Phenotypic variation in transgenic tobacco expressing mutated geminivirus movement/pathogenicity (BC1) proteins

Synergy between an emerging monopartite begomovirus and a DNA-B component

In recent decades, a legion of monopartite begomoviruses transmitted by the whitefly Bemisia tabaci has emerged as serious threats to vegetable crops in Africa. Recent studies in Burkina Faso (West Africa) reported the predominance of pepper yellow vein Mali virus (PepYVMLV) and its frequent association with a previously unknown DNA-B component. To understand the role of this DNA-B component in the emergence of PepYVMLV, we assessed biological traits related to virulence, virus accumulation, location in the tissue and transmission. We demonstrate that the DNA-B component is not required for systemic movement and symptom development of PepYVMLV (non-strict association), but that its association produces more severe symptoms including growth arrest and plant death. The increased virulence is associated with a higher viral DNA accumulation in plant tissues, an increase in the number of contaminated nuclei of the phloem parenchyma and in the transmission rate by B. tabaci. Our results suggest that the association of a DNA-B component with the otherwise monopartite PepYVMLV is a key factor of its emergence.

Interaction between βC1 of satellite and coat protein of Chili leaf curl virus plays a crucial role in suppression of host RNA silencing

The monopartite Chili leaf curl virus (ChiLCV) and its β-satellite (ChiLCB) have been found to co-exist in infected plants. The ability of βC1 protein to suppress RNA silencing was investigated using an in-house developed in-planta reversal of silencing assay, using Nicotiana tabacum lines harboring green fluorescent protein (GFP) silenced by short hairpin GFP (ShGFP). Transient expression of recombinant βC1 complemented and increased the suppressor activity of ChiLCV coat protein (CP), and this was confirmed by molecular analysis. In silico analysis followed by a yeast two-hybrid screen-identified ChiLCV-CP as the interacting partner of the ChiLCB-βC1 protein. Subcellular localization through confocal analysis revealed that when βC1 and ChiLCV-CP were co-present, the fluorescence was localized in the cytoplasm indicating that nuclear localization of both proteins was obstructed. The cytoplasmic compartmentalization of the two viral suppressors of RNA silencing may be responsible for the enhanced suppression of the host gene silencing. This study presents evidence on the interaction of ChiLCV-CP and βC1 proteins and indicates that ChiLCB may support the ChiLCV in overcoming host gene silencing to cause Chili leaf curl disease. KEY POINTS: • CP of ChiLCV and βC1 of ChiLCB contain RNA silencing suppression activity • The RNA silencing suppression activity of ChiLCB-βC1 complements that of ChiLCV-CP • There is a direct interaction between ChiLCB-βC1 and ChiLCV-CP.

Phosphorylations of the Abutilon Mosaic Virus Movement Protein Affect Its Self-Interaction, Symptom Development, Viral DNA Accumulation, and Host Range

The genome of bipartite geminiviruses in the genus Begomovirus comprises two circular DNAs: DNA-A and DNA-B. The DNA-B component encodes a nuclear shuttle protein (NSP) and a movement protein (MP), which cooperate for systemic spread of infectious nucleic acids within host plants and affect pathogenicity. MP mediates multiple functions during intra- and intercellular trafficking, such as binding of viral nucleoprotein complexes, targeting to and modification of plasmodesmata, and release of the cargo after cell-to-cell transfer. For Abutilon mosaic virus (AbMV), phosphorylation of MP expressed in bacteria, yeast, and Nicotiana benthamiana plants, respectively, has been demonstrated in previous studies. Three phosphorylation sites (T221, S223, and S250) were identified in its C-terminal oligomerization domain by mass spectrometry, suggesting a regulation of MP by posttranslational modification. To examine the influence of the three sites on the self-interaction in more detail, MP mutants were tested for their interaction in yeast by two-hybrid assays, or by Förster resonance energy transfer (FRET) techniques in planta. Expression constructs with point mutations leading to simultaneous (triple) exchange of T221, S223, and S250 to either uncharged alanine (MP^AAA), or phosphorylation charge-mimicking aspartate residues (MP^DDD) were compared. MP^DDD interfered with MP-MP binding in contrast to MP^AAA. The roles of the phosphorylation sites for the viral life cycle were studied further, using plant-infectious AbMV DNA-B variants with the same triple mutants each. When co-inoculated with wild-type DNA-A, both mutants infected N. benthamiana plants systemically, but were unable to do so for some other plant species of the families Solanaceae or Malvaceae. Systemically infected plants developed symptoms and viral DNA levels different from those of wild-type AbMV for most virus-plant combinations. The results indicate a regulation of diverse MP functions by posttranslational modifications and underscore their biological relevance for a complex host plant-geminivirus interaction.

The Search for Resistance to Cassava Mosaic Geminiviruses: How Much We Have Accomplished, and What Lies Ahead

The cassava mosaic disease (CMD), which occurs in all cassava growing regions of Africa and the Indian subcontinent, is caused by cassava mosaic geminiviruses (CMGs). CMGs are considered to be the most damaging vector-borne plant pathogens. So far, the most successful approach used to control these viruses has been the transfer of a polygenic recessive resistance locus, designated CMD1, from wild cassava to cassava cultivars. Further progress in harnessing natural resistance to contain CMGs has come from the discovery of the dominant monogenic resistance locus, CMD2, in some West African cassava cultivars. CMD2 has been combined with CMD1 through genetic crosses. Because of the limitations of the cassava breeding approach, especially with regard to time required to produce a variety and the loss of preferred agronomic attributes, efforts have been directed toward the deployment of genetic engineering approaches. Most of these approaches have been centered on RNA silencing strategies, developed mainly in the model plant Nicotiana benthamiana. Early RNA silencing platforms assessed for CMG resistance have been use of viral genes for co-suppression, antisense suppression or for hairpin RNAs-mediated gene silencing. Here, progress and challenges in the deployment of these approaches in the control of CMGs are discussed. Novel functional genomics approaches with potential to overcome some of the drawbacks of the current strategies are also discussed.

