Cucumber mosaic virus

Molecular characterization of Penicillium chrysogenum virus: reconsideration of the taxonomy of the genus Chrysovirus

Molecular cloning and complete nucleotide sequencing of Penicillium chrysogenum virus (PcV) dsRNAs indicated that PcV virions contained four dsRNA segments with sizes of 3562, 3200, 2976 and 2902 bp. Each dsRNA segment had unique sequences and contained a single large open reading frame (ORF). In vitro translation of transcripts derived from full-length cDNA clones of PcV dsRNAs yielded single products of sizes similar to those predicted from the deduced amino acid sequences of the individual ORFs. Sequence similarity searches revealed that dsRNA1 encodes a putative RNA-dependent RNA polymerase. In this study, it was determined that dsRNA2 encodes the major capsid protein and that p4, encoded by dsRNA4, is virion-associated as a minor component. All four dsRNAs of PcV, like the genomic segments of viruses with multipartite genomes, were found to have extended regions of highly conserved terminal sequences at both ends. In addition to the strictly conserved 5'-terminal 10 nt, a second region consisting of reiteration of the sequence CAA was found immediately upstream of the AUG initiator codon. These (CAA)(n) repeats are reminiscent of the translational enhancer elements of tobamoviruses. The 3'-terminal 14 nt were also strictly conserved. As PcV and related viruses with four dsRNA segments (genus Chrysovirus) have not been previously characterized at the molecular level, they were provisionally classified in the family Partitiviridae, comprising viruses with bipartite genomes. This study represents the first report on molecular characterization of a chrysovirus and the results suggest the creation of a new family of mycoviruses with multipartite dsRNA genomes to accommodate PcV and related viruses.

The necrotic pathotype of the cucumber mosaic virus (CMV) ns strain is solely determined by amino acid 461 of the 1a protein

The unique Ns isolate of Cucumber mosaic virus (CMV) induces necrotic lesions on several Nicotiana spp. in contrast to other strains that cause systemic mosaic on these plants. By using biologically active RNA transcripts from cDNAs of Ns-CMV and a reference subgroup I strain Rs-CMV, we confined the genetic determinant solely responsible for necrosis induction to amino acid 461 of the la protein translated from genomic RNA1. An Arg to Cys change at this position (R461C) rendered Rs-CMV necrotic, whereas the reciprocal C461R mutation reverted the necrotic phenotype of Ns-CMV. Necrotic (Ns-CMV, R461C) and non-necrotic (Rs-CMV and C461R) viruses accumulated to similar levels in Nicotiana clevelandii protoplasts. Deletion of the residue at position 461 abolished replicase activity of the Ns-CMV 1a protein. The R461C mutation also was introduced into the 1a protein of Trk7-CMV, a subgroup II isolate. Symptoms induced by the Trk7/R461C mutant were identical to those caused by wild-type Trk7-CMV, even when the mutant Trk7 RNA1 was co-inoculated with RNA2 and 3 of the necrotic Ns strain.

The Arabidopsis cucumovirus multiplication 1 and 2 loci encode translation initiation factors 4E and 4G

The cum1 and cum2 mutations of Arabidopsis thaliana inhibit cucumber mosaic virus (CMV) multiplication. In cum1 and cum2 protoplasts, CMV RNA and the coat protein accumulated to wild-type levels, but the accumulation of the 3a protein of CMV, which is necessary for cell-to-cell movement of the virus, was strongly reduced compared with that in wild-type protoplasts. In cum2 protoplasts, the accumulation of turnip crinkle virus (TCV)-related RNA and proteins was also reduced. Positional cloning demonstrated that CUM1 and CUM2 encode eukaryotic translation initiation factors 4E and 4G, respectively. Unlike most cellular mRNA, the CMV RNA lacks a poly(A) tail, whereas the TCV RNA lacks both a 5'-terminal cap and a poly(A) tail. In vivo translation analyses, using chimeric luciferase mRNA carrying the terminal structures and untranslated sequences of the CMV or TCV RNA, demonstrated that these viral untranslated sequences contain elements that regulate the expression of encoded proteins positively or negatively. The cum1 and cum2 mutations had different effects on the action of these elements, suggesting that the cum1 and cum2 mutations cause inefficient production of CMV 3a protein and that the cum2 mutation affects the production of TCV-encoded proteins.

Molecular population genetics of Cucumber mosaic virus in California: evidence for founder effects and reassortment

The structure and genetic diversity of a California Cucumber mosaic virus (CMV) population was assessed by single-strand conformation polymorphism and nucleotide sequence analyses of genomic regions 2b, CP, MP, and the 3' nontranslated region of RNA3. The California CMV population exhibited low genetic diversity and was composed of one to three predominant haplotypes and a large number of minor haplotypes for specific genomic regions. Extremely low diversity and close evolutionary relationships among isolates in a subpopulation suggested that founder effects might play a role in shaping the genetic structure. Phylogenetic analysis indicated a naturally occurring reassortant between subgroup IA and IB isolates and potential reassortants between subgroup IA isolates, suggesting that genetic exchange by reassortment contributed to the evolution of the California CMV population. Analysis of various population genetics parameters and distribution of synonymous and nonsynonymous mutations revealed that different coding regions and even different parts of coding regions were under different evolutionary constraints, including a short region of the 2b gene for which evidence suggests possible positive selection.

Comparative analysis of expressed sequence tags in resistant and susceptible ecotypes of Arabidopsis thaliana infected with cucumber mosaic virus

Arabidopsis thaliana ecotype Columbia (Col-0) is susceptible to the yellow strain of cucumber mosaic virus [CMV(Y)], whereas ecotype C24 is resistant to CMV(Y). Comprehensive analyses of approximately 9,000 expressed sequence tags in ecotypes Col-0 and C24 infected with CMV(Y) suggested that the gene expression patterns in the two ecotypes differed. At 6, 12, 24 and 48 h after CMV(Y) inoculation, the expression of 6, 30, 85 and 788 genes, respectively, had changed in C24, as opposed to 20, 80, 53 and 150 genes in CMV(Y)-infected Col-0. At 12, 24 and 48 h after CMV(Y) inoculation, the abundance of 3, 10 and 55 mRNAs was altered in both ecotypes. However, at 6 h after CMV(Y) inoculation, no genes were co-induced or co-suppressed in both ecotypes. This differential pattern of gene expression between the two ecotypes at an early stage of CMV(Y) infection indicated that the cellular response for resistance may differ from that resulting in susceptibility at the level detectable by the macroarray. According to the expression pattern at various stages of infection, the expression of many genes could be grouped into clusters using cluster analysis. About 100 genes that encode proteins involved in chloroplast function were categorized into clusters 1 and 4, which had a differentially lower expression in CMV(Y)-inoculated C24. The expression of various genes encoding proteins in the endomembrane system belonged to clusters 2 and 4, which were induced in CMV(Y)-inoculated C24 and Col-0 leaves. Characterization of CMV(Y)-altered gene expression in the two ecotypes will contribute to a better understanding of the molecular basis of compatible and incompatible interactions between virus and host plants.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

