Geographically and temporally distant natural recombinant isolates of Plum pox virus (PPV) are genetically very similar and form a unique PPV subgroup

New Cherry-Adapted Plum Pox Virus Phylogroups Discovered in Russia

Plum pox virus (PPV) is the most pathogenic virus of stone fruit crops worldwide. Unusual PPV isolates were discovered on sour cherry (Prunus cerasus L.) and steppe cherry (P. fruticosa Pall.) in the Republic of Tatarstan and the Middle Ural region, Russia. They induced typical sharka symptoms and tested positive for PPV by ELISA and RT-PCR, but were not detected by PCR using known strain-specific primers. Their complete genomes were determined using high-throughput sequencing. Phylogenetic analysis allocated new isolates to four clearly distinguished lineages (SC, TAT, Y, Tat-26) within a cluster of PPV cherry-adapted strains. The phylogroups SC and TAT had 84.5 to 86.9% average nucleotide identity to each other and strain CR, with which they comprised a common subcluster. Isolates from the Middle Ural region (group Y) were closer to strain C, sharing 96.9% identity. The fourth lineage is represented by the isolate Tat-26, which was a recombinant of strain CR and C isolates as major and minor parents, respectively. These results show that the genetic diversity of PPV is higher than thought and may contribute to a better understanding of the origin and evolution of cherry-adapted strains of the virus. P. fruticosa was reported as a new natural PPV host for the first time.

Genome characterization of zucchini yellow mosaic virus infecting cucurbits reveals the presence of a new genotype in Trinidad and Tobago in the Caribbean region

Zucchini yellow mosaic virus (ZYMV) is a member of the genus Potyvirus that is becoming a serious pathogen of pumpkin and other cucurbits in Trinidad and Tobago and the entire Caribbean region. In this study, four ZYMV isolates infecting pumpkin in Trinidad and Tobago were characterized by complete genome sequencing. Phylogenetic analysis showed 5.9-6.0% nt and 7.7-7.9% aa sequence divergence in comparison to the most closely related isolates NAT and AG from Israel and SE04T from Slovakia. Based on the variations in the complete genome sequence as well as individual gene sequences, a new genotype, designated ZYMV-Trini, is proposed for these isolates. Among the gene sequences of ZYMV-Trini isolates, the greatest variation was observed in the HC-Pro gene, with 20.8% aa sequence divergence from their closest relatives, whereas the least variation was observed in the NIb, P3, and CP genes, with 1.8-2.2% aa sequence divergence. This study also showed that transmission of ZYMV can occur through seeds, but this was less common than transmission via the aphid Aphis gossypii. The progression of ZYMV in pumpkin seedlings was quantified by RT-qPCR, which showed a rapid surge in viral load after 37 days. From recombination analysis, it could be concluded that the isolates SE04T from Slovakia, NAT from Israel, and AG from Israel have made major contributions to the genome architecture of ZYMV-Trini isolates.

Different potato virus Y strains frequently co-localize in single epidermal leaf cells and in the aphid stylet

Single aphids can simultaneously or sequentially acquire and transmit multiple potato virus Y (PVY) strains. Multiple PVY strains are often found in the same field and occasionally within the same plant, but little is known about how PVY strains interact in plants or in aphid stylets. Immuno-staining and confocal microscopy were used to examine the spatial and temporal dynamics of PVY strain mixtures (PVYO and PVYNTN or PVYO and PVYN) in epidermal leaf cells of 'Samsun NN' tobacco and 'Goldrush' potato. Virus binding and localization was also examined in aphid stylets following acquisition. Both strains systemically infected tobacco and co-localized in cells of all leaves examined; however, the relative amounts of each virus changed over time. Early in the tobacco infection, when mosaic symptoms were observed, PVYO dominated the infection although PVYNTN was detected in some cells. As the infection progressed and vein necrosis developed, PVYNTN was prevalent. Co-localization of PVYO and PVYN was also observed in epidermal cells of potato leaves with most cells infected with both viruses. Furthermore, two strains could be detected binding to the distal end of aphid stylets following virus acquisition from a plant infected with a strain mixture. These data are in contrast with the traditional belief of spatial separation of two closely related potyviruses and suggest apparent non-antagonistic interaction between PVY strains that could help explain the multitude of emerging recombinant PVY strains discovered in potato in recent years.

Genetic Diversity of Potyviruses Associated with Tulip Breaking Syndrome

Tulip breaking is economically the most important viral disease of modern-day tulip growing. It is characterized by irregular flame and feather-like patterns in the flowers and mosaic on the foliage. Thirty-two leaf samples were collected from cultivated tulip plants showing tulip breaking syndrome from Hungary in 2017 and 2018. Virus identification was performed by serological (ELISA) and molecular (RT-PCR) methods. All samples proved to be infected with a potyvirus and evidence was provided that three potyvirus species could be identified in the samples: Lily mottle virus (LMoV), Tulip breaking virus (TBV) and Rembrandt tulip-breaking virus (ReTBV). Recombination prediction accomplished with Recombination Detection Program (RDP) v4.98 revealed potential intraspecies recombination in the case of TBV and LMoV. Phylogenetic analyses of the coat protein (CP) regions proved the monophyletic origin of these viruses and verified them as three different species according to current International Committee on Taxonomy of Viruses (ICTV) species demarcation criteria. Based on these results, we analyzed taxonomic relations concerning potyviruses associated with tulip breaking syndrome. We propose the elevation of ReTBV to species level, and emergence of two new subgroups in ReTBV.

Efficient Confirmation of Plant Viral Proteins and Identification of Specific Viral Strains by nanoLC-ESI-Q-TOF Using Single-Leaf-Tissue Samples

Plant viruses are important pathogens that cause significant crop losses. A plant protein extraction protocol that combines crushing the tissue by a pestle in liquid nitrogen with subsequent crushing by a roller-ball crusher in urea solution, followed by RuBisCO depletion, reduction, alkylation, protein digestion, and ZipTip purification allowed us to substantially simplify the sample preparation by removing any other precipitation steps and to detect viral proteins from samples, even with less than 0.2 g of leaf tissue, by a medium resolution nanoLC-ESI-Q-TOF. The presence of capsid proteins or polyproteins of fourteen important viruses from seven different families (Geminiviridae, Luteoviridae, Bromoviridae, Caulimoviridae, Virgaviridae, Potyviridae, and Secoviridae) isolated from ten different economically important plant hosts was confirmed through many identified pathogen-specific peptides from a protein database of host proteins and potential pathogen proteins assembled separately for each host and based on existing online plant virus pathogen databases. The presented extraction protocol, combined with a medium resolution LC-MS/MS, represents a cost-efficient virus protein confirmation method that proved to be effective at identifying virus strains (as demonstrated for PPV, WDV) and distinct disease species of BYDV, as well as putative new viral protein sequences from single-plant-leaf tissue samples. Data are available via ProteomeXchange with identifier PXD022456.