The Agrobacterium tumefaciens Ti Plasmid Virulence Gene virE2 Reduces Sri Lankan Cassava Mosaic Virus Infection in Transgenic Nicotiana benthamiana Plants

Cassava mosaic disease is a major constraint to cassava cultivation worldwide. In India, the disease is caused by Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV). The Agrobacterium Ti plasmid virulence gene virE2, encoding a nuclear-localized, single-stranded DNA binding protein, was introduced into Nicotiana benthamiana to develop tolerance against SLCMV. Leaf discs of transgenic N. benthamiana plants, harboring the virE2 gene, complemented a virE2 mutation in A. tumefaciens and produced tumours. Three tested virE2 transgenic plants displayed reduction in disease symptoms upon agroinoculation with SLCMV DNA A and DNA B partial dimers. A pronounced reduction in viral DNA accumulation was observed in all three virE2 transgenic plants. Thus, virE2 is an effective candidate gene to develop tolerance against the cassava mosaic disease and possibly other DNA virus diseases.

RNAi mediated gene silencing against betasatellite associated with Croton yellow vein mosaic begomovirus

Plant viruses encode suppressors of posttranscriptional gene silencing, an adaptive antiviral defense responses that confines virus infection. Previously, we identified single-stranded DNA satellite (also known as DNA-β) of ~1,350 nucleotides in length associated with Croton yellow vein mosaic begomovirus (CYVMV) in croton plants. The expression of genes from DNA-β requires the begomovirus for packaged, replication, insect transmission and movement in plants. The present study demonstrates the effect of the βC1 gene on the silencing pathway as analysed by using both transgenic systems and transient Agrobacterium tumefaciens based delivery. Plants that carry an intron-hairpin construct covering the βC1 gene accumulated cognate small-interfering RNAs and remained symptom-free after exposure to CYVMV and its satellite. These results suggest that βC1 interferes with silencing mechanism.

Infection of tomato leaf curl New Delhi virus (ToLCNDV), a bipartite begomovirus with betasatellites, results in enhanced level of helper virus components and antagonistic interaction between DNA B and betasatellites

Tomato leaf curl New Delhi virus (ToLCNDV) (Geminiviridae) is an important pathogen that severely affects tomato production. An extensive survey was carried out during 2003-2010 to study the diversity of begomoviruses found in tomato, potato, and cucurbits that showed symptoms of leaf puckering, distortion, curling, vein clearing, and yellow mosaic in various fields in different regions of India. Ten begomovirus isolates were cloned from infected samples and identified as belonging to the species ToLCNDV. A total of 44 % of the samples showed association of betasatellites, with CLCuMuB and LuLDB being the most frequent. The ToLCNDV cloned component DNA A and DNA B were agroinoculated on Nicotiana benthamiana and tomato (Solanum lycopersicum) plants with or without betasatellites, CLCuMuB or LuLDB. The viral genome levels were then monitored by real-time polymerase chain reaction at different time points of disease development. Plants co-inoculated with betasatellites showed enhanced symptom severity in both N. benthamiana and tomato, as well as increases in helper viral DNA A and DNA B levels. The DNA B and betasatellites acted antagonistically to each other, so that the level of DNA B was 16-fold greater in the presence of betasatellites, while accumulation of betasatellites, CLCuMuB and LuLDB, were reduced by 60 % in the presence of DNA B. DNA B-mediated symptoms predominated in CLCuMuB-inoculated plants, whereas betasatellite-mediated leaf abnormalities were prominent in LuLDB-co-inoculated plants. Inoculation with the cloned components will be a good biotechnological tool in resistance breeding program.

Recombination in eukaryotic single stranded DNA viruses

Although single stranded (ss) DNA viruses that infect humans and their domesticated animals do not generally cause major diseases, the arthropod borne ssDNA viruses of plants do, and as a result seriously constrain food production in most temperate regions of the world. Besides the well known plant and animal-infecting ssDNA viruses, it has recently become apparent through metagenomic surveys of ssDNA molecules that there also exist large numbers of other diverse ssDNA viruses within almost all terrestrial and aquatic environments. The host ranges of these viruses probably span the tree of life and they are likely to be important components of global ecosystems. Various lines of evidence suggest that a pivotal evolutionary process during the generation of this global ssDNA virus diversity has probably been genetic recombination. High rates of homologous recombination, non-homologous recombination and genome component reassortment are known to occur within and between various different ssDNA virus species and we look here at the various roles that these different types of recombination may play, both in the day-to-day biology, and in the longer term evolution, of these viruses. We specifically focus on the ecological, biochemical and selective factors underlying patterns of genetic exchange detectable amongst the ssDNA viruses and discuss how these should all be considered when assessing the adaptive value of recombination during ssDNA virus evolution.

Severe stunting in blackgram caused by the Mungbean yellow mosaic virus (MYMV) KA27 DNA B component is ameliorated by co-infection or post-infection with the KA22 DNA B: MYMV nuclear shuttle protein is the symptom determinant

Mungbean yellow mosaic virus-[India:Vigna] (MYMV-[IN:Vig]), a blackgram isolate of MYMV, has five variable and infective DNA B components of which KA22 and KA27 DNA Bs share only 72% nucleotide sequence identity between them. Agroinoculation of blackgram with partial dimers of DNA A and KA27 DNA B caused severe stunting and an inordinate delay in flowering. Interestingly, co-agroinoculation of KA27+KA22 DNA B components along with DNA A ameliorated severe stunting, rescued from the delay in flowering and caused the appearance of yellow mosaic symptom characteristic of KA22 DNA B. Post-agroinoculation of KA27 DNA B-infected blackgram plants with KA22 DNA B also resulted in the amelioration from severe stunting and in the alleviation from the delay in flowering. Alleviation from KA27 DNA B-type of symptom by co-infection or post-infection with KA22 DNA B did not result in a corresponding reduction in KA27 DNA B levels. Swapping of KA27 DNA B with the nuclear shuttle protein gene (NSP) of KA22 DNA B abolished severe stunting and caused the appearance of mild yellow symptom, suggesting that the NSP is the major symptom determinant in MYMV DNA B.

