Polymorphism of a specific region in gene p23 of Citrus tristeza virus allows discrimination between mild and severe isolates

Transcriptome analysis of genes involved in the pathogenesis mechanism of potato virus Y in potato cultivar YouJin

Introduction

Potatoes (Solanum tuberosum L.) can be infected by various viruses, but out of all of viruses, the potato virus Y (PVY) is the most detrimental. Research shows that the potato cultivar YouJin is especially vulnerable to PVY and displays severe symptoms, including leaf vein chlorosis, curled leaf margins, large necrotic spots on the leaf blades, and the growth of small new leaves.

Methods

PVY infection in potato cultivar YouJin was confirmed through symptom observation, RT-PCR, and Western blot analysis. Transcriptome sequencing was used to analyze the genes associated with PVY pathogenesis in this cultivar.

Result

Transcriptome analysis of differential genes was conducted in this study to examine the pathogenesis of PVY on YouJin. The results showed that 1,949 genes were differentially regulated, including 853 upregulated genes and 1,096 downregulated genes. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that carbohydrate synthesis and metabolism pathways were suppressed, and electron transferase and hydrolase activities were reduced. Moreover, there were increased expression levels of protein kinase genes. By focusing on plant-pathogen interaction pathways, six core genes all upregulating the WARK family of transcription factors were obtained. Additionally, a constructed PPI network revealed the identification of key modular differential genes, such as downregulated photosynthesis-related protein genes and upregulated AP2/ERF-ERF transcription factors. Functional network enrichment analysis revealed that PVY infection limited RNA metabolism, glutathionylation, and peroxiredoxin activity while triggering the expression of associated defense genes in YouJin. After analyzing the above, 26 DEGs were screened and 12 DEGs were confirmed via RT-qPCR.

Conclusion

These results establish a hypothetical framework for clarifying the pathogenesis of PVY in the YouJin variety of potatoes, which will help design the disease resistance of YouJin.

Minor Variants of Orf1a, p33, and p23 Genes of VT Strain Citrus Tristeza Virus Isolates Show Symptomless Reactions on Sour Orange and Prevent Superinfection of Severe VT Isolates

The control of tristeza quick decline (QD) of citrus is based on the use of rootstocks that are tolerant or resistant to the Citrus tristeza virus (CTV), but some of them show bio-agronomic limits. The application of cross-protection (CP) has been insufficiently explored. The present study examined the possibility of QD control by cross-protection (CP) following reports showing the dependence of the CP strategy on the close genetic relationships between the protective and challenging CTV isolates. Taking advantage of deep sequencing technologies, we located six naturally infected trees harboring no-seedling yellow (no-SY) and no QD decline (mild) VT isolates and used these for challenge inoculation with three QD VT isolates. Symptom monitoring showed that all six Sicilian mild no-SY isolates, based on their genomic relatedness and mild symptoms reactions, provide effective protection against the three severe local VT isolates. The differences between the six mild and three severe isolates were confined to just a few nucleotide variations conserved in eight positions of three CTV genes (p23, p33, and Orf1a). These results confirm that the superinfection exclusion (SIE mechanism) depends on close genetic relatedness between the protective and challenging severe VT strain isolates. Ten years of investigation suggest that CP could turn into an efficient strategy to contain CTV QD infections of sweet orange trees on SO rootstock.

From the smallest to the largest subcellular plant pathogen: Citrus tristeza virus and its unique p23 protein

Knowledge on diseases caused by Citrus tristeza virus (CTV) has greatly increased in last decades after their etiology was demonstrated in the past seventies. Professor Ricardo Flores substantially contributed to these advances in topics like: i) improvement of virus purification to obtain biologically active virions, ii) sequencing mild CTV isolates for genetic comparisons with sequences of moderate or severe isolates and genetic engineering, iii) analysis of genetic variation of both CTV genomic RNA ends and features of the highly variable 5' end that allow accommodating this variation within a conserved secondary structure, iv) studies on the structure, subcellular localization and biological functions of the CTV-unique p23 protein, and v) potential use of p23 and other 3'-proximal regions of the CTV genome to develop transgenic citrus resistant to the virus. Here we review his main achievements on these topics and how they contributed to deeper understanding of CTV biology and to new potential measures for disease control.