Intra-strain biological and epidemiological characterization of plum pox virus

Plum pox virus (PPV) is one of the most important plant viruses causing serious economic losses. Thus far, strain typing based on the definition of 10 monophyletic strains with partially differentiable biological properties has been the sole approach used for epidemiological characterization of PPV. However, elucidating the genetic determinants underlying intra-strain biological variation among populations or isolates remains a relevant but unexamined aspect of the epidemiology of the virus. In this study, based on complete nucleotide sequence information of 210 Japanese and 47 non-Japanese isolates of the PPV-Dideron (D) strain, we identified five positively selected sites in the PPV-D genome. Among them, molecular studies showed that amino acid substitutions at position 2,635 in viral replicase correlate with viral titre and competitiveness at the systemic level, suggesting that amino acid position 2,635 is involved in aphid transmission efficiency and symptom severity. Estimation of ancestral genome sequences indicated that substitutions at amino acid position 2,635 were reversible and peculiar to one of two genetically distinct PPV-D populations in Japan. The reversible amino acid evolution probably contributes to the dissemination of the virus population. This study provides the first genomic insight into the evolutionary epidemiology of PPV based on intra-strain biological variation ascribed to positive selection.

Evolving by deleting: patterns of molecular evolution of Apple stem pitting virus isolates from Poland

In this study, 267 coat protein gene (CP) sequences from 48 Polish isolates of Apple stem pitting virus (ASPV) were determined. The genetic structure of the virus population was analysed and possible mechanisms of molecular evolution explored. We found evidence of recombination within the ASPV population and the presence of 17 ASPV molecular variants that differ in the length, number and arrangement of deletions in the CP. Population genetic analyses showed significant variation among isolates from pear and apple trees, between isolates from the same host species and, more interestingly, within isolates, supporting the existence of significant levels of variability within individual hosts, as expected by a quasispecies population structure. In addition, different tests support that selection might have been an important force driving diversification within isolates: positive selection was found acting upon certain amino acids. Phylogenetic analyses also showed that isolates did not classify according to the host species (pear or apple trees) but according to the pattern of deletions, suggesting a possible role for deletions during clade diversification.

The global phylogeny of Plum pox virus is emerging

The 206 complete genomic sequences of Plum pox virus in GenBank (January 2019) were downloaded. Their main open reading frames (ORF)s were compared by phylogenetic and population genetic methods. All fell into the nine previously recognized strain clusters; the PPV-Rec and PPV-T strain ORFs were all recombinants, whereas most of those in the PPV-C, PPV-CR, PPV-CV, PPV-D, PPV-EA, PPV-M and PPV-W strain clusters were not. The strain clusters ranged in size from 2 (PPV-CV and PPV-EA) to 74 (PPV-D). The isolates of eight of the nine strains came solely from Europe and the Levant (with an exception resulting from a quarantine breach), but many PPV-D strain isolates also came from east and south Asia and the Americas. The estimated time to the most recent common ancestor (TMRCA) of all 134 non-recombinant ORFs was 820 (865-775) BCE. Most strain populations were only a few decades old, and had small intra-strain, but large inter-strain, differences; strain PPV-W was the oldest. Eurasia is clearly the 'centre of emergence' of PPV and the several PPV-D strain populations found elsewhere only show evidence of gene flow with Europe, so have come from separate introductions from Europe. All ORFs and their individual genes show evidence of strong negative selection, except the positively selected pipo gene of the recently migrant populations. The possible ancient origins of PPV are discussed.

Sequence and phylogenetic analysis of novel porcine parvovirus 7 isolates from pigs in Guangxi, China

Parvoviruses are a diverse group of viruses that infect a wide range of animals and humans. In recent years, advances in molecular techniques have resulted in the identification of several novel parvoviruses in swine. In this study, porcine parvovirus 7 (PPV7) isolates from clinical samples collected in Guangxi, China, were examined to understand their molecular epidemiology and co-infection with porcine circovirus type 2 (PCV2). In this study, among the 385 pig serum samples, 105 were positive for PPV7, representing a 27.3% positive detection rate. The co-infection rate of PPV7 and PCV2 was 17.4% (67/385). Compared with the reference strains, we noted 93.9%-97.9% similarity in the NS1 gene and 87.4%-95.0% similarity in the cap gene. Interestingly, compared with the reference strains, sixteen of the PPV7 strains in this study contained an additional 3 to 15 nucleotides in the middle of the cap gene. Therefore, the Cap protein of fourteen strains encoded 474 amino acids, and the Cap protein of the other two strains encoded 470 amino acids. However, the Cap protein of the reference strain PPV7 isolate 42 encodes 469 amino acids. This is the first report of sequence variation within the cap gene, confirming an increase in the number of amino acids in the Cap protein of PPV7. Our findings provide new insight into the prevalence of PPV7 in swine in Guangxi, China, as well as sequence data and phylogenetic analysis of these novel PPV7 isolates.

Update on Distribution and Genetic Variability of Plum pox virus Strains in Bulgaria

Field surveys for Plum pox virus (PPV) infection were conducted in stone fruit orchards all over Bulgaria. In total, 1168 out of 3020 leaf samples from cultivated Prunus spp. and wildly growing P. cerasifera trees reacted positive for PPV in DASI-ELISA with the universal monoclonal antibody (MAb) 5B. Further ELISA analyses showed that 987 and 127 isolates belonged to PPV-M and PPV-D serotypes, respectively. The plum and P. cerasifera showed 82.0% and 50.5% levels of infection, respectively followed by the peach (40.0%) and the apricot (32.0%). Five hundred fifty one PPV isolates were further typed by IC-RT-PCR with PPV-Rec, -M and -D-specific primers, targeting (Cter)NIb-(Nter) CP genome region, as 125 isolates were sequenced. The results revealed the presence of PPV-Rec, PPV-M and PPV-D and mixed infections of these strains. PPV-Rec was the most prevalent strain (49.0%), followed by PPV-M (40.1%), while PPV-D was the less spread strain (8.2%). PPV-Rec was the most common strain in plums, including the eight "old-aged" trees from the region of the first Sharka discovery. PPV-M was the most prevalent strain in peach and apricot. Phylogenetic analyses on (Cter)NIb-(Nter)CP of the isolates were performed. PPV-Rec isolates formed a homogeneous group, while PPV-M isolates split into PPV-Ma and PPV-Mb subgroups. Five separated clades were formed by the analyzed PPV-D isolates. Nucleotide sequences of the partial CP coding region of the analyzed isolates revealed a slightly higher intra-strain genetic variability in PPV-Rec and PPV-M isolates, while that of PPV-D strain isolates was higher from the reported for these strains.