C4 protein of Beet severe curly top virus is a pathomorphogenetic factor in Arabidopsis

The Curtovirus C4 protein is required for symptom development during infection of Arabidopsis. Transgenic Arabidopsis plants expressing C4 from either Beet curly top virus or Beet severe curly top virus produced phenotypes that were similar to symptoms seen during infection with wild-type viruses. The pseudosymptoms caused by C4 protein alone were novel to transgenic Arabidopsis and included bumpy trichomes, severe enations, disorientation of vascular bundles and stomata, swelling, callus-like structure formation, and twisted siliques. C4 induced abnormal cell division and altered cell fate in a variety of tissues depending on the C4 expression level. C4 protein expression increased the expression levels of cell-cycle-related genes CYCs, CDKs and PCNA, and suppressed ICK1 and the retinoblastoma-related gene RBR1, resulting in activation of host cell division. These results suggest that the Curtovirus C4 proteins are involved actively in host cell-cycle regulation to recruit host factors for virus replication and symptom development.

Cell-free construction of disarmed Abutilon mosaic virus-based gene silencing vectors

The bipartite Abutilon mosaic virus (AbMV) was engineered as a versatile silencing vector in which the coat protein gene of DNA A was deleted and replaced by sequences of interest. Plants transgenic for the dimeric AbMV DNA B component were used as test hosts to minimize the risk of unintended release of the recombinant DNA. The vector construct was stable genetically upon systemic infection and, in common with the parental virus, the vector remained phloem-limited. For virus-induced gene silencing (VIGS), a phytoene desaturase gene fragment was isolated from Nicotiana benthamiana (NbPDS) and inserted into the vector. After agroinfection, phytoene desaturase silencing was triggered efficiently in all leaf tissues without interference by viral symptoms. In order to facilitate further the use of the system, a technique for cell-free construction of recombinants was established using rolling circle amplification and biolistic inoculation of DNA B-transgenic plants. This novel procedure provides a convenient and safe way for delivering VIGS constructs for functional genomics.

Bean dwarf mosaic virus: a model system for the study of viral movement

TAXONOMY

Bean dwarf mosaic virus-[Colombia:1987] (BDMV-[CO:87]) is a single-stranded plant DNA virus, a member of the genus Begomovirus of the family Geminiviridae.

PHYSICAL PROPERTIES

BDMV virions are twinned incomplete isosahedra measuring 18 x 30 nm. The viral particle is composed of 110 subunits of coat protein, organized as 22 pentameric capsomers. Each subunit has a molecular mass of approximately 29 kDa. BDMV possesses two DNA components (designated DNA-A and DNA-B), each approximately 2.6 kb in size.

HOST RANGE

The natural and most important host of BDMV is the common bean (Phaseolus vulgaris). Nicotiana benthamiana is often used as an experimental host. Common bean germplasm can be divided into two major gene pools: Andean materials, which are mostly susceptible to BDMV, and Middle American materials, which are mostly resistant to BDMV.

DISEASE SYMPTOMS

The symptom intensity in common bean plants depends on the stage of infection. Early infection of susceptible bean seedlings will result in severe stunting and dwarfing, leaf distortion and mottling or mosaic, as well as chlorotic or yellow spots or blotches. BDMV-infected plants usually abort their flowers or produce severely distorted pods. Late infection of susceptible plants or early infection of moderately resistant genotypes may show a mild mosaic, mottle and crumpling or an irregular distribution of variegated patches. BIOLOGICAL PROPERTIES: As a member of the Begomovirus group, BDMV is transmitted from plant to plant by the whitefly Bemisia tabaci. BDMV is a nonphloem-limited virus and can replicate and move in the epidermal, cortical and phloem cells. As a nonphloem-limited virus, it is sap-transmissible.

Expression dynamics and ultrastructural localization of epitope-tagged Abutilon mosaic virus nuclear shuttle and movement proteins in Nicotiana benthamiana cells

The geminivirus Abutilon mosaic virus (AbMV) encodes two proteins which are essential for viral spread within plants. The nuclear shuttle protein (NSP) transfers viral DNA between the nucleus and cytoplasm, whereas the movement protein (MP) facilitates transport between cells through plasmodesmata and long-distance via phloem. An inducible overexpression system for epitope-tagged NSP and MP in plants yielded unprecedented amounts of both proteins. Western blots revealed extensive posttranslational modification and truncation for MP, but not for NSP. Ultrastructural examination of Nicotiana benthamiana tissues showed characteristic nucleopathic alterations, including fibrillar rings, when epitope-tagged NSP and MP were simultaneously expressed in leaves locally infected with an AbMV DNA A in which the coat protein gene was replaced by a green fluorescent protein encoding gene. Immunogold labelling localized NSP in the nucleoplasm and in the fibrillar rings. MP appeared at the cell periphery, probably the plasma membrane, and plasmodesmata.

Genetic determinants of symptoms on viral DNA satellites

Begomovirus-DNA-beta disease complexes induce different symptom phenotypes in their hosts. To investigate the genetic determinants of the phenotypic differences, Nicotiana spp. and tomato plants were inoculated with infectious clones of Tobacco curly shoot virus (TbCSV)/TbCSV DNA-beta (TbCSB) and Tomato yellow leaf curl China virus (TYLCCNV)/TYLCCNV DNA-beta (TYLCCNB) pseudorecombinants and showed that TYLCCNB induced characteristic vein-thickening and enation symptoms, while TbCSB only slightly exacerbated the leaf-curling symptoms, regardless of the helper virus being used. The roles of DNA-beta-encoded betaC1 and a 430-nucleotide fragment containing the A-rich region and the putative betaC1 promoter region of the betaC1 gene (referred to as AP) in symptom development were further investigated by constructing hybrid satellites in which the betaC1 coding region or AP was exchanged between the two satellite molecules. A TYLCCNB hybrid with TbCSB betaC1 lost the ability to elicit the vein-thickening and enation phenotypes. TbCSB hybrids containing the TYLCCNB betaC1 or AP fragment failed to induce the characteristic vein thickening and enations. A TYLCCNB hybrid having the TbCSB AP fragment produced the enations, but the number of enations was less and their sizes were reduced. Differently from the phloem-specific pattern of the TYLCCNB promoter, a full-length fragment upstream of the TbCSB betaC1 gene confers a constitutive beta-glucuronidase expression pattern in transgenic tobacco plants. The above results indicate that the DNA-beta-encoded betaC1 protein is the symptom determinant, but the promoter of the betaC1 gene has influence on symptom production.

