Characterization of horizontal transmission of blueberry latent spherical virus by pollen

Nicotiana benthamiana plants became infected with blueberry latent spherical virus (BLSV) after pollination with pollen grains produced by BLSV-infected N. benthamiana plants. Interestingly, pollen grains produced by BLSV-infected Vaccinium corymbosum (blueberry), Nicotiana alata, and Petunia × hybrida (petunia) plants also transmitted the virus to healthy N. benthamiana plants after pollination. As seen using aniline blue staining and fluorescence microscopy, pollen grains from BLSV-infected blueberry, N. alata, and petunia plants germinated on stigmas of N. benthamiana, and the pollen tubes penetrated the stigmas in a manner similar to that of N. benthamiana pollen grains on N. benthamiana stigmas. Whole-mount in situ hybridization and chromogenic in situ hybridization analysis showed that infected blueberry and N. benthamiana pollen grains germinated on N. benthamiana stigmas, and virus-containing pollen tubes penetrated the stigmas. Tissue blot hybridization analysis revealed that the initial infection sites were the N. benthamiana stigmas pollinated with infected pollen grains from blueberry and N. benthamiana. In addition, the virus spread from the initial infection sites to the phloem in the stigma and style. Taken together, we suggest that penetrating pollen tubes that harbored the virus results in infection foci in the stigma, and the virus then moves to the vascular tissues in the stigma and style and eventually establishes systemic infection.

Disturbance of floral colour pattern by activation of an endogenous pararetrovirus, petunia vein clearing virus, in aged petunia plants

White areas of star-type bicolour petals of petunia (Petunia hybrida) are caused by post-transcriptional gene silencing (PTGS) of the key enzyme of anthocyanin biosynthesis. We observed blotched flowers and a vein-clearing symptom in aged petunia plants. To determine the cause of blotched flowers, we focused on an endogenous pararetrovirus, petunia vein clearing virus (PVCV), because this virus may have a suppressor of PTGS (VSR). Transcripts and episomal DNAs derived from proviral PVCVs accumulated in aged plants, indicating that PVCV was activated as the host plant aged. Furthermore, DNA methylation of CG and CHG sites in the promoter region of proviral PVCV decreased in aged plants, suggesting that poor maintenance of DNA methylation activates PVCV. In parallel, de novo DNA methylation of CHH sites in its promoter region was also detected. Therefore, both activation and inactivation of PVCV occurred in aged plants. The accumulation of PVCV transcripts and episomal DNAs in blotched regions and the detection of VSR activity support a mechanism in which suppression of PTGS by PVCV causes blotched flowers.

LreEF1A4, a Translation Elongation Factor from Lilium regale, Is Pivotal for Cucumber Mosaic Virus and Tobacco Rattle Virus Infections and Tolerance to Salt and Drought

Eukaryotic translation elongation factors are implicated in protein synthesis across different living organisms, but their biological functions in the pathogenesis of cucumber mosaic virus (CMV) and tobacco rattle virus (TRV) infections are poorly understood. Here, we isolated and characterized a cDNA clone, LreEF1A4, encoding the alpha subunit of elongation factor 1, from a CMV-elicited suppression subtractive hybridization library of Lilium regale. The infection tests using CMV remarkably increased transcript abundance of LreEF1A4; however, it also led to inconsistent expression profiles of three other LreEF1A homologs (LreEF1A1-3). Protein modelling analysis revealed that the amino acid substitutions among four LreEF1As may not affect their enzymatic functions. LreEF1A4 was ectopically overexpressed in petunia (Petunia hybrida), and transgenic plants exhibited delayed leaf and flower senescence, concomitant with increased transcription of photosynthesis-related genes and reduced expression of senescence-associated genes, respectively. A compromised resistance to CMV and TRV infections was found in transgenic petunia plants overexpressing LreEF1A4, whereas its overexpression resulted in an enhanced tolerance to salt and drought stresses. Taken together, our data demonstrate that LreEF1A4 functions as a positive regulator in viral multiplication and plant adaption to high salinity and dehydration.