Analysis of Genotype Composition of Citrus tristeza virus Populations Using Illumina Miseq Technology

Recent research describing the strain-specific mechanisms underlying experimental CTV superinfection exclusion has far-reaching implications for the manner in which cross-protecting sources should be selected for. The strain composition of both cross-protecting sources and field populations needs to be sufficiently characterized to improve control of severe stem-pitting and decline isolates. Many of the biological, serological, and molecular techniques used in previous studies yield very limited information about the strain composition of populations and the relative titer of their components. In this chapter we describe a protocol for the characterization of CTV populations, based on the use of the next-generation sequencing Illumina MiSeq platform of p33 gene amplicons.

Citrus tristeza virus: Host RNA Silencing and Virus Counteraction

To dissect the host RNA silencing response incited by citrus tristeza virus (CTV, genus Closterovirus), a (+) ssRNA of ~19300 nt, and the counter reaction deployed by the virus via its three RNA silencing suppressors (RSS), the small RNAs (sRNAs) of three virus-host combinations were deep sequenced. The subsequent analysis indicated that CTV sRNAs (1) constitute more than half of the total sRNAs in the susceptible Mexican lime and sweet orange, while only 3.5% in the restrictive sour orange; (2) are mostly of 21-22 nt, with those of (+) sense predominating slightly; and (3) derive from all the CTV genome, as evidenced by its entire recomposition from viral sRNA contigs but adopt an asymmetric pattern with a hotspot mapping at the 3'-terminal ~2500 nt. The citrus homologues of Arabidopsis Dicer-like (DCL) 4 and 2 most likely generate the 21 and 22 nt CTV sRNAs, respectively, by dicing the gRNA and the 3' co-terminal sgRNAs and, particularly, their double-stranded forms accumulating in infected cells. The plant sRNA profile, very similar and dominated by the 24 nt sRNAs in the three mock-inoculated controls, displayed a major reduction of the 24 nt sRNAs in Mexican lime and sweet orange, but not in sour orange. CTV infection also influences the levels of certain microRNAs.The high accumulation of CTV sRNAs in two of the citrus hosts examined suggests that it is not their synthesis, but their function, the target of the RSS encoded by CTV: p25 (intercellular), p23 (intracellular) and p20 (both). The two latter might block the loading of CTV sRNAs into the RNA silencing complex or interfere with it through alternative mechanisms. Of the three CTV RSS, p23 is the one that has been more thoroughly studied. It is a multifunctional RNA-binding protein with a putative Zn finger domain and basic motifs that (1) has no homologues in other closteroviruses, (2) accumulates in the nucleolus and plasmodesmata, (3) regulates the asymmetric balance of CTV (+) and (-) RNA strands, and (4) induces CTV syndromes and stimulates systemic infection in certain citrus species when expressed as a transgene ectopically or in phloem-associated cells.

Citrus tristeza virus co-opts glyceraldehyde 3-phosphate dehydrogenase for its infectious cycle by interacting with the viral-encoded protein p23

KEY MESSAGE

Citrus tristeza virus encodes a unique protein, p23, with multiple functional roles that include co-option of the cytoplasmic glyceraldehyde 3-phosphate dehydrogenase to facilitate the viral infectious cycle. The genome of citrus tristeza virus (CTV), genus Closterovirus family Closteroviridae, is a single-stranded (+) RNA potentially encoding at least 17 proteins. One (p23), an RNA-binding protein of 209 amino acids with a putative Zn-finger and some basic motifs, displays singular features: (i) it has no homologues in other closteroviruses, (ii) it accumulates mainly in the nucleolus and Cajal bodies, and in plasmodesmata, and (iii) it mediates asymmetric accumulation of CTV RNA strands, intracellular suppression of RNA silencing, induction of some CTV syndromes and enhancement of systemic infection when expressed as a transgene ectopically or in phloem-associated cells in several Citrus spp. Here, a yeast two-hybrid screening of an expression library of Nicotiana benthamiana (a symptomatic experimental host for CTV), identified a transducin/WD40 domain protein and the cytosolic glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as potential host interactors with p23. Bimolecular fluorescence complementation corroborated the p23-GAPDH interaction in planta and showed that p23 interacts with itself in the nucleolus, Cajal bodies and plasmodesmata, and with GAPDH in the cytoplasm (forming aggregates) and in plasmodesmata. The latter interaction was preserved in a p23 deletion mutant affecting the C-terminal domain, but not in two others affecting the Zn-finger and one internal basic motif. Virus-induced gene silencing of GAPDH mRNA resulted in a decrease of CTV titer as revealed by real-time RT-quantitative PCR and RNA gel-blot hybridization. Thus, like other viruses, CTV seems to co-opt GAPDH, via interaction with p23, to facilitate its infectious cycle.