Recent Advances on Detection and Characterization of Fruit Tree Viruses Using High-Throughput Sequencing Technologies

Perennial crops, such as fruit trees, are infected by many viruses, which are transmitted through vegetative propagation and grafting of infected plant material. Some of these pathogens cause severe crop losses and often reduce the productive life of the orchards. Detection and characterization of these agents in fruit trees is challenging, however, during the last years, the wide application of high-throughput sequencing (HTS) technologies has significantly facilitated this task. In this review, we present recent advances in the discovery, detection, and characterization of fruit tree viruses and virus-like agents accomplished by HTS approaches. A high number of new viruses have been described in the last 5 years, some of them exhibiting novel genomic features that have led to the proposal of the creation of new genera, and the revision of the current virus taxonomy status. Interestingly, several of the newly identified viruses belong to virus genera previously unknown to infect fruit tree species (e.g., Fabavirus, Luteovirus) a fact that challenges our perspective of plant viruses in general. Finally, applied methodologies, including the use of different molecules as templates, as well as advantages and disadvantages and future directions of HTS in fruit tree virology are discussed.

Analysis of Genetic Diversity of Russian Sour Cherry Plum pox virus Isolates Provides Evidence of a New Strain

Plum pox virus (PPV) exists as a complex of nine strains adapted to different Prunus hosts. Unusual PPV isolates that do not belong to the known cherry-adapted strains were discovered on sour cherry in Russia. Here, two complete genomes of isolates Tat-2 and Tat-4 were determined by sequencing on the Illumina HiSeq 2500 platform. Both were composed of 9,792 nucleotides, excluding the poly(A) tail, with the organization typical of PPV and had 99.4 and 99.7% identity between each other at the nucleotide and amino acid levels. The sequence identities between Tat-2/Tat-4 and known PPV strains ranged from 77.6 to 83.3% for genomic RNA and from 80.0 to 93.8% for polyprotein. Phylogenetic analysis placed Tat-2 and Tat-4 in a separate clade, distinct from the C and CR strains. Three more Tat-2/Tat-4-like isolates were detected in local cherry plantings using the newly developed, specific RT-PCR assay. Based on the phylogenetic analysis, sequence identities, and environmental distribution, Tat-2, Tat-4, and related isolates represent a new cherry-adapted PPV strain for which the name PPV-CV (Cherry Volga) is proposed.

Molecular characterization of Plum pox virus Rec isolates from Russia suggests a new insight into evolution of the strain

Field isolates of Plum pox virus (PPV), belonging to the strain Rec, have been found for the first time in Russia. Full-size genomes of the isolates K28 and Kisl-1pl from myrobalan and plum, respectively, were sequenced on the 454 platform. Analysis of all known PPV-Rec complete genomes using the Recombination Detection Program (RDP4) revealed yet another recombination event in the 5'-terminal region. This event was detected by seven algorithms, implemented in the RDP4, with statistically significant P values and supported by a phylogenetic analysis with the bootstrap value of 87%. A putative PPV-M-derived segment, encompassing the C-terminus of the P1 gene and approximately two-thirds of the HcPro gene, is bordered by breakpoints at positions 760-940 and 1838-1964, depending on the recombinant isolate. The predicted 5'-distal breakpoint for the isolate Valjevka is located at position 2804. The Dideron (strain D) and SK68 (strain M) isolates were inferred as major and minor parents, respectively. Finding of another recombination event suggests more complex evolutionary history of PPV-Rec than previously assumed. Perhaps the first recombination event led to the formation of a PPV-D variant harboring the PPV-M-derived fragment within the 5'-proximal part of the genome. Subsequent recombination of its descendant with PPV-M in the 3'-proximal genomic region resulted in the emergence of the evolutionary successful strain Rec.

New highly divergent Plum pox virus isolates infecting sour cherry in Russia

Unusual Plum pox virus (PPV) isolates (named Tat isolates) were discovered on sour cherry (Prunus cerasus) in Russia. They failed to be recognized by RT-PCR using commonly employed primers specific to the strains C or CR (the only ones that proved able to infect sour cherry) as well as to the strains M and W. Some of them can be detected by RT-PCR using the PPV-D-specific primers P1/PD or by TAS-ELISA with the PPV-C-specific monoclonal antibody AC. Phylogenetic analysis of the 3'-terminal genomic region assigned the Tat isolates into the cluster of cherry-adapted strains. However, they grouped separately from the C and CR strains and from each other as well. The sequence divergence of the Tat isolates is comparable to the differences between the known PPV strains. They may represent new group(s) of cherry-adapted isolates which do not seem to belong to any known strain of the virus.

Genome Sequence Analysis of New Isolates of the Winona Strain of Plum pox virus and the First Definitive Evidence of Intrastrain Recombination Events

Plum pox virus (PPV) is genetically diverse with nine different strains identified. Mutations, indel events, and interstrain recombination events are known to contribute to the genetic diversity of PPV. This is the first report of intrastrain recombination events that contribute to PPV's genetic diversity. Fourteen isolates of the PPV strain Winona (W) were analyzed including nine new strain W isolates sequenced completely in this study. Isolates of other strains of PPV with more than one isolate with the complete genome sequence available in GenBank were included also in this study for comparison and analysis. Five intrastrain recombination events were detected among the PPV W isolates, one among PPV C strain isolates, and one among PPV M strain isolates. Four (29%) of the PPV W isolates analyzed are recombinants; one of which (P2-1) is a mosaic, with three recombination events identified. A new interstrain recombinant event was identified between a strain M isolate and a strain Rec isolate, a known recombinant. In silico recombination studies and pairwise distance analyses of PPV strain D isolates indicate that a threshold of genetic diversity exists for the detectability of recombination events, in the range of approximately 0.78×10(-2) to 1.33×10(-2) mean pairwise distance. RDP4 analyses indicate that in the case of PPV Rec isolates there may be a recombinant breakpoint distinct from the obvious transition point of strain sequences. Evidence was obtained that indicates that the frequency of PPV recombination is underestimated, which may be true for other RNA viruses where low genetic diversity exists.

Simultaneous visualization of two Citrus tristeza virus genotypes provides new insights into the structure of multi-component virus populations in a host

Complex Citrus tristeza virus (CTV) populations composed of mixtures of different strains of the virus are commonly found in citrus trees in the field. At present, little is known about how these populations are formed, maintained, and how they are structured within a host. Here we used a novel in situ hybridization approach allowing simultaneous visualization of two different RNA targets with high sensitivity and specificity to examine the distribution of two isolates, T36 and T68-1, representing phylogenetically distinct strains of CTV, in a citrus host in single and mixed infections. Remarkably, in doubly inoculated plants the two virus variants appeared to be well mixed within the infected tissue and showed no spatial segregation. In addition, both CTV variants were often found occupying the same cells. Possible mechanisms involved in shaping CTV populations and the biological significance of the observed lack of structural separation of the individual components are discussed.