Analysis of mechanisms involved in the Cucumber mosaic virus satellite RNA-mediated transgenic resistance in tomato plants

Transgenic tomato (Lycopersicon esculentum Mill. cv. UC82) plants expressing a benign variant of Cucumber mosaic virus satellite RNA (CMV Tfn-satRNA) were generated. The transformed plants did not produce symptoms when challenged with a satRNA-free strain of CMV (CMV-FL). The same plant lines initially were susceptible to necrosis elicited by a CMV strain supporting a necrogenic variant of satRNA (CMV-77), but a phenotype of total recovery from the necrosis was observed in the newly developing leaves. The features of the observed resistance were analyzed and are consistent with two different mechanisms of resistance. In transgenic plants inoculated with CMV-FL strain, the symptomless phenotype was correlated to the down-regulation of CMV by Tfn-satRNA, amplified from the transgene transcripts, as the first resistance mechanism. On the other hand, the delayed resistance to CMV-77 in transgenic tomato lines was mediated by a degradation process that targets satRNAs in a sequence-specific manner. Evidence is provided for a correlation between a reduced accumulation level of transgenic messenger Tfn-satRNA, the accumulation of small (approximately 23 nucleotides) RNAs with sequence homology to satRNAs, the progressively reduced accumulation of 77-satRNA in infected tissues, and the transition in infected plants from diseased to healthy. Thus, events leading to the degradation of satRNA sequences indicate a role for RNA silencing as the second mechanism determining resistance of transgenic tomato lines.

Involvement of RNA2 for systemic infection of Cucumber mosaic virus isolated from lily on zucchini squash

A lily strain of Cucumber mosaic virus (LK-CMV) was not able to systemically infect zucchini squash (Cucurbita pepo), while Fny strain of CMV (Fny-CMV) caused systemic mosaic and stunting symptom at 4 days post-inoculation on the same host species. The pathogenicity of LK-CMV in zucchini squash was investigated by reassortments of genomic RNAs of LK-CMV and Fny-CMV for infection, as well as by pseudorecombinants generated from biologically active transcripts of cDNA clones of LK-CMV and Fny-CMV, respectively. The assessments of pathogenicity for LK-CMV indicated that RNA2 of LK-CMV was responsible for systemic infection in zucchini squash. In the protoplast of zucchini squash, the RNA accumulations of all constructed pseudorecombinants were indistinguishable and LK-CMV replication was slightly lower than that of Fny-CMV, suggesting that the inability of LK-CMV to infect squash plants was responsible for the poor efficiency of virus movement, rather than the reduction of replication function.

Rhizobacteria-Mediated Growth Promotion of Tomato Leads to Protection Against Cucumber mosaic virus

ABSTRACT We evaluated combinations of two strains of plant growth-promoting rhizobacteria (PGPR) formulated with the carrier chitosan for the ability to induce growth promotion of tomato plants and resistance to infection by Cucumber mosaic virus (CMV). Each PGPR combination included GB03 (Bacillus subtilis) and one of the following PGPR strains: SE34 (B. pumilus), IN937a (B. amyloliquefaciens), IN937b (B. subtilis), INR7 (B. pumilus), or T4 (B. pumilus). The PGPR combinations formulated with chitosan are referred to as biopreparations. Tomato plants treated with each of the biopreparations appeared phenotypically and developmentally similar to nonbacterized control plants that were 10 days older (referred to as the older control). When plants were challenged with CMV, all plants in the biopreparation treatments and the older control treatment had significantly greater height, fresh weight, and flower and fruit numbers than that of plants in the CMV-inoculated same age control treatment. CMV disease severity ratings were significantly lower for biopreparation-treated and older control tomato plants than for that of same age control plants at 14 and 28 days postinoculation (dpi). CMV accumulation in young noninoculated leaves was significantly less for all biopreparation-treated plants and those in the older control than for the same age control plants at 14 dpi and for four of the five biopreparation treatments at 28 dpi. In those tomato plants shown to be infected, the amount of CMV in noninoculated leaves was significantly lower for three of the biopreparation treatments and the older control treatment at 14 dpi and biopreparation G/INR7 treatment at 28 dpi when compared with the control treatment. These data show that treatment of tomato plants with biopreparations results in significant enhancement of growth and protection against infection by CMV.

An analysis of the durability of resistance to plant viruses

ABSTRACT Genetic resistance often fails because a resistance-breaking (RB) pathogen genotype increases in frequency. On the basis of an analysis of cellular plant pathogens, it was recently proposed that the evolutionary potential of a pathogen is a major determinant of the durability of resistance. We test this hypothesis for plant viruses, which differ substantially from cellular pathogens in the nature, size, and expression of their genomes. Our analysis was based on 29 plant virus species that provide a good representation of the genetic and biological diversity of plant viruses. These 29 viruses were involved in 35 pathosystems, and 50 resistance factors deployed against them were analyzed. Resistance was found to be durable more often than not, in contrast with resistance to cellular plant pathogens. In a third of the analyzed pathosystems RB strains have not been reported, and in another third RB strains have been reported but have not become prevalent in the virus population. The evolutionary potential of the viruses in the 35 pathosystems was evaluated with a compound risk index based on three evolutionary factors: the population of the pathogen, the degree of recombination, and the amount of gene and genotype flow. Our analysis indicates that evolutionary potential may be an important determinant of the durability of resistance against plant viruses.

Interaction of replicase components between Cucumber mosaic virus and Peanut stunt virus

Cucumber mosaic virus (CMV) and Peanut stunt virus (PSV) each have genomes consisting of three single-stranded RNA molecules: RNA 1, 2 and 3. RNAs 1 and 2 encode the 1a and 2a proteins, respectively, which are necessary for replication of the viral genome. Although RNA 3 is exchangeable between CMV and PSV, exchange of RNA 1 and 2 between the two viruses has been unsuccessful. In this study, reassortants containing PSV RNA 1 and CMV RNA 2 together with RNA 3 of CMV or PSV were shown to be able to replicate their genomic RNA, but not to transcribe subgenomic RNA 4 in tobacco protoplasts. Conversely, the reassortant consisting of CMV RNA 1 and PSV RNA 2 together with RNA 3 of CMV or PSV could not replicate. Subsequently, a yeast two-hybrid system was used to analyse the in vivo interaction between the 1a and 2a proteins. The C-terminal half of PSV-1a protein interacted with the N-terminal region of 2a protein of both PSV and CMV, but the C-terminal half of CMV-1a and the N-terminal region of PSV-2a did not interact. These results suggest that RNA replication in the interspecific reassortant between CMV and PSV requires compatibility between the C-terminal half of the 1a protein and the N-terminal region of the 2a protein, and this compatibility is insufficient for transcription of subgenomic RNA 4.