Three C-terminal phosphorylation sites in the Abutilon mosaic virus movement protein affect symptom development and viral DNA accumulation

The Abutilon mosaic virus (AbMV, Geminiviridae) DNA B component encodes a movement protein (MP), which facilitates viral transport within plants and affects pathogenicity. The presence of phosphorylated serine and threonine residues was confirmed for MP expressed in yeast and Nicotiana benthamiana by comparative Western blot analysis using phospho-amino acid- and MP-specific immunodetection. Mass spectrometry of yeast-derived MP identified three phosphorylation sites located in the C-terminal domain (Thr-221, Ser-223 and Ser-250). To assess their functional relevance in plants, several point mutations were generated in the MP gene of DNA B, which replace Thr-221, Ser-223 and Ser-250, either singly or in combinations, with either an uncharged alanine or a phosphorylation-mimicking aspartate residue. When co-inoculated with DNA A, all mutants were infectious. In systemically infected plants the symptoms and/or viral DNA accumulation were significantly altered for several of the mutants.

Geminiviruses

Plant pathogenic geminiviruses have been proliferating worldwide and have, therefore, attracted considerable scientific interest during the past three decades. Current knowledge concerning their virion and genome structure, their molecular biology of replication, recombination, transcription, and silencing, as well as their transport through plants and dynamic competition with host responses are summarized. The topics are chosen to provide a comprehensive introduction for animal virologists, emphasizing similarities and differences to the closest functional relatives, polyomaviruses and circoviruses.

Pathogenicity of a naturally occurring recombinant DNA satellite associated with tomato yellow leaf curl China virus

Recombinant DNA β molecules (RecDNA-Aβ) comprising parts of DNA A and DNA β associated with tomato yellow leaf curl China virus (TYLCCNV) have been identified in naturally infected tobacco plants. Several examples of the recombinant DNA have been cloned and characterized by sequence analysis. All are approximately half the size of TYLCCNV genomic DNA, and all contain the βC1 gene and the A-rich region from TYLCCNV DNA β as well as intergenic region sequences and the 5′ terminus of the AC1 gene from TYLCCNV DNA A. RecDNA-Aβ was detected by PCR in five of 25 TYLCCNV isolates. Co-inoculation of TYLCCNV DNA A and RecDNA-Aβ induced symptoms indistinguishable from those induced by TYLCCNV DNA A and DNA β in Nicotiana benthamiana, Nicotiana glutinosa, Solanum lycopersicum and Petunia hybrida plants, and Southern blot hybridization results showed that RecDNA-Aβ could replicate stably in N. benthamiana plants.

Post-translational modifications of Abutilon mosaic virus movement protein (BC1) in fission yeast

The movement protein (MP) of Abutilon mosaic virus (AbMV, Geminiviridae) exhibited a complex band pattern upon gel electrophoresis indicating its post-translational modification when expressed in AbMV-infected plants or, ectopically, in fission yeasts. High-resolution separation according to charge and molecular weight in acetic acid/urea polyacrylamide or sodium dodecyl sulphate polyacrylamide gels followed by western blot analysis revealed a pattern of AbMV MP from infected plants more related to that from fission yeast than from bacteria. For this reason, expression in fission yeast was established as an experimental system to study post-translational modifications of AbMV MP. Metabolic labelling with 32P-orthophosphate confirmed a phosphorylation of all MP variants suggesting multiple phosphorylation sites. Treatment with calf intestinal alkaline phosphatase removed this label completely, but was unable to eliminate all protein bands with lower electrophoretic mobility. Thus, multiple phosphorylations contribute to the complex migration behaviour of MP, but additional post-translational modifications may occur as well.

RNAi-mediated resistance to Bean golden mosaic virus in genetically engineered common bean (Phaseolus vulgaris)

Bean golden mosaic virus (BGMV) is transmitted by the whitefly Bemisia tabaci in a persistent, circulative manner, causing the golden mosaic of common bean (Phaseolus vulgaris L.). The characteristic symptoms are yellow-green mosaic of leaves, stunted growth, or distorted pods. The disease is the largest constraint to bean production in Latin America and causes severe yield losses (40 to 100%). Here, we explored the concept of using an RNA interference construct to silence the sequence region of the AC1 viral gene and generate highly resistant transgenic common bean plants. Eighteen transgenic common bean lines were obtained with an intron-hairpin construction to induce post-transcriptional gene silencing against the AC1 gene. One line (named 5.1) presented high resistance (approximately 93% of the plants were free of symptoms) upon inoculation at high pressure (more than 300 viruliferous whiteflies per plant during the whole plant life cycle) and at a very early stage of plant development. Transgene-specific small interfering RNAs were detected in both inoculated and non-inoculated transgenic plants. A semiquantitative polymerase chain reaction analysis revealed the presence of viral DNA in transgenic plants exposed to viruliferous whiteflies for a period of 6 days. However, when insects were removed, no virus DNA could be detected after an additional period of 6 days.

Abutilon mosaic virus DNA B component supports mechanical virus transmission, but does not counteract begomoviral phloem limitation in transgenic plants

Different Nicotiana benthamiana lines stably transformed with Abutilon mosaic virus (AbMV) dimeric DNA B were capable of systemically spreading complete bipartite AbMV genomes, following agroinoculation of DNA A alone. Constitutively expressed viral movement protein (BC1) did not induce any persistent disease phenotype, but plants developed transient morphological abnormalities such as radially symmetric leaves after kanamycin withdrawal. Systemic AbMV infection produced symptoms and virus titers indistinguishable from those in non-transgenic plants. In systemically invaded leaves, the begomovirus remained phloem-limited, whereas the plants' susceptibility to mechanical transmission of AbMV was enhanced by a factor of three to five, as compared to non-transgenic controls. Hence, DNA B-encoded movement functions can complement local movement to the phloem after mechanical transmission, but fail to support viral invasion of non-phloem cells in systemically infected organs, indicating that the phloem restriction of AbMV does not result predominantly from a lack of transport competence in mesophyll tissues.