Aphid Transmission of the Ontario Isolate of Plum Pox Virus

Utilization of timed virus acquisition access probes in studies of plum pox virus (PPV) transmission by aphids demonstrated that endemic species transmitted the virus readily from plum, Prunus domestica (L.) Batsch; peach, P. persica (L.); or dwarf flowering almond, P. glandulosa Thunberg., to peach seedlings. The green peach aphid, Myzus persicae (Sulzer), was shown to be the most efficient vector. Acquisition of virus by green peach aphids from infected peach leaves resulted in 18-28% infected peach seedlings, while aphids previously fed on infected leaves of plum transferred virus to 36% of peach seedlings. Although the spirea aphid, Aphis spiraecola (Patch), was a less efficient vector than M. persicae it is perhaps more important for the spread of PPV due to its greater abundance and occurrence earlier in the season when peach trees are thought to be more susceptible to infection. Virus transmission rates varied depending on the virus source and healthy test plant species. In contrast to many previous studies, aphid inoculation of the experimental host Nicotiana benthamiana Domin occurred at a low rate, never exceeding 4%. Acquisition of PPV by M. persicae from infected peach fruit was greatly reduced compared with acquisition from leaves. The results of this research indicate that the Ontario isolate of PPV-D is readily transmissible by aphids to peach and natural spread of the virus needs to be considered in future management or eradication programs.

Recombination analysis of Maize dwarf mosaic virus (MDMV) in the Sugarcane mosaic virus (SCMV) subgroup of potyviruses

Recombination among RNA viruses is a natural phenomenon that appears to have played a significant role in the species development and the evolution of many strains. It also has particular significance for the risk assessment of plants which have been genetically modified for disease resistance by incorporating viral sequences into their genomes. However, the exact recombination events taking place in viral genomes are not investigated in detail for many virus groups. In this analysis, different single-stranded positive-sense RNA potyviruses were compared using various in silico recombination detection methods and new recombination events in the Sugarcane mosaic virus (SCMV) subgroup were detected. For an extended in silico recombination analysis, two of the analyzed Maize dwarf mosaic virus full-length genomes were sequenced additionally during this work. These results strengthen the evidence that recombination is a major driving force in virus evolution, and the emergence of new virus variants in the SCMV subgroup, paired with mutations, could generate viruses with altered biological properties. The intra- and interspecific homolog recombinations seem to be a general trait in this virus group, causing little or no changes to the amino acid of the progenies. However, we found a few breakpoints between the members of SCMV subgroup and the weed-infecting distant relatives, but only a few methods of the RDP3 package predicted these events with low significance level.

Differential RNAi responses of Nicotiana benthamiana individuals transformed with a hairpin-inducing construct during Plum pox virus challenge

Gene silencing and large-scale small RNA analysis can be used to develop RNA interference (RNAi)-based resistance strategies for Plum pox virus (PPV), a high impact disease of Prunus spp. In this study, a pPPViRNA hairpin-inducing vector harboring two silencing motif-rich regions of the PPV coat protein (CP) gene was evaluated in transgenic Nicotiana benthamiana (NB) plants. Wild-type NB plants infected with a chimeric PPV virus (PPV::GFP) exhibited affected leaves with mosaic chlorosis congruent to GFP fluorescence at 21 day post-inoculation; transgenic lines depicted a range of phenotypes from fully resistant to susceptible. ELISA values and GFP fluorescence intensities were used to select transgenic-resistant (TG-R) and transgenic-susceptible (TG-S) lines for further characterization of small interfering RNAs (siRNAs) by large-scale small RNA sequencing. In infected TG-S and untransformed (WT) plants, the observed siRNAs were nearly exclusively 21- and 22-nt siRNAs that targeted the whole PPV::GFP genome; 24-nt siRNAs were absent in these individuals. Challenged TG-R plants accumulated a full set of 21- to 24-nt siRNAs that were primarily associated with the selected motif-rich regions, indicating that a trans-acting siRNAs process prevented viral multiplication. BLAST analysis identified 13 common siRNA clusters targeting the CP gene. 21-nt siRNA sequences were associated with the 22-nt siRNAs and the scarce 23- and 24-nt molecules in TG-S plants and with most of the observed 22-, 23-, and 24-nt siRNAs in TG-R individuals. These results validate the use of a multi-hot spot silencing vector against PPV and elucidate the molecules by which hairpin-inducing vectors initiate RNAi in vivo.

Genetic variation in potato virus M isolates infecting pepino (Solanum muricatum) in China

Potato virus M (PVM, genus Carlavirus, family Betaflexviridae) is considered to be one of most economically important pathogens of pepino in China. However, the details and the mechanisms underlying PVM evolution are unknown. In this study, we determined and analyzed 40 TGB 1 gene sequences, 67 TGB 2 and TGB 3 gene sequences, and 88 CP and NABP gene sequences from viruses isolated from 19 samples of pepino (Solanum muricatum) and one sample of tomato (S. lycopersicum) collected from different areas of China. Recombination analysis identified only one clear recombinant in the TGB2-TGB3-CP region, but no recombinants were detected for each of the five individual genes. Phylogenetic analysis showed that all PVM isolates could be divided into at least two lineages in trees derived from the TGB 2, CP, and NABP gene sequences, and the lineages seemed to reflect geographical origin. The five PVM genes in this study were found to be under strong negative selection pressure. The PVM isolates examined showed frequent gene flow between the Chinese and European populations, and also within the Chinese population. Clear star phylogenies and the neutral equilibrium model test showed that pepino isolates of PVM appear to be experiencing a new expansion after a recent introduction into China, and these isolates display low levels of genetic diversity. To our knowledge, this study is the first report describing genetic structure, recombination, and gene flow in PVM populations, and it provides strong evolutionary evidence for the virus populations from different geographic regions of China.

Plum pox virus and sharka: a model potyvirus and a major disease

TAXONOMIC RELATIONSHIPS

Plum pox virus (PPV) is a member of the genus Potyvirus in the family Potyviridae. PPV diversity is structured into at least eight monophyletic strains.

GEOGRAPHICAL DISTRIBUTION

First discovered in Bulgaria, PPV is nowadays present in most of continental Europe (with an endemic status in many central and southern European countries) and has progressively spread to many countries on other continents.

GENOMIC STRUCTURE

Typical of potyviruses, the PPV genome is a positive-sense single-stranded RNA (ssRNA), with a protein linked to its 5' end and a 3'-terminal poly A tail. It is encapsidated by a single type of capsid protein (CP) in flexuous rod particles and is translated into a large polyprotein which is proteolytically processed in at least 10 final products: P1, HCPro, P3, 6K1, CI, 6K2, VPg, NIapro, NIb and CP. In addition, P3N-PIPO is predicted to be produced by a translational frameshift.

PATHOGENICITY FEATURES

PPV causes sharka, the most damaging viral disease of stone fruit trees. It also infects wild and ornamental Prunus trees and has a large experimental host range in herbaceous species. PPV spreads over long distances by uncontrolled movement of plant material, and many species of aphid transmit the virus locally in a nonpersistent manner.