Population structure and diversity of citrus tristeza virus (CTV) isolates in Hunan province, China

Stem-pitting (SP) is the main type of citrus tristeza virus (CTV) that causes severe damage to citrus trees, especially those of sweet orange, in Hunan province, China. Understanding the local CTV population structure should provide clues for effective mild strain cross-protection (MSCP) of the SP strain of CTV. In this study, markers for the p23 gene, multiple molecular markers (MMMs), and sequence analysis of the three silencing suppressor genes (p20, p23 and p25) were employed to analyze the genetic diversity and genotype composition of the CTV population based on 51 CTV-positive samples collected from 14 citrus orchards scattered around six major citrus-growing areas of Hunan. The results indicated that the CTV population structure was extremely complex and that infection was highly mixed. In total, p23 gene markers resulted in six profiles, and MMMs demonstrated 25 profiles. The severe VT and T3 types appeared to be predominantly associated with SP, while the mild T30 and RB types were related to asymptomatic samples. Based on phylogenetic analysis of the amino acid sequences of p20, p23 and p25, 19 representative CTV samples were classified into seven recently established CTV groups and a potentially novel one. A high level of genetic diversity, as well as potential recombination, was revealed among different CTV isolates. Five pure SP severe and two pure mild strains were identified by genotype composition analysis. Taken together, the results update the genetic diversity of CTV in Hunan with the detection of one possible novel strain, and this information might be applicable for the selection of appropriate mild CTV strains for controlling citrus SP disease through cross-protection.

PCR bias associated with conserved primer binding sites, used to determine genotype diversity within Citrus tristeza virus populations

Citrus tristeza virus (CTV) is present in almost all of the major citrus production areas where it continues to reduce the profitability of citriculture. The accurate characterisation of CTV populations, which are usually made up of a number of disparate strains, requires the use of robust PCR protocols. Mismatches between primers and their corresponding binding sites may introduce primer-associated bias during amplification. The primer-associated bias of four sets of CTV specific primers, targeting the A and F regions and the p33 and p23 genes, were evaluated. This was done through the amplification of defined templates followed by their characterisation using the sequencing of multiple clones, as well as Illumina next generation sequencing. High levels of bias were found to be associated with the primer pairs targeting the A and F regions. The p33 gene primers were found to be biased against two genotypes and suggestions for preventing this apparent bias are discussed. The primer pair targeting the conserved p23 gene was found to have very little associated bias. Primers should undergo rigorous screening before being used to characterize virus populations that are known to exhibit high levels of variation, especially within primer binding sites.

Application of a simple and affordable protocol for isolating plant total nucleic acids for RNA and DNA virus detection

Standard molecular methods for plant virus diagnosis require the purification of RNA or DNA extracts from a large number of samples, with sufficient concentration and quality for their use in PCR, RT-PCR, or qPCR analysis. Most methods are laborious and use either hazardous and/or costly chemicals. A previously published protocol for RNA isolation from several plant species yields high amounts of good quality RNA-DNA mixture in a simple, safe and inexpensive manner. In the present work, this method was tested to obtain RNA-DNA extracts from leaves of tomato, potato and three species of citrus, and was compared with two commercial kits. The results demonstrated that this protocol offers at least comparable nucleic acid quality, quantity and purity to those provided by commercial phenol-based or spin column systems and that are suitable to be used in PCR, RT-PCR and qPCR for virus and viroid detection. Because of its easy implementation and the use of safe and inexpensive reagents, it can be easily implemented to work in plant virus and viroid detection in different plant species.

Symptoms induced by transgenic expression of p23 from Citrus tristeza virus in phloem-associated cells of Mexican lime mimic virus infection without the aberrations accompanying constitutive expression

Citrus tristeza virus (CTV) is phloem restricted in natural citrus hosts. The 23-kDa protein (p23) encoded by the virus is an RNA silencing suppressor and a pathogenicity determinant. The expression of p23, or its N-terminal 157-amino-acid fragment comprising the zinc finger and flanking basic motifs, driven by the constitutive 35S promoter of cauliflower mosaic virus, induces CTV-like symptoms and other aberrations in transgenic citrus. To better define the role of p23 in CTV pathogenesis, we compared the phenotypes of Mexican lime transformed with p23-derived transgenes from the severe T36 and mild T317 CTV isolates under the control of the phloem-specific promoter from Commelina yellow mottle virus (CoYMV) or the 35S promoter. Expression of the constructs restricted to the phloem induced a phenotype resembling CTV-specific symptoms (vein clearing and necrosis, and stem pitting), but not the non-specific aberrations (such as mature leaf epinasty and yellow pinpoints, growth cessation and apical necrosis) observed when p23 was ectopically expressed. Furthermore, vein necrosis and stem pitting in Mexican lime appeared to be specifically associated with p23 from T36. Phloem-specific accumulation of the p23Δ158-209(T36) fragment was sufficient to induce the same anomalies, indicating that the region comprising the N-terminal 157 amino acids of p23 is responsible (at least in part) for the vein clearing, stem pitting and, possibly, vein corking in this host.

