Molecular characterization of a structural epitope that is largely conserved among severe isolates of a plant virus

Serological and Molecular Detection of Citrus Tristeza Virus: A Review

Citrus tristeza virus (CTV) is a globally pervasive and economically significant virus that negatively impacts citrus trees, leading to substantial reductions in fruit yield. CTV occurs within the phloem of infected plants, causing a range of disease phenotypes, such as stem pitting (SP), quick decline (QD), and other detrimental diseases. Research on CTV is challenging due to the large size of its RNA genome and the diversity of CTV populations. Comparative genomic analyses have uncovered genetic diversity in multiple regions of CTV isolates' genomes, facilitating the classification of the virus into distinct genotypes. Despite these challenges, notable advancements have been made in identifying and controlling CTV strains through serological and molecular methods. The following review concentrates on the techniques of nucleic acid identification and serological analysis for various CTV isolates, assisting in the comparison and evaluation of various detection methods, which are crucial for the effective management of CTV diseases, and so contributes to the innovation and development of CTV detection methods.

Amino acid, sugar, phenolic, and terpenoid profiles are capable of distinguishing Citrus tristeza virus infection status in citrus cultivars: Grapefruit, lemon, mandarin, and sweet orange

Citrus tristeza virus (CTV) is the most severe viral disease for citrus production. Many strains of CTV have been characterized and their symptomology widely varies, ranging from asymptomatic or mild infections to severe symptomology that results in substantial yield loss or host death. The capacity of the different CTV strains to affect the biochemistry of different citrus species has remained largely unstudied, despite that associated metabolomic shifts would be relevant toward symptom development. Thus, amino acid, sugar, phenolic, and terpenoid levels were assessed in leaves of healthy and CTV-infected grapefruit, lemon, mandarin, and two different sweet orange cultivars. Both mild [VT-negative (VT-)] and severe [VT-positive (VT+)] CTV genotype strains were utilized. When looking at overall totals of these metabolite classes, only amino acid levels were significantly increased by infection of citrus with severe CTV strains, relative to mild CTV strains or healthy plants. No significant trends of CTV infection on summed amounts of all sugar, phenolic, or terpenoid compounds were observed. However, individual compound levels were affected by CTV infections. Subsequent canonical discriminant analysis (CDA) that utilized profiles of individual amino acids, terpenoids, or phenolics successfully distinguished leaf samples to specific citrus varieties and identified infection status with good accuracy. Collectively, this study reveals biochemical patterns associated with severity of CTV infections that can potentially be utilized to help identify in-field CTV infections of economic relevance.

Spread of Citrus Tristeza Virus in Citrus Orchards in Central California

In California, citrus tristeza virus (CTV) is regulated by a State Interior Quarantine. In CTV abatement districts in central California, trees with CTV that react to MCA13 (MCA13-positive [MCA13+]), a strain-discriminating monoclonal antibody, are rogued to prevent virus spread. The Tulare County Pest Control District, however, does not participate in this abatement program except for a 1.6-km² zone around the Lindcove Research and Extension Center, Exeter, CA. To quantify CTV spread under these two disparate management programs, CTV surveys were conducted in abatement plots with mandatory aphid control and nonabatement plots. Abatement plot surveys used hierarchical sampling of 25% of trees with samples pooled from four adjacent trees. Detection of MCA13+ CTV in a sample prompted resampling and testing of individual trees. From 2008 to 2018, incidence of CTV increased by an average of 3.9%, with only two MCA13+ samples detected. In contrast, in nonabatement plots, incidence of CTV increased by an average of 4.6% between 2015 and 2018. Increase in MCA13-negative (MCA-) isolates was 11 times greater than that of MCA13+ isolates, with the number of MCA13+ trees increasing by 19 trees between 2015 and 2018. MCA13- isolates were more randomly distributed, suggesting primary spread, whereas MCA13+ CTV isolates were more aggregated, suggesting some secondary spread. These results suggest that spread of MCA13+ isolates was limited by a combination of tree removal and aphid vector suppression. MCA13+ samples were VT isolates with some mixtures with T30 isolates. Despite the presence of VT isolates, all CTV-infected trees were asymptomatic.

