Stunting induced by cucumber mosaic cucumovirus-infected Nicotiana glutinosa is determined by a single amino acid residue in the coat protein

Peanut stunt virus movement protein is the limiting factor in Capsicum annuum infection

Cucumber mosaic virus (CMV) is one of the most devastating plant viruses, with more than 1,200 species of host plants. The host range and economic importance of peanut stunt virus (PSV) are mostly limited to legumes, despite the similar taxonomy and genome structure with CMV. Since no data are available on the background of the limited host range of PSV, RNA 3 recombinant and reassortant viruses were generated (C12P3, P12C3, C12CP3, C12PC3, C12PΔC3) to study their infection phenotype on a common host (Nicotiana benthamiana) and on a selective host (Capsicum annuum cv. Brody). The PSV movement protein (MP) was not able to function with the coat protein (CP) of CMV unless the C-terminal 42 amino acids were deleted from the PSV MP. As a result of the inoculation experiments, MP was considered the protein influencing symptom phenotypes on N. benthamiana and responsible for the host range difference on the pepper. Since plasmodesmata (PD) localization of viral MPs is essential for cell-to-cell movement, subcellular localization of GFP-tagged MPs (CMV-MP-eGFP, PSV-MP-eGFP) was observed. In the case of CMV-MP-eGFP, clear colocalization with PD was detected in both hosts, but PSV-MP-eGFP was not tightly connected to the PD in N. benthamiana and barely localized to the PD in C. annuum epidermal cells. Measuring Pearson correlation coefficients (PCCs) also supported the visual observation.

The Amino-Proximal Region of the Coat Protein of Cucumber Vein Yellowing Virus (Family Potyviridae) Affects the Infection Process and Whitefly Transmission

Most plant viruses rely on vector transmission for their spread and specific interactions between vector and virus have evolved to regulate this relationship. The whitefly Bemisia tabaci- transmitted cucumber vein yellowing virus (CVYV; genus Ipomovirus, family Potyviridae) is endemic in the Mediterranean Basin, where it causes significant losses in cucurbit crops. In this study, the role of the coat protein (CP) of CVYV for B. tabaci transmission and plant infection was investigated using a cloned and infectious CVYV cDNA and a collection of point and deletion mutants derived from this clone. Whitefly transmission of CVYV was abolished in a deletion mutant lacking amino acids in position 93-105 of the CP. This deletion mutant caused more severe disease symptoms compared to the cDNA clone representing the wild-type (wt) virus and movement efficiency was likewise affected. Two virus mutants carrying a partially restored CP were transmissible and showed symptoms comparable to the wt virus. Collectively, our data demonstrate that the N-terminus of the CVYV CP is a determinant for transmission by the whitefly vector and is involved in plant infection and symptom expression.

Efficient and high-throughput pseudorecombinant-chimeric Cucumber mosaic virus-based VIGS in maize

Virus-induced gene silencing (VIGS) is a versatile and attractive approach for functional gene characterization in plants. Although several VIGS vectors for maize (Zea mays) have been previously developed, their utilities are limited due to low viral infection efficiency, insert instability, short maintenance of silencing, inadequate inoculation method, or abnormal requirement of growth temperature. Here, we established a Cucumber mosaic virus (CMV)-based VIGS system for efficient maize gene silencing that overcomes many limitations of VIGS currently available for maize. Using two distinct strains, CMV-ZMBJ and CMV-Fny, we generated a pseudorecombinant-chimeric (Pr) CMV. Pr CMV showed high infection efficacy but mild viral symptoms in maize. We then constructed Pr CMV-based vectors for VIGS, dubbed Pr CMV VIGS. Pr CMV VIGS is simply performed by mechanical inoculation of young maize leaves with saps of Pr CMV-infected Nicotiana benthamiana under normal growth conditions. Indeed, suppression of isopentenyl/dimethylallyl diphosphate synthase (ZmIspH) expression by Pr CMV VIGS resulted in non-inoculated leaf bleaching as early as 5 d post-inoculation (dpi) and exhibited constant and efficient systemic silencing over the whole maize growth period up to 105 dpi. Furthermore, utilizing a ligation-independent cloning (LIC) strategy, we developed a modified Pr CMV-LIC VIGS vector, allowing easy gene cloning for high-throughput silencing in maize. Thus, our Pr CMV VIGS system provides a much-improved toolbox to facilitate efficient and long-duration gene silencing for large-scale functional genomics in maize, and our pseudorecombination-chimera combination strategy provides an approach to construct efficient VIGS systems in plants.

