Biolistic inoculation of plants with Tomato yellow leaf curl virus DNA

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.

Quantification of a legume begomovirus to evaluate soybean genotypes for resistance to yellow mosaic disease

Mungbean yellow mosaic India virus (MYMIV) infecting soybean and other legumes causes yellow mosaic disease (YMD). Evaluation of soybean genotypes for YMD resistance involves field screening at disease hot spots or in a protected environment using infectious clones or viruliferous whiteflies as sources of virus inocula. Development of efficient virus inoculation and quantification protocols to screen soybean genetic stocks against YMD is imperative for breeding resistant varieties. Binary plasmids harbouring complete, tandem dimeric genomic components DNA A and DNA B of MYMIV-soybean isolate were engineered. The infectivity of the clones was demonstrated in soybean genotypes JS335 and UPSM534 that display contrasting YMD resistance. As a follow-up, soybean germplasm lines, breeding lines, and representative cultivars that were initially screened at an YMD hot-spot were then subjected to Agrobacterium-based infection with MYMIV. Quantitative real time polymerase chain reaction (qRT-PCR) based copy number analysis of MYMIV genomic components allowed soybean genotypes to be classified into three discrete categories; resistant, moderately resistant and susceptible to the viral infection. Thus, a soybean germplasm disease screening system based on agro-infection and qRT-PCR based quantification of MYMIV was developed to facilitate breeding YMD resistant soybean. The implications of this study for obtaining YMD resistant soybean cultivars are discussed.

Infectivity of cloned begomoviral DNAs: an appraisal

Infectivity of cloned begomoviral DNAs is an important criterion to establish the etiology of the disease it causes, to study viral gene functions and host-virus interactions. Three main methods have been employed to study infectivity; mechanical inoculation with cloned viral DNA using abrasives, Agrobacterium-mediated inoculation (agroinoculation) of cloned viral DNA and bombardment using microprojectiles coated with cloned viral DNA (biolistics). Each method has its own advantages and disadvantages and the adoption of one over the other for demonstrating infectivity depends on various factors. This review compares the various features associated with the above three methods.

Tomato Apical Leaf Curl Virus: A Novel, Monopartite Geminivirus Detected in Tomatoes in Argentina

Plant viruses that are members of the Geminiviridae family have circular single-stranded DNA (ssDNA) genome and are responsible for major crop diseases worldwide. We have identified and characterized a novel monopartite geminivirus infecting tomato in Argentina. The full-length genome was cloned and sequenced. The genome-wide pairwise identity calculation that resulted in a maximum of 63% identity with all of other known geminiviruses indicated that it is a new geminivirus species. Biolistic infected plants presented interveinal yellowing, apical leaf curling and extreme root hypotrophy. Thus, the name proposed for this species is tomato apical leaf curl virus (ToALCV). The phylogenetic inferences suggested different evolutionary relationships for the replication-associated protein (Rep) and the coat protein (CP). Besides, the sequence similarity network (SSN) protein analyses showed that the complementary-sense gene products (RepA, Rep and C3) are similar to capulavirus while the viron-sense gene products (CP, MP and V3) are similar to topocuvirus, curtovirus and becurtovirus. Based on the data presented, ToALCV genome appears to have “modular organization” supported by its recombination origin. Analyses of the specificity-determining positions (SDPs) of the CP of geminiviruses defined nine subgroups that include geminiviruses that share the same type of insect vector. Our sequences were clustered with the sequences of topocuvirus, whose vector is the treehopper, Micrutalis malleifera. Also, a set of the highest scored amino acid residues was predicted for the CP, which could determine differences in virus transmission specificity. We predict that a treehopper could be the vector of ToALCV, but transmission assays need to be performed to confirm this. Given everything we demonstrate in this paper, ToALCV can be considered a type member of a new putative genus of the Geminiviridae family.

TYLCV-Is movement in planta does not require V2 protein

Tomato yellow leaf curl virus (TYLCV), a major tomato pathogen causing extensive crop losses, is a whitefly-transmitted geminivirus. V2 mutants of TYLCV-Is and related viruses tend to induce symptomless infection with attenuated viral DNA levels, while accumulating close to wild-type DNA levels in protoplasts, suggesting V2 as a movement protein. The discovery of plant-silencing mechanisms and viral silencing suppressors, V2 included, led us to reconsider V2׳s involvement in viral movement. We studied two mutant versions of the virus, one impaired in V2 silencing-suppression activity, and another carrying a non-translatable V2. While both mutant viruses spread in the infected plant to newly emerged leaves at the same rate as the wild-type virus, their DNA-accumulation levels were tenfold lower than in the wild-type virus. Thus, we suggest that the setback in virus proliferation, previously ascribed to a movement impediment, is due to lack of silencing-suppression activity.