Analysis of potato virus Y coat protein epitopes recognized by three commercial monoclonal antibodies

BACKGROUND

Potato virus Y (PVY, genus Potyvirus) causes substantial economic losses in solanaceous plants. Routine screening for PVY is an essential part of seed potato certification, and serological assays are often used. The commercial, commonly used monoclonal antibodies, MAb1128, MAb1129, and MAb1130, recognize the viral coat protein (CP) of PVY and distinguish PVYN strains from PVYO and PVYC strains, or detect all PVY strains, respectively. However, the minimal epitopes recognized by these antibodies have not been identified.

METHODOLOGY/PRINCIPAL FINDINGS

SPOT peptide array was used to map the epitopes in CP recognized by MAb1128, MAb1129, and MAb1130. Then alanine replacement as well as N- and C-terminal deletion analysis of the identified peptide epitopes was done to determine critical amino acids for antibody recognition and the respective minimal epitopes. The epitopes of all antibodies were located within the 30 N-terminal-most residues. The minimal epitope of MAb1128 was 25NLNKEK30. Replacement of 25N or 27N with alanine weakened the recognition by MAb1128, and replacement of 26L, 29E, or 30K nearly precluded recognition. The minimal epitope for MAb1129 was 16RPEQGSIQSNP26 and the most critical residues for recognition were 22I and 23Q. The epitope of MAb1130 was defined by residues 5IDAGGS10. Mutation of residue 6D abrogated and mutation of 9G strongly reduced recognition of the peptide by MAb1130. Amino acid sequence alignment demonstrated that these epitopes are relatively conserved among PVY strains. Finally, recombinant CPs were produced to demonstrate that mutations in the variable positions of the epitope regions can affect detection with the MAbs.

CONCLUSIONS/SIGNIFICANCE

The epitope data acquired can be compared with data on PVY CP-encoding sequences produced by laboratories worldwide and utilized to monitor how widely the new variants of PVY can be detected with current seed potato certification schemes or during the inspection of imported seed potatoes as conducted with these MAbs.

The epitope structure of Citrus tristeza virus coat protein mapped by recombinant proteins and monoclonal antibodies

It has been known that there exists serological differentiation among Citrus tristeza virus (CTV) isolates. The present study reports three linear epitopes (aa 48-63, 97-104, and 114-125) identified by using bacterially expressed truncated coat proteins and ten monoclonal antibodies against the native virions of CTV-S4. Site-directed mutagenesis analysis demonstrated that the mutation D98G within the newly identified epitope (97)DDDSTGIT(104) abolished its reaction to MAbs 1, 4, and 10, and the presence of G98 in HB1-CP also resulted in its failure to recognize the three MAbs. Our results suggest that the conformational differences in the epitope I (48)LGTQQNAALNRDLFLT(63) between the CPs of isolates S4 and HB1 might contribute to the different reactions of two isolates to MAbs 5 and 6. This study provides new information for the antigenic structures of CTV, and will extend the understanding of the processes required for antibody binding and aid the development of epitope-based diagnostic tools.

Mild strain cross protection of tristeza: a review of research to protect against decline on sour orange in Florida

Tristeza, caused by Citrus tristeza virus (CTV), has long been present in Florida but outbreaks of decline on sour orange rootstock were occasional events until the late 1970s. Sour orange rootstock was valued for the high quality of fruit produced and was widely used because of its tolerance of citrus blight, a disease of unknown etiology. Research was directed towards the selection and screening of mild strains of CTV which could protect against sour orange decline strains. Following the introduction of Toxoptera citricida (also known as the brown citrus aphid) in 1995 there was a greater concern for maintaining production of existing blocks of citrus on sour orange rootstock. Availability of the CTV genome sequence around the same time as well as molecular characterization of in planta CTV populations led to the selection of mild CTV isolates which when inoculated into existing field trees, extended the productive life of the groves and enabled a more graduate replanting of trees on CTV-tolerant rootstocks. The history of CTV in Florida and the methods developed to select mild isolates for use for mild strain cross protection will be reviewed.