Cucumber mosaic virus coat protein: The potential target of 1, 4-pentadien-3-one derivatives

Cucumber mosaic virus coat protein (CMV CP) plays a key role in cell-to-cell movement in host organisms. 1,4-Pentadien-3-one derivatives have excellent antiviral activities. In this study, we cloned, expressed and purified a CP recombinant protein. Then, we studied the binding interactions of CMV CP and 1, 4-pentadien-3-one derivatives N1-N20. Microscale thermophoresis experiments showed that N12 and N16 bound to CMV CP with dissociation constants of 0.071 and 0.11 μM, respectively. Docking and site-directed mutagenesis studies provided further insights into the interactions of N12 and N16 with Ile210, Thr69 and Ser213of CMV CP. Thus, these CMV CP residues may be important binding sites for the 1,4-pentadien-3-one derivatives N12 and N16. The data are important for designing and synthesizing new pentadienone derivatives.

Molecular Modeling for Better Understanding of Cucumovirus Pathology

Cucumber mosaic virus (CMV) is a small RNA virus capable of infecting a wide variety of plant species. The high economic losses due to the CMV infection made this virus a relevant subject of scientific studies, which were further facilitated by the small size of the viral genome. Hence, CMV also became a model organism to investigate the molecular mechanism of pathogenesis. All viral functions are dependent on intra- and intermolecular interactions between nucleic acids and proteins of the virus and the host. This review summarizes the recent data on molecular determinants of such interactions. A particular emphasis is given to the results obtained by utilizing molecular-based planning and modeling techniques.

Movement protein of broad bean wilt virus 2 serves as a determinant of symptom severity in pepper

Broad bean wilt virus 2 (BBWV2, genus Fabavirus, family Secoviridae) has a wide host range and infects many economically important crops. Various isolates of BBWV2 have been identified from diverse host plants, and their molecular and biological characteristics have been investigated. In our previous study, we demonstrated that BBWV2 RNA2 contains a symptom determinant(s) capable of enhancing symptom severity by utilizing infectious full-length cDNA clones of two distinct strains of BBWV2, pBBWV2-PAP1 (a severe strain) and pBBWV2-RP1 (a mild strain). In the present study, to identify the symptom determinant(s) of BBWV2, we exploited disease responses of pBBWV2-PAP1- and pBBWV2-RP1-derived chimeric viruses and amino acid substitution mutant viruses in Nicotiana benthamiana and pepper (Capsicum annuum Quarri) and demonstrated that the movement protein (MP) encoded in BBWV RNA2 is the determinant of disease symptom severity in both plants. A single amino acid substitution in the MP was sufficient for changing symptom severity of BBWV2. Our finding provides a role for the MP as a symptom determinant in BBWV2 and increases the understanding of the basis of molecular interactions between host plants and BBWV2.

Pseudorecombination between Two Distinct Strains of Cucumber mosaic virus Results in Enhancement of Symptom Severity

Recently, a Cucumber mosaic virus (CMV) strain, named as CMV-209, was isolated from Glycine soja. In this study, symptom expression of CMV-209 was analyzed in detail in Nicotiana benthamiana by comparing with that of CMV-Fny, which is a representative strain of CMV. Using infectious cDNA clones of CMV strains 209 and Fny, symptom expression of various pseudorecombinants between these two strains were examined in the early and late infection stages. In the early infection stage, the pseudorecombinants containing Fny-RNA2 induced stunting and leaf distortion on the newly emerged leaves whereas the pseudorecombinants containing 209-RNA2 caused no obvious symptoms. In the late infection stage, the pseudorecombinants containing 209-RNA1 and Fny-RNA2 induced severe leaf distortion and stunting, while CMV-209 induced mild symptom and CMV-Fny caused typical mosaic, general stunting, and leaf distortion symptoms, indicating that RNA 2 encodes a symptom determinant(s) of CMV, which is capable of enhancing symptoms. Furthermore, our results support the possibility that natural recombination between compatible viruses can result in emergence of novel viruses causing severe damages in crop fields.