Development and validation of a multiplex reverse transcription quantitative PCR (RT-qPCR) assay for the rapid detection of Citrus tristeza virus, Citrus psorosis virus, and Citrus leaf blotch virus

A single real-time multiplex reverse transcription quantitative polymerase chain reaction (RT-qPCR) assay for the simultaneous detection of Citrus tristeza virus (CTV), Citrus psorosis virus (CPsV), and Citrus leaf blotch virus (CLBV) was developed and validated using three different fluorescently labeled minor groove binding qPCR probes. To increase the detection reliability, coat protein (CP) genes from large number of different isolates of CTV, CPsV and CLBV were sequenced and a multiple sequence alignment was generated with corresponding CP sequences from the GenBank and a robust multiplex RT-qPCR assay was designed. The capacity of the multiplex RT-qPCR assay in detecting the viruses was compared to singleplex RT-qPCR designed specifically for each virus and was assessed using multiple virus isolates from diverse geographical regions and citrus species as well as graft-inoculated citrus plants infected with various combination of the three viruses. No significant difference in detection limits was found and specificity was not affected by the inclusion of the three assays in a multiplex RT-qPCR reaction. Comparison of the viral load for each virus using singleplex and multiplex RT-qPCR assays, revealed no significant differences between the two assays in virus detection. No significant difference in Cq values was detected when using one-step and two-step multiplex RT-qPCR detection formats. Optimizing the RNA extraction technique for citrus tissues and testing the quality of the extracted RNA using RT-qPCR targeting the cytochrome oxidase citrus gene as an RNA specific internal control proved to generate better diagnostic assays. Results showed that the developed multiplex RT-qPCR can streamline viruses testing of citrus nursery stock by replacing three separate singleplex assays, thus reducing time and labor while retaining the same sensitivity and specificity. The three targeted RNA viruses are regulated pathogens for California's mandatory "Section 3701: Citrus Nursery Stock Pest Cleanliness Program". Adopting a compatible multiplex RT-qPCR testing protocol for these viruses as well as other RNA and DNA regulated pathogens will provide a valuable alternative tool for virus detection and efficient program implementation.

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.

Genetic variability and evolutionary dynamics of viruses of the family Closteroviridae

RNA viruses have a great potential for genetic variation, rapid evolution and adaptation. Characterization of the genetic variation of viral populations provides relevant information on the processes involved in virus evolution and epidemiology and it is crucial for designing reliable diagnostic tools and developing efficient and durable disease control strategies. Here we performed an updated analysis of sequences available in Genbank and reviewed present knowledge on the genetic variability and evolutionary processes of viruses of the family Closteroviridae. Several factors have shaped the genetic structure and diversity of closteroviruses. (I) A strong negative selection seems to be responsible for the high genetic stability in space and time for some viruses. (2) Long distance migration, probably by human transport of infected propagative plant material, have caused that genetically similar virus isolates are found in distant geographical regions. (3) Recombination between divergent sequence variants have generated new genotypes and plays an important role for the evolution of some viruses of the family Closteroviridae. (4) Interaction between virus strains or between different viruses in mixed infections may alter accumulation of certain strains. (5) Host change or virus transmission by insect vectors induced changes in the viral population structure due to positive selection of sequence variants with higher fitness for host-virus or vector-virus interaction (adaptation) or by genetic drift due to random selection of sequence variants during the population bottleneck associated to the transmission process.