In spite of induced multiple defense responses, tomato plants infected with Cucumber mosaic virus and D satellite RNA succumb to systemic necrosis

Cucumber mosaic virus (CMV) D satellite RNA (satRNA) attenuates the symptoms induced by CMV in most plants, but causes leaf epinasty and systemic necrosis in tomato plants, where programmed cell death (PCD) is involved. However, our understanding of the cellular and molecular responses to the infection of CMV D satRNA that result in this lethal disease remains limited. In this article, we show for the first time, by histochemical and molecular analysis, that multiple defense responses are specifically induced in CMV and D satRNA (CMV/D satRNA)-infected tomato plants but not in mock-inoculated or CMV-infected plants. These responses include callose deposition and hydrogen peroxide accumulation in infected plants. Furthermore, the transcription of several tomato defense-related genes (e.g., PR-1a1, PR-1b1, PR-2, and PR-10) were activated, and the expression of tomato PR-5 and some abiotic and biotic stress-responsive genes (e.g., catalase II and tomato analogs of Arabidopsis AtBI-1 and tobacco hsr203j) are enhanced. The activation and increase in expression of these genes is correlated with the appearance of leaf epinasty and the development of systemic necrosis in infected tomato plants, while increased expression of the hsr203j analog precedes the development of any disease symptoms. The spatial and temporal expression patterns of these genes as detected by RNA in situ hybridization point to the involvement of a complex developmental program that accompanies disease development resulting from CMV/D satRNA infection.

Pea Tissue Necrosis Induced by Cucumber mosaic virus Alone or Together with Watermelon mosaic virus

Necrotic diseases of the stems, petioles, and leaves of pea plants (Pisum sativumL.), leading to wilting and death, occur in the Wakayama and Mie Prefectures of Japan. Based on host range, symptomatology, electron microscopy, and serological relationships, Watermelon mosaic virus (WMV) and three Cucumber mosaic virus (CMV) isolates (PE2, PE3A, and PB1) were isolated from diseased plants in the Wakayama Prefecture. In the Mie Prefecture, CMV (PEAN) also was isolated from pea plants with similar symptoms. Single infection with CMV (PB1 or PEAN) caused stem necrosis and eventual death of pea plants. Similar symptoms developed after double infection with WMV and PE2 or PE3A, whereas single infection with PE2 and PE3A induced symptomless infection in pea plants. We concluded either CMV alone or synergistic effects of mixed infection with CMV and WMV induced pea plant stem necrosis.

The evolution of virulence in a plant virus

The evolution of virulence is a rapidly growing field of research, but few reports deal with the evolution of virulence in natural populations of parasites. We present here an observational and experimental analysis of the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) during an epidemic on tomato in eastern Spain. Three types of CMV isolates were found that caused in tomato plants either a systemic necrosis (N isolates), stunting and a severe reduction of leaf lamina (Y isolates), or stunting and leaf curl (A isolates). These phenotypes were due to the presence of satellite RNAs (satRNAs) necrogenic (in N isolates) or attenuative (in A isolates) of the symptoms caused by CMV without satRNA (Y isolates). For these three types of isolates, parameters of virulence and transmission were estimated experimentally. For virulence the ranking of isolates was N > Y > A, for transmissibility, Y > A > N. The predictions of theoretical models for the evolution of virulence were analyzed with these parameters and compared with observations from the field. A single-infection model predicted adequately the observed long-term evolution of the CMV population to intermediate levels of virulence. A coinfection model that considered competition between isolates with an effect on transmission explained the invasion of the CMV population by N isolates at the beginning of the epidemic, and its predictions also agreed with field data on the long-term evolution of the CMV population. An important conclusion from both models was that the density of the aphid vector's population is a major factor in the evolution of CMV virulence. This may be relevant for the design of control strategies for CMV-induced diseases.

RNA recombination between cucumoviruses: possible role of predicted stem-loop structures and an internal subgenomic promoter-like motif

We previously analyzed hybrids of Cucumber mosaic virus (CMV) and Tomato aspermy virus (TAV) that contained CMV RNA2 with the 3'-terminal sequence from TAV RNA2. In this article, we scrutinized the RNA3 molecules in these hybrid viruses by Northern hybridization and RT-PCR and found some recombinant CMV RNA3 molecules and various recombinant RNA4 molecules whose 3'-termini were derived from TAV RNA1 or 2. Sequence analyses revealed that most of the crossover sites for recombination were located near putative stem-loop structures and an internal subgenomic promoter-like motif. We inoculated in vitro transcripts synthesized from cDNA clones of the recombinant RNA3 onto N. benthamiana along with either CMV RNA1 and 2 or TAV RNA1 and 2. Although all of the hybrids were infectious, many sequence deletions and nucleotide substitutions were found when RNA1 and 2 from TAV were used, which suggests that fidelity of TAV replicase was lower than that of CMV replicase. The possible role of secondary structures and an internal subgenomic promoter-like motif in RNA recombination is discussed.

Two amino acids on 2a polymerase of Cucumber mosaic virus co-determine hypersensitive response on legumes

The hypersensitive response (HR) is one of the most important defense responses during the incompatible interaction between plant and pathogen. The viral determinant of HR on legumes induced by Cucumber mosaic virus (CMV) was studied, and our previous results showed that 243 nucleotides on 2a polymerase gene of CMV were involved in the induction of HR on legumes. With further analysis of the nucleotides and amino acids in this region, the amino acids Phe and Ala at positions 631 and 641 in the 2a polymerase of CMV-Fny, a legume local necrotic strain, were specifically exchanged to Tyr and Ser, respectively and simultaneously, in the 2a polymerase of CMV-P1, a legume systemic infecting strain, and three point mutants were constructed. The point mutant Fny-F/Y (Phe631 to Tyr) induced large necrotic lesions instead of pinpoint lesions, and the size of lesions could enlarge from initial sites. The point mutant Fny-A/S (Ala641 to Ser) induced similar symptoms as CMV-Fny. The double-point mutant Fny-FA/YS (Phe631 to Tyr and Ala641 to Ser) infected the legumes systemically without HR. These data indicate that the induction of HR on legumes is co-determined by two amino acids at positions 631 and 641 in CMV 2a polymerase.