Quantitative transcriptional changes associated with chlorosis severity in mosaic leaves of tobacco plants infected with Cucumber mosaic virus

Cucumber mosaic virus (CMV) causes mosaic disease in inoculated tobacco plants. Coat protein (CP) is one of the major virulence determinants of CMV, and an amino acid substitution at residue 129 in CP alters the severity of chlorosis, such as pale green chlorosis and white chlorosis, in symptomatic tissues of mosaic leaves of infected tobacco. In this study, we compared the transcriptomes of chlorotic tissues infected with the wild-type pepo strain of CMV and two strains carrying CP mutants with diverse chlorosis severity. Differential gene expression analysis showed that CMV inoculation appeared to have similar effects on the transcriptional expression profiles of the symptomatic chlorotic tissues, and only the magnitude of expression differed among the different CMVs. Gene ontology analysis with biological process and cellular component terms revealed that many nuclear genes related to abiotic stress responses, including responses to cadmium, heat, cold and salt, were up-regulated, whereas chloroplast- and photosynthesis-related genes (CPRGs) were down-regulated, in the chlorotic tissues. Interestingly, the level of CPRG down-regulation was correlated with the severity of chlorosis. These results indicate that CP mutation governs the repression level and mRNA accumulation of CPRGs, which are closely associated with the induction of chlorosis.

Analysis of two strains of Peanut stunt virus: satRNA-associated and satRNA free

Peanut stunt virus (PSV) is a pathogen of legumes, vegetables, trees, and weeds occurring worldwide. The species is characterized by significant genetic variability. PSV strains are classified into four subgroups on the basis of their nucleotide sequence homology. Here, we are presenting two further, fully sequenced PSV strains-PSV-Ag and PSV-G, that could be considered as I subgroup representatives. However, their sequence homology with other typical I subgroups members, similarly as another strain-PSV-P, characterized by our group previously, is lower than 90%. This lead us to propose further subdivision of the I subgroup into IA, IB, and IC units, and to classify PSV-Ag and PSV-G strains to the last one. In this article, we are showing that identity level of PSV-Ag and PSV-G is very high and apart from the presence of satRNA in the first one, they differ only by a few nucleotides in their genomic RNAs. Nevertheless, symptoms they cause on host plants might differ significantly, just as the levels in infected plants. Effect of single amino acid changes between strains on the three-dimensional structure of viral proteins was analyzed. Differences occur mainly on the protein surfaces which can possibly affect protein-protein interaction in infected cells, which is discussed.

Cucumber mosaic virus: viral genes as virulence determinants

TAXONOMIC RELATIONSHIPS

Cucumber mosaic virus (CMV) is the type species of the genus Cucumovirus in the family Bromoviridae, which also encompasses the Peanut stunt virus (PSV) and the Tomato aspermy virus (TAV). Nucleotide sequence similarity among these three cucumoviruses is 60%-65%. CMV strains are divided into three subgroups, IA, IB and II, based on the sequence of the 5' untranslated region of the genomic RNA 3. Overall nucleotide sequence similarity among CMV strains is approximately 70%-98%. GEOGRAPHICAL DISTRIBUTION, HOST RANGE AND SYMPTOMATOLOGY: CMV is distributed worldwide, primarily in temperate to tropical climate zones. CMV infects more than 1200 species of 100 plant families, including monocot and dicot plants. Symptoms caused by CMV infection vary with the host species and/or CMV strain, and include mosaic, stunt, chlorosis, dwarfing, leaf malformation and systemic necrosis. CMV disease is spread primarily by aphid transmission in a nonpersistent manner.