SOURCES OF RESISTANCE

A few natural sources of resistance to PPV have been found so far in Prunus species, which are being used in classical breeding programmes. Different genetic engineering approaches are being used to generate resistance to PPV, and a transgenic plum, 'HoneySweet', transformed with the viral CP gene, has demonstrated high resistance to PPV in field tests in several countries and has obtained regulatory approval in the USA.

Detection and characterisation of Plum pox virus (PPV) isolates from Eastern Slovakia revealed the presence of three main viral strains

Plum pox virus (PPV), the agent responsible for Sharka disease, is the most important viral pathogen of stone fruit trees world-wide, having an endemic status in Slovakia. To increase knowledge of PPV diversity in Slovakia, a set of 11 isolates, originated from the eastern part of the country, was characterised. The isolates were chip-budded from their original Prunus hosts to the susceptible GF305 indicators, exhibiting the symptoms of variable severity. A genomic region encompassing the partial NIb and the hypervariable 5´terminal region of the CP gene was amplified from all 11 isolates in RT-PCR and directly sequenced. The phylogenetic analysis revealed the grouping of the 11 Slovak isolates into 3 distinct clusters, representing the PPV-M (2 isolates), D (7 isolates) and Rec strains (2 isolates). The strain affiliation of isolates was further confirmed by strain-specific RT-PCR, using which the presence of additional mixed infection by minor PPV variants was detected in 2 samples. The results further contribute to the understanding of PPV diversity in Slovakia and confirm the specificity and sensitivity of molecular approaches used for the virus strain determination.

Silencing of the host factor eIF(iso)4E gene confers plum pox virus resistance in plum

Plum pox virus (PPV) causes the most economically-devastating viral disease in Prunus species. Unfortunately, few natural resistance genes are available for the control of PPV. Recessive resistance to some potyviruses is associated with mutations of eukaryotic translation initiation factor 4E (eIF4E) or its isoform eIF(iso)4E. In this study, we used an RNA silencing approach to manipulate the expression of eIF4E and eIF(iso)4E towards the development of PPV resistance in Prunus species. The eIF4E and eIF(iso)4E genes were cloned from plum (Prunus domestica L.). The sequence identity between plum eIF4E and eIF(iso)4E coding sequences is 60.4% at the nucleotide level and 52.1% at the amino acid level. Quantitative real-time RT-PCR analysis showed that these two genes have a similar expression pattern in different tissues. Transgenes allowing the production of hairpin RNAs of plum eIF4E or eIF(iso)4E were introduced into plum via Agrobacterium-mediated transformation. Gene expression analysis confirmed specific reduced expression of eIF4E or eIF(iso)4E in the transgenic lines and this was associated with the accumulation of siRNAs. Transgenic plants were challenged with PPV-D strain and resistance was evaluated by measuring the concentration of viral RNA. Eighty-two percent of the eIF(iso)4E silenced transgenic plants were resistant to PPV, while eIF4E silenced transgenic plants did not show PPV resistance. Physical interaction between PPV-VPg and plum eIF(iso)4E was confirmed. In contrast, no PPV-VPg/eIF4E interaction was observed. These results indicate that eIF(iso)4E is involved in PPV infection in plum, and that silencing of eIF(iso)4E expression can lead to PPV resistance in Prunus species.

Molecular Analysis of a Plum pox virus W Isolate in Plum Germplasm Hand Carried into the USA from the Ukraine Shows a Close Relationship to a Latvian Isolate

Four of 19 Prunus germplasm accessions hand carried from the Ukraine into the United States without authorization were found to be infected with Plum pox virus (PPV). Of the three isolates characterized, isolates UKR 44189 and UKR 44191 were confirmed to be isolates of PPV strain W, and UKR 44188 was confirmed to be an isolate of PPV strain D. UKR 44189 and UKR 44191 are very closely related to the PPV strain W isolate LV-145bt (HQ670748) from Latvia. Nucleotide and amino acid sequence identities between these three isolates were greater than 99%. This indicates that the isolates are very closely related and likely originated from a common source. The high genetic diversity among PPV-W strain isolates allowed the identification of potential recombination events between PPV isolates. It appears also that GF 305 peach and Prunus tomentosa are not hosts for the PPV isolate UKR 44189.

An integrated badnavirus is prevalent in fig germplasm

Fig mosaic occurs worldwide and is the most common and important viral disease of fig. In the quest to identify the causal agent of the disease, several new viruses have been identified, including a new DNA virus, the subject of this communication. Phylogenetic analysis placed the virus, provisionally named Fig badnavirus-1 (FBV-1), in the genus Badnavirus, family Caulimoviridae. The experimental host range of FBV-1 was evaluated and the virus was mechanically transmitted to several herbaceous hosts. FBV-1 was detected in the National Clonal Germplasm Repository fig collection and additional samples from Arkansas, California, Florida, Michigan, Ohio, Oregon, and South Carolina, suggesting its wide distribution in the United States. Further tests revealed the presence of FBV-1 in seedlings and meristem tissue culture plants. Forty-four isolates were used in a study evaluating the population structure of the virus in the United States. Evidence that FBV-1 is integrated in the fig genome is presented and discussed.

The 3'-proximal part of the Plum pox virus P1 gene determinates the symptom expression in two herbaceous host plants

Three major strains of the Plum pox virus (PPV) are the most important in Europe: PPV-D, PPV-M, and PPV-Rec. By combining the genomes of two different strains of PPV (PPV-D with PPV-Rec; PPV-D with PPV-M), 20 inter-strain chimeric infectious clones (CICPPV) were constructed. Biological properties of CICPPV were tested by inoculating them on different herbaceous host species susceptible to PPV. Four of the seven species tested, exhibited visible symptoms. In Nicotiana benthamiana all CICPPV induced systemic mosaic and leaf malformation. Pisum sativum showed a broad range of symptom severity (systemic chlorotic and necrotic lesions) but neither qualitative nor quantitative aspects of symptomatology were related to a single PPV genome locus. Nicotiana occidentalis and Nicandra physaloides proved to be suitable for symptom-based differentiation. Depending on the virus strain/chimera, N. occidentalis showed two types of symptoms: mild systemic chlorotic spots or local necrotic lesions/systemic vein necroses. N. physaloides reacted to the PPV infection either symptomless or by local necrotic lesions. Our results demonstrated that the P1/HC-pro region of the PPV genome appears to be the determinant of the symptom manifestation in these host plants. In silico analysis mapped it to the 3'-proximal part of the P1 gene.