Genetic diversity and biological variation among California isolates of Cucumber mosaic virus

Genetic diversity and biological variation were compared for California isolates of Cucumber mosaic virus (CMV). These fell into five pathotypes based on their reactions on three cucurbits including a susceptible squash, a melon with conventional resistance and a commercial CMV-resistant transgenic squash. Thirty-three isolates infected and caused symptoms on CMV-resistant transgenic squash. Forty-two isolates infected the CMV-resistant melon, but only 25 isolates infected both. Single-strand conformation polymorphism (SSCP) analysis was used to differentiate 81 California isolates into 14 groups, and the coat protein (CP) genes of 27 isolates with distinct and indistinguishable SSCP patterns were sequenced. Fourteen isolates corresponding to the different SSCP patterns were also used for phylogenetic analysis. Seventy-nine isolates belonged to CMV subgroup IA, but two belonged to CMV subgroup IB. This is the first report of subgroup IB isolates in the Americas. All CMV isolates had a nucleotide identity greater than or equal to 93.24 %. There was no correlation between CP gene variation and geographical origin, collection year, original host plant, or between the degree of CP amino acid sequence identity and the capacity to overcome transgenic and/or conventional resistance. SSCP and sequence analyses were used to compare 33 CMV isolates on CMV-resistant transgenic squash and susceptible pumpkin plants. One isolate showed sequence differences between these two hosts, but this was not due to recombination or selection pressure of transgenic resistance. CMV isolates capable of infecting cucurbits with conventional and transgenic CMV resistance were present in California, even before CMV transgenic material was available.

Optimization of trans-splicing ribozyme efficiency and specificity by in vivo genetic selection

Trans-splicing ribozymes are RNA-based catalysts capable of splicing RNA sequences from one transcript specifically into a separate target transcript. In doing so, a chimeric mRNA can be produced, and new gene activities triggered in living cells dependent on the presence of the target mRNA. Based on this ability of trans-splicing ribozymes to deliver new gene activities, a simple and versatile plating assay was developed in Saccharomyces cerevisiae for assessing and optimizing constructs in vivo. Trans-splicing ribozymes were used to splice sequences encoding a GAL4-derived transcription activator into a target transcript from a prevalent viral pathogen. The transcription activator translated from this new mRNA in turn triggered the expression of genes under the regulatory control of GAL4 upstream-activating sequences. Two of the activated genes complemented metabolic deficiencies in the host strain, and allowed growth on selective media. A simple genetic assay based on phenotypic conversion from auxotrophy to prototrophy was established to select efficient and specific trans-splicing ribozymes from a ribozyme library. This simple assay may prove valuable for selecting optimal target sites for therapeutic agents such as ribozymes, antisense RNA and antisense oligodeoxyribonucleotides, and for optimizing the design of the therapeutic agents themselves, in higher eukaryotes.

RCY1, an Arabidopsis thaliana RPP8/HRT family resistance gene, conferring resistance to cucumber mosaic virus requires salicylic acid, ethylene and a novel signal transduction mechanism

The dominant locus, RCY1, in the Arabidopsis thaliana ecotype C24 confers resistance to the yellow strain of cucumber mosaic virus (CMV-Y). The RCY1 locus was mapped to a 150-kb region on chromosome 5. Sequence comparison of this region from C24 and a CMV-Y-susceptible C24 mutant predicts that the RCY1 gene encodes a 104-kDa CC-NBS-LRR-type protein. The RCY1 gene from C24, when expressed in the susceptible ecotype Wassilewskija (Ws), restricted the systemic spread of virus. RCY1 is allelic to the resistance genes RPP8 from the ecotype Landsberg erecta and HRT from the ecotype Dijon-17, which confer resistance to Peronospora parasitica biotype Emco5 and turnip crinkle virus (TCV), respectively. Examination of RCY1 plants defective in salicylic acid (SA), jasmonic acid (JA) and ethylene signaling revealed a requirement for SA and ethylene signaling in mounting a resistance response to CMV-Y. The RCY1 nahG etr1 double mutants exhibited an intermediate level of susceptibility to CMV-Y, compared to the resistant ecotype C24 and the susceptible ecotypes Columbia and Nossen. This suggests that in addition to SA and ethylene, a novel signaling mechanism is associated with the induction of resistance in CMV-Y-infected C24 plants. Moreover, our results suggest that the signaling pathways downstream of the RPP8, HRT, and RCY1 have evolved independently.

An antibody to the putative aphid recognition site on cucumber mosaic virus recognizes pentons but not hexons

Cucumber mosaic virus (CMV), the type member of the genus Cucumovirus (family Bromoviridae), is transmitted by aphids in a nonpersistent manner. Mutagenesis experiments identified the betaH-betaI loop of the capsid subunit as a potential key motif responsible for interactions with the insect vector. To further examine the functional characteristics of this motif, we generated monoclonal antibodies that bound to native virions but not to betaH-betaI mutants. Fab fragments from these antibodies were complexed with wild-type CMV and the virus-Fab structure was determined to 12-A resolution by using electron cryomicroscopy and image reconstruction techniques. The electron density attributed to the bound antibody has a turret-like appearance and protrudes from each of the 12 fivefold axes of the icosahedral virus. Thus, the antibody binds only to the pentameric clusters (pentons) of A subunits of the T=3 quasisymmetric virus and does not appear to bind to any of the B and C subunits that occur as hexameric clusters (hexons) at the threefold (quasi-sixfold) axes. Modeling and electron density comparisons were used to analyze the paratope-epitope interface and demonstrated that the antibody binds to three betaH-betaI loops in three adjacent A subunits in each penton. This antibody can discriminate between A and B/C subunits even though the betaH-betaI loop adopts the same structure in all 180 capsid subunits and is therefore recognizing differences in subunit arrangements. Antibodies with such character have potential use as probes of viral assembly. Our results may provide an additional rationale for designing synthetic vaccines by using symmetrical viral particles.

A conserved capsid protein surface domain of Cucumber mosaic virus is essential for efficient aphid vector transmission

A prominent feature on the surfaces of virions of Cucumber mosaic virus (CMV) is a negatively charged loop structure (the beta H-beta I loop). Six of 8 amino acids in this capsid protein loop are highly conserved among strains of CMV and other cucumoviruses. Five of these amino acids were individually changed to alanine or lysine (an amino acid of opposite charge) to create nine mutants (the D191A, D191K, D192A, D192K, L194A, E195A, E195K, D197A, and D197K mutants). Transcripts of cDNA clones were infectious when they were mechanically inoculated onto tobacco, giving rise to symptoms of a mottle-mosaic typical of the wild-type virus (the D191A, D191K, D192A, E195A, E195K, and D197A mutants), a systemic necrosis (the D192K mutant), or an atypical chlorosis with necrotic flecking (the L194A mutant). The mutants formed virions and accumulated to wild-type levels, but eight of the nine mutants were defective in aphid vector transmission. The aspartate-to-lysine mutation at position 197 interfered with infection; the only recovered progeny (the D197K(*) mutant) harbored a second-site mutation (denoted by the asterisk) of alanine to glutamate at position 193, a proximal site in the beta H-beta I loop. Since the disruption of charged amino acid residues in the beta H-beta I loop reduces or eliminates vector transmissibility without grossly affecting infectivity or virion formation, we hypothesize that this sequence or structure has been conserved to facilitate aphid vector transmission.