The First Identified Citrus tristeza virus Isolate of Turkey Contains a Mixture of Mild and Severe Strains

The presence of Citrus tristeza virus (CTV) has previously been reported in citrus growing regions of Turkey. All serologically and biologically characterized isolates including Iğdır, which was the first identified CTV isolates from Turkey, were considered mild isolates. In this study, molecular characteristics of the Iğdır isolate were determined by different methods. Analysis of the Iğdır isolate by western blot and BD-RT-PCR assays showed the presence of MCA13 epitope, predominantly found in severe isolates, in the Iğdır isolate revealing that it contains a severe component. For further characterization, the coat protein (CP) and the RNA-dependent RNA polymerase (RdRp) genes representing the 3' and 5' half of CTV genome, respectively, were amplified from dsRNA by RT-PCR. Both genes were cloned separately and two clones for each gene were sequenced. Comparisons of nucleotide and deduced amino acid sequences showed that while two CP gene sequences were identical, two RdRp clones showed only 90% and 91% sequence identity in their nucleotide and amino acid sequences, respectively, suggesting a mixed infection with different strains. Phylogenetic analyses of the CP and RdRp genes of Iğdır isolate with previously characterized CTV isolates from different citrus growing regions showed that the CP gene was clustered with NZRB-TH30, a resistance breaking isolate from New Zealand, clearly showing the presence of severe component. Furthermore, two different clones of the RdRp gene were clustered separately with different CTV isolates with a diverse biological activity. While the RdRp-1 was clustered with T30 and T385, two well-characterized mild isolates from Florida and Spain, respectively, the RdRp-2 was most closely related to NZRB-G90 and NZRB-TH30, two well-characterized resistance breaking and stem pitting (SP) isolates from New Zealand confirming the mixed infection. These results clearly demonstrated that the Iğdır isolate, which was previously described as biologically a mild isolate, actually contains a mixture of mild and severe strains.

Infection with strains of Citrus tristeza virus does not exclude superinfection by other strains of the virus

Superinfection exclusion or homologous interference, a phenomenon in which a primary viral infection prevents a secondary infection with the same or closely related virus, has been observed commonly for viruses in various systems, including viruses of bacteria, plants, and animals. With plant viruses, homologous interference initially was used as a test of virus relatedness to define whether two virus isolates were "strains" of the same virus or represented different viruses, and subsequently purposeful infection with a mild isolate was implemented as a protective measure against isolates of the virus causing severe disease. In this study we examined superinfection exclusion of Citrus tristeza virus (CTV), a positive-sense RNA closterovirus. Thirteen naturally occurring isolates of CTV representing five different virus strains and a set of isolates originated from virus constructs engineered based on an infectious cDNA clone of T36 isolate of CTV, including hybrids containing sequences from different isolates, were examined for their ability to prevent superinfection by another isolate of the virus. We show that superinfection exclusion occurred only between isolates of the same strain and not between isolates of different strains. When isolates of the same strain were used for sequential plant inoculation, the primary infection provided complete exclusion of the challenge isolate, whereas isolates from heterologous strains appeared to have no effect on replication, movement or systemic infection by the challenge virus. Surprisingly, substitution of extended cognate sequences from isolates of the T68 or T30 strains into T36 did not confer the ability of resulting hybrid viruses to exclude superinfection by those donor strains. Overall, these results do not appear to be explained by mechanisms proposed previously for other viruses. Moreover, these observations bring an understanding of some previously unexplained fundamental features of CTV biology and, most importantly, build a foundation for the strategy of selecting mild isolates that would efficiently exclude severe virus isolates as a practical means to control CTV diseases.