Protein-protein interactions and nuclear trafficking of coat protein and betaC1 protein associated with Bhendi yellow vein mosaic disease

Bhendi yellow vein mosaic disease (BYVMD) is caused by a complex consisting of a monopartite begomovirus BYVMV and a satellite DNA beta component. BYVMV represents a new member of the emerging group of monopartite begomoviruses requiring a satellite component for symptom induction. Here we report the results of the transient expression of green fluorescent protein (GFP) fused with the betaC1 and coat protein (CP) coding regions, in the epidermal cells of Nicotiana benthamiana. GFPCP was found to be targeted into the nucleus whereas GFPbetaC1 was localized towards the periphery of the cell. The sub-cellular localization of the betaC1 protein has been compared with that of the CP in yeast cells using a genetic system for detection of protein nuclear import and export. Expression of betaC1 ORF in transgenic N. benthamiana under the control of the Cauliflower mosaic virus 35S promoter produced severe developmental abnormalities in the plant, like distorted stem, leaves and stunting of the plant. We also present the results on the interaction of CP and betaC1 proteins using yeast two hybrid analysis, suggesting a collaborative role in the inter- and intracellular dynamics of BYVMD.

Identification and characterization of a host reversibly glycosylated peptide that interacts with the Tomato leaf curl virus V1 protein

Monopartite geminiviruses of the genus Begomovirus have two virion-sense genes, V1 and V2. V2 encodes the viral coat protein, but the function of V1 is largely unknown, although some studies suggest that it may play a role in cell-to-cell movement. Yeast two-hybrid technology was used to identify possible host binding partners of V1 from Tomato leaf curl virus (TLCV) to better understand its function. A protein closely related to a family of plant reversibly glycosylated peptides, designated SlUPTG1, was found to interact with V1 in yeast and in vitro. SlUPTG1 may function endogenously in the synthesis of cell wall polysaccharides, since a bacterially expressed form of the protein acted as an autocatalytic glycosyltransferase in vitro, a SlUPTG1:GFP fusion protein localized to the cell wall, and expression of SlUPTG1 appeared to be highest in actively dividing tissues. However, expression of SlUPTG1 in a transient TLCV replication assay increased the accumulation of viral DNA, suggesting that this host factor also plays a role in viral infection. Together, these data provide new insight into the role of V1 in TLCV infection and reveal another host pathway which geminiviruses may manipulate to achieve an efficient infection.

A Begomovirus DNAbeta-encoded protein binds DNA, functions as a suppressor of RNA silencing, and targets the cell nucleus

Our previous results demonstrated that the DNAbeta satellite (Y10beta) associated with Tomato yellow leaf curl China virus Y10 isolate (TYLCCNV-Y10) is essential for induction of leaf curl symptoms in plants and that transgenic expression of its betaC1 gene in Nicotiana plants induces virus-like symptoms. In the present study, in vitro DNA binding activity of the betaC1 proteins of Y10beta and DNAbeta (Y35beta) found in the Tobacco curly shoot virus Y35 isolate (TbCSV-Y35) were studied following their expression as six-His fusion proteins in Escherichia coli. Electrophoretic mobility shift assays and UV cross-linking experiments revealed that betaC1 proteins could bind both single-stranded and double-stranded DNA without size or sequence specificity. Suppression of green fluorescent protein (GFP) transgene silencing was observed with the new leaves of GFP-expressing Nicotiana benthamiana plants coinoculated by TYLCCNV-Y10 plus Y10beta or by TbCSV-Y35 plus Y35beta. In a patch agroinfiltration assay, the transiently expressed betaC1 gene of Y10beta or Y35beta was able to suppress host RNA silencing activities and permitted the accumulation of high levels of GFP mRNA in the infiltrated leaf patches of GFP transgenic N. benthamiana plants. The betaC1 protein of Y10beta accumulated primarily in the nuclei of plant and insect cells when fused with beta-glucuronidase or GFP and immunogold labeling showed that the betaC1 protein is present in the nuclei of infected N. benthamiana plants. A mutant version of Y10beta carrying the mutations within the putative nuclear localization sequence of the Y10 betaC1 protein failed to induce disease symptoms, suppress RNA silencing, or accumulate in the nucleus, suggesting that nuclear localization of the betaC1 protein is a key requirement for symptom induction and silencing suppression.

Tobacco plants respond to the constitutive expression of the tospovirus movement protein NS(M) with a heat-reversible sealing of plasmodesmata that impairs development

Viral infection often results in typical symptoms, the biological background of which has remained elusive. We show that constitutive expression of the NSM viral movement protein (MP) of tomato spotted wilt virus in Nicotiana tabacum is sufficient to induce severe, infection-like symptoms, including pronounced deficiencies in root and shoot development. Leaves failed to expand and were arranged in a rosette due to the absence of internode elongation. Following the sink-source transition they accumulated excessive amounts of starch and developed fusing chlorotic patches in the mesophyll, resembling virus-induced chlorotic lesions. Eventually, the leaves became entirely white and brittle. With a combination of techniques, including photosystem II quantum-yield measurements, iontophoresis of symplasmic tracers, bombardment with pPVX.GFP and double immunolabelling it was shown that these symptoms correlated with the obstruction of NSM-targeted mesophyll plasmodesmata (Pd) in source tissues by depositions of 1,3-beta-D-glucan (GLU) or callose. Temperature-shift treatments (TST; 22-->32 degrees C), known to abolish chlorotic local lesions, also abolished the chlorotic 'superlesions' of transgenic plants and rescued plant development, by restoring the transport capacity of Pd through the action of 1,3-beta-D-glucanase (GLU-h) or callase. Return of these elongated, TST-recovered plants to 22 degrees C reintroduced superlesions and arrested shoot elongation, resulting in the formation of a rosette of clustered leaves at the shoot tip. Collectively, this indicates that the symptoms of NSM plants are self-inflicted and due to a basal defence response that counteracts prolonged interference of the MP with Pd functioning. This type of defence may also play a role in the formation of symptoms during viral infection.