Influence of the terminal left domain on horizontal and vertical transmissions of tomato planta macho viroid and potato spindle tuber viroid through pollen

Viroids can be transmitted vertically and/or horizontally by pollen. Tomato planta macho viroid (TPMVd) has a high rate of horizontal transmission by pollen, whereas potato spindle tuber viroid (PSTVd) does not. To specify the domain(s) involved in horizontal transmission, four viroid chimeras were created by exchanging the terminal left (TL) and/or pathogenicity (P) domains between PSTVd and TPMVd. PSTVd-based chimeras containing TPMVd-TL and P, or TPMVd-TL alone, displayed a high rate of horizontal transmission. TPMVd-based chimeras containing PSTVd-TL and P lost infectivity, and those containing PSTVd-TL alone displayed a low rate of horizontal transmission. In addition, the vertical transmission rate was also higher in the mutants containing TPMVd-TL than in the others. These findings indicate that the sequences or structures in the TL and P (although the role is limited) domains are important not only for horizontal but also for vertical transmission by pollen.

Characterization, phylogeny and recombination analysis of Pedilanthus leaf curl virus-Petunia isolate and its associated betasatellite

BACKGROUND

Geminiviruses cause major losses to several economically important crops. Pedilanthus leaf curl virus (PeLCV) is a pathogenic geminivirus that appeared in the last decade and is continuously increasing its host range in Pakistan and India. This study reports the identification and characterization of PeLCV-Petunia from ornamental plants in Pakistan, as well as geographical, phylogenetic, and recombination analysis.

METHODS

Viral genomes and associated satellites were amplified, cloned, and sequenced from Petunia atkinsiana plants showing typical geminivirus infection symptoms. Virus-satellite complex was analyzed for phylogenetic and recombination pattern. Infectious clones of isolated virus and satellite molecules were constructed using a partial dimer strategy. Infectivity analysis of PeLCV alone and in combination with Digera yellow vein betasatellite (DiYVB) was performed by Agrobacterium infiltration of Nicotiana benthamiana and Petunia atkinsiana plants with infectious clones.

RESULTS

PeLCV, in association with DiYVB, was identified as the cause of leaf curl disease on P. atkinsiana plants. Sequence analysis showed that the isolated PeLCV is 96-98% identical to PeLCV from soybean, and DiYVB has 91% identity to a betasatellite identified from rose. Infectivity analysis of PeLCV alone and in combination with DiYVB, performed by Agrobacterium infiltration of infectious clones in N. benthamiana and P. atkinsiana plants, resulted in mild and severe disease symptoms 14 days after infiltration, respectively, demonstrating that these viruses are natural disease-causing agents. Southern blot hybridization indicated successful replication of the virus-betasatellite complex in the infected plants. Phylogenetic analysis suggests that PeLCV originated from Pakistan and later spread to India. Recombination analysis predicted that PeLCV is a donor parent for recombination and evolution of two important begomoviruses, Papaya leaf curl virus (PaLCuV) and Radish leaf curl virus (RaLCuV). The molecular phylogeny of genes encoding coat protein (CP) and replication associated protein (Rep) depict a complex evolutionary pattern of the viruses, with wide diversity in both of the genes.

CONCLUSIONS

This study presents PeLCV and DiYVB as a new natural combination resulting in leaf curl disease on P. atkinsiana plants. Phylogenetic analysis, in addition to recent agricultural reports, identify PeLCV as an emerging broad host range Begomovirus that is resident in Pakistan and, more recently, has also spread to India. Recombination analysis showed that PeLCV was involved in a natural recombinational event leading to the evolution of two recombinant begomoviruses, RaLCuV and PaLCuV.

RETRACTED: WRKY71 and TGA1a physically interact and synergistically regulate the activity of a novel promoter isolated from Petunia vein-clearing virus

Caulimoviral promoters have become excellent tools for efficient transgene expression in plants. However, the transcriptional framework controlling their systematic regulation is poorly understood. To understand this regulatory mechanism, we extensively studied a novel caulimoviral promoter, PV8 (-163 to +138, 301 bp), isolated from Petunia vein-clearing virus (PVCV). PVCV was found to be Salicylic acid (SA)-inducible and 2.5-3.0 times stronger than the widely used CaMV35S promoter. In silico analysis of the PV8 sequence revealed a unique clustering of two stress-responsive cis-elements, namely, as-11 and W-box1-2, located within a span of 31 bp (-74 to -47) that bound to the TGA1a and WRKY71 plant transcription factors (TFs), respectively. We found that as-1 (TTACG) and W-box (TGAC) elements occupied both TGA1a and WRKY71 on the PV8 backbone. Mutational studies demonstrated that the combinatorial influence of as-1 (-57) and W-box1-2 (-74 and -47) on the PV8 promoter sequence largely modulated its activity. TGA1a and WRKY71 physically interacted and cooperatively enhanced the transcriptional activity of the PV8 promoter. Biotic stress stimuli induced PV8 promoter activity by ~1.5 times. We also established the possible pathogen-elicitor function of AtWRKY71 and NtabWRKY71 TFs. Altogether, this study elucidates the interplay between TFs, biotic stress and caulimoviral promoter function.