Molecular characterization of watermelon chlorotic stunt virus (WmCSV) from Palestine

The incidence of watermelon chlorotic stunt disease and molecular characterization of the Palestinian isolate of Watermelon chlorotic stunt virus (WmCSV-[PAL]) are described in this study. Symptomatic leaf samples obtained from watermelon Citrullus lanatus (Thunb.), and cucumber (Cucumis sativus L.) plants were tested for WmCSV-[PAL] infection by polymerase chain reaction (PCR) and Rolling Circle Amplification (RCA). Disease incidence ranged between 25%-98% in watermelon fields in the studied area, 77% of leaf samples collected from Jenin were found to be mixed infected with WmCSV-[PAL] and SLCV. The full-length DNA-A and DNA-B genomes of WmCSV-[PAL] were amplified and sequenced, and the sequences were deposited in the GenBank. Sequence analysis of virus genomes showed that DNA-A and DNA-B had 97.6%-99.42% and 93.16%-98.26% nucleotide identity with other virus isolates in the region, respectively. Sequence analysis also revealed that the Palestinian isolate of WmCSV shared the highest nucleotide identity with an isolate from Israel suggesting that the virus was introduced to Palestine from Israel.

Molecular Evidence for the Occurrence of Abutilon mosaic virus, A New World Begomovirus in India

During an investigation in the year 2010, on the weed reservoir of begomovirus, Abutilon pictum showing bright yellow mosaic symptoms was observed in Udhagamandalam, Tamil Nadu, India. The complete bipartite genome of a begomovirus was cloned and sequenced which revealed association of Abutilon mosaic virus (AbMV). Nicotiana benthamiana plants inoculated biolistically with the concatemers generated through rolling circle amplification of the cloned DNAs were asymptomatic; however three out of nine plants showed presence of viral DNA A. A recombination event in the ORF BC1 with ToLCNDV DNA B (HM989846) was detected. This is the first molecular evidence of AbMV in India.

HandGun-mediated inoculation of plants with viral pathogens for mechanistic studies

Particle bombardment is an efficient method for virus inoculation of intact plants. This technique enables inoculation with full-length infectious clone cDNA, PCR products, virus from sap or virus preparation, and in vitro viral transcripts. The inoculation of some phloem-limited RNA and circular DNA viruses is also possible. The technique of bombardment without the use of vacuum permits the inoculation of soft-leaved plants that do not usually survive bombardment inoculation, the investigation of viral recombination in planta, promoter analysis, monitoring virus movement using an infectious clone bearing a reporter gene and the inoculation of large numbers of plants. The inoculation of whitefly-borne circular DNA begomoviruses is now possible due to direct genome amplification by Rolling Circle Amplification (RCA), followed by bombardment using a device that does not require a vacuum for operation. Here we describe the inoculation of intact plants with (a) RNA virus infective clones and (b) begomoviruses after direct genome amplification by RCA, using a handheld bombardment device.

Predominance of tomato leaf curl Gujarat virus as a monopartite begomovirus: association with tomato yellow leaf curl Thailand betasatellite

Tomato leaf curl is a serious malady in the state of Maharashtra, India, causing nearly 100 % yield loss. An extensive survey was done in the affected fields of tomato in the year 2008, and members of three species of begomoviruses were identified as causing the disease. More than 60 % of the samples from diseased plants were infected with tomato leaf curl Gujarat virus (ToLCGuV). Isolates collected from these fields differed from the Varanasi isolate of ToLCGuV in not having a DNA B component. Instead, they were like typical Old World monopartite begomoviruses in that they were associated with only one betasatellite, tomato yellow leaf curl Thailand betasatellite (TYLCTHB). ToLCGuV alone is readily infectious, expressing systemic symptoms in Nicotiana benthamiana and tomato. Co-inoculation of ToLCGuV with TYLCTHB, increased symptom severity and reduced the incubation time required for symptom expression. ToLCGuV successfully interacted with heterologous DNA B component of ToLCNDV [IN:Pun:JID:08], and co-inoculation of these two resulted in yellow mottling symptoms that were typical of DNA B.