Differential virulence by strains of Cucumber mosaic virus is mediated by the 2b gene

The approximately 12-kDa 2b protein, encoded by all cucumoviruses, had been shown to play an important role in viral long-distance movement, hypervirulence, and suppression of post-transcriptional gene silencing. The role of the 2b gene in the hypervirulence of Cucumber mosaic virus (CMV) and whether hypervirulence was linked to movement were analyzed using a hybrid virus (CMV-qw), generated by replacing the 2b gene in a subgroup II strain, Q-CMV, with the 2b gene from a subgroup IA strain, WAII-CMV. CMV-qw was more virulent than Q-CMV or WAII-CMV on most of the host plant species tested. Northern blot and nucleotide sequence analyses demonstrated that CMV-qw was stably maintained during the course of infection and upon passage. Kinetic studies revealed that the hypervirulence induced by the hybrid virus was associated with neither increased viral RNA accumulation nor more rapid viral movement per se, suggesting that other functions of the 2b protein are important in determining the hypervirulence.

The occurrence of CMV-specific short Rnas in transgenic tobacco expressing virus-derived double-stranded RNA is indicative of resistance to the virus

Expression or introduction of double-stranded (ds)RNA in eukaryotic cells can trigger sequence-specific gene silencing of transgenes, endogenes, and viruses. Transgenic plants producing dsRNAs with homology to viral sequences are likely to exhibit pathogen-derived resistance to the virus. Cucumber mosaic virus (CMV), a very widespread virus with over 1,000 host species, has the natural ability to suppress silencing in order to establish infection. Here, we report the generation of transgenic tobacco lines, where a DNA transgene containing an inverted repeat of CMV cDNA had been introduced. Expression of this DNA construct delivered an RNA transcript that is able to form an intramolecular double strand. Transgenic plants were challenged with CMV. Three categories of plants could be discriminated: susceptible plants, which typically reacted with milder symptoms than the wild-type control; a "recovery" phenotype, in which newly emerging leaves were free of symptoms; and plants that showed complete resistance. Northern analysis showed that the expression of CMV dsRNA caused, in some transgenic lines, the generation of short RNAs characteristic of posttranscriptional gene silencing. Those lines were CMV resistant. The correlation between the detection of short RNAs and virus resistance provides a molecular marker that makes it possible to predict success in attempts to engineer virus resistance by dsRNA.

Virulence and differential local and systemic spread of cucumber mosaic virus in tobacco are affected by the CMV 2b protein

A mutant of the Cucumber mosaic virus subgroup IA strain Fny (Fny-CMV) lacking the gene encoding the 2b protein (Fny-CMVdelta2b) induced a symptomless systemic infection in tobacco. Both the accumulation of Fny-CMVdelta2b in inoculated tissue and the systemic movement of the virus appeared to proceed more slowly than for wild-type Fny-CMV. The influence of the 2b protein on virus movement in the inoculated leaf was examined using viral constructs derived from Fny-CMV and Fny-CMVdelta2b expressing the green fluorescent protein. Laser scanning confocal microscopy was used to visualize the movement of these viruses. Whereas the wild-type virus spread between the epidermal cells as well as the mesophyll cells, the mutant virus spread less efficiently through the epidermal layer and moved preferentially through the mesophyll. Thus, the 2b protein of Fny-CMV influences the dynamics of movement of the virus both within the inoculated leaf and through the whole plant. We propose that this altered movement profile of Fny-CMVdelta2b results in the absence of disease symptoms in tobacco.

Phosphorylation of cucumber mosaic virus RNA polymerase 2a protein inhibits formation of replicase complex

The 2a (polymerase) protein of cucumber mosaic virus (CMV) was shown to be phosphorylated both in vivo and in vitro. In vitro assays using 2a protein mutants and tobacco protein kinases showed that the 2a protein has at least three phosphorylation sites, one of which is located within the N-terminal 126 amino acid region. This region is essential and sufficient for interaction with the CMV 1a protein. When phosphorylated in vitro, the 2a protein N-terminal region failed to interact with the 1a protein. Since the 1a-2a interaction is essential for the replication of CMV, this suggests that phosphorylation of the N-terminal region of the 2a protein negatively modulates the interaction in vivo, and may have a regulatory role acting directly in viral infection.

Heterologous movement protein strongly modifies the infection phenotype of cucumber mosaic virus

A hybrid virus (CMVcymMP) constructed by replacing the movement protein (MP) of cucumber mosaic cucumovirus (CMV) with that of cymbidium ringspot tombusvirus (CymRSV) was viable and could efficiently spread both cell to cell and long distance in host plants. The hybrid virus was able to move cell to cell in the absence of functional CP, whereas CP-deficient CMV was restricted to single inoculated cells. In several Chenopodium and Nicotiana species, the symptom phenotype of the hybrid virus infection was clearly determined by the foreign MP gene. In Nicotiana debneyi and Nicotiana tabacum cv. Xanthi, the hybrid virus could move systemically, contrary to CymRSV.

Inter- and intramolecular recombinations in the cucumber mosaic virus genome related to adaptation to alstroemeria

In four distinct alstroemeria-infecting cucumber mosaic virus (CMV) isolates, additional sequences of various lengths were present in the 3' nontranslated regions of their RNAs 2 and 3, apparently the result of intra- and intermolecular recombination events. Competition experiments revealed that these recombined RNA 2 and 3 segments increased the biological fitness of CMV in alstroemeria.