Seed Transmission of Beet Curly Top Virus and Beet Curly Top Iran Virus in a Local Cultivar of Petunia in Iran

Beet curly top virus (BCTV) and beet curly top Iran virus (BCTIV) are known as the causal agents of curly top disease in beet and several other dicotyledonous plants in Iran. These viruses are transmitted by Circulifer species, and until now, there has been no confirmed report of their seed transmission. A percentage (38.2–78.0%) of the seedlings developed from the seeds of a petunia local cultivar under insect-free conditions showed stunting, interveinal chlorosis, leaf curling, and vein swelling symptoms, and were infected by BCTV when tested by PCR. Presence of BCTV in seed extracts of petunia local cultivar was confirmed by PCR and IC-PCR, followed by sequencing. Agroinoculation of curly top free petunia plants with a BCTV infectious clone resulted in BCTV infection of plants and their developed seeds. These results show the seed infection and transmission of BCTV in a local cultivar of petunia. Similar experiments performed with BCTIV showed that this virus is also seed transmissible in the same cultivar of petunia, although with a lower rate (8.8–18.5%). Seed transmission of curly top viruses may have significant implications in the epidemiology of these viruses.

PhOBF1, a petunia ocs element binding factor, plays an important role in antiviral RNA silencing

Virus-induced gene silencing (VIGS) is a common reverse genetics strategy for characterizing the function of genes in plants. The detailed mechanism governing RNA silencing efficiency triggered by viruses is largely unclear. Here, we reveal that a petunia (Petunia hybrida) ocs element binding factor, PhOBF1, one of the basic leucine zipper (bZIP) transcription factors, was up-regulated by Tobacco rattle virus (TRV) infection. Simultaneous silencing of PhOBF1 and a reporter gene, phytoene desaturase (PDS) or chalcone synthase (CHS), by TRV-based VIGS led to a failure of the development of leaf photobleaching or the white-corollas phenotype. PhOBF1 silencing caused down-regulation of RNA silencing-related genes, including RNA-dependent RNA polymerases (RDRs), Dicer-like RNase III enzymes (DCLs), and Argonautes (AGOs). After inoculation with the TRV-PhPDS, PhOBF1-RNAi lines exhibited a substantially impaired PDS silencing efficiency, whereas overexpression of PhOBF1 resulted in a recovery of the silencing phenotype (photobleaching) in systemic leaves. A compromised resistance to TRV and Tobacco mosaic virus was found in PhOBF1-RNAi lines, while PhOBF1-overexpressing lines displayed an enhanced resistance to their infections. Compared with wild-type plants, PhOBF1-silenced plants accumulated lower levels of free salicylic acid (SA), salicylic acid glucoside, and phenylalanine, contrarily to higher levels of those in plants overexpressing PhOBF1. Furthermore, transcripts of a number of genes associated with the shikimate and phenylpropanoid pathways were decreased or increased in PhOBF1-RNAi or PhOBF1-overexpressing lines, respectively. Taken together, the data suggest that PhOBF1 regulates TRV-induced RNA silencing efficiency through modulation of RDRs, DCLs, and AGOs mediated by the SA biosynthesis pathway.

A petunia ethylene-responsive element binding factor, PhERF2, plays an important role in antiviral RNA silencing