Sweepoviruses cause disease in sweet potato and related Ipomoea spp.: fulfilling Koch's postulates for a divergent group in the genus begomovirus

Sweet potato (Ipomoea batatas) and related Ipomoea species are frequently infected by monopartite begomoviruses (genus Begomovirus, family Geminiviridae), known as sweepoviruses. Unlike other geminiviruses, the genomes of sweepoviruses have been recalcitrant to rendering infectious clones to date. Thus, Koch's postulates have not been fullfilled for any of the viruses in this group. Three novel species of sweepoviruses have recently been described in Spain: Sweet potato leaf curl Lanzarote virus (SPLCLaV), Sweet potato leaf curl Spain virus (SPLCSV) and Sweet potato leaf curl Canary virus (SPLCCaV). Here we describe the generation of the first infectious clone of an isolate (ES:MAL:BG30:06) of SPLCLaV. The clone consisted of a complete tandem dimeric viral genome in a binary vector. Successful infection by agroinoculation of several species of Ipomoea (including sweet potato) and Nicotiana benthamiana was confirmed by PCR, dot blot and Southern blot hybridization. Symptoms observed in infected plants consisted of leaf curl, yellowing, growth reduction and vein yellowing. Two varieties of sweet potato, 'Beauregard' and 'Promesa', were infected by agroinoculation, and symptoms of leaf curl and interveinal loss of purple colouration were observed, respectively. The virus present in agroinfected plants was readily transmitted by the whitefly Bemisia tabaci to I. setosa plants. The progeny virus population present in agroinfected I. setosa and sweet potato plants was isolated and identity to the original isolate was confirmed by sequencing. Therefore, Koch's postulates were fulfilled for the first time for a sweepovirus.

Watermelon chlorotic stunt virus (WmCSV): a serious disease threatening watermelon production in Jordan

The incidence of watermelon chlorotic stunt disease and the molecular characterization of the Jordanian isolate of Watermelon chlorotic stunt virus (WmCSV-[JO]) are described in this study. Symptomatic leaf samples obtained from watermelon (Citrullus lanatus Thunb.), melon (Cucumis melo L.), squash (Cucurbita pepo), cucumber (Cucumis sativus L.), and bottle gourd (Lagenaria siceraria) plants were tested for WmCSV-[JO] infection by PCR. The virus could be detected in 8 melon and 87 watermelon samples obtained from Ghor Assafi (southern part of Jordan Valley). Three samples collected from Mafraq (eastern part of Jordan) were found mixed infected with WmCSV-[JO] and Squash leaf curl virus. The full-length DNA-A and DNA-B genomes of WmCSV-[JO] were amplified, and sequences were deposited in the GenBank under accession numbers EU561237 and EU561236, respectively. Sequence analysis reveals that WmCSV-[JO] is closely related to other virus isolates from Israel (WmCSV-[IL]), Yemen (WmCSV-[YE]), Iran (WmCSV-[IR]), Lebanon (WmCSV-[LB]), and Sudan (WmCSV-[SD]). DNA-A of WmCSV-[JO] showed highest nucleotide identity (99.42%) with WmCSV-[IL], while DNA-B had highest nucleotide identity (95.52%) with WmCSV-[YE]. Data of this study demonstrate that digestion of DNA-B genome of WmCSV isolates with ApaI enzyme can discriminate between these isolates at the molecular level. Infectious clones of WmCSV-[JO] were constructed and agroinoculated to Nicotiana benthamiana plants. Inoculated plants developed mild disease symptoms 4 weeks post inoculation, while watermelon plants biolistically inoculated with WmCSV-[JO] developed characteristic mottling, yellowing and severe leaf curling symptoms 3 weeks post inoculation.

Effect of a single amino acid substitution in the NLS domain of Tomato yellow leaf curl virus-Israel (TYLCV-IL) capsid protein (CP) on its activity and on the virus life cycle

The capsid protein (CP) of Tomato yellow leaf curl virus-Israel (TYLCV-IL), encoded by the v1 gene, is the only known component of the viral capsid. Three point mutations introduced into the conserved NLS region of the CP were investigated. One mutant, in which the Arg at position 19 was converted to Leu, had the most significant effect on the CP-CP homotypic interaction as well as on CP's interaction with its nuclear receptor karyopherin α1 and with the protein GroEL. The latter has been suggested to protect the virions in the insect vector hemolymph. These effects were first observed by yeast two-hybrid assay and then confirmed in tobacco protoplasts by measuring fluorescence resonance energy transfer (FRET) between YFP- and CFP-tagged proteins. Most importantly, when the point mutation converting Arg 19 to Leu was introduced into the full-length TYLCV genome, it disrupted its ability to cause symptoms.