Citrus tristeza virus p23: a unique protein mediating key virus-host interactions

The large RNA genome of Citrus tristeza virus (CTV; ca. 20 kb) contains 12 open reading frames, with the 3′-terminal one corresponding to a protein of 209 amino acids (p23) that is expressed from an abundant subgenomic RNA. p23, an RNA-binding protein with a putative zinc-finger domain and some basic motifs, is unique to CTV because no homologs have been found in other closteroviruses, including the type species of the genus Beet yellows virus (despite both viruses having many homologous genes). Consequently, p23 might have evolved for the specific interaction of CTV with its citrus hosts. From a functional perspective p23 has been involved in many roles: (i) regulation of the asymmetrical accumulation of CTV RNA strands, (ii) induction of the seedling yellows syndrome in sour orange and grapefruit, (iii) intracellular suppression of RNA silencing, (iv) elicitation of CTV-like symptoms when expressed ectopically as a transgene in several Citrus spp., and (v) enhancement of systemic infection (and virus accumulation) in sour orange and CTV release from the phloem in p23-expressing transgenic sweet and sour orange. Moreover, transformation of Mexican lime with intron-hairpin constructs designed for the co-inactivation of p23 and the two other CTV silencing suppressors results in complete resistance against the homologous virus. From a cellular point of view, recent data indicate that p23 accumulates preferentially in the nucleolus, being the first closterovirus protein with such a subcellular localization, as well as in plasmodesmata. These major accumulation sites most likely determine some of the functional roles of p23.

Citrus tristeza virus p23: determinants for nucleolar localization and their influence on suppression of RNA silencing and pathogenesis

Citrus tristeza virus (CTV) encodes a singular protein (p23, 209 amino acids) with multiple functions, including RNA silencing suppression (RSS). Confocal laser-scanning microscopy of green fluorescent protein (GFP)-p23 agroexpressed in Nicotiana benthamiana revealed its accumulation in the nucleolus, Cajal bodies, and plasmodesmata. To dissect the nucleolar localization signal (NoLS) typically associated with basic motifs, seven truncated and 10 point-mutated versions of p23 were assayed. Deletion mutants showed that regions 50 to 86 and 100 to 157 (excluding fragment 106 to 114), both with basic motifs and the first with a zinc-finger, contain the (bipartite) NoLS. Alanine substitutions delimited this signal to three cysteines of the Zn-finger and some basic amino acids. RSS activity of p23 in N. benthamiana was abolished by essentially all mutants, indicating that it involves most p23 regions. The necrotic-inducing ability of p23 when launched in N. benthamiana from Potato virus X was only retained by deletion mutant 158-209 and one substitution mutant, showing that the Zn-finger and flanking basic motifs form part of the pathogenic determinant. Ectopic expression of p23 and some deletion mutants in transgenic Mexican lime demarcated a similar determinant, suggesting that p23 affects related pathways in citrus and N. benthamiana. Both RSS activity and pathogenicity of p23 appear related to its nucleolar localization.

Development and application of a multiplex reverse-transcription polymerase chain reaction assay for screening a global collection of Citrus tristeza virus isolates

The emerging diversity of Citrus tristeza virus (CTV) genotypes has complicated detection and diagnostic measures and prompted the search for new differentiation methods. To simplify the identification and differentiation of CTV genotypes, a multiplex reverse-transcription polymerase chain reaction (RT-PCR) technique for the screening of CTV isolates was developed. Variable regions within the open reading frame (ORF)-1a of diverse CTV genotypes were identified to develop first a simplex (S) and then a hexaplex (H) RT-PCR. CTV isolates have been grouped previously into five genotypes (namely, T3, T30, T36, VT, and B165) based on the nucleotide sequence comparisons and phylogenetic analyses. Nucleotide sequences from GenBank were used to design species and genotype-specific primers (GSPs). The GSPs were initially used for reliable detection of all CTV genotypes using S-RT-PCR. Furthermore, detection of all five recognized CTV genotypes was established using the H-RT-PCR. Six amplicons, one generic to all CTV isolates and one for each of the five recognized genotypes, were identified on the basis of their size and were confirmed by sequence analysis. In all, 175 CTV isolates from 29 citrus-growing countries were successfully analyzed by S- and H-RT-PCR. Of these, 97 isolates contained T36 genotypes, 95 contained T3 genotypes, 76 contained T30 genotypes, 71 contained VT genotypes, and 24 contained B165 genotype isolates. In total, 126 isolates contained mixed infections of 2 to 5 of the known CTV genotypes. Two of the CTV isolates could not be assigned to a known genotype. H-RT-PCR provides a sensitive, specific, reliable, and rapid way to screen for CTV genotypes compared with other methods for CTV genotype detection. Efficient identification of CTV genotypes will facilitate a better understanding of CTV isolates, including the possible interaction of different genotypes in causing or preventing diseases. The methods described can also be used in virus-free citrus propagation programs and in the development of CTV-resistant cultivars.

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.