Evolutionary history of Cucumber mosaic virus deduced by phylogenetic analyses

Cucumber mosaic virus (CMV) is an RNA plant virus with a tripartite genome and an extremely broad host range. Previous evolutionary analyses with the coat protein (CP) and 5' nontranslated region (NTR) of RNA 3 suggested subdivision of the virus into three groups, subgroups IA, IB, and II. In this study 15 strains of CMV whose nucleotide sequences have been determined were used for a complete phylogenetic analysis of the virus. The trees estimated for open reading frames (ORFs) located on the different RNAs were not congruent and did not completely support the subgrouping indicated by the CP ORF, indicating that different RNAs had independent evolutionary histories. This is consistent with a reassortment mechanism playing an important role in the evolution of the virus. The evolutionary trees of the 1a and 3a ORFs were more compact and displayed more branching than did those of the 2a and CP ORFs. This may reflect more rigid host-interactive constraints exerted on the 1a and 3a ORFs. In addition, analysis of the 3' NTR that is conserved among all RNAs indicated that evolutionary constraints on this region are specific to the RNA component rather than the virus isolate. This indicates that functions other than replication are encoded in the 3' NTR. Reassortment may have led to the genetic diversity found among CMV strains and contributed to its enormous evolutionary success.

tRNA elements mediate the assembly of an icosahedral RNA virus

tRNAs, the adapter molecules in protein synthesis, also serve as metabolic cofactors and as primers for viral RNA-directed DNA synthesis. The genomic and subgenomic RNAs of some plant viruses have a 3'-terminal tRNA-like structure (TLS) that can accept a specific amino acid and serve as a site for initiation of replication and as a simple telomere. We report a previously undescribed role for the TLS of brome mosaic virus (BMV), and potentially for cellular tRNA, in mediating the assembly of its icosahedral virions. BMV genomic RNAs and subgenomic RNA lacking the TLS failed to assemble into virions when incubated with purified BMV coat protein. Assembly was restored by addition of a 201-nt RNA containing the BMV TLS. TLSs from two other plant viruses as well as tRNAs from wheat germ and yeast were similarly active in the BMV virion assembly reaction, but ribosomal RNA and polyadenylate did not facilitate assembly. Surprisingly, virions assembled from TLS-less BMV RNA in the presence of tRNAs or TLS-containing short RNA did not incorporate the latter molecules. Consistent with a critical role for the BMV TLS in virion assembly, mutations in the BMV genomic RNAs that were designed to disrupt the folding of the TLS also abolished virion assembly. We discuss the likely roles of the TLS in early stages of virion assembly.

Characterization of Synergy Between Cucumber mosaic virus and Potyviruses in Cucurbit Hosts

ABSTRACT Mixed infections of cucurbits by Cucumber mosaic virus (CMV) and potyviruses exhibit a synergistic interaction. Zucchini squash and melon plants coinfected by the potyvirus Zucchini yellow mosaic virus (ZYMV) and either Fny-CMV (subgroup IA) or LS-CMV (subgroup II) displayed strong synergistic pathological responses, eventually progressing to vascular wilt and plant death. Accumulation of Fny- or LS-CMV RNAs in a mixed infection with ZYMV in zucchini squash was slightly higher than infection with CMV strains alone. There was an increase in CMV (+) strand RNA levels, but no increase in CMV (-) RNA3 levels during mixed infection with ZYMV. Moreover, only the level of capsid protein from LS-CMV increased in mixed infection. ZYMV accumulated to similar levels in singly and mixed infected zucchini squash and melon plants. Coinfection of squash with the potyvirus Watermelon mosaic virus (WMV) and CMV strains increased both the Fny-CMV RNA levels and the LS-CMV RNA levels. However, CMV (-) strand RNA3 levels were increased little or not at all for CMV on coinfection with WMV. Infection of CMV strains (LS and Fny) containing satellite RNAs (WL47-sat RNA and B5*-sat RNA) reduced the accumulation of the helper virus RNA, except when B5*-sat RNA was mixed with LS- CMV. However, mixed infection containing ZYMV and the CMV strains with satellites reversed the suppression effect of satellite RNAs on helper virus accumulation and increased satellite RNA accumulation. The synergistic interaction between CMV and potyviruses in cucurbits exhibited different features from that documented in tobacco, indicating there are differences in the mechanisms of potyvirus synergistic phenomena.

Lessons learned from the core RNA promoters of Brome mosaic virus and Cucumber mosaic virus

summary RNA core promoters are nucleotide sequences needed to direct proper initiation of viral RNA synthesis by the viral replicase. Minimal length core promoter-templates that can direct accurate initiation of the genomic plus-, genomic minus-, and subgenomic RNAs of Brome mosaic virus and Cucumber mosaic virus were characterized in previous works. Several common themes and differences were observed in how each of the core promoters directed the initiation of viral RNA synthesis in vitro. These observations are summarized and compared in this short review.

Conversion in the requirement of coat protein in cell-to-cell movement mediated by the cucumber mosaic virus movement protein

Plant viruses have movement protein (MP) gene(s) essential for cell-to-cell movement in hosts. Cucumber mosaic virus (CMV) requires its own coat protein (CP) in addition to the MP for intercellular movement. Our present results using variants of both CMV and a chimeric Brome mosaic virus with the CMV MP gene revealed that CMV MP truncated in its C-terminal 33 amino acids has the ability to mediate viral movement independently of CP. Coexpression of the intact and truncated CMV MPs extremely reduced movement of the chimeric viruses, suggesting that these heterogeneous CMV MPs function antagonistically. Sequential deletion analyses of the CMV MP revealed that the dispensability of CP occurred when the C-terminal deletion ranged between 31 and 36 amino acids and that shorter deletion impaired the ability of the MP to promote viral movement. This is the first report that a region of MP determines the requirement of CP in cell-to-cell movement of a plant virus.

Differentiation of Peanut Seedborne Potyviruses and Cucumoviruses by RT-PCR

Seedborne peanut viruses pose important constraints to peanut production and safe movement of germ plasm. They also pose a risk of accidental introduction into previously disease-free regions. We have developed reverse transcription-polymerase chain reaction (RT-PCR) assays based on identical cycling parameters which identified peanut stripe, Peanut mottle, Peanut stunt, and Cucumber mosaic viruses through production of specific DNA fragments of 234 bp, 327 bp, 390 bp, and 133 bp, respectively. Assay sensitivity in the picogram range was achieved. The two potyviruses and two cucumoviruses could be differentiated using duplex RT-PCR assays. These assays should be useful for testing peanut leaves or seeds for virus identification in epidemiological studies, seed testing or in post-entry quarantine.

Field Survey of Cucumber mosaic virus Subgroups I and II in Crop Plants in Costa Rica

Leaf samples were collected from cucurbit and solanaceous crop plants and Musa spp. in 28 locations in five provinces of Costa Rica during the period from January to October 1996. Sampling sites were selected in dry, humid, and moist tropical regions ranging in altitude from 50 to 2,100 m above sea level. RNA-enriched total nucleic acid solutions were spotted onto nylon membranes and hybridized to RNA probes specific for Cucumber mosaic virus (CMV) subgroups I or II. The presence of CMV was confirmed in 13 crops in 23 of the 28 sampling sites. CMV subgroup I was found to predominate in Costa Rica. CMV subgroup II was detected in the Atlantic region only, and in only 1 out of 113 CMV-positive samples.

