Cloning biologically active geminivirus DNA using PCR and overlapping primers

Ahmed MA, M. Al-Sadi A. Infectious clone construction and pathogenicity confirmation of Cotton leaf curl Multan virus (CLCuMuV), Ramie mosaic virus (RamV) and Corchorus yellow vein Vietnam virus (CoYVV) by southern blot analysis

Geminiviruses are insect-transmissible, economically vital group of plant viruses, which cause significant losses to crop production and ornamental plants across the world. During this study, infectious clones of three devastating begomoviruses, i.e., Cotton leaf curl Multan virus (CLCuMuV), Ramie mosaic virus (RamV) and Corchorus yellow vein Vietnam virus (CoYVV) were constructed by following novel protocol. All infectious clones were confirmed by cloning and sequencing. All of the infectious clones were agro-inoculated in Agrobacterium. After the agro-infiltrations, all clones were injected into Nicotiana benthamiana and jute plants under controlled condition. After 28 days of inoculation, plants exhibited typical symptoms of their corresponding viruses. All the symptomatic and asymptomatic leaves were collected from inoculated plants for further analysis. The southern blot analysis was used to confirm the infection of studied begomoviruses. At the end, all the products were sequenced and analyzed.

The AC4 Protein of a Cassava Geminivirus Is Required for Virus Infection

Geminiviruses (family Geminiviridae) are among the most devastating plant viruses worldwide, causing severe damage in crops of economic and subsistence importance. These viruses have very compact genomes and many of the encoded proteins are multifunctional. Here, we investigated the role of the East African cassava mosaic Cameroon virus (EACMCV) AC4 on virus infectivity in Nicotiana benthamiana. Results showed that plants inoculated with EACMCV containing a knockout mutation in an AC4 open reading frame displayed symptoms 2 to 3 days later than plants inoculated with wild-type virus, and these plants recovered from infection, whereas plants inoculated with the wild-type virus did not. Curiously, when an additional mutation was made in the knockout mutant, the resulting double mutant virus completely failed to cause any apparent symptoms. Interestingly, the role of AC4 on virus infectivity appeared to be dependent on an encoded N-myristoylation motif that mediates cell membrane binding. We previously showed that EACMCV containing the AC4T38I mutant produced virus progeny characterized by second-site mutations and reversion to wild-type virus. These results were confirmed in this study using additional mutations. Together, these results show involvement of EACMCV AC4 in virus infectivity; they also suggest a role for the combined action of mutation and selection, under prevailing environmental conditions, on begomovirus genetic variation and diversity.

Crystal Structure of the Regulatory Domain of AphB from Vibrio vulnificus, a Virulence Gene Regulator

The transcriptional activator AphB has been implicated in acid resistance and pathogenesis in the food borne pathogens Vibrio vulnificus and Vibrio cholerae. To date, the full-length AphB crystal structure of V. cholerae has been determined and characterized by a tetrameric assembly of AphB consisting of a DNA binding domain and a regulatory domain (RD). Although acidic pH and low oxygen tension might be involved in the activation of AphB, it remains unknown which ligand or stimulus activates AphB at the molecular level. In this study, we determine the crystal structure of the AphB RD from V. vulnificus under aerobic conditions without modification at the conserved cysteine residue of the RD, even in the presence of the oxidizing agent cumene hydroperoxide. A cysteine to serine amino acid residue mutant RD protein further confirmed that the cysteine residue is not involved in sensing oxidative stress in vitro. Interestingly, an unidentified small molecule was observed in the inter-subdomain cavity in the RD when the crystal was incubated with cumene hydroperoxide molecules, suggesting a new ligand-binding site. In addition, we confirmed the role of AphB in acid tolerance by observing an aphB-dependent increase in cadC transcript level when V. vulnificus was exposed to acidic pH. Our study contributes to the understanding of the AphB molecular mechanism in the process of recognizing the host environment.

