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

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.

Historical Review of Sugarcane Streak Mosaic Virus that Has Recently Emerged in Africa

Sugarcane streak mosaic virus (SCSMV), now assigned to the genus Poacevirus of the family Potyviridae, was reported for the first time in 1932 in Louisiana and was believed to be strain F of sugarcane mosaic virus (SCMV) for more than six decades. SCMV-F was renamed SCSMV in 1998 after partial sequencing of its genome and phylogenetic investigations. Following the development of specific molecular diagnostic methods in the 2000s, SCSMV was recurrently found in sugarcane exhibiting streak mosaic symptoms in numerous Asian countries but not in the Western hemisphere or in Africa. In this review, we give an overview of the current knowledge on this disease and the progression in research on SCSMV. This includes symptoms, geographical distribution and incidence, diagnosis and genetic diversity of the virus, epidemiology, and control. Finally, we highlight future challenges, as sugarcane streak mosaic has recently been found in Africa, where this disease represents a new threat to sugarcane production.

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.

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".

Development and Evaluation of Stable Sugarcane Mosaic Virus Mild Mutants for Cross-Protection Against Infection by Severe Strain

Sugarcane mosaic virus (SCMV; genus Potyvirus) induces maize dwarf mosaic disease that has caused serious yield losses of maize in China. Cross-protection is one of the efficient strategies to fight against severe virus strains. Although many mild strains have been identified, the spontaneous mutation is one of the challenging problems affecting their application in cross-protection. In this study, we found that the substitution of cysteine (C) at positions 57 or 60 in the zinc finger-like motif of HC-Pro with alanine (A; C57A or C60A) significantly reduced its RNA silencing suppression activity and SCMV virulence. To reduce the risk of mild strains mutating to virulent ones by reverse or complementary mutations, we obtained attenuated SCMV mutants with double-mutations in the zinc finger-like and FRNK motifs of HC-Pro and evaluated their potential application in cross-protection. The results showed that the maize plants infected with FKNK/C60A double-mutant showed symptomless until 95 days post-inoculation and FKNK/C60A cross-protected plants displayed high resistance to severe SCMV strain. This study provides theoretical and material bases for the control of SCMV through cross-protection.

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: 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.

The Interface Between Wheat and the Wheat Curl Mite, Aceria tosichella, the Primary Vector of Globally Important Viral Diseases

Wheat production and sustainability are steadily threatened by pests and pathogens in both wealthy and developing countries. This review is focused on the wheat curl mite (WCM), Aceria tosichella, and its relationship with wheat. WCM is a major pest of wheat and other cereals and a vector of at least four damaging plant viruses (Wheat streak mosaic virus, High plains wheat mosaic virus, Brome streak mosaic virus, and Triticum mosaic virus). The WCM-virus pathosystem causes considerable yield losses worldwide and its severity increases significantly when mixed-virus infections occur. Chemical control strategies are largely ineffective because WCM occupies secluded niches on the plant, e.g., leaf sheaths or curled leaves in the whorl. The challenge of effectively managing this pest-virus complex is exacerbated by the existence of divergent WCM lineages that differ in host-colonization and virus-transmission abilities. We highlight research progress in mite ecology and virus epidemiology that affect management and development of cereal cultivars with WCM- and virus-resistance genes. We also address the challenge of avoiding both agronomically deleterious side effects and selection for field populations of WCM that can overcome these resistance genes. This report integrates the current state of knowledge of WCM-virus-plant interactions and addresses knowledge gaps regarding the mechanisms driving WCM infestation, viral epidemics, and plant responses. We discuss the potential application of molecular methods (e.g., transcriptomics, epigenetics, and whole-genome sequencing) to understand the chemical and cellular interface between the wheat plant and WCM-virus complexes.

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.

2b or Not 2b: Experimental Evolution of Functional Exogenous Sequences in a Plant RNA Virus

Horizontal gene transfer (HGT) is pervasive in viruses and thought to be a key mechanism in their evolution. On the other hand, strong selective constraints against increasing genome size are an impediment for HGT, rapidly purging horizontally transferred sequences and thereby potentially hindering evolutionary innovation. Here, we explore experimentally the evolutionary fate of viruses with simulated HGT events, using the plant RNA virus Tobacco etch virus (TEV), by separately introducing two functional, exogenous sequences to its genome. One of the events simulates the acquisition of a new function though HGT of a conserved AlkB domain, responsible for the repair of alkylation or methylation damage in many organisms. The other event simulates the acquisition of a sequence that duplicates an existing function, through HGT of the 2b RNA silencing suppressor from Cucumber mosaic virus. We then evolved these two viruses, tracked the maintenance of the horizontally transferred sequences over time, and for the final virus populations, sequenced their genome and measured viral fitness. We found that the AlkB domain was rapidly purged from the TEV genome, restoring fitness to wild-type levels. Conversely, the 2b gene was stably maintained and did not have a major impact on viral fitness. Moreover, we found that 2b is functional in TEV, as it provides a replicative advantage when the RNA silencing suppression domain of HC-Pro is mutated. These observations suggest a potentially interesting role for HGT of short functional sequences in ameliorating evolutionary constraints on viruses, through the duplication of functions.