PHYSICAL PROPERTIES

In tobacco sap, the thermal inactivation point of the viral infectivity is approximately 70 °C (10 min), the dilution end-point is approximately 10(-4) and viral infectivity is lost after a few days of exposure to 20 °C. Viral infectivity can be retained in freeze-dried tissues and in the form of virions purified using 5 mm sodium borate, 0.5 mm ethylenediaminetetraacetic acid and 50% glycerol (pH 9.0) at -20 °C. CMV particles are isometric, approximately 28-30 nm in diameter and are composed of 180 capsid subunits arranged in pentamer-hexamer clusters with T= 3 symmetry. The sedimentation coefficient (s(20) ,(w) ) is c. 98 S and the particle weight is (5.8-6.7) × 10(6) Da. The virions contain 18% RNA. The RNA-protein interactions that stabilize the CMV virions are readily disrupted by sodium dodecylsulphate or neutral chloride salts. GENOMIC PROPERTIES: The genomic RNAs are single-stranded messenger sense RNAs with 5' cap and 3' tRNA-like structures containing at least five open reading frames. The viral RNA consists of three genomic RNAs, RNA 1 (c. 3.3 kb), RNA 2 (c. 3.0 kb) and RNA 3 (c. 2.2 kb), and two subgenomic RNAs, RNA 4 (c. 1.0 kb) and RNA 4A (c. 0.7 kb). The 3' untranslated regions are conserved across all viral RNAs. CMV is often accompanied by satellite, noncoding, small, linear RNA that is nonhomologous to the helper CMV.

Relationship between symptoms and gene expression induced by the infection of three strains of Rice dwarf virus

Background

Rice dwarf virus (RDV) is the causal agent of rice dwarf disease, which often results in severe yield losses of rice in East Asian countries. The disease symptoms are stunted growth, chlorotic specks on leaves, and delayed and incomplete panicle exsertion. Three RDV strains, O, D84, and S, were reported. RDV-S causes the most severe symptoms, whereas RDV-O causes the mildest. Twenty amino acid substitutions were found in 10 of 12 virus proteins among three RDV strains.

Methodology/Principal Findings

We analyzed the gene expression of rice in response to infection with the three RDV strains using a 60-mer oligonucleotide microarray to examine the relationship between symptom severity and gene responses. The number of differentially expressed genes (DEGs) upon the infection of RDV-O, -D84, and -S was 1985, 3782, and 6726, respectively, showing a correlation between the number of DEGs and symptom severity. Many DEGs were related to defense, stress response, and development and morphogenesis processes. For defense and stress response processes, gene silencing-related genes were activated by RDV infection and the degree of activation was similar among plants infected with the three RDV strains. Genes for hormone-regulated defense systems were also activated by RDV infection, and the degree of activation seemed to be correlated with the concentration of RDV in plants. Some development and morphogenesis processes were suppressed by RDV infection, but the degree of suppression was not correlated well with the RDV concentration.

Conclusions/Significance

Gene responses to RDV infection were regulated differently depending on the gene groups regulated and the strains infecting. It seems that symptom severity is associated with the degree of gene response in defense-related and development- and morphogenesis-related processes. The titer levels of RDV in plants and the amino acid substitutions in RDV proteins could be involved in regulating such gene responses.

The 2b protein and the C-terminus of the 2a protein of cucumber mosaic virus subgroup I strains both play a role in viral RNA accumulation and induction of symptoms

Two chimeras of cucumber mosaic virus (CMV), FCb7(2b)-CMV and FRad35(2b)-CMV, with the 2b genes of strains Cb7-CMV and Rad35-CMV, respectively, in an Fny-CMV background, gave different responses on Nicotiana glutinosa: FCb7(2b)-CMV induced systemic necrosis while FRad35(2b)-CMV caused only mild mosaic. This differential virulence was attributable to the nature of amino acid 55 of their 2b proteins. However, sequence analysis revealed that Leu(55) of the 2b protein was necessary but not sufficient for FCb7(2b)-CMV to induce systemic necrosis. Surprisingly, inhibition of translation of the 2a/2b overlapping region of the 2a protein in FCb7(2b)-CMV led to a loss of systemic necrosis and a reduction in accumulation of viral progeny RNAs. The 2a/2b overlapping region of Fny-CMV had a similar effect on virulence and viral accumulation. Thus, the 2a protein C-terminus of subgroup I strains, as well as the 2b protein, play a role in symptom induction and accumulation of viral RNAs.