The suppressor of transgene RNA silencing encoded by Cucumber mosaic virus interferes with salicylic acid-mediated virus resistance

The Cucumber mosaic virus (CMV)-encoded 2b protein (Cmv2b) is a nuclear protein that suppresses transgene RNA silencing in Nicotiana benthamiana. Cmv2b is an important virulence determinant but nonessential for systemic spread in N. glutinosa, in contrast to its indispensable role for systemic infections in cucumber. Here, we report that Cmv2b became essential for systemic infections in older N. glutinosa plants or in young seedlings pretreated with salicylic acid (SA). Expression of Cmv2b from the genome of either CMV or Tobacco mosaic virus significantly reduced the inhibitory effect of SA on virus accumulation in inoculated leaves and systemic leaves. A close correlation is demonstrated between Cmv2b expression and a reduced SA-dependent induction of the alternative oxidase gene, a component of the recently proposed SA-regulated antiviral defense. These results collectively reveal a novel activity of Cmv2b in the inhibition of SA-mediated virus resistance. We used a N. tabacum line expressing a bacterial nahG transgene that degrades SA to provide evidence for a Cmv2b-sensitive antiviral defense mechanism in tobacco in which SA acts as a positive modifier but not as an essential component. We propose that SA induces virus resistance by potentiating a RNA-silencing antiviral defense that is targeted by Cmv2b.

An early tobacco mosaic virus-induced oxidative burst in tobacco indicates extracellular perception of the virus coat protein

Induction of reactive oxygen species (ROS) was observed within seconds of the addition of exogenous tobacco mosaic virus (TMV) to the outside of tobacco (Nicotiana tabacum cv Samsun NN, EN, or nn) epidermal cells. Cell death was correlated with ROS production. Infectivity of the TMV virus was not a prerequisite for this elicitation and isolated coat protein (CP) subunits could also elicit the fast oxidative burst. The rapid induction of ROS was prevented by both inhibitors of plant signal transduction and inhibitors of NAD(P)H oxidases, suggesting activation of a multi-step signal transduction pathway. Induction of intracellular ROS by TMV was detected in TMV-resistant and -susceptible tobacco cultivars isogenic for the N allele. The burst was also detected with strains of virus that either elicit (ToMV) or fail to elicit (TMV U1) N' gene-mediated responses. Hence, early ROS generation is independent or upstream of known genetic systems in tobacco that can mediate hypersensitive responses. Analysis of other viruses and TMV CP mutants showed marked differences in their ability to induce ROS showing specificity of the response. Thus, initial TMV-plant cell interactions that lead to early ROS induction occur outside the plasma membrane in an event requiring specific CP epitopes.

Mapping the virus and host genes involved in the resistance response in cucumber mosaic virus-Infected Arabidopsis thaliana

A yellow strain of cucumber mosaic virus (CMV) [CMV(Y)] induces a resistance response characterized by inhibition of virus systemic movement with development of necrotic local lesions in the virus-inoculated leaves of Arabidopsis thaliana ecotype C24. In this report, the avirulence determinant in the virus genome was defined and the resistance gene (RCY1) of C24 was genetically mapped. The response of C24 to CMV containing the chimeric RNA3 between CMV(Y) and a virulent strain of CMV indicated that the coat protein gene of CMV(Y) determined the localization of the virus in the inoculated leaves of C24. The RCY1 locus was mapped between two CAPS markers, DFR and T43968, which were located in the region containing genetically defined disease resistance genes and their homologues. These results indicate that the resistance response to CMV(Y) in C24 is determined by the combination of the coat protein gene and RCY1 on chromosome 5.

Cucumber mosaic virus-plant interactions: identification of 3a protein sequences affecting infectivity, cell-to-cell movement, and long-distance movement

Mutants of the Cucumber mosaic virus (CMV) movement protein (MP) were generated and analyzed for their effects on virus movement and pathogenicity in vivo. Similar to the wild-type MP, mutants M1, M2, and M3, promoted virus movement in eight plant species. Mutant M3 showed some differences in pathogenicity in one host species. Mutant M8 showed some host-specific alterations in movement in two hypersensitive hosts of CMV. Mutant M9 showed altered pathogenicity on three hosts and was temperature sensitive for long-distance movement, demonstrating that cell-to-cell and long-distance movement are distinct movement functions for CMV. Four mutants (M4, M5, M6, and M7) were debilitated from movement in all hosts tested. Mutants M4, M5, and M6 could be complemented in trans by the wild-type MP expressed transgenically, although not by each other or by mutant M9 (at the restrictive temperature). Mutant M7 showed an inability to be complemented in trans. From these mutants, different aspects of the CMV movement process could be defined and specific roles for particular sequence domains assigned. The broader implications of these functions are discussed.

Efficient and specific initiation of subgenomic RNA synthesis by cucumber mosaic virus replicase in vitro requires an upstream RNA stem-loop

We defined the minimal core promoter sequences responsible for efficient and accurate initiation of cucumber mosaic virus (CMV) subgenomic RNA4. The necessary sequence maps to positions -28 to +15 relative to the initiation cytidylate used to initiate RNA synthesis in vivo. Positions -28 to -5 contain a 9-bp stem and a 6-nucleotide purine-rich loop. Considerable changes in the stem and the loop are tolerated for RNA synthesis, including replacement with a different stem-loop. In a template competition assay, the stem-loop and the initiation cytidylate are sufficient to interact with the CMV replicase. Thus, the mechanism of core promoter recognition by the CMV replicase appears to be less specific in comparison to the minimal subgenomic core promoter of the closely related brome mosaic virus.

The ecology of Cucumber mosaic virus and sustainable agriculture

An account is given of the ecology of Cucumber mosaic virus (CMV) as a pertinent example of how a virus can affect the sustainability of an important crop. It is now generally accepted that the technologies used in modern agriculture should ensure that production systems are operated in such a way that the quality of the produce is maintained year after year without causing degradation of the environment. Recent experiences in countries of the Mediterranean basin demonstrate that the benefits expected from the introduction of new and highly productive plant varieties may be quickly eroded by the concomitant introduction of new virus strains which can greatly change the structure of the resident virus population. Quarantine inspection of plant propagules and genetic engineering are suggested as powerful tools to help achieve sustainability.