Sequence diversity on four ORFs of citrus tristeza virus correlates with pathogenicity

The molecular characterization of isolates of citrus tristeza virus (CTV) from eight locations in Mexico was undertaken by analyzing five regions located at the opposite ends of the virus genome. Two regions have been previously used to study CTV variability (coat protein and p23), while the other three correspond to other genomic segments (p349-B, p349-C and p13). Our comparative nucleotide analyses included CTV sequences from different geographical origins already deposited in the GenBank databases. The largest nucleotide differences were located in two fragments located at the 5' end of the genome (p349-B and p349-C). Phylogenetic analyses on those five regions showed that the degree of nucleotide divergence among strains tended to correlate with their pathogenicity. Two main groups were defined: mild, with almost no noticeable effects on the indicator plants and severe, with drastic symptoms. Mild isolates clustered together in every analyzed ORF sharing a genetic distance below 0.022, in contrast with the severe isolates, which showed a more disperse distribution and a genetic distance of 0.276. Analyses of the p349-B and p349-C regions evidenced two lineages within the severe group: severe common subgroup (most of severe isolates) and severe divergent subgroup (T36-like isolates). This study represents the first attempt to analyze the genetic variability of CTV in Mexico by constructing phylogenetic trees based on new genomic regions that use group-specific nucleotide and amino acid sequences. These results may be useful to implement specific assays for strain discrimination. Moreover, it would be an excellent reference for the CTV situation in México to face the recent arrival of brown citrus aphid.

Contribution of recombination and selection to molecular evolution of Citrus tristeza virus

The genetic variation of Citrus tristeza virus (CTV) was analysed by comparing the predominant sequence variants in seven genomic regions (p33, p65, p61, p18, p13, p20 and p23) of 18 pathogenically distinct isolates from seven different countries. Analyses of the selective constraints acting on each codon suggest that most regions were under purifying selection. Phylogenetic analysis shows diverse patterns of molecular evolution for different genomic regions. A first clade composed of isolates that are genetically close to the reference mild isolates T385 or T30 was inferred from all genomic regions. A second clade, mostly comprising virulent isolates, was defined from regions p33, p65, p13 and p23. For regions p65, p61, p18, p13 and p23, a third clade that mostly included South American isolates could not be related to any reference genotype. Phylogenetic relationships among isolates did not reflect their geographical origin, suggesting significant gene flow between geographically distant areas. Incongruent phylogenetic trees for different genomic regions suggested recombination events, an extreme that was supported by several recombination-detecting methods. A phylogenetic network incorporating the effect of recombination showed an explosive radiation pattern for the evolution of some isolates and also grouped isolates by virulence. Taken together, the above results suggest that negative selection, gene flow, sequence recombination and virulence may be important factors driving CTV evolution.

Discrimination between mild and severe Citrus tristeza virus isolates with a rapid and highly specific real-time reverse transcription-polymerase chain reaction method using TaqMan LNA probes

Severe isolates of Citrus tristeza virus (CTV) inducing seedling yellows (SY) and/or stem pitting (SP) in grapefruit or sweet orange are a major threat for the citrus industry worldwide. Identification of these CTV variants was achieved by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) using a general primer set and three TaqMan locked nucleic acids (LNA) probes targeting sequences characteristic of severe, mild (non-SY, non-SP), and T36-like isolates. Successful amplification was achieved from fresh or silica-desiccated CTV-infected samples and all isolates but one reacted with one or more probes. Standard curves using RNA transcripts homologous to the three probes allowed a reproducible quantitative assay, with a wide dynamic range of detection starting with 10(2) copies. RT-PCR assays with homologous and heterologous transcript RNA mixes demonstrated that each probe reacted only with its cognate sequence which was detected even at ratios below 2.5%. Analysis of 56 pathogenically distinct CTV isolates from 20 countries showed that mild isolates reacted only with the mild probe, whereas severe SP and SY isolates reacted with the severe-SP or the T36-like probes, respectively, and often with a second probe. This procedure can be useful to identify and control potentially dangerous CTV isolates in areas affected only by mild isolates.

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.