Comparisons of complete RNA-2 sequences, pathological and serological features among three Japanese isolates of Arabis mosaic virus

Arabis mosaic virus lily and narcissus isolates (ArMV-Li and ArMV-Na) induced severe necrotic spots on Chenopodium quinoa, whereas ArMV butterbur isolate (ArMV-Bu) caused symptomless infection in the plant. The accumulation level of ArMV-Bu in upper non-inoculated leaves of C. quinoa was comparable to that of ArMV-Li or -Na. The agar gel double-diffusion test using an antiserum against ArMV-Li showed ArMV-Li was closely related to ArMV-Na, but not to ArMV-Bu. The RNAs-2 of ArMV-Li, -Na, and -Bu consist of 3707, 3709, and 3789 nucleotides, and they contain one open reading frame encoding a putative polyprotein of 1083, 1084, and 1122 amino acids, respectively. The overall identity of RNA-2 of ArMV-Li displayed more than 90% with ArMV-Na, but less than 70% with ArMV-Bu. A phylogenetic analysis of 2A sequences from ArMV isolates revealed ArMV-Bu was not categorized in any cluster. ArMV-Bu is a unique isolate from the point of view of pathological and serological features, and nucleotide sequence.

The sequence and model structure analysis of three Polish peanut stunt virus strains

Peanut stunt virus (PSV) belongs to the Cucumovirus genus of the family Bromoviridae and is widely distributed worldwide, also in Poland. PSV is a common pathogen of a wide range of economically important plants. Its coat protein (CP), similarly as in other viruses, plays an important role in many processes during viral life cycle and has great impact on the infectivity. In this study, we present the results of sequence-structure analysis of CP derived from three Polish strains of PSV: PSV Ag, G, and P. Sequences were determined using RT-PCR amplification followed by sequencing and compared with each other and also with CP from other known PSV viruses. We analyzed their phylogenetic relationship, based on CP sequence, using bioinformatic tools as well as their spatial model using homology-modeling approach with combination of ROSETTA algorithm for de novo modeling. We compared our model with those recently obtained for other cucumoviruses including PSV-Er. Our results have shown that all Polish strains probably belong to the first subgroup of PSV viruses. Homology level between strains Ag and G proved very high. Using theoretical modeling approach we obtained a model very similar to the one resolved previously with the differences caused by slightly different amino acid sequence. We have also undertaken an attempt to analyze its distant regions; however, results are not unequivocal. Analysis of symptoms and their correlation with specific amino acid position was also performed on the basis of results published elsewhere. The definite interpretation is impeded by the presence of satellite RNAs in Ag and P strains modulating symptoms' severity, though.

Variation analysis of two cucumber mosaic viruses and their associated satellite RNAs from sugar beet in China

Two cucumber mosaic virus (CMV) isolates XJ1 and XJ2 were obtained from sugar beet showing yellow mosaic symptom in Shihezi, Xinjiang Uigur municipality of China. The coat protein gene of the two CMV isolates and their associated satellite RNAs were amplified by reverse transcriptase polymerase chain reaction (RT-PCR) and were cloned and sequenced. Comparison of CP gene sequences showed that XJ1 and XJ2 have the highest sequence identity with that of CMV-Danshen (97.8%) and CMV-SD (98.7%), respectively. Two types of satellite RNAs (XJs1 and XJs2) were found to be associated with the two CMV isolates consisting of 384 nucleotides and 336 nucleotides, respectively. Sequence comparisons revealed that XJs1 and XJs2 were most closely related to CS2-sat and CS1-sat, respectively, with 98.9% and 98.5% nucleotide sequence identity. Phylogenetic analysis of nucleotide sequence and deduced amino acid sequence of coat protein gene revealed that XJ1 and XJ2 belong to subgroup IB but there exist some variation between them. Parallel analyses of nucleotide sequence of XJsl and XJs2 suggested that these two satellite RNAs probably originated from China.