Virus-induced RNA silencing is involved in plant antiviral defense and requires key enzyme components, including RNA-dependent RNA polymerases (RDRs), Dicer-like RNase III enzymes (DCLs), and Argonaute proteins (AGOs). However, the transcriptional regulation of these critical components is largely unknown. In petunia (Petunia hybrida), an ethylene-responsive element binding factor, PhERF2, is induced by Tobacco rattle virus (TRV) infection. Inclusion of a PhERF2 fragment in a TRV silencing construct containing reporter fragments of phytoene desaturase (PDS) or chalcone synthase (CHS) substantially impaired silencing efficiency of both the PDS and CHS reporters. Silencing was also impaired in PhERF2- RNAi lines, where TRV-PhPDS infection did not show the expected silencing phenotype (photobleaching). In contrast, photobleaching in response to infiltration with the TRV-PhPDS construct was enhanced in plants overexpressing PhERF2 Transcript abundance of the RNA silencing-related genes RDR2, RDR6, DCL2, and AGO2 was lower in PhERF2-silenced plants but higher in PhERF2-overexpressing plants. Moreover, PhERF2-silenced lines showed higher susceptibility to Cucumber mosaic virus (CMV) than wild-type (WT) plants, while plants overexpressing PhERF2 exhibited increased resistance. Interestingly, growth and development of PhERF2-RNAi lines were substantially slower, whereas the overexpressing lines were more vigorous than the controls. Taken together, our results indicate that PhERF2 functions as a positive regulator in antiviral RNA silencing.

Distribution of potato spindle tuber viroid in reproductive organs of petunia during its developmental stages

Embryo infection is important for efficient seed transmission of viroids. To identify the major pattern of seed transmission of viroids, we used in situ hybridization to histochemically analyze the distribution of Potato spindle tuber viroid (PSTVd) in each developmental stage of petunia (flowering to mature seed stages). In floral organs, PSTVd was present in the reproductive tissues of infected female × infected male and infected female × healthy male but not of healthy female × infected male before embryogenesis. After pollination, PSTVd was detected in the developed embryo and endosperm in all three crosses. These findings indicate that PSTVd is indirectly delivered to the embryo through ovule or pollen during the development of reproductive tissues before embryogenesis but not directly through maternal tissues as cell-to-cell movement during embryogenesis.

Virus-aided gene expression and silencing using TRV for functional analysis of floral scent-related genes

Flower scent is a composite character determined by a complex mixture of low-molecular-weight volatile molecules. Despite the importance of floral fragrance, our knowledge on factors regulating these pathways remains sketchy. Virus-induced gene silencing (VIGS) and virus-aided gene expression (VAGE) are characterized by a simple inoculation procedure and rapid results as compared to transgenesis, allowing screening and characterization of scent-related genes. Here, we describe methods using TRV as a VIGS/VAGE vector for the characterization of scent-related genes, protein compartmentalization studies, and protein subcellular targeting.

Virus-induced gene silencing using begomovirus satellite molecules

Virus-induced gene silencing (VIGS) has emerged as a powerful method for studying gene function. VIGS is induced by infecting a plant with a plant virus that has had its genome modified to include a sequence from the host gene to be silenced. DNAβ and DNA1 are satellite and single-stranded DNA molecules associated with begomoviruses (family Geminiviridae). We converted DNAβ and DNA1 into gene-silencing vectors. The VIGS vectors can induce silencing efficiently in many solanaceous plants. Here, we describe procedures for the use of these two gene-silencing vectors for VIGS in different hosts.

Modified expression of alternative oxidase in transgenic tomato and petunia affects the level of tomato spotted wilt virus resistance

BACKGROUND

Tomato spotted wilt virus (TSWV) has a very wide host range, and is transmitted in a persistent manner by several species of thrips. These characteristics make this virus difficult to control. We show here that the over-expression of the mitochondrial alternative oxidase (AOX) in tomato and petunia is related to TSWV resistance.

RESULTS

The open reading frame and full-length sequence of the tomato AOX gene LeAox1au were cloned and introduced into tomato 'Healani' and petunia 'Sheer Madness' using Agrobacterium-mediated transformation. Highly expressed AOX transgenic tomato and petunia plants were selfed and transgenic R1 seedlings from 10 tomato lines and 12 petunia lines were used for bioassay. For each assayed line, 22 to 32 tomato R1 progeny in three replications and 39 to 128 petunia progeny in 13 replications were challenged with TSWV. Enzyme-Linked Immunosorbent Assays showed that the TSWV levels in transgenic tomato line FKT4-1 was significantly lower than that of wild-type controls after challenge with TSWV. In addition, transgenic petunia line FKP10 showed significantly less lesion number and smaller lesion size than non-transgenic controls after inoculation by TSWV.

CONCLUSION

In all assayed transgenic tomato lines, a higher percentage of transgenic progeny had lower TSWV levels than non-transgenic plants after challenge with TSWV, and the significantly increased resistant levels of tomato and petunia lines identified in this study indicate that altered expression levels of AOX in tomato and petunia can affect the levels of TSWV resistance.