A Rapid and Efficient Method for Construction of an Infectious Clone of Tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV), a member of the genus Begomovirus, is responsible for one of the most devastating viral diseases in tomato-growing countries and is becoming a serious problem in many subtropical and tropical countries. The climate in Korea is getting warmer and developing subtropical features in response to global warming. These changes are being accompanied by TYLCV, which is now becoming a large problem in the Korean tomato industry. The most effective way to reduce damage caused by TYLCV is to breed resistant varieties of tomatoes. To accomplish this, it is necessary to establish a simple inoculation technique for the efficient evaluation of resistance to TYLCV. Here, we present the rolling circle amplification (RCA) method, which employs a bacteriophage using phi-29 DNA polymerase for construction of infectious TYLCV clones. The RCA method is simple, does not require sequence information for cloning, and is less expensive and time consuming than conventional PCR based-methods. Furthermore, RCA-based construction of an infectious clone can be very useful to other emerging and unknown geminiviruses in Korea.

Characterization and distribution of tomato yellow margin leaf curl virus, a begomovirus from Venezuela

A begomovirus causing mottling and leaf deformation in tomato from the State of Mérida was cloned and sequenced. The virus has a bipartite genome comprised of a DNA-A (2,572 nucleotides) and a DNA-B (2,543 nucleotides) with a genome organization typical of New World begomoviruses. Both components share a common region of 115 nucleotides with 98 % sequence identity. Phylogenetic analysis indicated that while no virus sequences were closely related, the A component was distantly related to those of two other tomato-infecting viruses, tomato leaf deformation virus and Merremia mosaic virus; and the DNA-B, to those of pepper huasteco yellow vein virus and Rhynchosia golden mosaic Yucatan virus. The DNA-A and DNA-B sequences were submitted to GenBank (accession no. AY508993 and AY508994, respectively) and later accepted by the International Committee on Taxonomy of Viruses as the genome of a member of a unique virus species with the name Tomato yellow margin leaf curl virus (TYMLCV). Tomato (Solanum lycopersicum L. 'Fl. Lanai') plants inoculated with cloned TYMLCV DNA-A and DNA-B became systemically infected and showed chlorotic margins and leaf curling. The distribution of TYMLCV in tomato-producing states in Venezuela was determined by nucleic acid spot hybridization analysis of 334 tomato leaf samples collected from ten states using a TYMLCV-specific probe and confirmed by PCR and sequencing of the PCR fragment. TYMLCV was detected in samples from the states of Aragua, Guárico, and Mérida, suggesting that TYMLCV is widely distributed in Venezuela.

Agroinoculation of Nicotiana benthamiana with cloned honeysuckle yellow vein virus isolated from Lonicera japonica

A new honeysuckle yellow vein geminivirus (HYVV) isolate (2,763 nucleotides) and the HYVV-β satellite (1,227 nucleotides) from Korea were cloned and characterized from symptomatic leaves of Lonicera japonica with a yellow net mosaic appearance. Phylogenetic analysis of HYVV and 13 other begomoviruses revealed that HYVV has the highest nucleotide sequence homology to HYVV-UK2. Grafting challenge of a virus-free L. japonica (scion) on an HYVV-infected L. japonica (stock) resulted in the appearance of typical HYVV disease symptoms on the newly developed leaves of the scion. Two pMon521 plasmids containing 1.3 copies and 2.0 copies of the cloned HYVV isolate, respectively, were infectious and produced disease symptoms in Nicotiana benthamiana after agroinoculation. We conclude that the causal agent of yellow net mosaic in L. japonica is a new Korean isolate of HYVV, based on sequence comparisons, agroinoculation-induced disease symptoms, electron microscopy, and phylogenetic analysis.

Genetic diversity in curtoviruses: a highly divergent strain of Beet mild curly top virus associated with an outbreak of curly top disease in pepper in Mexico

A full-length curtovirus genome was PCR-amplified and cloned from peppers in Mexico with symptoms of curly top disease. The cloned DNA of this isolate, MX-P24, replicated in Nicotiana tabacum protoplasts and was infectious in N. benthamiana plants. Sequence analysis revealed that the MX-P24 isolate had a typical curtovirus genome organization and was most similar to beet mild curly top virus (BMCTV). However, sequence identities were at the threshold value for establishment of a new curtovirus species. To further investigate the biological properties of MX-P24, an agroinoculation system was generated. Agroinoculated shepherd's purse plants developed typical curly top symptoms, and virus from these plants was transmissible by the beet leafhopper (Circulifer tenellus). The host range of MX-P24 was similar to that of BMCTV, with curly top symptoms induced in common bean, pepper, pumpkin, shepherd's purse and tomato plants and mild or no symptoms induced in sugar beet plants. Together, these results indicate that MX-P24 is a highly divergent strain of BMCTV associated with an outbreak of curly top disease in peppers in Mexico.