Homology modelling and protein structure based functional analysis of five cucumovirus coat proteins

Coat proteins (CP) of five cucumovirus isolates, Cucumber mosaic virus (CMV) strains R, M and Trk7, Tomato aspermy virus (TAV) strain P and Peanut stunt virus (PSV) strain Er, were constructed by homology modelling. The X-ray structure of the Fny-CMV CP subunit B was used as a template. Models of cucumovirus CPs were built by the MODELLER program. Model refinements were carried out using the Kollman molecular mechanical force field. Models were analyzed by the PROCHECK programs. Electrostatic potential calculations were applied to all models and functional site search was performed with the PROSITE software, a web based tool for searching biologically significant sites. Symptom determinants published up to the present were compared with the PROSITE hits in the light of 3D models and electrostatic information. In all cases, we analyzed the effect of mutations on the structure, electrostatic potential patterns and function of CPs, respectively. We found that high flexibility of the betaE-alphaEF loop starting with the residue 129 is required, but it is not sufficient for the symptom appearance. Furthermore, phosphorylation of the CP is prospective to be important in the host response mechanism. All analyzed mutations were related to the modifications of the predicted phosphorylation sites. Based on our conclusions we predicted the infectivity of the examined viruses.

Phylogenetic characteristics, genomic heterogeneity and symptomatic variation of five closely related Japanese strains of Potato virus X

To elucidate the genomic determinants of Potato virus X (PVX) strains, which cause diverse responses in host plants, we determined the complete genomic RNA sequences of four Japanese PVX strains: PVX-BS, -BH, -OG, and -TO. These four strains, plus the previously sequenced PVX-OS strain, differ in their pathogenicity in wild potato (Solanum demissum) and tobacco (Nicotiana tabacum cv. Samsun NN). The genomic sequences of these five PVX strains were highly homologous (i.e., the nucleotide sequence identity ranged from 95.4 to 98.5%). Phylogenetic analysis indicated that the Japanese PVX strains originated from an ancestral PVX strain in the European group, and that the virulence of these strains in both S. demissum and tobacco is not correlated with their phylogenetic relationships, suggesting that the pathogenicity of each strain in these host plants is determined by a relatively small number of nucleotides and can easily be altered independent of phylogenetic relationships. Particularly, OS, BH, and BS, which respectively produce markedly contrasting ringspot, mosaic, and asymptomatic infections in tobacco leaves, were the most closely related, suggesting that these three strains are an attractive model for analyzing the genetic determinants causing these symptoms. A possible correlation between the genomic and biological differences of these strains is discussed.

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.

Compatibility of the movement protein and the coat protein of cucumoviruses is required for cell-to-cell movement

For the cell-to-cell movement of cucumoviruses both the movement protein (MP) and the coat protein (CP) are required. These are not reversibly exchangeable between Cucumber mosaic virus (CMV) and Tomato aspermy virus (TAV). The MP of CMV is able to function with the TAV CP (chimera RT), but TAV MP is unable to promote the cell-to-cell movement in the presence of CMV CP (chimera TR). To gain further insight into the non-infectious nature of the TR recombinant, RNA 3 chimeras were constructed with recombinant MPs and CPs. The chimeric MP and one of the CP recombinants were infectious. The other recombinant CP enabled virus movement only after the introduction of two point mutations (Glu-->Lys and Lys-->Arg at aa 62 and 65, respectively). The mutations served to correct the CP surface electrostatic potential that was altered by the recombination. The infectivity of the TR virus on different test plants was restored by replacing the sequence encoding the C-terminal 29 aa of the MP with the corresponding sequence of the CMV MP gene or by exchanging the sequence encoding the C-terminal 15 aa of the CP with the same region of TAV. The analysis of the recombinant clones suggests a requirement for compatibility between the C-terminal 29 aa of the MP and the C-terminal two-thirds of the CP for cell-to-cell movement of cucumoviruses.