Evaluation of the genetic diversity of Plum pox virus in a single plum tree

Genetic diversity of Plum pox virus (PPV) and its distribution within a single perennial woody host (plum, Prunus domestica) has been evaluated. A plum tree was triply infected by chip-budding with PPV-M, PPV-D and PPV-Rec isolates in 2003 and left to develop untreated under open field conditions. In September 2010 leaf and fruit samples were collected from different parts of the tree canopy. A 745-bp NIb-CP fragment of PPV genome, containing the hypervariable region encoding the CP N-terminal end was amplified by RT-PCR from each sample and directly sequenced to determine the dominant sequence. In parallel, the PCR products were cloned and a total of 105 individual clones were sequenced. Sequence analysis revealed that after 7 years of infection, only PPV-M was still detectable in the tree and that the two other isolates (PPV-Rec and PPV-D) had been displaced. Despite the fact that the analysis targeted a relatively short portion of the genome, a substantial amount of intra-isolate variability was observed for PPV-M. A total of 51 different haplotypes could be identified from the 105 individual sequences, two of which were largely dominant. However, no clear-cut structuration of the viral population by the tree architecture could be highlighted although the results obtained suggest the possibility of intra-leaf/fruit differentiation of the viral population. Comparison of the consensus sequence with the original source isolate showed no difference, suggesting within-plant stability of this original isolate under open field conditions.

Sequence variability, recombination analysis, and specific detection of the W strain of Plum pox virus

Plum pox virus (PPV), a member of the genus Potyvirus, is the causal agent of Sharka, the most detrimental disease of stone-fruit trees worldwide. PPV isolates are grouped into seven distinct strains. The minor PPV-W strain was established recently for the divergent W3174 isolate found in Canada. Here, the partial or complete genomic sequences of four PPV-W isolates from Latvia have been determined. The completely sequenced isolates LV-141pl and LV-145bt share 93.1 and 92.1% nucleotide identity, respectively, with isolate W3174, with two regions of higher (>20%) divergence in the P1/HC-Pro and NIa (VPg) regions. Further analyses demonstrated that these two regions correspond to two independent recombination events in the W3174 genome, one involving PPV-M (approximate genome positions 692 to 1424) and the other PPV-D (nucleotides 5672 to 5789). The LV-141pl and LV-145bt isolates appear to be representatives of the "ancestral" PPV-W strain, not affected by recombination. The PPV-W intrastrain variability is substantially higher than that of all other PPV strains, with potential implications for the serological detection of PPV-W isolates. A PPV-W-specific primer pair has been developed, allowing the specific reverse-transcription polymerase chain reaction detection of all five presently available W isolates. The characterization of these new PPV-W isolates sheds light on PPV-W evolutionary history, further supports the hypothesis of its East-European origin, and opens the way for the biological and epidemiological characterization of this poorly known PPV strain.

Molecular variability of Tobacco vein banding mosaic virus populations

The incidence of Tobacco vein banding mosaic virus (TVBMV) on tobacco increases dramatically in China recently and it has caused great economic losses. To gain insights into the evolutionary mechanisms of TVBMV, a total of 40 TVBMV isolates were collected from different tobacco production regions in China and their genomic regions encoding helper component-proteinase (HC-Pro), the third protein (P3), the first 6K protein (6K1) and coat protein (CP) were sequenced. Phylogenetic analyses revealed that TVBMV isolates can be divided into two evolutionary divergent groups based on P3, the frame-shifting pipo and 6K1 genes, and three groups on HC-Pro and CP genes. The populations from most parts of mainland China (MC) showed frequent gene flow; those from Yunnan province in south western China always formed a separate group (YN) and also had frequent within-group gene flow. However, the gene flow between groups MC and YN was uncommon. Our results revealed that all the tested TVBMV genes were under negative selection and the HC-Pro gene was under the strongest constraints. Recombination events were identified in 13 of the 42 analyzed isolates. This study suggested that negative selection, gene flow and recombination were important evolutionary factors driving the genetic diversification of TVBMV.

Molecular, ultrastructural, and biological characterization of Pennsylvania isolates of Plum pox virus

Plum pox virus (PPV) was identified in Pennsylvania in 1999. The outbreak was limited to a four-county region in southern Pennsylvania. Initial serological and molecular characterization indicated that the isolates in Pennsylvania belong to the D strain of PPV. The Pennsylvania isolates were characterized by sequence analysis, electron microscopy, host range, and vector transmission to determine how these isolates related to their previously studied European counterparts. Genetically, Pennsylvania (PPV-Penn) isolates were more closely related to each other than to any other PPV-D strains, and isolates from the United States, Canada, and Chile were more closely related to each other than to European isolates. The PPV-Penn isolates exist as two clades, suggesting the possibility of multiple introductions. Electron microscopy analysis of PPV-Penn isolates, including cytopathological studies, indicated that the virions were similar to other Potyvirus spp. PPV-Penn isolates had a herbaceous host range similar to that of European D isolates. There were distinct differences in the transmission efficiencies of the two PPV-Penn isolates using Myzus persicae and Aphis spiraecola as vectors; however, both PPV-Penn isolates were transmitted by M. persicae more efficiently than a European D isolate but less efficiently than a European M isolate.

Analysis of the epitope structure of Plum pox virus coat protein

Typing of the particular Plum pox virus (PPV) strain responsible in an outbreak has important practical implications and is frequently performed using strain-specific monoclonal antibodies (MAbs). Analysis in Western blots of the reactivity of 24 MAbs to a 112-amino-acid N-terminal fragment of the PPV coat protein (CP) expressed in Escherichia coli showed that 21 of the 24 MAbs recognized linear or denaturation-insensitive epitopes. A series of eight C-truncated CP fragments allowed the mapping of the epitopes recognized by the MAbs. In all, 14 of them reacted to the N-terminal hypervariable region, defining a minimum of six epitopes, while 7 reacted to the beginning of the core region, defining a minimum of three epitopes. Sequence comparisons allowed the more precise positioning of regions recognized by several MAbs, including those recognized by the 5B-IVIA universal MAb (amino acids 94 to 100) and by the 4DG5 and 4DG11 D serogroup-specific MAbs (amino acids 43 to 64). A similar approach coupled with infectious cDNA clone mutagenesis showed that a V74T mutation in the N-terminus of the CP abolished the binding of the M serogroup-specific AL MAb. Taken together, these results provide a detailed positioning of the epitopes recognized by the most widely used PPV detection and typing MAbs.

Molecular epidemiology of Plum pox virus in Japan

For a molecular epidemiological study based on complete genome sequences, 37 Plum pox virus (PPV) isolates were collected from the Kanto region in Japan. Pair-wise analyses revealed that all 37 Japanese isolates belong to the PPV-D strain, with low genetic diversity (less than 0.8%). In phylogenetic analysis of the PPV-D strain based on complete nucleotide sequences, the relationships of the PPV-D strain were reconstructed with high resolution: at the global level, the American, Canadian, and Japanese isolates formed their own distinct monophyletic clusters, suggesting that the routes of viral entry into these countries were independent; at the local level, the actual transmission histories of PPV were precisely reconstructed with high bootstrap support. This is the first description of the molecular epidemiology of PPV based on complete genome sequences.

Recombination in natural populations of watermelon mosaic virus: new agronomic threat or damp squib?