An efficient in vitro-inoculation method for Tomato yellow leaf curl virus

Background

Tomato yellow leaf curl virus (TYLCV) is a member of the family Geminiviridae, genus Begomovirus. To test the infectivity of TYLCV in tomato plants, an improved protocol for inoculation of in vitro-cultured tomato plants was developed.

Results

A TYLCV isolate was cloned, sequenced and used to construct a 1.8-mer infectious clone. Three weeks old microshoots of TYLCV-susceptible tomato plants were inoculated with Agrobacterium tumefaciens harboring the infectious clone for the TYLCV isolate. After two weeks, the TYLCV symptoms started to appear on the in vitro-inoculated plants and the symptoms became more severe and pronounced eight weeks post-inoculation. The method was used efficiently to uncover the resistance mechanism against TYLCV in Solanum habrochaites accession LA 1777, a wild tomato known for its high resistance to whitefly and TYLCV.

Conclusions

The reported in vitro-inoculation method can be used to screen tomato genotypes for their responses to TYLCV under controlled conditions and it will be a useful tool for better understanding of the TYLCV biology in tomato plants.

Soybean cultivar resistant to Mungbean Yellow Mosaic India Virus infection induces viral RNA degradation earlier than the susceptible cultivar

Yellow mosaic disease caused by whitefly-transmitted bipartite Geminiviruses is one of the major constraints on productivity of a number of pulse crops. We have cloned the bipartite genome of Mungbean Yellow Mosaic India Virus isolated from infected Soybean. We report here that agroinfection of Soybean seedlings with a single uncut recombinant binary plasmid containing tandem dimers of both DNA A and DNA B resulted in 100% infectivity in susceptible varieties. To understand the mechanism of natural resistance in a Soybean variety, we compared the abundance of the viral RNAs in a resistant and a susceptible variety at the early time points after agroinfection. Whilst the resistant variety displayed synthesis but rapid degradation of the early viral RNAs; the degradation in the susceptible variety was delayed resulting in accumulation of those transcripts later in infection. Accumulation of the late viral transcripts and DNA replication were detectable only in the susceptible variety. This indicates that rapid degradation of the early viral transcripts, possibly through siRNA mechanism, is one of the probable mechanisms of natural resistance against geminivirus.

A severe symptom phenotype in tomato in Mali is caused by a reassortant between a novel recombinant begomovirus (Tomato yellow leaf curl Mali virus) and a betasatellite

Tomato production in West Africa has been severely affected by begomovirus diseases, including yellow leaf curl and a severe symptom phenotype, characterized by extremely stunted and distorted growth and small deformed leaves. Here, a novel recombinant begomovirus from Mali, Tomato yellow leaf curl Mali virus (TYLCMLV), is described that, alone, causes tomato yellow leaf curl disease or, in combination with a betasatellite, causes the severe symptom phenotype. TYLCMLV is an Old World monopartite begomovirus with a hybrid genome composed of sequences from Tomato yellow leaf curl virus-Mild (TYLCV-Mld) and Hollyhock leaf crumple virus (HoLCrV). A TYLCMLV infectious clone induced leaf curl and yellowing in tomato, leaf curl, crumpling and yellowing in Nicotiana benthamiana and common bean, mild symptoms in N. glutinosa, and a symptomless infection in Datura stramonium. In a field-collected sample from a tomato plant showing the severe symptom phenotype in Mali, TYLCMLV was detected together with a betasatellite, identified as Cotton leaf curl Gezira betasatellite (CLCuGB). Tomato plants co-agroinoculated with TYLCMLV and CLCuGB developed severely stunted and distorted growth and small crumpled leaves. These symptoms were more severe than those induced by TYLCMLV alone, and were similar to the severe symptom phenotype observed in the field in Mali and in other West African countries. TYLCMLV and CLCuGB also induced more severe symptoms than TYLCMLV in the other solanaceous hosts, but not in common bean. The increased symptom severity was associated with hyperplasia of phloem-associated cells, but relatively little increase in TYLCMLV DNA levels. In surveys of tomato virus diseases in West Africa, TYLCMLV was commonly detected in plants with leaf curl and yellow leaf curl symptoms, whereas CLCuGB was infrequently detected and always in association with the severe symptom phenotype. Together, these results indicate that TYLCMLV causes tomato yellow leaf curl disease throughout West Africa, whereas TYLCMLV and CLCuGB represent a reassortant that causes the severe symptom phenotype in tomato.