Infectivity of Euphorbia leaf curl virus and interaction with Tomato yellow leaf curl China betasatellite

To investigate the infectivity of Euphorbia leaf curl virus (EuLCV), an infectious clone was constructed and tested by agroinoculation and whitefly inoculation. EuLCV infected Nicotiana benthamiana, N. glutinosa, Solanum lycopersicum, Petunia hybrida efficiently upon agroinoculation and induced leaf curling, vein swelling and stunting in these plants but no symptoms in N. tabacum. Co-inoculation of EuLCV with a betasatellite DNA from an unrelated begomovirus enhanced symptoms in N. benthamiana, N. glutinosa, N. tabacum, S. lycopersicum and P. hybrida plants but had no effect on the accumulation of EuLCV DNA. Euphorbia pulcherrima plants were only infectable by insect transmission from agro-infected P. hybrida as a source. This is the first report about a monopartite begomovirus that has been reintroduced into a plant of the genus Euphorbia.

Virus-induced gene silencing in ornamental plants

Virus-induced gene silencing (VIGS) provides an attractive tool for high-throughput analysis of the functional effects of gene knockdown. Virus genomes are engineered to include fragments of target host genes, and the infected plant recognizes and silences the target genes as part of its viral defense mechanism. The consequences of gene inactivation, even of key metabolic, regulatory, or embryo-lethal genes, can thus be readily analyzed. A number of viral vectors have been developed for VIGS; one of the most frequently employed is based on tobacco rattle virus (TRV) due to its wide host range, efficiency, ease of application, and limited disease symptoms. TRV-based VIGS comprises two vectors. One (RNA2) includes a multiple cloning site into which fragments of target genes can be inserted. We have shown that the TRV/VIGS system can simultaneously silence as many as five independent genes. TRV is a mosaic-type virus, and silencing also occurs in a mosaic pattern. It is therefore desirable to have a reporter that can show where target genes have been silenced. The photobleaching induced by silencing phytoene desaturase (PDS) and the loss of purple pigmentation induced by silencing chalcone synthase (CHS) have successfully been used to indicate the location of coordinate silencing of other target genes. In this chapter, we outline our protocols for the use of VIGS for analysis of gene function, focusing particularly on the use of TRV with petunia and tomato.

Molecular characterization and experimental host-range of two begomoviruses infecting Clerodendrum cyrtophyllum in China

Two begomovirus isolates (YX2-I and YX2-II) were identified from Clerodendrum cyrtophyllum showing yellow mosaic symptoms collected in Jiangsu province of China. Sequence analysis reveals that YX2-I is a distinct begomovirus species for which the name Clerodendrum golden mosaic Jiangsu virus (ClGMJSV-[CN:YX2:08]) is proposed. YX2-II is an isolate of bipartite begomovirus Clerodendrum golden mosaic China virus (ClGMCNV-[CN:YX2:08]). Infectious clones of the two viruses were constructed and agroinoculated into Nicotiana benthamiana, N. glutinosa, N. tabacum Samsun, Petunia hybrida, Solanum lycopersicum, Capsicum annuum, S. melongena, Glycine max and Gossypium hirsutum plants. ClGMJSV induced leaf curling and stunting symptoms in N. benthamiana, N. glutinosa, N. tabacum Samsun, and P. hybrida, and ClGMCNV infected N. benthamiana, N. glutinosa, and N. tabacum Samsun with severe symptoms and P. hybrida without obvious symptom. Latent infection in N. benthamiana, N. glutinosa, N. tabacum Samsun, and P. hybrida plants was observed when plants were inoculated with ClGMCNV DNA-A alone. In addition, we illustrated that ClGMCNV DNA-A was capable of interacting with the betasatellite associated with Tobacco curly shoot virus (TbCSB) to produce symptoms in N. benthamiana and N. glutinosa plants, and ClGMJSV could interact with TbCSB but not with ClGMCNV DNA-B in N. benthamiana plants.