Molecular epidemiology of Cucumber mosaic virus and its satellite RNA

Molecular analysis of viral isolates can yield information that facilitates an understanding of virus epidemiology and has been termed molecular epidemiology. This approach has only recently been applied to plant viruses. Results on the molecular epidemiology of Cucumber mosaic virus (CMV) and its satellite RNA (satRNA) in Spain, where CMV is endemic in vegetable crops are presented here. To characterise the genetic structure of CMV populations, c. 300 isolates, representing 17 outbreaks (i.e. sub-populations) in different crops, regions and years, were compared. Genetic analyses of CMV isolates were done by ribonuclease protection assay of cRNA probes representing RNA1, RNA2 and the two open reading frames in RNA3. All isolates belonged to one of three genetic types: Sub-group II and two types of Sub-group I. The genetic structure of the 17 sub-populations varied randomly, without correlation with location, year, or host plant species. Thus, CMV in Spain shows a metapopulation structure with local extinction and random recolonisation from local or distant virus reservoirs. The frequency of mixed infections and of new genetic types generated by reassortment of genomic segments or by recombination was also estimated. Results indicate that heterologous genetic combinations are not favoured. About 30% of CMV isolates were supporting a satRNA. The frequency of CMV isolates with a satRNA differed for each sub-population, being c. 1 in eastern Spain in 1990 and decreasing to c. 0 in distant regions and in subsequent years. Molecular analyses of CMV-satRNA isolates show high genetic diversity, due both to the accumulation of point mutations and to recombination. The CMV-satRNA population is a single, unstructured one. Thus, the CMV-satRNA population has a genetic structure and dynamics different from those of its helper virus. This indicates that CMV-satRNA has spread epidemically on the extant virus population from an original reservoir in eastern Spain. The relevance of these results for the control of CMV infections is discussed.

Solanaceous Weeds as Possible Sources of Cucumber mosaic virus in Southern Illinois for Aphid Transmission to Pepper

Over 5,000 individual plants representing approximately 55 species from an area in southern Illinois where Cucumber mosaic virus (CMV) has been a major problem in pepper (Capsicum annuum) were tested for the presence of CMV by enzyme-linked immunosorbent assay (ELISA). Representative ELISA-positive samples were checked by western blot tests to confirm virus-specific reactions. Nearly all of the infected plants detected were either Solanum ptycanthum (eastern black nightshade) or Physalis spp. (principally P. heterophylla, groundcherry). Over 1,000 pepper transplants and approximately 500 tomato transplants, collected prior to planting, were negative for CMV by ELISA. In aphid transmission (arena) experiments, all five aphid species tested were capable of transmitting CMV from nightshade to pepper: Aphis fabae subsp. solanella, Aphis gossypii, Myzus persicae, Rhopalosiphum padi, and Sitobion avenae. Aphis fabae subsp. solanella, A. gossypii, and A. nerii were able to transmit CMV from P. heterophylla to pepper. Aphis fabae subsp. solanella was commonly found colonizing nightshade from May through October in southern Illinois.

Recognition of the core RNA promoter for minus-strand RNA synthesis by the replicases of Brome mosaic virus and Cucumber mosaic virus

Replication of viral RNA genomes requires the specific interaction between the replicase and the RNA template. Members of the Bromovirus and Cucumovirus genera have a tRNA-like structure at the 3' end of their genomic RNAs that interacts with the replicase and is required for minus-strand synthesis. In Brome mosaic virus (BMV), a stem-loop structure named C (SLC) is present within the tRNA-like region and is required for replicase binding and initiation of RNA synthesis in vitro. We have prepared an enriched replicase fraction from tobacco plants infected with the Fny isolate of Cucumber mosaic virus (Fny-CMV) that will direct synthesis from exogenously added templates. Using this replicase, we demonstrate that the SLC-like structure in Fny-CMV plays a role similar to that of BMV SLC in interacting with the CMV replicase. While the majority of CMV isolates have SLC-like elements similar to that of Fny-CMV, a second group displays sequence or structural features that are distinct but nonetheless recognized by Fny-CMV replicase for RNA synthesis. Both motifs have a 5'CA3' dinucleotide that is invariant in the CMV isolates examined, and mutational analysis indicates that these are critical for interaction with the replicase. In the context of the entire tRNA-like element, both CMV SLC-like motifs are recognized by the BMV replicase. However, neither motif can direct synthesis by the BMV replicase in the absence of other tRNA-like elements, indicating that other features of the CMV tRNA can induce promoter recognition by a heterologous replicase.

Transmissibility of Cucumber mosaic virus by Aphis gossypii Correlates with Viral Accumulation and Is Affected by the Presence of Its Satellite RNA

ABSTRACT Satellite RNAs (satRNAs) are associated with Cucumber mosaic virus (CMV) in tomato, most often causing severe epidemics of necrotic plants, and not associated with specific host symptoms. Laboratory studies on virus transmission by the aphid vector Aphis gossypii were performed to better understand the dynamics of field populations of CMV. The presence of satRNAs correlated with lower concentrations of virus in infected plants and with a decrease in the efficiency of transmission from satRNA-infected plants. Both the concentration of virus in CMV-infected tomato and the efficiency of transmission varied more extensively with nonnecrogenic satRNAs than with necrogenic satRNAs. A negative effect of satRNAs on virus accumulation can account, in part, for a decrease in the field transmission and recovery of CMV + satRNAs. Aphids behaved differently and probed less readily on plants infected with CMV + necrogenic satRNAs compared with plants containing non-necrogenic satRNAs. Aphid-mediated satRNA-free CMV infections were observed in test plants when aphids were fed on source plants containing CMV + nonnecrogenic satRNA; no comparable satRNA-free test plants occurred when aphids were fed on source plants containing necrogenic satRNAs. These results indicate that factors associated with transmission can be a determinant in the evolution of natural populations of CMV and its satRNA.

The structure of cucumber mosaic virus and comparison to cowpea chlorotic mottle virus

The structure of cucumber mosaic virus (CMV; strain Fny) has been determined to a 3.2-A resolution using X-ray crystallography. Despite the fact that CMV has only 19% capsid protein sequence identity (34% similarity) to cowpea chlorotic mottle virus (CCMV), the core structures of these two members of the Bromoviridae family are highly homologous. As suggested by a previous low-resolution structural study, the 305-A diameter (maximum) of CMV is approximately 12 A larger than that of CCMV. In CCMV, the structures of the A, B, and C subunits are nearly identical except in their N termini. In contrast, the structures of two loops in subunit A of CMV differ from those in B and C. These loops are 6 and 7 residues longer than the analogous regions in CCMV. Unlike that of CCMV, the capsid of CMV does not undergo swelling at pH 7.0 and is stable at pH 9.0. This may be partly due to the fact that the N termini of the B and C subunits form a unique bundle of six amphipathic helices oriented down into the virion core at the threefold axes. In addition, while CCMV has a cluster of aspartic acid residues at the quasi-threefold axis that are proposed to bind metal in a pH-dependent manner, this cluster is replaced by complementing acids and bases in CMV. Finally, this structure clearly demonstrates that the residues important for aphid transmission lie at the outermost portion of the betaH-betaI loop and yields details of the portions of the virus that are hypothesized to mediate binding to aphid mouthparts.