Biology and Molecular Characterization of Cucurbit leaf crumple virus, an Emergent Cucurbit-Infecting Begomovirus in the Imperial Valley of California

Cucurbit leaf crumple virus (CuLCrV) is an emergent and potentially economically important bipartite begomovirus first identified in volunteer watermelon plants in the Imperial Valley of southern California in 1998. Field surveys indicated that CuLCrV has become established in the Imperial Valley; and field plot studies revealed that CuLCrV primarily infects cucurbits, including cantaloupe, squash, and watermelon. Full-length DNA-A and DNA-B clones of an Imperial Valley isolate of CuLCrV were obtained by polymerase chain reaction (PCR) with overlapping primers. These clones were infectious in various cucurbits and common bean (cv. Topcrop); symptoms included stunted growth and leaf crumple, curl, and chlorosis. CuLCrV was not sap-transmissible, and immunolocalization and DNA in situ hybridization studies revealed that it is phloem-limited. A CuLCrV agroinoculation system was generated, and host range studies revealed differential susceptibility in cucurbits, with squash, watermelon, cantaloupe, and honeydew melon being most to least susceptible, respectively. Germplasm screening studies identified a number of resistant cantaloupe and honeydew melon cultivars. The genome organization of this CuLCrV isolate (CuLCrV-CA) is similar to other bipartite begomoviruses, and phylogenetic analysis placed CuLCrV in the Squash leaf curl virus (SLCV) cluster of New World bipartite begomoviruses. A CuLCrV-specific PCR test was developed which allows for differentiation from other begomoviruses, including SLCV.

Evidence of local evolution of tomato-infecting begomovirus species in West Africa: characterization of tomato leaf curl Mali virus and tomato yellow leaf crumple virus from Mali

Tomato yellow leaf curl (TYLC) and tomato leaf curl (ToLC) diseases are serious constraints to tomato production in Mali and other countries in West Africa. In 2003 and 2004, samples of tomato showing virus-like symptoms were collected during a survey of tomato virus diseases in Mali. Three predominant symptom phenotypes were observed: (1) TYLC/ToLC (stunted upright growth and upcurled leaves with interveinal yellowing and vein purpling), (2) yellow leaf crumple and (3) broccoli or bonsai (severe stunting and distorted growth). Squash blot (SB) hybridization with a general begomovirus probe and/or SB/PCR analyses revealed begomovirus infection in plants with each of these symptom phenotypes and no evidence of phytoplasma infection. Sequence analysis of PCR-amplified begomovirus fragments revealed two putative new begomovirus species associated with the TYLC/ToLC and yellow leaf crumple symptom phenotypes, respectively. Full-length clones of these begomoviruses were obtained using PCR and overlapping primers. When introduced into N. benthamiana and tomato plants, these clones induced upward leaf curling and crumpling (the TYLC/ToLC-associated begomovirus) or downward leaf curl/yellow mottle (yellow leaf crumple-associated begomovirus) symptoms. Thus, these begomoviruses were named tomato leaf curl Mali virus (ToLCMLV) and tomato yellow leaf crumple virus (ToYLCrV). The genome organization of both viruses was similar to those of other monopartite begomoviruses. ToLCMLV and ToYLCrV were most closely related to each other and to tobacco leaf curl Zimbabwe virus (TbLCZV-[ZW]) and tomato curly stunt virus from South Africa (ToCSV-ZA). Thus, these likely represent tomato-infecting begomoviruses that evolved from indigenous begomoviruses on the African continent. Mixed infections of ToLCMLV and ToYLCrV in N. benthamiana and tomato plants resulted in more severe symptoms than in plants infected with either virus alone, suggesting a synergistic interaction. Agroinoculation experiments indicated that both viruses induced symptomatic infections in tomato and tobacco, whereas neither virus induced disease symptoms in pepper, common bean, small sugar pumpkin, African eggplant, or Arabidopsis. Virus-specific PCR primers were developed for detection of ToLCMLV and ToYLCrV and will be used to further investigate the distribution and host range of these viruses.