The pathogenicity determinant of Citrus tristeza virus causing the seedling yellows syndrome maps at the 3'-terminal region of the viral genome

Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) causes some of the more important viral diseases of citrus worldwide. The ability to map disease-inducing determinants of CTV is needed to develop better diagnostic and disease control procedures. A distinctive phenotype of some isolates of CTV is the ability to induce seedling yellows (SY) in sour orange, lemon and grapefruit seedlings. In Florida, the decline isolate of CTV, T36, induces SY, whereas a widely distributed mild isolate, T30, does not. To delimit the viral sequences associated with the SY syndrome, we created a number of T36/T30 hybrids by substituting T30 sequences into different regions of the 3' half of the genome of an infectious cDNA of T36. Eleven T36/T30 hybrids replicated in Nicotiana benthamiana protoplasts. Five of these hybrids formed viable virions that were mechanically transmitted to Citrus macrophylla, a permissive host for CTV. All induced systemic infections, similar to that of the parental T36 clone. Tissues from these C. macrophylla source plants were then used to graft inoculate sour orange and grapefruit seedlings. Inoculation with three of the T30/T36 hybrid constructs induced SY symptoms identical to those of T36; however, two hybrids with T30 substitutions in the p23-3' nontranslated region (NTR) (nucleotides 18 394-19 296) failed to induce SY. Sour orange seedlings infected with a recombinant non-SY p23-3' NTR hybrid also remained symptomless when challenged with the parental virus (T36), demonstrating the potential feasibility of using engineered constructs of CTV to mitigate disease.

Characterization of the mixture of genotypes of a Citrus tristeza virus isolate by reverse transcription-quantitative real-time PCR

A multiplex real-time PCR assay was developed to detect and quantify the Citrus tristeza virus (CTV) genotypic mixture present in infected plants. CTV isolate FS627, a complex Florida isolate containing T36, T30 and VT genotypes and its aphid transmitted subisolates was used. The relative quantitative assay was carried out using specific primers and probes developed from the genotypes of three CTV virus isolates and included the coat protein region of isolate T36 and the 5' end, ORF 1a and ORF 2 region of isolates T36, T30 and VT. Among the three genotypes present in the aphid transmitted subisolates, the T30 genotype showed higher overall relative quantitation in all specific regions compared to other isolates. The profiles of the some aphid transmitted subisolates were different from the parent source from which they transmitted. The 2(-DeltaDeltaCt) method (the amount of target, normalized to an endogenous control and relative to a calibrator) was used to analyze the relative titers of the three reference genotypes in the aphid transmitted plants infected with FS627. This protocol enabled assessments of CTV genetic diversity in the aphid transmitted subisolates. This simple quantitative assay was sensitive, efficient, and took less time than other existing methods. This relative quantitative assay will be a reliable tool for diagnosis, detection and genetic diversity studies on CTV.

Antigenic properties of the coat of Cucumber mosaic virus using monoclonal antibodies

The coat protein (CP) of Cucumber mosaic virus (CMV) was characterized by antigen-capture-ELISA using a panel of monoclonal antibodies (mAbs) which were produced against Pepo-CMV-CP. Comparative analysis of three mAbs with four different strains by competitive ELISA revealed that the binding affinity of the mAb decreased about 10-fold with both MY17- and Y-CMV than with Pepo-CMV. The CP of these three strains showed high homology (approximately 98%) following comparison in the GenBank database. CMV has a negatively charged loop structure, the betaH-betaI loop, although the amino acid at position 193 is not conserved. In addition, an amino acid residue identified within the variable region spanning amino acids 191-198, specifically at position 194, showed significant changes in Threonine, Alanine, Alanine, and Lysine of the Pepo-, MY17-, Y-, and M2-CMV strains, respectively. Evidence from competitive ELISA and GenBank database amino acid residues, when taken together, provide strong support suggesting that the dominant epitope site of CMV-CP-specific mAbs is the betaH-betaI loop 191-198. The four mAbs were chosen because they represent distinct, overlapping epitopes within the group-specific determinant located on the CMV-CP and because they all recognize linear epitopes. Knowledge of specific immunoglobulin genes for a common epitope may lead to insight on pathogen-host co-evolution and may help prevent virus infection in plants.

