proteins as products of genome expression are quite necessary for the proposal the new antiviral strategies. This review brings to view the members of Potyviridae family with respect to plum pox virus. The genome of potyviruses is discussed with respect to protein products of its expression and their function. Plum pox virus distribution, genome organization, transmission and biochemical changes in infected plants are introduced. In addition, techniques used in PPV detection are accentuated and discussed, especially with respect to new modern techniques of nucleic acids isolation, based on the nanotechnological approach. Finally, perspectives on the future of possibilities for nanotechnology application in PPV determination/identification are outlined.

Caladenia virus A, an unusual new member of the family Potyviridae from terrestrial orchids in Western Australia

An isolate of a new virus, Caladenia virus A (CalVA), was identified infecting Australian terrestrial orchids. The complete genome of 9,847 nucleotides encodes 11 gene products typical of most members of the family Potyviridae. Sequence comparisons of the polyprotein revealed that CalVA shared highest sequence identity (37.5-39.6 %) with members of the genus Poacevirus. Although a vector for CalVA was not identified, a mite transmission motif was present in the helper component protease, indicating that, like other poaceviruses, mites may transmit it. CalVA is the only proposed member of the genus Poacevirus not isolated from a poaceous host.

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

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

New records of eriophyoid mites (Acari: Prostigmata: Eriophyoidea) from Saudi Arabia

A field study was conducted to investigate eriophyoid mites associated with some fruit trees in Riyadh, Saudi Arabia. The survey was carried out in four localities (El-Waseel, Al-Beer, Al-Haiyer and El-Deriya) and included seven species of fruit trees, namely olives (Olea europea), fig (Ficus carica), grapes (Vitis vinifera), apple (Malus domestica), citrus (Citrus spp.), pomegranate (Punica granatum) and pear (Pyrus communis). Seven new records of eriophyoid species (Aceria benghalensis Soliman and Abou-Awad, A. olivi Zaher and Abou-Awad, Caleptrimerus baileyi K., Colomerus oculivitis (Attiah), Oxycenus niloticus (Zaher and Abou-Awad), Rhynchaphytoptusficifolia (Keifer) and Tegolophus hassani (Keifer)), belonging two families, Eriophyidae and Diptilomiopidae, were collected from four species of fruit crops covering four different production localities in Riyadh. An illustrated identification key for these mites is provided. The present study is the first scientific study on Saudi eriophyoid mites.

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

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

Efficient and stable expression of GFP through Wheat streak mosaic virus-based vectors in cereal hosts using a range of cleavage sites: formation of dense fluorescent aggregates for sensitive virus tracking

A series of Wheat streak mosaic virus (WSMV)-based expression vectors were developed by engineering a cycle 3 GFP (GFP) cistron between P1 and HC-Pro cistrons with several catalytic/cleavage peptides at the C-terminus of GFP. WSMV-GFP vectors with the Foot-and-mouth disease virus 1D/2A or 2A catalytic peptides cleaved GFP from HC-Pro but expressed GFP inefficiently. WSMV-GFP vectors with homologous NIa-Pro heptapeptide cleavage sites did not release GFP from HC-Pro, but efficiently expressed GFP as dense fluorescent aggregates. However, insertion of one or two spacer amino acids on either side of NIb/CP heptapeptide cleavage site or deletion in HC-Pro cistron improved processing by NIa-Pro. WSMV-GFP vectors were remarkably stable in wheat for seven serial passages and for 120 days postinoculation. Mite transmission efficiencies of WSMV-GFP vectors correlated with the amount of free GFP produced. WSMV-GFP vectors infected the same range of cereal hosts as wild-type virus, and GFP fluorescence was detected in most wheat tissues.