Geminivirus: biolistic inoculation and molecular diagnosis

The Geminiviridae family is a large family of plant viruses that has single-stranded DNA genomes and infects a large variety of crop species. In this chapter, we describe a biolistic inoculation protocol that has been successfully used to propagate new species of geminivirus in permissive hosts with total DNA extracted from infected plants. This allows us to directly investigate the biological properties of uncloned and not sap-transmissible geminiviruses.

Begomoviruses: molecular cloning and identification of replication origin

The Begomovirus genus is the largest genus of the Geminiviridae family and comprises the whitefly transmitted geminiviruses that infect dicotyledonous plants. They can be either mono or bipartite. In this chapter, we describe the cloning of begomovirus replication modules and the subsequent functional characterization of geminivirus replication origins.

Evaluation of Cotton Germ Plasm for Resistance to the Whitefly and Cotton Leaf Crumple (CLCr) Disease and Etiology of CLCr in California's Imperial Valley

Cotton (Gossypium hirsutum) entries were evaluated for resistance to the whitefly (Bemisia tabaci biotype B) and cotton leaf crumple (CLCr) disease during the 1999 to 2001 growing seasons in the Imperial Valley of California. Entries were evaluated for densities of whitefly adults and nymphs, and for CLCr, by visual rating and squash/dot blot hybridization analyses. Differences in whitefly densities were detected among entries, but none were highly resistant, nor was there any correlation with CLCr disease severity. Entries AP 4103 and AP 6101 had relatively low whitefly densities and were highly susceptible (high CLCr disease severity ratings and viral titers), whereas NK 2387C and DPX 1883 also had low whitefly densities but were highly resistant (no symptoms or detectable viral titers). Other entries showed moderate CLCr resistance, which was independent of whitefly density. Geminivirus DNA-A and DNA-B components were consistently detected in cotton leaves with CLCr symptoms by polymerase chain reaction (PCR) with degenerate begomovirus primers, and full-length DNA-A and DNA-B clones were obtained. Cotton seedlings inoculated with these cloned DNAs by particle bombardment developed CLCr symptoms, and progeny virus was whitefly-transmissible. Sequence analysis revealed that these clones comprised the genome of a California isolate of the bipartite begomovirus Cotton leaf crumple virus (CLCrV-CA). Thus, CLCr disease in the Imperial Valley is caused by CLCrV-CA, and cotton entries with high levels of resistance were identified.

Identification of regions of the Beet mild curly top virus (family Geminiviridae) capsid protein involved in systemic infection, virion formation and leafhopper transmission

Plant viruses in the genus Curtovirus (family Geminiviridae) are vectored by the beet leafhopper (Circulifer tenellus) and cause curly top disease in a wide range of dicotyledonous plants. An infectious clone of an isolate of Beet mild curly top virus (BMCTV-[W4]), associated with an outbreak of curly top in pepper and tomato crops, was characterized and used to investigate the role of the capsid protein (CP) in viral biology and pathogenesis. Frameshift mutations were introduced into the overlapping CP and V2 genes, and a series of CP alanine scanning mutations were generated. All mutants replicated in tobacco protoplasts or systemically infected plants, consistent with these gene products not being required for viral DNA replication. The CP frameshift mutant and most C-terminal alanine scanning mutants did not systemically infect Nicotiana benthamiana plants or form detectable virions, and were not leafhopper-transmitted. In contrast, most N-terminal alanine scanning mutants systemically infected N. benthamiana and induced disease symptoms, formed virions and were leafhopper-transmissible; thus, these substitution mutations did not significantly alter the functional properties of this region. One N-terminal mutant (CP49-51) systemically infected N. benthamiana, but did not form detectable virions; whereas another (CP25-28) systemically infected N. benthamiana and formed virions, but was not insect-transmissible. These mutants may reveal regions involved in virus movement through the plant and/or leafhopper vector. Together, these results indicate an important role for virions in systemic infection (long-distance movement) and insect transmission, and strongly suggest that virions are the form in which BMCTV moves, long distance, in the phloem.