Citrus tristeza virus: a pathogen that changed the course of the citrus industry

Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) is the causal agent of devastating epidemics that changed the course of the citrus industry. Adapted to replicate in phloem cells of a few species within the family Rutaceae and to transmission by a few aphid species, CTV and citrus probably coevolved for centuries at the site of origin of citrus plants. CTV dispersal to other regions and its interaction with new scion varieties and rootstock combinations resulted in three distinct syndromes named tristeza, stem pitting and seedling yellows. The first, inciting decline of varieties propagated on sour orange, has forced the rebuilding of many citrus industries using tristeza-tolerant rootstocks. The second, inducing stunting, stem pitting and low bearing of some varieties, causes economic losses in an increasing number of countries. The third is usually observed by biological indexing, but rarely in the field. CTV polar virions are composed of two capsid proteins and a single-stranded, positive-sense genomic RNA (gRNA) of approximately 20 kb, containing 12 open reading frames (ORFs) and two untranslated regions (UTRs). ORFs 1a and 1b, encoding proteins of the replicase complex, are directly translated from the gRNA, and together with the 5' and 3'UTRs are the only regions required for RNA replication. The remaining ORFs, expressed via 3'-coterminal subgenomic RNAs, encode proteins required for virion assembly and movement (p6, p65, p61, p27 and p25), asymmetrical accumulation of positive and negative strands during RNA replication (p23), or suppression of post-transcriptional gene silencing (p25, p20 and p23), with the role of proteins p33, p18 and p13 as yet unknown. Analysis of genetic variation in CTV isolates revealed (1) conservation of genomes in distant geographical regions, with a limited repertoire of genotypes, (2) uneven distribution of variation along the gRNA, (3) frequent recombination events and (4) different selection pressures shaping CTV populations. Measures to control CTV damage include quarantine and budwood certification programmes, elimination of infected trees, use of tristeza-tolerant rootstocks, or cross protection with mild isolates, depending on CTV incidence and on the virus strains and host varieties predominant in each region. Incorporating resistance genes into commercial varieties by conventional breeding is presently unfeasible, whereas incorporation of pathogen-derived resistance by plant transformation has yielded variable results, indicating that the CTV-citrus interaction may be more specific and complex than initially thought. A deep understanding of the interactions between viral proteins and host and vector factors will be necessary to develop reliable and sound control measures.

Occurrence of genetic bottlenecks during citrus tristeza virus acquisition by Toxoptera citricida under field conditions

In this study, we address the involvement of T. citricida in strain segregation and genetic bottleneck events by comparing the nucleotide diversity of CTV coat protein (CP) gene variants present in field-grown trees with that of variants retrieved from single apterous aphids. Plant material and aphids were collected in orange orchards in the northern part of Portugal. Shoots from two trees that were found to be positive using ELISA and twenty-four apterous aphids from these same trees were selected for individual molecular assays. CTV was detected in 60% of the aphids by amplification of a 417-bp fragment of the CP gene. Analysis of molecular variance (AMOVA) of this fragment revealed that most of the variation of the virus was found among individual aphids (FSC: 0.766) within each location. Nucleotide diversity comparison between the pool of sequences obtained from a given shoot and sequences obtained from individual aphids present on that shoot showed a reduction of more than one order of magnitude in most cases. Computer simulations of random virus acquisition by single aphids showed that in 54% of the cases only a single CP gene phylogenetic group was acquired. However, a small number of aphids (e.g. 6) was enough to acquire the full complement of phylogenetic groups present.

Detection and Isolate Differentiation of Citrus tristeza virus in Infected Field Trees Based on Reverse Transcription-Polymerase Chain Reaction

Reverse transcription-polymerase chain reaction (RT-PCR) was compared with enzyme-linked immunosorbent assay (ELISA) and direct tissue blot immunoassay (DTBIA) for detection of non-decline-inducing and decline-inducing isolates of Citrus tristeza virus (CTV) in 21 field sweet orange and grapefruit plants on sour orange rootstock in Fort Pierce, FL. Among these samples, seven, six, and eight were infected with decline-inducing, non-decline-inducing, and both decline-inducing and non-decline-inducing isolates of CTV, respectively. However, there was not a good correlation between field symptoms and detection of the decline-inducing isolate. The results confirmed that RT-PCR is not only able to detect and differentiate decline-inducing and non-decline-inducing isolates of CTV in Florida, but also can detect both isolate types in a single field sweet orange or grapefruit tree. For most samples, results from RT-PCR, ELISA, and DTBIA were the same. However, the 320-bp fragments produced only from decline-inducing isolates were amplified from two sweet orange and two grapefruit samples that did not react with decline-inducing CTV-specific monoclonal antibody MCA13 in ELISA or DTBIA, indicating that RT-PCR has a higher sensitivity than these immunological tests for field sweet orange or grapefruit samples. Thus, RT-PCR is a simple, rapid, and specific procedure for CTV identification applicable to both research and diagnostic needs.