Since their introduction in south-eastern France around 1999, new, 'emerging' (EM) strains of watermelon mosaic virus (WMV) coexist with the 'classic' (CL) strains present for more than 40 years. This situation constitutes a unique opportunity to estimate the frequency of recombinants appearing in the few years following introduction of new strains of a plant RNA virus. Molecular analyses performed on more than 1000 isolates from epidemiological surveys (2004-2008) and from experimental plots (2009-2010), and targeting only recombinants that became predominant in at least one plant, revealed at least seven independent CL/EM or EM/EM recombination events. The frequency of recombinants involving at least one EM parent in the natural populations tested was on the order of 1 %. No new recombinant was detected for more than 1 year, and none but one in more than one location. In tests comparing host range and aphid transmissibility, the new recombinants did not display a better fitness than their 'parental' isolates. No recombinant was detected from artificial mixed infections of CL and EM isolates of various hosts after testing more than 1500 subcultures obtained after single-aphid transmission. These results constitute one of the first estimations of the frequency of recombinants in natural conditions for a plant RNA virus. This suggests that although viable recombinants of WMV are not rare, and although recombination may potentially lead to new highly damaging strains, the new recombinants observed so far had a lower fitness than the parental strains and did not emerge durably in the populations.

Sequence diversity and potential recombination events in the coat protein gene of Apple stem pitting virus

The variability of the Apple stem pitting virus (ASPV) coat protein (CP) gene was investigated. The CP gene of ten virus isolates from apple and pear trees was sequenced. Comparison of all sequenced virus isolates revealed high diversity of the CP gene (70.7-93.5% at the nucleotide level and 77.8-98.7% at the amino acid level). Additionally, one or two deletions in the N-terminal part of the coat protein gene of the studied virus isolates were identified. The ratio of nonsynonymous to synonymous polymorphic sites indicated that purifying selection has acted to eliminate deleterious mutations in coding sites. Moreover, the evidences for recombination in analyzed sequences were provided. It is likely that recombination, along with selection, enhances the speed of elimination of deleterious mutations in ASPV, following the mutational deterministic hypothesis of Kondrashov.

Silencing of Plum pox virus 5'UTR/P1 sequence confers resistance to a wide range of PPV strains

An effective disease-control strategy should protect the host from the major economically important and geographically widespread variants of a pathogen. Plum pox virus (PPV) is the causal agent of sharka, the most devastating viral disease of Prunus species. We have shown previously that the hairpin RNA expression driven by h-UTR/P1, h-P1/HCPro, h-HCPro and h-HCPro/P3 constructs, derived from the PPV-M ISPaVe44 isolate, confers resistance to the homologous virus in Nicotiana benthamiana plants. Since the production of transgenic stone fruits and their evaluation for PPV resistance would take several years, the ISPaVe44-resistant plant lines were used to evaluate which construct would be the best candidate to be transferred to Prunus elite cultivars. To do that, nine PPV isolates of the D, M, Rec, EA and C strains originally collected from five Prunus species in different geographical areas, were typed by sequencing and used to challenge the transgenic N. benthamiana lines; 464 out of 464 virus-inoculated plants of lines h-UTR/P1, h-HCPro and h-HCPro/P3 showed complete and long-lasting resistance to the seven PPV isolates of D, M and Rec strains. Moreover, the h-UTR/P1 plants were also fully resistant to PPV-C and -EA isolates. Our data suggest that the h-UTR/P1 construct is of particular practical interest to obtain stone fruit plants resistant to the sharka disease.

Putative recombination events and evolutionary history of five economically important viruses of fruit trees based on coat protein-encoding gene sequence analysis

To enhance the knowledge of recombination as an evolutionary process, 267 accessions retrieved from GenBank were investigated, all belonging to five economically important viruses infecting fruit crops (Plum pox, Apple chlorotic leaf spot, Apple mosaic, Prune dwarf, and Prunus necrotic ringspot viruses). Putative recombinational events were detected in the coat protein (CP)-encoding gene using RECCO and RDP version 3.31beta algorithms. Based on RECCO results, all five viruses were shown to contain potential recombination signals in the CP gene. Reconstructed trees with modified topologies were proposed. Furthermore, RECCO performed better than the RDP package in detecting recombination events and exhibiting their evolution rate along the sequences of the five viruses. RDP, however, provided the possible major and minor parents of the recombinants. Thus, the two methods should be considered complementary.

An Amino Acid Deletion in Wheat streak mosaic virus Capsid Protein Distinguishes a Homogeneous Group of European Isolates and Facilitates Their Specific Detection

The tritimovirus Wheat streak mosaic virus (WSMV) is widespread throughout the world and represents a severe threat to cereal crop production. To increase knowledge of genetic diversity of WSMV in Europe, until now scarce, capsid protein (CP) sequences of several Czech, French, Italian, Slovak, and Turkish isolates have been determined. A multiple alignment of CP nucleotide sequences using available WSMV sequences revealed only limited sequence variation among 3 previously sequenced European isolates and the 14 European isolates sequenced in this study. Moreover, these isolates were characterized by an identical 3-nucleotide deletion, resulting in the lack of the Gly₂₇₆₁ codon within the CP region of the polyprotein. The results indicate that this monophyletic group of isolates (designated as WSMV-ΔE) is common and widely dispersed throughout the European continent. The close relationship of WSMV-ΔE isolates implies a single common ancestor and, presumably, subsequent dispersal throughout Europe from a single focus. We developed two simple assays for specific and accurate detection of WSMV-ΔE isolates. First, a conserved ClaI restriction site in the core CP gene sequence unique to WSMV-ΔE isolates was used for restriction fragment length polymorphism analysis of amplified polymerase chain reaction (PCR) products. Second, the conserved and specific codon gap in WSMV-ΔE sequences was used as a target to design specific primers functional in one-step reverse-transcription PCR detection of WSMV-ΔE isolates.

Molecular variability and genetic structure of the population of soybean mosaic virus based on the analysis of complete genome sequences

The complete genomes of 30 Soybean mosaic virus (SMV) isolates and strains were sequenced in this study. Together with fourteen previously reported sequences, we analyzed the genetic structure of the SMV population. Analyses of genetic diversity showed that different genomic regions of SMV are under different evolutionary constraints and that there was no significant genetic differentiation between East Asian and North American populations of SMV. Phylogenetic analyses revealed a significant correlation between phylogeny of the cylindrical inclusion (CI) gene of SMV and SMV resistance gene 3 (Rsv3)-relating pathogenicity of SMV, suggesting CI might be a pathogenic determinant in Rsv3-mediated disease response. Interestingly, recombination analyses identified 19 'clear' recombination events in the SMV population. Furthermore, as several resistance-breaking strains were identified as recombinants, it appears that recombination might contribute to overcome host resistance in SMV-soybean pathosystem. Our finding suggests that recombination as well as mutation is an important evolutionary process in the genetic diversification of SMV population.