Pepper (Capsicum annuum) Is a Dead-End Host for Tomato yellow leaf curl virus

ABSTRACT Tomato yellow leaf curl (TYLC) is one of the most devastating pathogens affecting tomato (Lycopersicon esculentum) worldwide. The disease is caused by a complex of begomovirus species, two of which, Tomato yellow leaf curl Sardinia virus (TYLCSV) and Tomato yellow leaf curl virus (TYLCV), are responsible for epidemics in Southern Spain. TYLCV also has been reported to cause severe damage to common bean (Phaseolus vulgaris) crops. Pepper (Capsicum annuum) plants collected from commercial crops were found to be infected by isolates of two TYLCV strains: TYLCV-Mld[ES01/99], an isolate of the mild strain similar to other TYLCVs isolated from tomato crops in Spain, and TYLCV-[Alm], an isolate of the more virulent TYLCV type strain, not previously reported in the Iberian Peninsula. In this work, pepper, Nicotiana benthamiana, common bean, and tomato were tested for susceptibility to TYLCV-Mld[ES01/99]and TYLCV-[Alm] by Agrobacterium tumefaciens infiltration, biolistic bombardment, or Bemisia tabaci inoculation. Results indicate that both strains are able to infect plants of these species, including pepper. This is the first time that infection of pepper plants with TYLCV clones has been shown. Implications of pepper infection for the epidemiology of TYLCV are discussed.

South West Indian Ocean islands tomato begomovirus populations represent a new major monopartite begomovirus group

Biological and molecular properties of Tomato leaf curl Madagascar virus isolates from Morondova and Toliary (ToLCMGV-[Tol], -[Mor]), Tomato leaf curl Mayotte virus isolates from Dembeni and Kahani (ToLCYTV-[Dem], -[Kah]) and a Tomato yellow leaf curl virus isolate from Reunion (TYLCV-Mld[RE]) were determined. Full-length DNA components of the five isolates from Madagascar, Mayotte and Reunion were cloned and sequenced and, with the exception of ToLCMGV-[Tol], were shown to be both infectious in tomato and transmissible by Bemisia tabaci. Sequence analysis revealed that these viruses had genome organizations of monopartite begomoviruses and that both ToLCMGV and ToLCYTV belong to the African begomoviruses but represent a distinct monophyletic group that we have tentatively named the South West islands of the Indian Ocean (SWIO). All of the SWIO isolates examined were apparently complex recombinants. None of the sequences within the recombinant regions closely resembled that of any known non-SWIO begomovirus, suggesting an isolation of these virus populations.

A simple method for cloning the complete begomovirus genome using the bacteriophage phi29 DNA polymerase

The bacteriophage phiDNA polymerase amplifies circular DNA in a rolling circle amplification mechanism. This characteristic was applied to amplify and clone the complete circular DNA genome of a begomovirus. Total DNA extracted from infected tissue was used as the template of an amplification reaction using the commercial kit TempliPhi (Amersham Biosciences). The amplified DNA could be used for direct sequencing and was cloned after digestion with a single cutting restriction endonuclease. The use of this enzyme simplified the cloning steps and increased the cloning efficiency of the complete genome of a circular plant DNA virus.

Tomato yellow leaf curl virus in the Dominican Republic: Characterization of an Infectious Clone, Virus Monitoring in Whiteflies, and Identification of Reservoir Hosts