The nuclear shuttle protein of Tomato leaf curl New Delhi virus is a pathogenicity determinant

The role of the movement protein (MP) and nuclear shuttle protein (NSP) in the pathogenicity of Tomato leaf curl New Delhi virus (ToLCNDV), a bipartite begomovirus, was studied. Both genes were expressed in Nicotiana benthamiana, Nicotiana tabacum, and Lycopersicon esculentum plants with the Potato virus X (PVX) expression vector or by stable transformation of gene constructs under the control of the 35S promoter in N. tabacum. No phenotypic changes were observed in any of the three species when the MP was expressed from the PVX vector or constitutively expressed in transgenic plants. Expression of the ToLCNDV NSP from the PVX vector in N. benthamiana resulted in leaf curling that is typical of the disease symptoms caused by ToLCNDV in this species. Expression of NSP from PVX in N. tabacum and L. esculentum resulted in a hypersensitive response (HR), demonstrating that the ToLCVDV NSP is a target of host defense responses in these hosts. The NSP, when expressed as a transgene under the control of the 35S promoter, resulted in necrotic lesions in expanded leaves that initiated from a point and then spread across the leaf. The necrotic response was systemic in all the transgenic plants. Deletion of 100 amino acids from the C terminus did not compromise the HR response, suggesting that this region has no role in HR. Deletion of 60 or 100 amino acids from the N terminus of NSP abolished the HR response, suggesting that these sequences are required for the HR response. These findings demonstrate that the ToLCNDV NSP is a pathogenicity determinant as well as a target of host defense responses.

Exploiting chinks in the plant's armor: evolution and emergence of geminiviruses

The majority of plant-infecting viruses utilize an RNA genome, suggesting that plants have imposed strict constraints on the evolution of DNA viruses. The geminiviruses represent a family of DNA viruses that has circumvented these impediments to emerge as one of the most successful viral pathogens, causing severe economic losses to agricultural production worldwide. The genetic diversity reflected in present-day geminiviruses provides important insights into the evolution and biology of these pathogens. To maximize replication of their DNA genome, these viruses acquired and evolved mechanisms to manipulate the plant cell cycle machinery for DNA replication, and to optimize the number of cells available for infection. In addition, several strategies for cell-to-cell and long-distance movement of the infectious viral DNA were evolved and refined to be compatible with the constraints imposed by the host endogenous macromolecular trafficking machinery. Mechanisms also evolved to circumvent the host antiviral defense systems. Effectively combatting diseases caused by geminiviruses represents a major challenge and opportunity for biotechnology.

A single complementary-sense transcript of a geminiviral DNA beta satellite is determinant of pathogenicity

Small circular single-stranded DNA satellites, termed DNAbeta, have recently been found associated with some geminivirus infections. The DNA beta associated with Cotton leaf curl virus is responsible for symptom expression of a devastating disease in Pakistan. Mutagenesis of DNA beta revealed that the complementary-sense open reading frame (ORF) betaC1 is required for inducing disease symptoms in Nicotiana tabacum. An ORF present on the virion-sense strand betaV1 appeared to have no role in pathogenesis. Tobacco plants transformed with a betaC1 ORF under the control of the Cauliflower mosaic virus 35S promoter or with a dimeric DNA beta exhibited severe disease-like phenotypes, while plants transformed with a mutated version of betaC1 appeared normal. Northern blot analysis of RNA from the transgenic plants, using strand-specific probes, identified a single complementary-sense transcript. The transcript carries the full betaC1 ORF encoding a 118-amino acid product. It maps to the DNA beta at nucleotide position 186 to 563 and contains a polyadenylation signal 18 nt upstream of the stop codon. A TATA box is located 43 nt upstream of the start codon. Our results indicate that betaC1 protein is responsible for DNA beta-induced disease symptoms.

A DNAbeta associated with Tomato yellow leaf curl China virus is required for symptom induction

We report here that all 25 isolates of Tomato yellow leaf curl China virus (TYLCCNV) collected from tobacco, tomato, or Siegesbeckia orientalis plants in different regions of Yunnan Province, China, were associated with DNAbeta molecules. To investigate the biological role of DNAbeta, full-length infectious clones of viral DNA and DNAbeta of TYLCCNV isolate Y10 (TYLCCNV-Y10) were agroinoculated into Nicotiana benthamiana, Nicotiana glutinosa, Nicotiana. tabacum Samsun (NN or nn), tomato, and petunia plants. We found that TYLCCNV-Y10 alone could systemically infect these plants, but no symptoms were induced. TYLCCNV-Y10 DNAbeta was required, in addition to TYLCCNV-Y10, for induction of leaf curl disease in these hosts. Similar to TYLCCNV-Y10, DNAbeta of TYLCCNV isolate Y64 was also found to be required for induction of typical leaf curl diseases in the hosts tested. When the betaC1 gene of TYLCCNV-Y10 DNAbeta was mutated, the mutants failed to induce leaf curl symptoms in N. benthamiana when coinoculated with TYLCCNV-Y10. However, Southern blot hybridization analyses showed that the mutated DNAbeta molecules were replicated. When N. benthamiana and N. tabacum plants were transformed with a construct containing the betaC1 gene under the control of the Cauliflower mosaic virus 35S promoter, many transgenic plants developed leaf curl symptoms similar to those caused by a virus, the severity of which paralleled the level of betaC1 transcripts, while transgenic plants transformed with the betaC1 gene containing a stop codon after the start codon remained symptomless. Thus, expression of a betaC1 gene is adequate for induction of symptoms of viral infection in the absence of virus.