Inoculation of plants with begomoviruses by particle bombardment without cloning: Using rolling circle amplification of total DNA from infected plants and whiteflies

A new system for inoculation of plants with begomoviral DNA without cloning or the use insect vectors is described. Total DNA extracted from begomovirus-infected plants was amplified by rolling circle amplification (RCA) using the bacteriophage phi29 DNA polymerase, and inoculated to plants by particle bombardment. Infection rates of up to 100% were obtained using this technique. This technique successfully inoculated all the begomoviruses evaluated: five bipartite (Bean golden yellow mosaic virus, Cabbage leaf curl virus, Squash leaf curl virus, Tomato mottle virus, Watermelon chlorotic stunt virus) as well as one monopartite (Tomato yellow leaf curl virus). The success of the technique was not dependent upon plant species. Four species from three plant families [Phaseolus vulgaris (bean), Solanum lycopersicum (tomato), Cucurbita pepo (squash), and Citrullus lanatus (watermelon)], could all be inoculated by this technique. The success of the method was not dependent upon either the type or the age of the source of virus. Infectious DNA was obtained successfully from fresh, freeze-dried or desiccated plant material, from squashes of plant leaves on FTA cards, as well as from the insect vector. Plant material collected and dried as long as 25 years ago yielded infectious DNA by this method. In summary, this method can be used to obtain infectious DNA of single-stranded circular DNA viruses that can be activated for purposes of completing Koch's postulates, for preservation of pure virus cultures, and for many other applications where infectious DNA is required.

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.

A novel cloning strategy for isolating, genotyping and phenotyping genetic variants of geminiviruses

BACKGROUND

Viruses of the genus Begomovirus (Geminiviridae) are emerging economically important plant viruses with a circular, single-stranded DNA genome. Previous studies have shown that geminiviruses and RNA viruses exhibit similar mutation frequencies, although geminiviruses are replicated by host DNA polymerases and RNA viruses by their own virus-encoded error-prone RNA-dependent RNA-polymerase. However, the phenotypic effects of naturally occurring mutations have never been extensively investigated in geminiviruses, particularly because, to be infectious, cloned viral genomes usually require sub-cloning as complete or partial tandem repeats into a binary vector from Agrobacterium tumefaciens.

RESULTS

Using Tomato yellow leaf curl virus (TYLCV), we show here that infectivity can be obtained when only a 41-nucleotide region containing a highly conserved stem-loop is repeated. A binary vector containing this 41-nt region and a unique restriction site was created, allowing direct cloning of infectious monomeric viral genomes provided that they harbour the same restriction site at the corresponding nucleotide position. This experimental system, which can be transferable to other geminiviruses, was validated by analysis of the phenotypic effect of mutations appearing in TYLCV genomes in a single tomato host plant originally inoculated with a unique viral sequence. Fourteen full-length infectious genomes extracted from this plant were directly cloned and sequenced. The mutation frequency was 1.38 x 10-4 mutation per nucleotide sequenced, similar to that found previously for another begomovirus by sequencing PCR-amplified partial sequences. Interestingly, even in this minimal pool of analysed genomes, mutants with altered properties were readily identified, one of them being fitter and reducing plant biomass more drastically than the parental clone.

CONCLUSION

The cloning strategy presented here is useful for any extensive phenotyping of geminivirus variants and particularly of artificially generated mutants or recombinants.

A simplified method of constructing infectious clones of begomovirus employing limited restriction enzyme digestion of products of rolling circle amplification

Most infectious clones of geminiviruses consist of (partial) tandem repeats of viral genomes in the vectors, which usually involve tedious, multi-step assemblies of genomic fragments in the construction process. A simplified procedure was devised to circumvent these problems, which employs limited restriction digestion of multimeric viral genomes produced by rolling circle amplification (RCA), followed by direct cloning into appropriate vectors. The efficiency of the procedure, and infectivity of the dimeric constructs it produced, were demonstrated using three different geminiviruses, namely ageratum yellow vein virus, tomato leaf curl virus, and squash leaf curl virus.