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.

Cucumber mosaic virus Establishes Systemic Infection at Increased Temperature Following Viral Entrance Into the Phloem Pathway of Tetragonia expansa

ABSTRACT A potential regulatory site for Cucumber mosaic virus (CMV, pepo strain) movement necessary to establish systemic infection was identified through immunological and hybridization studies on Tetragonia expansa, which was systemically infected by CMV at 36 degrees C but not at 24 degrees C. In inoculated leaves, cell-to-cell movement of CMV was enhanced at 36 degrees C compared with that observed at 24 degrees C. CMV was distributed in the phloem cells of minor veins as well as epidermal and mesophyll cells at both 36 and 24 degrees C. CMV was detected in the petioles of inoculated leaves, stems, and petioles of uninoculated upper leaves at 36 degrees C, whereas CMV was detected only in the petioles of inoculated leaves and in stems at 24 degrees C. CMV moved into the phloem and was transported to the stem within 24 h postinoculation (hpi) at 36 degrees C. However, it did not accumulate in the petioles of the upper leaves until 36 hpi. In petioles of inoculated leaves at 24 degrees C, CMV was detected in the external phloem but not in the internal phloem. From these results, we conclude that systemic infection is established after viral entrance into the phloem pathway in T. expansa at 36 degrees C.

A single silent substitution in the genome of Apple stem grooving virus causes symptom attenuation

Among randomly mutagenized clones derived from an infectious cDNA copy of genomic RNA of Apple stem grooving virus (ASGV), we previously identified a clone, pRM21, whose in vitro transcript (ASGV-RM21) does not induce any symptoms characteristic of the original (wild-type) cDNA clone (ASGV-wt) in several host plants. Interestingly, ASGV-RM21 has only a single, translationally silent nucleotide substitution, U to C, at nucleotide 4646 of the viral genome within open reading frame (ORF) 1. Here, we characterize and verify this unprecedented silent-mutation-induced attenuation of symptoms in infected plants. Northern and Western blot analyses showed that less ASGV-RM21 accumulates in host plants than ASGV-wt. In addition, two more silent substitutions, U to A and U to G, constructed by site-directed mutagenesis at the same nucleotide (4646), also induced attenuated symptoms. This is the first report that a single silent substitution attenuates virus-infection symptoms and implicates a novel determinant of disease symptom severity.

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.

The dialogue between viruses and hosts in compatible interactions

Understanding the biological principles behind virus-induced symptom expression in plants remains a longstanding challenge. By dissecting the compatible host-virus relationship temporally and genetically, we have begun to map out the relationships of its component parts. The picture that emerges is one in which host gene expression and physiology are under tight temporal control during infection.

Cymbidium ringspot tombusvirus coat protein coding sequence acts as an avirulent RNA

Avirulent genes either directly or indirectly produce elicitors that are recognized by specific receptors of plant resistance genes, leading to the induction of host defense responses such as hypersensitive reaction (HR). HR is characterized by the development of a necrotic lesion at the site of infection which results in confinement of the invader to this area. Artificial chimeras and mutants of cymbidium ringspot (CymRSV) and the pepper isolate of tomato bushy stunt (TBSV-P) tombusviruses were used to determine viral factors involved in the HR resistance phenotype of Datura stramonium upon infection with CymRSV. A series of constructs carrying deletions and frameshifts of the CymRSV coat protein (CP) undoubtedly clarified that an 860-nucleotide (nt)-long RNA sequence in the CymRSV CP coding region (between nt 2666 and 3526) is the elicitor of a very rapid HR-like response of D. stramonium which limits the virus spread. This finding provides the first evidence that an untranslatable RNA can trigger an HR-like resistance response in virus-infected plants. The effectiveness of the resistance response might indicate that other nonhost resistance could also be due to RNA-mediated HR. It is an appealing explanation that RNA-mediated HR has evolved as an alternative defense strategy against RNA viruses.

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.