A real-time RT-PCR assay for detection and absolute quantitation of Citrus tristeza virus in different plant tissues

A real-time RT-PCR assay using SYBR Green was developed for specific and reliable quantitative detection of Citrus tristeza virus (CTV) in infected plants. A general primer set designed from conserved sequences in ORFs 1b and 2 enabled amplification of the genomic RNA (gRNA) while excluding most subgenomic and defective RNAs. Single RT-PCR products of 204 bp (isolate T36) or 186 bp (other isolates) were obtained with no primer-dimer or non-specific amplifications detected. Melting curve analysis revealed distinct melting temperature peaks (T(m)) for severe and mild isolates. External standard curves using RNA transcripts of the selected target allowed a reproducible quantitative assay, with a wide dynamic range of detection starting with 10(2) gRNA copies and with very low variation coefficient values. This protocol enabled reliable assessments of CTV accumulation in different tissues and from different citrus species, grown in the greenhouse or under field conditions, and infected with CTV isolates differing in their pathogenicity. CTV accumulation was higher in bark and fruits than in roots or leaves and showed minimal differences among several susceptible citrus species, but it was significantly lower in sour orange. This quantitative detection assay will be a valuable tool for diagnosis and molecular studies on CTV biology.

Improved Efficiency for Quantitative and Qualitative Indexing for Citrus tristeza virus and Citrus psorosis virus

Reverse transcription-polymerase chain reaction (RT-PCR) assays were developed for the detection of Citrus tristeza virus (CTV; genus Closterovirus) and Citrus psorosis virus (CPsV; genus Ophiovirus) in citrus trees. Real-time TaqMan RT-PCR was also developed for the detection of CTV. Three different sample preparation methods were compared. The total RNA extraction method by Qiagen was found to be more reliable than the other two methods consisting of crude plant sap extraction and total nucleic acid trapping on a silica bed. Of 287 samples tested for CTV, 210 samples tested positive by RT-PCR and 198 samples by enzyme-linked immunosorbent assay (ELISA). Furthermore, the results from monthly tests of a selected number of field-grown CTV-infected trees showed that RT-PCR detected the virus in 100% of the infected trees in winter and summer, whereas ELISA did not. The one-tube RT-PCR detection was developed for CPsV and was more sensitive than ELISA. Notably, three of 10 CPsV isolates were not detected by ELISA. As demonstrated here, our approach allows the efficacious detection of different viruses in citrus plants using a minimal amount of tissue during all seasons. The molecular methods described could be used in citrus certification programs and to test trees in nurseries and commercial orchards.

Preferential accumulation of severe variants of Citrus tristeza virus in plants co-inoculated with mild and severe variants

The viral population in sweet orange plants, either healthy or pre-inoculated with the asymptomatic isolate of Citrus tristeza virus (CTV) T32, and then graft- or aphid-inoculated with the stem-pitting isolate T318, was characterized with respect to symptom expression, reaction with monoclonal antibody MCA13, single-strand conformation polymorphism (SSCP) of genes p18 and p20, bi-directional RT-PCR, and dot-blot hybridisation. All plants inoculated with T318, with or without pre-inoculation, showed stem pitting, reacted with MCA13, had the SSCP profile characteristic of this isolate, and in bi-directional RT-PCR yielded a 450-bp DNA product associated with severe isolates, indicating that T32 afforded no protection against T318. The latter isolate had two main sequence variants, the minor one of which was indistinguishable from the main T32 sequence, and both were detected in most plants that were graft-inoculated with T318. However, the T32 variant was not detected in plants that were aphid-inoculated only with T318 and also showed stem pitting. This suggested an association of symptoms with the major T318 sequence and preferential transmission of this variant by aphids. The T318-specific variant accumulated more than the T32 variant in plants in which both were replicating, suggesting a higher fitness of the former. Our results clearly emphasize the potential threat of severe CTV variants in areas where mild isolates are presently predominant.

Evolutionary analysis of genetic variation observed in citrus tristeza virus (CTV) after host passage

We have studied the genetic variability in two genes (p18 and p20) from two groups of Citrus tristeza virus (CTV) isolates. One group (isolates T385, T317, T318, and T305) was derived from a Spanish source by successive host passages while the other (isolates T388 and T390) was obtained after aphid transmission from a Japanese source. A total of 274 sequences were obtained for gene p18 and 451 for p20. In the corresponding phylogenetic trees, sequences derived from the severe isolates (T318, T305, and T388) clustered together and separately from those derived from mild or moderate isolates (T385, T317, and T390), regardless of their geographic origin. Hierarchical analyses of molecular variance showed that up to 53% of the total genetic variability in p18 and up to 87% of the variation in p20 could be explained by differences in the pathogenicity features of the isolates. Neutrality tests revealed that different selection forces had been acting between isolates and between genes, with purifying selection being suggested for p18 from isolates T385 and T390 and for p20 from isolates T385, T317, and T388, and balancing selection for p18 from isolates T318, T305, and T388 and for p20 from isolates T318 and T390. Furthermore, several models of codon selection were observed, with purifying selection being the most notable one, compatible with low effective population size of the virus populations resulting from transmission bottlenecks. We found no evidence of recombination playing a significant role during p18 and p20 evolution in these isolates. These results suggest that hosts can be an important evolutionary factor for CTV isolates.