Progress on strain differentiation of Citrus tristeza virus and its application to the epidemiology of citrus tristeza disease

Citrus tristeza virus (CTV) occurs in most citrus producing regions of the world, and it is the most serious viral pathogen of citrus. With the recent establishment of the brown citrus aphid, Toxoptera citricida, its most efficient vector, on Madeira Island (Portugal) and in Florida (USA) and the countries of the Caribbean Basin, the impact of CTV is likely to increase in these regions. Since there are many strains of CTV and CTV infections frequently occur as mixtures of several strains, it is necessary to be able to distinguish the strains for regulatory purposes, disease management and epidemiology. We describe the evolution of techniques developed to detect CTV and to differentiate the individual strains, and present the results of tests using these latest methods on CTV isolates from mainland Portugal, Madeira Island and Florida. Mild and decline-inducing strains of CTV were detected in mainland Portugal and mild, decline-inducing and severe stem pitting strains on Madeira Island. In Florida we demonstrated the presence of infections that reacted with probes made against stem pitting strains not previously detected there. It is concluded that CTV presents a significant threat to citrus production in mainland Portugal, on Madeira Island and in the neighbouring countries of the Mediterranean Basin, as well as in Florida, elsewhere in the USA and throughout the Caribbean Basin, especially following the widespread establishment of T. citricida throughout the region.

Differentiation of citrus tristeza virus isolates by serological analysis of p25 coat protein peptide maps

A procedure was developed to purify rapidly and easily a sufficient quantity of native p25 coat protein (CP) to allow comparison of five isolates of citrus tristeza virus (CTV) by serological analysis of peptide maps, using monoclonal and polyclonal antibodies. CTV particles were concentrated by centrifugation and purified by agarose gel electrophoresis. The CP was extracted from gel slices riched in virions and protein yields were about three times greater than those obtained previously and of comparable purity. The purified CP was partially digested with either V8 or papain endo-protease, and the peptides generated were separated and electroblotted to a membrane. Protein blots were tested with four monoclonal antibodies and one source of polyclonal antibodies. The serological maps generated by papain allowed differentiation of all the isolates examined, and those generated by V8 endoprotease allowed discrimination of four of the five isolates tested. Some of these isolates had been indistinguishable based on their reactivity in DASI-ELISA, dsRNA pattern and biological characterization. Serological analysis of peptide maps, as described below, allowed accurate comparison of CTV isolates with minimum amounts of p25 CP and proved superior to other techniques for discriminating CTV isolates.

Serological Differentiation of the Citrus Tristeza Virus Isolates Causing Stem Pitting in Sweet Orange

Citrus tristeza virus (CTV) complex comprises a number of isolates or strains producing several economically important disease syndromes in commercial Citrus spp. The stem pitting syndrome is the most important, and causes substantial losses in many citrus-producing regions of the world. In an attempt to develop a serological tool to rapidly differentiate stem pitting isolates of CTV, we evaluated many combinations of trapping and detecting antibodies in an indirect double-antibody sandwich (I-DAS) enzyme-linked immunosorbent assay (ELISA). Two combinations of trapping and detecting antibodies were found suitable for differentiating stem pitting isolates in extracts of infected sweet orange plants. One used a polyclonal serum raised against bacterially expressed CTV coat protein (CP) for trapping and a conformational monoclonal antibody 3E10 for detection, and the other used two polyclonal antisera generated against bacterially expressed CTV CP. Seventy-six CTV isolates from 20 countries, including 35 that cause stem pitting in sweet orange plants, were analyzed in I-DAS-ELISA using different combinations of polyclonal and monoclonal antibodies for trapping and as intermediate detecting antibodies. The ELISA format developed produces a strong positive signal for CTV isolates that cause stem pitting in sweet orange plants and a negative ELISA signal for CTV isolates that do not cause stem pitting. When combined with data on a universal ELISA format, i.e., reacting with a broad range of CTV isolates, these selective ELISA formats allowed reliable serological differentiation of CTV isolates that caused stem pitting in infected sweet orange plants.