The dialogue between viruses and hosts in compatible interactions

Understanding the biological principles behind virus-induced symptom expression in plants remains a longstanding challenge. By dissecting the compatible host-virus relationship temporally and genetically, we have begun to map out the relationships of its component parts. The picture that emerges is one in which host gene expression and physiology are under tight temporal control during infection.

Functional analysis of proteins involved in movement of the monopartite begomovirus, Tomato yellow leaf curl virus

The functional properties of proteins [capsid protein (CP), V1, and C4] potentially involved with movement of the monopartite begomovirus, Tomato yellow leaf curl virus (TYLCV), were investigated using microinjection of Escherichia coli expressed proteins and transient expression of GFP fusion proteins. The TYLCV CP localized to the nucleus and nucleolus and acted as a nuclear shuttle, facilitating import and export of DNA. Thus, the CP serves as the functional homolog of the bipartite begomovirus BV1. The TYLCV V1 localized around the nucleus and at the cell periphery and colocalized with the endoplasmic reticulum, whereas C4 was localized to the cell periphery. Together, these patterns of localization were similar to that of the bipartite begomovirus BC1, known to mediate cell-to-cell movement. However, in contrast to BC1, V1 and C4, alone or in combination, had a limited capacity to move and mediate macromolecular trafficking through mesophyll or epidermal plasmodesmata. Immunolocalization and in situ PCR experiments, conducted with tomato plants at three stages of development, established that TYLCV infection was limited to phloem cells of shoot apical, leaf, stem, and floral tissues. Thus, the V1 and/or C4 may be analogs of the bipartite begomovirus BC1 that have evolved to mediate TYLCV movement within phloem tissue.

Pathogenicity of a natural recombinant associated with ageratum yellow vein disease: implications for geminivirus evolution and disease aetiology

Yellow vein disease of Ageratum conyzoides is caused by a viral DNA complex consisting of the genomic component (DNA A) of the monopartite begomovirus Ageratum yellow vein virus (AYVV, family: Geminiviridae) and a small satellite-like DNA beta component. AYVV DNA A is unable to induce symptoms in this host alone but can systemically infect A. conyzoides in which it accumulates to low levels. Here, we demonstrate that the yellow vein phenotype can also be produced by co-inoculating A. conyzoides with AYVV DNA A and recDNA-Abeta17, a naturally occurring recombinant of approximately the same size as DNA beta that contains sequences from both DNA A and DNA beta. Symptoms induced by DNA A and recDNA-Abeta17 in A. conyzoides and Nicotiana glutinosa are qualitatively similar to those associated with DNA A and DNA beta although milder. Recombination between DNA A and DNA beta to produce a chimera resembling recDNA-Abeta17 was observed after whitefly transmission of the disease in A. conyzoides. Hence, such recombination events are likely to occur frequently, implying that recombinants will normally be associated with this type of disease complex in the field. Possible implications of these findings for the evolution of begomoviruses and the aetiology of their diseases are discussed.

Transgenic citrus plants expressing the citrus tristeza virus p23 protein exhibit viral-like symptoms

Summary The 23 kDa protein (p23) coded by the 3'-terminal gene of Citrus tristeza virus (CTV), a member of the genus Closterovirus with the largest genome among plant RNA viruses, is an RNA-binding protein that contains a motif rich in cysteine and histidine residues in the core of a putative zinc-finger domain. On this basis, a regulatory role for CTV replication or gene expression has been suggested for p23. To explore whether over-expression of this protein in transgenic plants could affect the normal CTV infection process, transgenic Mexican lime plants were generated carrying the p23 transgene, or a truncated version thereof, under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Constitutive expression of p23 induced phenotypic aberrations that resembled symptoms incited by CTV in non-transgenic lime plants, whereas transgenic plants expressing the p23 truncated version were normal. The onset of CTV-like symptoms in p23-transgenic plants was associated with the expression of p23, and its accumulation level paralleled the intensity of the symptoms. This demonstrates that p23 is involved in symptom development and that it most likely plays a key role in CTV pathogenesis. This is the first case in which a protein encoded by a woody plant-infecting RNA virus has been identified as being directly involved in pathogenesis in its natural host. This finding also delimits a small region of the large CTV genome for the future mapping of specific pathogenic determinants.

The distinct disease phenotypes of the common and yellow vein strains of Tomato golden mosaic virus are determined by nucleotide differences in the 3'-terminal region of the gene encoding the movement protein

In Nicotiana benthamiana, the common strain of the bipartite geminivirus Tomato golden mosaic virus (csTGMV) induces extensive chlorosis whereas the yellow vein strain (yvTGMV) produces veinal chlorosis on systemically infected leaves. In Datura stramonium, csTGMV produces leaf distortion and a severe chlorotic mosaic whereas yvTGMV produces only small chlorotic lesions on systemically infected leaves. Genetic recombination and site-directed mutagenesis studies using infectious clones of csTGMV and yvTGMV have identified a role in symptom production for the gene encoding the movement protein (MP). The MP amino acid at position 272, either valine (csTGMV) or isoleucine (yvTGMV), influenced symptoms in both hosts by inducing an intermediate phenotype when exchanged between the two strains. Exchange of an additional strain-specific MP amino acid at position 288, either glutamine (csTGMV) or lysine (yvTGMV), resulted in the change of symptom phenotype to that of the other strain. In situ hybridization analysis in N. benthamiana demonstrated that there was no qualitative difference in the tissue distribution of the two strains although csTGMV accumulated in higher amounts, suggesting that the efficiency of virus movement rather than distinct differences in tissue specificity of the strains is responsible for the symptom phenotypes.