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

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

Synergistic interaction between the Potyvirus, Turnip mosaic virus and the Crinivirus, Lettuce infectious yellows virus in plants and protoplasts

Lettuce infectious yellows virus (LIYV), the type member of the genus Crinivirus in the family Closteroviridae, is specifically transmitted by the sweet potato whitefly (Bemisia tabaci) in a semipersistent manner. LIYV infections result in a low virus titer in plants and protoplasts, impeding reverse genetic efforts to analyze LIYV gene/protein functions. We found that synergistic interactions occurred in mixed infections of LIYV and Turnip mosaic virus (TuMV) in Nicotiana benthamiana plants, and these resulted in enhanced accumulation of LIYV. Furthermore, we examined the ability of transgenic plants and protoplasts expressing only the TuMV P1/HC-Pro sequence to enhance the accumulation of LIYV. LIYV RNA and protein titers increased by as much as 8-fold in these plants and protoplasts relative to control plants. LIYV infections remained phloem-limited in P1/HC-Pro transgenic plants, suggesting that enhanced accumulation of LIYV in these plants was due primarily to increased replication efficiency, not to greater spread.

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

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

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.

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

ABSTRACT The tritimovirus Wheat streak mosaic virus (WSMV) and the machlomovirus Maize chlorotic mottle virus (MCMV) each cause systemic chlorosis in infected maize plants. Infection of maize with both viruses produces corn lethal necrosis disease (CLND). Here, we report that complete deletion of the WSMV helper component-proteinase (HC-Pro) coding region had no effect on induction of CLND symptoms following coinoculation of maize with WSMV and MCMV. We further demonstrated that elevation of virus titers in double infections, relative to single infections, also was independent of WSMV HC-Pro. Thus, unlike potyvirus HC-Pro, WSMV HC-Pro was dispensable for disease synergism. Because disease synergism involving potyviruses requires HC-Pro-mediated suppression of posttranscriptional gene silencing (PTGS), we hypothesized that WSMV HC-Pro may not be a suppressor of PTGS. Indeed, WSMV HC-Pro did not suppress PTGS of a green fluorescent protein (GFP) transgene in an Agrobacterium-mediated coinfiltration assay in which potyvirus HC-Pro acted as a strong suppressor. Furthermore, coinfiltration with potyvirus HC-Pro, but not WSMV HC-Pro, resulted in elevated levels of the GFP target mRNA under conditions which trigger PTGS. Collectively, these results revealed significant differences in HC-Pro function among divergent genera of the family Potyviridae and suggest that the tritimovirus WSMV utilizes a gene other than HC-Pro to suppress PTGS and mediate synergistic interactions with unrelated viruses.

Substitution of conserved cysteine residues in wheat streak mosaic virus HC-Pro abolishes virus transmission by the wheat curl mite

Substitutions in the amino-proximal region of wheat streak mosaic virus (WSMV) HC-Pro were evaluated for effects on transmission by the wheat curl mite (Aceria tosichella Keifer). Alanine substitution at cysteine residues 16, 46 and 49 abolished vector transmission. Although alanine substitution at Cys(20) had no effect, substitution with arginine reduced vector transmission efficiency. Random substitutions at other positions (Lys(7) to Asn, Asn(19) to Ile, and Arg(45) to Lys) did not affect vector transmission. These results suggest that a zinc-finger-like motif (His(13)-X2-Cys(16)-X29-Cys(46)-X2-Cys(49)) in WSMV HC-Pro is essential for vector transmission.

Random mutagenesis of wheat streak mosaic virus HC-Pro: non-infectious interfering mutations in a gene dispensable for systemic infection of plants

Mutations within the HC-Pro coding region of Wheat streak mosaic virus (WSMV) were introduced by misincorporation during PCR and evaluated for phenotype within the context of an infectious clone. Nine synonymous substitutions and 15 of 25 non-synonymous substitutions had no phenotypic effect. Four non-synonymous substitutions, including one that reverted consistently to wild type, resulted in attenuated systemic infection. Six non-synonymous substitutions and one nonsense substitution abolished systemic infectivity. Mutants bearing the GUS reporter gene were evaluated for the ability to establish primary infection foci. All attenuated mutants and two systemic infection-deficient mutants produced localized regions of GUS expression on inoculated leaves 3 days post-inoculation. In vitro assays revealed that mutants able to establish infection foci retained HC-Pro proteinase activity. Among mutants unable to establish infection foci, HC-Pro proteinase activity was retained, reduced or absent. As a complete HC-Pro deletion mutant can infect plants systemically, certain substitutions in this dispensable gene probably prevented infection of WSMV via interference.