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.

Tall oatgrass mosaic virus (TOgMV): a novel member of the genus Tritimovirus infecting Arrhenatherum elatius

A novel tritimovirus of the family Potyviridae was isolated from tall oatgrass, Arrhenatherum elatius, exhibiting mosaic symptoms. The virus, for which the name tall oatgrass mosaic virus (TOgMV) is coined, has a filamentous particle of 720 nm and is associated with pinwheel inclusion bodies characteristic of members of the family Potyviridae. The virus was mechanically transmitted to tall oatgrass seedlings, which subsequently exhibited mosaic symptoms. The experimental host range was limited to a few monocot species. The complete genome sequence of TOgMV was determined to be 9359 nucleotides, excluding the 3' polyadenylated tail. The viral RNA encodes one large putative open reading frame of 3029 amino acids with a genome organization typical of monopartite potyvirids. Pairwise comparison of putative mature proteins and proteinase cleavage sites indicated that TOgMV is most closely related to members of the genus Tritimovirus. Phylogenetic analysis of the complete polyprotein and CP sequences of representative members of the family Potyviridae indicate that TOgMV is a distinct tritimovirus naturally infecting tall oatgrass.

Complete nucleotide sequence of rose yellow mosaic virus, a novel member of the family Potyviridae

The complete genomic sequence of rose yellow mosaic virus (RoYMV) was determined and found to have all the features that are characteristic of members of the family Potyviridae. The RoYMV genome is 9508 nucleotides long excluding the 3'-poly-(A) tail and contains a single open reading frame encoding a polyprotein of 3067 amino acids. The RoYMV P3 and CI cistrons are shorter than those of other members of the family Potyviridae, and the 6K1 cistron is completely absent. Comparative sequence analysis revealed that RoYMV had highest amino acid sequence identity across the entire genome sequence to brome streak mosaic virus (33 %) and to turnip mosaic virus (30 %) at the coat protein level. Based on its low sequence similarity to known members of the family Potyviridae and phylogenetic analysis, RoYMV appears to be a distinct, previously undescribed, member of this family.

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.

What's "cool" on eriophyoid mites?

Fundamental knowledge on the morphology, biology, ecology, and economic importance of Eriophyoidea has been exhaustively compiled by Lindquist et al. (Eriophyoid mites--their biology, natural enemies and control; Elsevier, 1996). Since that time, the number of recognized species and the economic importance of the taxon have increased substantially. The aim of this paper is to analyze and briefly review new findings from eriophyoid mites' literature after Lindquist et al. book, stressing persistent gaps and needs. Much recent attention has been given to sampling and detection, taxonomy and systematics, faunistic surveys, internal morphology, rearing techniques, biological and ecological aspects, biomolecular studies, and virus vectoring. Recommendations are made for integrating research and promoting broader dissemination of data among specialists and non-specialists.

Complete nucleotide sequence and taxonomy of Sugarcane streak mosaic virus, member of a novel genus in the family Potyviridae

The complete genomic sequence of a Pakistani isolate of Sugarcane streak mosaic virus (SCSMV-PAK) is determined to be 9782 nucleotides in length, excluding the 3' poly(A) tail, and it comprises a large open reading frame encoding a polyprotein of 3130 amino acid residues. The deduced polyprotein is likely to be cleaved at nine putative protease sites by three viral proteases to ten mature proteins. Conserved motifs of orthologous proteins of other potyviruses are identified in corresponding positions of SCSMV-PAK. The genomic organization is virtually identical to the genera Ipomovirus, Potyvirus, Rymovirus, and Tritimovirus in the family Potyviridae. Sequence analyses indicate that the SCSMV-PAK genomic sequence is different from those of Sugarcane mosaic virus and Sorghum mosaic virus, two viruses with very similar symptoms and host range to SCSMV-PAK. SCSMV-PAK shares 52.7% identity with Triticum mosaic virus (TriMV) and 26.4-31.5% identities with species of the existing genera and unassigned viruses in the Potyviridae at the polyprotein sequence level. Phylogenetic analyses of the polyprotein and deduced mature protein amino acid sequences reveal that SCSMV, together with TriMV, forms a distinct group in the family at the genus level. Therefore, SCSMV should represent a new genus, Susmovirus, in the Potyviridae.