Modulation of the antigenic reactivity of the citrus tristeza virus coat protein

Trapping properties of a panel of monoclonal antibodies (Mabs) raised against citrus tristeza virus (CTV) were analyzed in an indirect double-antibody sandwich ELISA (I-DAS-ELISA). These antibodies had been previously assigned by serological specificity into five groups (I to V). Mabs from group V, which are directed to conformational epitopes, trapped significant amounts of virus antigen from CTV-infected plant tissue at IgG concentration above 10 ng/ml. Mabs from groups I to IV, which are directed to linear, continuous epitopes, performed poorly as coating antibodies, even at a 1 microgram/ml concentration of the IgG's, indicating that the respective linear epitopes were inaccessible. However, when Mabs from groups I to IV were combined with a small amount of Mabs from group V, a substantial increase in trapping of the CTV antigen was recorded. In this 'two antibody-binding assay' previously cryptic, linear epitopes of the CTV CP apparently became accessible to the Mabs from groups I to IV. Modulation of the antigenic reactivity of the CTV CP was also recorded upon binding of the Mabs directed to the conformational epitopes in solution. Induced exposure of the linear epitopes of the CTV CP was revealed in 'two antibody-binding assays' with pairwise combinations of different mouse Mabs and several rabbit and chicken polyclonal antisera with different serological specificities, including antisera to bacterially expressed CP fragments. This mixed coating in I-DAS-ELISA resulted in substantially increased efficiency of the virus antigen trapping by antisera produced against bacterially expressed protein fragments and an increased sensitivity of the CTV detection after optimization of the ratio between conformational and linear antibodies.

Molecular characterization of coat protein genes of serologically distinct isolates of potato virus Y necrotic strain

The cost protein (CP) genes of two potato virus Y necrotic isolates (N27 and a mutant strain N27-92), which differed in their reactivity to a monoclonal antibody (mab), were characterized. Both isolates could be detected by mab 4E7, but mab VN295.5 selectively reacted to N27 and not to N27-92. The CP genes of both isolates coded for 267 amino acids with approximately 99.0% identity at both the nucleotide and the amino acid levels. nucleotide sequence comparison indicated five substitutions in N27-92 compared with N27. Three of these changes resulted in substitution of amino acids. Two transitions (A-->G) in N27-92 changed threonine to alanine and lysine to arginine at positions 7 and 55, respectively, whereas a A-->T transversion changed asparagine to isoleucine at positions 27. The surface probability curves of both the isolates could almost be superimposed, except at amino acid positions 7 and 27. Since amino acid substitution at position 55 is conservative, changes from polar to hydrophobic amino acids (threonine-->alanine and asparagine-->isoleucine) at positions 7 and 27 might have changed the epitope(s) of N27-92, abolishing its detection by mab VN295.5.

Principles of molecular organization, expression, and evolution of closteroviruses: over the barriers

This chapter focuses on the molecular organization, evolution, and expression of closterovirus genomes, as well as on their unique particle structure. The closterovirus group combines several positive-strand RNA viruses with very flexuous filamentous particles, of which beet yellows virus (BYV) is the type virus. Closteroviruses are distinct from other RNA viruses of plants in some important phenomenological aspects. They have genomes of up to 20 kilobases (kb), a value comparable only to those of the animal coronaviruses and toroviruses, which have the largest RNA genomes of all positive-strand RNA viruses. The existence of such genomes having a coding capacity several times that of an average RNA virus genome raises questions as to the trend whereby the long genomes have evolved and the possible novel functions they have acquired. The dramatic increase in the closterovirus genome coding capacity may be linked to the distinct ecological niche they occupy. Thus, closteroviruses are the only elongated plant viruses known so far to cause phloem-limited infections in plants and to persist in their insect vectors for many hours, in contrast to only minutes.