Variations in two gene sequences of Citrus tristeza virus after host passage

We estimated genetic variation in two groups of Citrus tristeza virus (CTV) isolates: one of them (isolates T385, T317, T318 and T305) derived from a Spanish source by successive host passages, and the other (isolates T388 and T390), obtained after aphid transmission of a Japanese source. The population structure of these isolates had been characterized by single-strand conformation polymorphism analysis of genes p18 and p20. The nucleotide sequences of representative haplotypes of each isolate and gene were used to estimate genetic diversity within and between isolates and to evaluate genetic differentiation between populations. Phylogenetic analysis of p18 and p20 sequence variants showed two main groups: one them included variants predominant in the severe isolates (T318, T305 and T388), and the other comprised variants present in both mild (T385, T317) and severe isolates. Most sequence variants of isolate T390 were not associated to these groups. In some isolates, within-isolate diversity was higher than diversity with other isolates because their population contained distantly related sequence variants, some of which were genetically close to variants predominant in the second isolate. Isolates T388 and T390 were genetically different for the two genes, as estimated by the F statistic. Furthermore, genetic differentiation between T385 and T317, T318 and T305 increased after each host passage. Our results suggest that aphid transmission and host passage may significantly alter the composition of CTV populations and thus be an important factor in their evolution.

Mechanisms of plant resistance to viruses

Plants have evolved in an environment rich with microorganisms that are eager to capitalize on the plants' biosynthetic and energy-producing capabilities. There are approximately 450 species of plant-pathogenic viruses, which cause a range of diseases. However, plants have not been passive in the face of these assaults, but have developed elaborate and effective defence mechanisms to prevent, or limit, damage owing to viral infection. Plant resistance genes confer resistance to various pathogens, including viruses. The defence response that is initiated after detection of a specific virus is stereotypical, and the cellular and physiological features associated with it have been well characterized. Recently, RNA silencing has gained prominence as an important cellular pathway for defence against foreign nucleic acids, including viruses. These pathways function in concert to result in effective protection against virus infection in plants.

The complete nucleotide sequence of a severe stem pitting isolate of Citrus tristeza virus from Spain: comparison with isolates from different origins

The genomic RNA of the severe stem pitting Citrus tristeza virus (CTV) isolate T318A from Spain (19252 nt) was completely sequenced. It showed strong sequence similarities with the severe isolates SY568 from California and NUagA from Japan, and distant relationships with mild non-stem pitting isolates T385 from Spain and T30 from Florida. Contrasting with other severe CTV isolates, T318A had a predominant sequence variant even in the highly variable 5'-terminal untranslated region, in which a unique sequence variant (type II) previously associated with severe stem pitting isolates was detected. The high homogeneity of the T318A population suggests that the sequence obtained is probably responsible for the symptoms induced and makes it a useful tool to delimit pathogenicity determinants.

Viral-like symptoms induced by the ectopic expression of the p23 gene of Citrus tristeza virus are citrus specific and do not correlate with the pathogenicity of the virus strain

Ectopic expression of the p23 gene from a severe (T36) strain of Citrus tristeza virus (CTV) induces viral-like symptoms in Mexican lime. Here, we report that expressing the same gene from a mild strain induced similar symptoms that correlated with accumulation of p23 protein irrespective of the source strain. CTV inoculation of transgenic limes showing CTV-like leaf symptoms and high p23 accumulation did not modify symptoms initially, with the virus titer being as in inoculated nontransgenic controls; however, at later stages, symptoms became attenuated. Transformation with p23-T36 of CTV-susceptible sweet and sour orange and CTV-resistant trifoliate orange also led to CTV-like leaf symptoms that did not develop when plants were transformed with a truncated p23 version. In transgenic citrus species and relatives other than Mexican lime, p23 was barely detectable, although symptom intensity correlated with levels of p23 transcripts. The lower accumulation of p23 in sweet and sour orange compared with Mexican lime also was observed in nontransgenic plants inoculated with CTV, suggesting that minimal p23 levels cause deleterious effects in the first two species. Conversely, transgenic expression of p23 in CTV nonhost Nicotiana spp. led to accumulation of p23 without phenotypic aberrations, indicating that p23 interferes with plant development only in citrus species and relatives.