Molecular characterization and infectivity of Papaya leaf curl China virus infecting tomato in China

Papaya leaf curl China virus (PaLCuCNV) was previously reported as a distinct begomovirus infecting papaya in southern China. Based on molecular diagnostic survey, 13 PaLCuCNV isolates were obtained from tomato plants showing leaf curl symptoms in Henan and Guangxi Provinces of China. Complete nucleotide sequences of 5 representative isolates (AJ558116, AJ558117, AJ704604, FN256260, and FN297834) were determined to be 2738-2751 nucleotides, which share 91.7%-97.9% sequence identities with PaLCuCNV isolate G2 (AJ558123). DNA-beta was not found to be associated with PaLCuCNV isolates. To investigate the infectivity of PaLCuCNV, an infectious clone of PaLCuCNV-[CN:HeNZM1] was constructed and agro-inoculated into Nicotiana benthamiana, N. tabacum Samsun, N. glutinosa, Solanum lycopersicum and Petunia hybrida plants, which induced severe leaf curling and crinkling symptoms in these plants. Southern blot analysis and polymerase chain reaction (PCR) indicated a systemic infection of test plants by the agro-infectious clone.

Molecular variability of five Cucumber mosaic virus isolates from China

Cucumber mosaic virus (CMV) isolates are currently divided into two main groups, I and II according to their genomic sequences. The group I is further divided into two subgroups IA and IB. We performed a phylogenetic analysis of the genome regions containing 1a, 2a, 2b, coat protein (CP), and movement protein (MP) genes of 5 CMV isolates from China and other 28 CMV isolates available in the GenBank. The results indicated that CMV isolates could be genetically divided into three groups I, II, and III according to the genes encoding MP, CP, 1a, and 2a proteins and to the 2 groups according to the gene 2b. Group I could be further divided into two subgroups (IA and IB) according to the genes encoding CP, MP, 2a, and 2b proteins and to the three subgroups (IA, IB, and IC) according to the gene encoding 1a protein. Four of 5 examined Chinese CMV isolates belonged to the subgroup IB, while the remaining isolate was a natural inter-subgroup reassortant. We found that the 2b gene of CMV was under positive selection, while the other genes were under negative selection. No evidence of the selection associated with a host adaptation or geographic distribution was found.

Reverse genetics of floral scent: application of tobacco rattle virus-based gene silencing in Petunia

Floral fragrance is responsible for attracting pollinators as well as repelling pathogens and pests. As such, it is of immense biological importance. Molecular dissection of the mechanisms underlying scent production would benefit from the use of model plant systems with big floral organs that generate an array of volatiles and that are amenable to methods of forward and reverse genetics. One candidate is petunia (Petunia hybrida), which has emerged as a convenient model system, and both RNAi and overexpression approaches using transgenes have been harnessed for the study of floral volatiles. Virus-induced gene silencing (VIGS) is characterized by a simple inoculation procedure and rapid results relative to transgenesis. Here, we demonstrate the applicability of the tobacco rattle virus-based VIGS system to studies of floral scent. Suppression of the anthocyanin pathway via chalcone synthase silencing was used as a reporter, allowing easy visual identification of anthocyaninless silenced flowers/tissues with no effect on the level of volatile emissions. Use of tobacco rattle virus constructs containing target genes involved in phenylpropanoid volatile production, fused to the chalcone synthase reporter, allowed simple identification of flowers with suppressed activity of the target genes. The applicability of VIGS was exemplified with genes encoding S-adenosyl-l-methionine:benzoic acid/salicylic acid carboxyl methyltransferase, phenylacetaldehyde synthase, and the myb transcription factor ODORANT1. Because this high-throughput reverse-genetics approach was applicable to both structural and regulatory genes responsible for volatile production, it is expected to be highly instrumental for large-scale scanning and functional characterization of novel scent genes.

Characterization of silencing suppressor 2b of cucumber mosaic virus based on examination of its small RNA-binding abilities

Double-stranded (ds) RNAs and imperfect hairpin RNAs of endogenous genes trigger post-transcriptional gene silencing (PTGS) and are cleaved by a Dicer-like nuclease into small interfering RNAs (siRNAs) and microRNs (miRNAs), respectively. Such small RNAs (siRNAs and miRNAs) then guide an RNA-induced silencing complex (RISC) for sequence-specific RNA degradation. While PTGS serves as an antiviral defense in plants, many plant viruses encode suppressors as a counter defense. Here we demonstrate that the PTGS suppressor (2b) of a severe strain (CM95R) of cucumber mosaic virus (CMV) can bind to in vitro synthesized siRNAs and even to long dsRNAs to a lesser extent. However, the 2b suppressor weakly bound to a miRNA (miR171) duplex in contrast to another small RNA-binding suppressor, p19 of tombusvirus that can effectively bind miRNAs. Because the 2b suppressor of an attenuated strain of CMV (CM95), which differs in a single amino acid from the 2b of CM95R, could barely bind siRNAs, we hypothesized that the weak suppressor activity of the attenuated strain resulted from a loss of the siRNA-binding property of 2b via a single amino acid change. Here we consider that 2b interferes with the PTGS pathway by directly binding siRNAs (or long dsRNA).