Transgenic plants expressing geminivirus movement proteins: abnormal phenotypes and delayed infection by Tomato mottle virus in transgenic tomatoes expressing the Bean dwarf mosaic virus BV1 or BC1 proteins

Transgenic tomato plants expressing wild-type or mutated BV1 or BC1 movement proteins from Bean dwarf mosaic virus (BDMV) were generated and examined for phenotypic effects and resistance to Tomato mottle virus (ToMoV). Fewer transgenic plants were recovered with the wild-type or mutated BC1 genes, compared with the wild-type or mutated BV1 genes. Transgenic tomato plants expressing the wild-type or mutated BV1 proteins appeared normal. Interestingly, although BDMV induces only a symptomless infection in tomato (i.e., BDMV is not well adapted to tomato), transgenic tomato plants expressing the BDMV BC1 protein showed a viral disease-like phenotype (i.e., stunted growth, and leaf mottling, curling, and distortion). This suggests that the symptomless phenotype of BDMV in tomato is not due to a host-specific defect in the BC1 protein. One transgenic line expressing the BC1 gene did not show the viral disease-like phenotype. This was associated with a deletion in the 3' region of the gene, which resulted in expression of a truncated BC1 protein. Several R0 plants, expressing either wild-type or mutated BV1 or BC1 proteins, showed a significant delay in ToMoV infection, compared with non-transformed plants. R1 progeny plants also showed a significant delay in ToMoV infection, but this delay was less than that in the R0 parents. These results also demonstrate that expression of viral movement proteins, in transgenic plants, can have deleterious effects on various aspects of plant development.

Programmed responses to virus replication in plants

Abstract Despite their economic importance, we understand very little about the mechanism leading to symptom formation in compatible virus infections. By applying a spatial analysis to advancing infection fronts, we have been able to relate molecular events in small groups of cells to a sequence of virus-induced changes. This sequence starts ahead of the main front of virus replication and virus protein accumulation and lasts beyond the time at which virus replication has ceased. The host changes include alterations in gene expression, physiology and cellular ultrastructure. The relationship between these effects has been analysed in comparative studies between different virus infections in different hosts and abiotic stress. The research points to there being common features for different viruses leading to common effects. Also, although many of the consequences of virus infection are similar to the effects of heat shock, there are sufficient differences to suggest that the two inducers use distinct control pathways. The immediate challenge for the future is to establish synchronous infections of tissues so that the complex relationship between the virus and the host can be investigated using temporal rather than spatial analyses.

Stunting induced by cucumber mosaic cucumovirus-infected Nicotiana glutinosa is determined by a single amino acid residue in the coat protein

R-CMV, a subgroup II strain of cucumber mosaic cucumovirus (CMV) induces a very strong stunting response in Nicotiana glutinosa plants, while Trk7-CMV causes green mosaic in this host. The genetic determinant of this phenotype was mapped to a 534-nucleotide region at the 3' end of RNA3 with biologically active, full-length cDNA clones of R-CMV and Trk7-CMV and RNA3 chimeras of the two strains. Within this region, R-CMV differs from Trk7-CMV by a single amino acid at position 193 in the coat protein. Changing the codon for Lys at this position to Asn or Ser, by site-directed mutagenesis, also changed the phenotype of the viruses from green mosaic to induction of stunting. Profound differences in both the spread and the accumulation of the viruses causing stunting and green mosaic were observed, although these did not correlate with the host specificity of stunting. Since expression of R-CMV coat protein with the PVX vector did not cause stunting, the data suggest that the presence of other CMV components is necessary for the induction of this symptom.

Identification of a novel circular single-stranded DNA associated with cotton leaf curl disease in Pakistan

Recent reports have suggested that cotton leaf curl virus (CLCuV), a geminivirus of the genus Begomovirus, may be responsible for cotton leaf curl disease in Pakistan. However, the causal agent of the disease remains unclear as CLCuV genomic components resembling begomovirus DNA A are unable to induce typical disease symptoms when reintroduced into plants. All attempts to isolate a genomic component equivalent to begomovirus DNA B have been unsuccessful. Here, we describe the isolation and characterisation of a novel circular single-stranded (ss) DNA associated with naturally infected cotton plants. In addition to a component resembling DNA A, purified geminate particles contain a smaller unrelated ssDNA that we refer to as DNA 1. DNA 1 was cloned from double-stranded replicative form of the viral DNA isolated from infected cotton plants. Blot hybridisation using probes specific for either CLCuV DNA or DNA 1 was used to demonstrate that both DNAs co-infect naturally infected cotton plants from different geographical locations. DNA 1 was detected in viruliferous Bemisia tabaci and in tobacco plants infected under laboratory conditions using B. tabaci, indicating that it is transmitted by whiteflies. Sequence analysis showed that DNA 1 is approximately half the size of CLCuV DNA but shares no homology, indicating that it is not a defective geminivirus component. DNA 1 has some homology to a genomic component of members of Nanoviridae, a family of DNA viruses that are normally transmitted by aphids or planthoppers. DNA 1 encodes a homologue of the nanovirus replication-associated protein (Rep) and has the capacity to autonomously replicate in tobacco. The data suggest that a nanovirus-like DNA has become whitefly-transmissible as a result of its association with a geminivirus and that cotton leaf curl disease may result from a mutually dependent relationship that has developed between members of two distinct DNA virus families that share a similar replication strategy.

Altered patterns of gene expression in Arabidopsis elicited by cauliflower mosaic virus (CaMV) infection and by a CaMV gene VI transgene

Cauliflower mosaic virus (CaMV) gene VI protein (P6) is an important determinant of symptom expression. Differential display polymerase chain reaction (PCR) was used to identify changes in gene expression in Arabidopsis elicited by a P6 transgene that causes a symptomatic phenotype. We used slot blot hybridization to measure the abundance of mRNAs complementary to 66 candidate PCR products in transgenic, CaMV-infected, and uninfected Arabidopsis plants. CaMV-infected and P6 transgenic plants showed broadly similar changes in abundance of mRNA species. In P6 transgenic plants we detected 18 PCR products that showed unambiguous changes in abundance plus another 15 that showed more limited changes (approximately twofold). CaMV-infected plants showed 17 unambiguous and 13 limited changes. Down-regulated species include those encoding a novel, phenol-like sulfotransferase, and a glycine-rich, RNA-binding protein. Up-regulated species included ones encoding an myb protein, glycine-rich and stress-inducible proteins, and a member of a previously unreported gene family. CaMV infection causes alterations in expression of many Arabidopsis genes. Transgene-mediated expression of P6 mimics virus infection in its effect on host gene expression, providing a potential mechanism for this process.