Quantitative trait loci meta-analysis of Plum pox virus resistance in apricot (Prunus armeniaca L.): new insights on the organization and the identification of genomic resistance factors

Plum pox virus (PPV) is responsible for sharka disease, one of the most detrimental stone fruit diseases affecting Prunus trees worldwide. Only a few apricot cultivars have been described as resistant, most originating from North American breeding programmes. Several PPV resistance quantitative trait loci (QTLs) have been mapped in various progenies, consistently highlighting the contribution to the resistance of the upper part of linkage group 1 (LG1). However, to date, no consensus has been reached on the precise number of QTLs linked to the resistance to PPV in apricot and P. davidiana or on their accurate position on the genetic linkage map. In the present study, the quantitative resistance of cultivar 'Harlayne' was analysed over five growth periods in a large F1 population. Four QTLs were identified, three mapping on LG1, explaining between 5% and 39% of the observed phenotypic variance. In an effort to further this analysis of PPV resistance in apricot, these results were merged in a single QTL meta-analysis with those of five other PPV resistance analyses available in the literature. Three consensus QTL regions were identified on LG1 and a putative fourth region on LG3. QTL meta-analysis also revealed the contribution of each resistant cultivar to metaQTLs, providing interesting comparative data on the resistance factors shared between the resistance sources used in the various studies. Finally, it was shown that one of the metaQTLs co-localizes with the eukaryotic translation initiation factor eIF4E, thus providing new hypotheses on the mechanisms of PPV resistance in apricot.

Further characterization of a new recombinant group of Plum pox virus isolates, PPV-T, found in orchards in the Ankara province of Turkey

Sixteen Plum pox virus (PPV) isolates collected in the Ankara region of Turkey were analyzed using available serological and molecular typing assays. Surprisingly, despite the fact that all isolates except one, which was a mix infection, were typed as belonging to the PPV-M strain in four independent molecular assays, nine of them (60%) reacted with both PPV-M specific and PPV-D specific monoclonal antibodies. Partial 5' and 3' genomic sequence analysis on four isolates demonstrated that irrespective of their reactivity towards the PPV-D specific monoclonal antibody, they were all closely related to a recombinant PPV isolate from Turkey, Ab-Tk. All three isolates for which the relevant genomic sequence was obtained showed the same recombination event as Ab-Tk in the HC-Pro gene, around position 1566 of the genome. Complete genomic sequencing of Ab-Tk did not provide evidence for additional recombination events in its evolutionary history. Taken together, these results indicate that a group of closely related PPV isolates characterized by a unique recombination in the HC-Pro gene is prevalent under field conditions in the Ankara region of Turkey. Similar to the situation with the PPV-Rec strain, we propose that these isolates represent a novel strain of PPV, for which the name PPV-T (Turkey) is proposed. Given that PPV-T isolates cannot be identified by currently available typing techniques, it is possible that their presence has been overlooked in other situations. Further efforts should allow a precise description of their prevalence and of their geographical distribution in Turkey and, possibly, in other countries.

Characterization of a natural Plum pox virus isolate bearing a truncated coat protein

Plum pox virus (PPV) isolates were collected in Hungary from plum varieties. PCR targeting the 3' genomic region resulted in a shorter PCR product in the case of the B1298 isolate bearing a 135-nucleotide deletion in frame in the N-terminal part of the coat protein (CP). The isolate was aphid-transmissible and the virion diameter was reduced compared to PPV-SK68. Detectability of this isolate by Western blot varied according to the antibody used. Integration of the deleted CP gene into an infectious PPV clone had no effect on infectivity and symptomatology. In competition experiments, B1298 had a considerable advantage in virus accumulation.

Molecular variability of sweet potato feathery mottle virus and other potyviruses infecting sweet potato in Peru

Several potyviruses are found infecting sweet potato (Ipomoea batatas) in Peru, of which sweet potato feathery mottle virus (SPFMV, genus Potyvirus) is the most common. However, sequence data for these viruses are not available from Peru. In this study, the 3'-terminal approximately 1,800 nucleotide sequences of 17 potyvirus samples collected from the six main sweet potato-producing areas of Peru over the past 20 years were determined and analyzed. Results of sequence comparisons and phylogenetic analysis showed that three of the four recognized SPFMV strain groups, including the East African strain, are established in Peru as well as two other potyviruses: sweet potato virus G (SPVG) and sweet potato virus 2 (SPV2). The analysis further revealed that SPFMV, SPVG and SPV2 are related and form an Ipomoea-specific phylogenetic lineage within the genus Potyvirus and identified for the first time recombination events between viruses from different strain groups of SPFMV.

Genetic structure of a population of Potato virus Y inducing potato tuber necrotic ringspot disease in Japan; comparison with North American and European populations

The structure of Potato virus Y (PVY) populations causing potato tuber necrotic ringspot disease (PTNRD) was analysed. The full-length sequences of the genomic RNAs of five geographically distinct isolates from Japan were determined. Recombination and phylogenetic analyses of European, North American and Japanese isolates of PVY showed that the world PVY population has three major lineages and two sublineages. Most recombinants were interlineage, and one isolate from Europe was identified as an intralineage recombinant. No recombinants were found among Japanese PTNRD isolates, which were most closely related to PTNRD isolates previously found in North America. Comparison of the within- and between population nucleotide diversities in the N lineage sequences from Japan, Europe and North America showed that Japanese population was distinct from the European and North American populations. The nucleotide sequences of the protein 1 and coat protein genes of a further 18 isolates were determined. One Japanese clade had radiated in a star burst as shown by its deviation from the neutral equilibrium model and its small nucleotide diversity. Our results suggest that PVY PTNRD was recently introduced into Japan more than once, and has expanded throughout Japan from founder populations.

A chimeric plum pox virus shows reduced spread and cannot compete with its parental wild-type viruses in a mixed infection

The effect of a recombination event in the genomic 3' end on the biological properties and competitiveness of plum pox virus (PPV) was investigated. Therefore, a fragment spanning the coat protein (CP) coding region and a part of the 3' non-translated region of a non-aphid-transmissible strain of PPV (PPV-NAT) was replaced by the corresponding region of a PPV sour cherry isolate (PPV-SoC). The resulting chimera (PPV-NAT/SoC) caused severe symptoms in Nicotiana benthamiana, resembling those of PPV-NAT. In mixed infections with either of the parental viruses, the chimera PPV-NAT/SoC was less competitive. Labelling experiments with DsRed showed that PPV-NAT/SoC (PPV-NAT/SoC-red) moved more slowly from cell to cell than PPV-NAT (PPV-NAT-red). In mixed infections of PPV-NAT/SoC-red with a green fluorescent protein-expressing PPV-NAT (PPV-NAT-AgfpS), spatial separation of the viruses was observed. These data suggest that, in PPV infections, symptom severity and competitiveness are independent aspects and that spatial separation may contribute to the displacement of a recombinant virus.