Distinct expression of endogenous Petunia vein clearing virus and the DNA transposon dTph1 in two Petunia hybrida lines is correlated with differences in histone modification and siRNA production

Endogenous viruses exist in all kingdoms. They usually have active mechanisms of integration, as in bacteriophage lambda and animal retroviruses, and sophisticated mechanisms to maintain a proviral state over decades and generations. Plant para retroviruses, however, neither have an integrase, nor genes for maintaining the proviral state. How are those elements controlled, and under what conditions can they be activated? Here we study the proviral state of endogenous petunia vein clearing virus (ePVCV). Our results support the hypothesis that the proviral state is associated with a host silencing mechanism manifested by DNA methylation, chromatin modification and production of small interfering (si) RNAs. PVCV may be induced by applying abiotic stress, leading to the development of viral symptoms and increased transcript and siRNA levels. Similar levels of ePVCV DNA methylation were observed in two different lines of Petunia hybrida, RdC (rose du ciel) and W138, the latter known for its active version of transposon dTph1. In contrast, significant differences in histone modification were detected. The predominant association of ePVCV sequences with histone H3 methylated at lysine 9 (H3mK9) in RdC and with about equal amounts of H3mK9 and H3mK4 in W138 indicates a less repressive proviral state in the latter cultivar.

Distinct expression of endogenous Petunia vein clearing virus and the DNA transposon dTph1 in two Petunia hybrida lines is correlated with differences in histone modification and siRNA production

Endogenous viruses exist in all kingdoms. They usually have active mechanisms of integration, as in bacteriophage lambda and animal retroviruses, and sophisticated mechanisms to maintain a proviral state over decades and generations. Plant para retroviruses, however, neither have an integrase, nor genes for maintaining the proviral state. How are those elements controlled, and under what conditions can they be activated? Here we study the proviral state of endogenous petunia vein clearing virus (ePVCV). Our results support the hypothesis that the proviral state is associated with a host silencing mechanism manifested by DNA methylation, chromatin modification and production of small interfering (si) RNAs. PVCV may be induced by applying abiotic stress, leading to the development of viral symptoms and increased transcript and siRNA levels. Similar levels of ePVCV DNA methylation were observed in two different lines of Petunia hybrida, RdC (rose du ciel) and W138, the latter known for its active version of transposon dTph1. In contrast, significant differences in histone modification were detected. The predominant association of ePVCV sequences with histone H3 methylated at lysine 9 (H3mK9) in RdC and with about equal amounts of H3mK9 and H3mK4 in W138 indicates a less repressive proviral state in the latter cultivar.

Detection and characterization of tobacco mild green mosaic virus (TMGMV) large type isolate from trailing petunia in France

During 2003 and 2004, unusual viral symptoms were observed on Surfinia trailing petunias in protected cultivations of Southern France. Symptoms consisted in yellow mosaic and distortion of the leaves accompanied by vein necrosis in some samples. The flowers were deformed and showed light colour break of the petals. Electron microscope observation of negatively stained leaf-dip from symptomatic leaves showed straight rod-shaped virus particles of about 300 nm in length. Sap extracts reacted in double-immunodiffusion tests by forming weak precipitin bands with antisera against Tomato mosaic virus (ToMV) and Tobacco mild green mosaic virus (TMGMV). However, symptoms developed on host range after mechanical inoculation suggested that ToMV was not involved in the disease. By using specific primer pairs designed to amplify the coat protein (CP) genes of ToMV and TMGMV in reverse transcriptase-polymerase chain reaction (RT-PCR), expected amplicon was obtained only with TMGMV primer pair. The identity of the virus was also confirmed by using a specific TMGMV riboprobe in dot-blot hybridization assays of symptomatic leaf extracts. The nucleotide sequence of TMGMV CP of the isolate from trailing petunia, named TMGMV-Pt, was determined and compared with those available from EMBL. The percentage of nucleotide identity was 97-98% compared with those of other isolates. Further molecular and biological characterization revealed that TMGMV-Pt belonging to the large type group of TMGMV isolates. In fact, the 3' UTR region of TMGMV-Pt consisted of 360 nucleotides, comprising of a 147 base repeat, as reported only for TMGMV large type isolates. Moreover, symptoms development observed on a differentially host range, used to distinguish between large type and small type isolates, confirmed that TMGMV-Pt belonging to the large type group of isolates. Only one commercial variety of trailing petunia out of 12 tested remained symptomless after mechanical inoculation with TMGMV-Pt. This highlights the potential risk that TMGMV could represent to petunia cultivations. To our knowledge this is the first report of a natural infection by TMGMV in trailing petunia.