ABSTRACT Epidemics of tomato yellow leaf curl disease (TYLCD) in the Dominican Republic in the early to mid-1990s resulted in catastrophic losses to processing tomato production. As part of an integrated management approach to TYLCD, the complete nucleotide sequence of a full-length infectious clone of an isolate of Tomato yellow leaf curl virus (TYLCV) from the Dominican Republic (TYLCV-[DO]) was determined. The TYLCV-[DO] genome was nearly identical in sequence (>97%) and genome organization to TYLCV isolates from Israel and Cuba. This established that TYLCV-[DO] is a bonafide TYLCV isolate (rather than a recombinant virus, such as isolates from Israel [Mild], Portugal, Japan, and Iran), and provided further evidence for the introduction of the virus from the eastern Mediterranean. A reduction in the incidence of TYLCV in the northern and southern processing tomato production areas of the Dominican Republic has been associated with the implementation of a mandatory 3-month whitefly host-free period (including tomato, common bean, cucurbits, eggplant, and pepper). Monitoring TYLCV levels in whiteflies, by polymerase chain reaction with TYLCV-specific primers, established that the incidence of TYLCV decreased markedly during the host-free period, and then gradually increased during the tomato-growing season. In contrast, TYLCV persisted in whiteflies and tomato plants in an area in which the host-free period was not implemented. Surveys for TYLCV reservoir hosts, conducted to identify where TYLCV persists during the host-free period, revealed symptomless infections in a number of weed species. The implications of these findings for TYLCV management in the Dominican Republic are discussed.

Analysis of the sequence of dioscorea Alata bacilliform virus: comparison to others members of the badnavirus group

The complete nucleotide sequence of the genome of Dioscorea alata bacilliform virus (DaBV) has been determined from cloned fragments. Features of the genome confirm DaBV to be a pararetrovirus of the genus Badnavirus which is more similar to other mealy-bug transmitted badnaviruses, in particular to cacao swollen shoot virus, than to rice tungro bacilliform virus. Sequence variability between cloned fragments suggests that the genetic variability of the virus may be quite high (up to 11% nucleotide sequence variation for some small regions of the genome) although the overall variability detected was 4.2% at the nucleotide level.

Increased pathogenicity in a pseudorecombinant bipartite geminivirus correlates with intermolecular recombination

Most whitefly-transmitted geminiviruses possess bipartite DNA genomes, and this feature may facilitate viral evolution through pseudorecombination and/or recombination. To test this hypothesis, the DNA-A and DNA-B components of the geminiviruses bean dwarf mosaic virus (BDMV) and tomato mottle virus (ToMoV) were exchanged, and the resultant pseudorecombinants were serially passaged through plants. Both pseudorecombinants were infectious in Nicotiana benthamiana but induced attenuated symptoms and had reduced DNA-B levels. Serial passage experiments revealed that the BDMV DNA-A plus ToMoV DNA-B pseudorecombinant could not be maintained beyond three passages. In contrast, the ToMoV DNA-A plus BDMV DNA-B pseudorecombinant was maintained during serial passage through N. benthamiana and Phaseolus vulgaris and, after three to five passages, became highly pathogenic. Furthermore, the increased pathogenicity of this pseudorecombinant was consistently associated with an increased level of DNA-B, which eventuated in equivalent levels of both components. Sequence analysis of the DNA-B component of the more pathogenic pseudorecombinant revealed that intermolecular recombination had taken place in which most of the BDMV DNA-B common region was replaced with the ToMoV DNA-A common region. This recombinant DNA-B component, which contained the ToMoV origin of replication, was the predominant DNA-B component associated with the more pathogenic pseudorecombinant. These results provide the first demonstration of recombination between distinct bipartite geminiviruses and establish that the bipartite genome can facilitate viral evolution through pseudorecombination and intermolecular recombination.

Two proteins of a plant DNA virus coordinate nuclear and plasmodesmal transport

Plant viruses establish a systemic infection by moving through plasmodesmata, but little is known of the mechanism(s) involved. The roles of two movement-associated proteins of a single-stranded DNA virus were investigated in vivo, using functional proteins expressed in E. coli and microinjection into plant cells. We report here that the BL1 protein of bean dwarf mosaic geminivirus moves extensively from cell to cell, increases mesophyll plasmodesmal size exclusion limit, and potentiates the movement of double-stranded DNA from cell to cell. Movement of single- and double-stranded DNA out of the nucleus is mediated by the BR1 protein. These results provide direct experimental evidence for intercellular macro-molecular transport in plants, and suggest that the BR1 and BL1 proteins coordinate the movement of viral DNA across both nuclear and plasmodesmal boundaries.