Nested deletion analysis of Wheat streak mosaic virus HC-Pro: Mapping of domains affecting polyprotein processing and eriophyid mite transmission

A series of in-frame and nested deletion mutations which progressively removed 5'-proximal sequences of the Wheat streak mosaic virus (WSMV) HC-Pro coding region (1152 nucleotides) was constructed and evaluated for pathogenicity to wheat. WSMV HC-Pro mutants with 5'-proximal deletions of 12 to 720 nucleotides systemically infected wheat. Boundary sequences flanking the deletions were stable and unaltered by passage through plants for all deletion mutants except HCD12 (lacking HC-Pro codons 3-6) that exhibited strong bias for G to A substitution at nucleotide 1190 in HC-Pro codon 2 (aspartic acid to asparagine). HC-Pro mutants with 5'-proximal deletions of up to 720 nucleotides retained autoproteolytic activity in vitro. In contrast, 5'-proximal deletion of 852 nucleotides of the HC-Pro coding region (HCD852) abolished both infectivity and in vitro proteolytic activity, confirming that the proteolytic domain of WSMV HC-Pro resides within the carboxy-terminal third of the protein and includes the cysteine proteinase motif (GYCY) conserved among four genera of the family Potyviridae. Inoculation of wheat with HC-Pro deletion mutants also bearing the GUS reporter gene revealed that HCD852 was unable to establish primary infection foci in inoculated leaves, indicating that processing of the P3 amino-terminus was essential. Deletion of as few as 24 nucleotides of HC-Pro (codons 3-10) eliminated transmission by the eriophyid mite vector Aceria tosichella Keifer. Collectively, these results demonstrated similar organization of proteinase and vector transmission functional domains among divergent HC-Pro homologues encoded by potyviruses and tritimoviruses.

Virus-vector interactions mediating nonpersistent and semipersistent transmission of plant viruses

Most plant viruses are absolutely dependent on a vector for plant-to-plant spread. Although a number of different types of organisms are vectors for different plant viruses, phloem-feeding Hemipterans are the most common and transmit the great majority of plant viruses. The complex and specific interactions between Hemipteran vectors and the viruses they transmit have been studied intensely, and two general strategies, the capsid and helper strategies, are recognized. Both strategies are found for plant viruses that are transmitted by aphids in a nonpersistent manner. Evidence suggests that these strategies are found also for viruses transmitted in a semipersistent manner. Recent applications of molecular and cell biology techniques have helped to elucidate the mechanisms underlying the vector transmission of several plant viruses. This review examines the fundamental contributions and recent developments in this area.

Complete deletion of Wheat streak mosaic virus HC-Pro: a null mutant is viable for systemic infection

A Wheat streak mosaic virus (WSMV) genome lacking HC-Pro was constructed and confirmed by reverse transcription-PCR to systemically infect wheat, oat, and corn. Coupled in vitro transcription/translation reactions indicated that WSMV P1 proteinase cleaved the polyprotein at the P1/P3 junction of the HC-Pro null mutant. The WSMV HC-Pro null mutant was competent for virion formation, but the virus titer was reduced 4.5-fold relative to that of the wild type. Collectively, these results indicate that WSMV HC-Pro is dispensable for replication and movement, two essential processes that are disrupted by point and small-insertion mutations introduced into potyvirus HC-Pro.