Induction of infectious petunia vein clearing (pararetro) virus from endogenous provirus in petunia

Infection by an endogenous pararetrovirus using forms of both episomal and chromosomal origin has been demonstrated and characterized, together with evidence that petunia vein clearing virus (PVCV) is a constituent of the Petunia hybrida genome. Our findings allow comparative and direct analysis of horizontally and vertically transmitted virus forms and demonstrate their infectivity using biolistic transformation of a provirus-free petunia species. Some integrants within the genome of P.hybrida are arranged in tandem, allowing direct release of virus by transcription. In addition to known inducers of endogenous pararetroviruses, such as genome hybridization, tissue culture and abiotic stresses, we observed activation of PVCV after wounding. Our data also support the hypothesis that the host plant uses DNA methylation to control the endogenous pararetrovirus.

Detection of petunia vein-clearing virus: model for the detection of DNA viruses in plants with homologous endogenous pararetrovirus sequences

A number of cases of plant virus sequence integration into host plant genome have been reported. In at least two cases, endogenous pararetrovirus sequences are correlated strongly with subsequent episomal virus infection and there is circumstantial evidence that this also occurs for Petunia vein-clearing virus (PVCV). The detection of viruses is a critical component of plant health and therefore, it is important to have diagnostic procedures that differentiate between the detection of encapsidated viral DNA and homologous sequences in the host genome. PCR-based detection methods targeted at PVCV DNA have been tested and particular attention was paid to design controls that would indicate the existence of host DNA in the reaction. The use of ion-exchange chromatography for the partial purification of plant viruses from other cellular components, including chromosomal DNA, is described. The methods tested for PVCV detection are used to illustrate general principles for the specific detection of virus infections in host plants that carry homologous virus sequences in their genomes.

Viral sequences integrated into plant genomes

Sequences of various DNA plant viruses have been found integrated into the host genome. There are two forms of integrant, those that can form episomal viral infections and those that cannot. Integrants of three pararetroviruses, Banana streak virus (BSV), Tobacco vein clearing virus (TVCV), and Petunia vein clearing virus (PVCV), can generate episomal infections in certain hybrid plant hosts in response to stress. In the case of BSV and TVCV, one of the parents contains the integrant but is has not been seen to be activated in that parent; the other parent does not contain the integrant. The number of integrant loci is low for BSV and PVCV and high in TVCV. The structure of the integrants is complex, and it is thought that episomal virus is released by recombination and/or reverse transcription. Geminiviral and pararetroviral sequences are found in plant genomes although not so far associated with a virus disease. It appears that integration of viral sequences is widespread in the plant kingdom and has been occurring for a long period of time.

Viral sequences integrated into plant genomes

Sequences of various DNA plant viruses have been found integrated into the host genome. There are two forms of integrant, those that can form episomal viral infections and those that cannot. Integrants of three pararetroviruses, Banana streak virus (BSV), Tobacco vein clearing virus (TVCV), and Petunia vein clearing virus (PVCV), can generate episomal infections in certain hybrid plant hosts in response to stress. In the case of BSV and TVCV, one of the parents contains the integrant but is has not been seen to be activated in that parent; the other parent does not contain the integrant. The number of integrant loci is low for BSV and PVCV and high in TVCV. The structure of the integrants is complex, and it is thought that episomal virus is released by recombination and/or reverse transcription. Geminiviral and pararetroviral sequences are found in plant genomes although not so far associated with a virus disease. It appears that integration of viral sequences is widespread in the plant kingdom and has been occurring for a long period of time.