Transactivation in a geminivirus: AL2 gene product is needed for coat protein expression

Transient expression analysis of promoters of okra enation leaf curl virus in Nicotiana benthamiana, cotton and okra plants

Viral promoters can be used to drive heterologous gene expression in transgenic plants. As part of our quest to look for new promoters, we have explored, for the first time, the promoters of okra enation leaf curl virus (OELCuV), a begomovirus infecting okra (Abelmoschus esculentus). The Rep and CP promoters of OELCuV fused with the gfp reporter gene, were expressed transiently in the natural host okra and the laboratory host cotton and Nicotiana benthamiana. The expression levels of the promoters were quantified through confocal laser scanning microscopy and GFP assay in N. benthamiana and okra. The results indicated that the Rep promoter was more active than the CP promoter, whose activity was similar to that of CaMV 35S promoter. Additionally, the Rep and CP promoters showed increase of expression, probably due to transactivation, when assayed following inoculation of OELCuV and betasatellite DNAs in cotton plants. A moderate increase in promoter activity in N. benthamiana was also seen, when assayed following the inoculation of the heterologous begomovirus Sri Lankan cassava mosaic virus.

Functional role of geminivirus encoded proteins in the host: Past and present

During plant-pathogen interaction, plant exhibits a strong defense system utilizing diverse groups of proteins to suppress the infection and subsequent establishment of the pathogen. However, in response, pathogens trigger an anti-silencing mechanism to overcome the host defense machinery. Among plant viruses, geminiviruses are the second largest virus family with a worldwide distribution and continue to be production constraints to food, feed, and fiber crops. These viruses are spread by a diverse group of insects, predominantly by whiteflies, and are characterized by a single-stranded DNA (ssDNA) genome coding for four to eight proteins that facilitate viral infection. The most effective means to managing these viruses is through an integrated disease management strategy that includes virus-resistant cultivars, vector management, and cultural practices. Dynamic changes in this virus family enable the species to manipulate their genome organization to respond to external changes in the environment. Therefore, the evolutionary nature of geminiviruses leads to new and novel approaches for developing virus-resistant cultivars and it is essential to study molecular ecology and evolution of geminiviruses. This review summarizes the multifunctionality of each geminivirus-encoded protein. These protein-based interactions trigger the abrupt changes in the host methyl cycle and signaling pathways that turn over protein normal production and impair the plant antiviral defense system. Studying these geminivirus interactions localized at cytoplasm-nucleus could reveal a more clear picture of host-pathogen relation. Data collected from this antagonistic relationship among geminivirus, vector, and its host, will provide extensive knowledge on their virulence mode and diversity with climate change.

Intracellular bottlenecking permits no more than three tomato yellow leaf curl virus genomes to initiate replication in a single cell

Viruses are constantly subject to natural selection to enrich beneficial mutations and weed out deleterious ones. However, it remains unresolved as to how the phenotypic gains or losses brought about by these mutations cause the viral genomes carrying the very mutations to become more or less numerous. Previous investigations by us and others suggest that viruses with plus strand (+) RNA genomes may compel such selection by bottlenecking the replicating genome copies in each cell to low single digits. Nevertheless, it is unclear if similarly stringent reproductive bottlenecks also occur in cells invaded by DNA viruses. Here we investigated whether tomato yellow leaf curl virus (TYLCV), a small virus with a single-stranded DNA genome, underwent population bottlenecking in cells of its host plants. We engineered a TYLCV genome to produce two replicons that express green fluorescent protein and mCherry, respectively, in a replication-dependent manner. We found that among the cells entered by both replicons, less than 65% replicated both, whereas at least 35% replicated either of them alone. Further probability computation concluded that replication in an average cell was unlikely to have been initiated with more than three replicon genome copies. Furthermore, sequential inoculations unveiled strong mutual exclusions of these two replicons at the intracellular level. In conclusion, the intracellular population of the small DNA virus TYLCV is actively bottlenecked, and such bottlenecking may be a virus-encoded, evolutionarily conserved trait that assures timely selection of new mutations emerging through error-prone replication.

The novel C5 protein from tomato yellow leaf curl virus is a virulence factor and suppressor of gene silencing

Tomato yellow leaf curl virus (TYLCV) is known to encode 6 canonical viral proteins. Our recent study revealed that TYLCV also encodes some additional small proteins with potential virulence functions. The fifth ORF of TYLCV in the complementary sense, which we name C5, is evolutionarily conserved, but little is known about its expression and function during viral infection. Here, we confirmed the expression of the TYLCV C5 by analyzing the promoter activity of its upstream sequences and by detecting the C5 protein in infected cells by using a specific custom-made antibody. Ectopic expression of C5 using a potato virus X (PVX) vector resulted in severe mosaic symptoms and higher virus accumulation levels followed by a burst of reactive oxygen species (ROS) in Nicotiana benthamiana plants. C5 was able to effectively suppress local and systemic post-transcriptional gene silencing (PTGS) induced by single-stranded GFP but not double-stranded GFP, and reversed the transcriptional gene silencing (TGS) of GFP. Furthermore, the mutation of C5 in TYLCV inhibited viral replication and the development of disease symptoms in infected plants. Transgenic overexpression of C5 could complement the virulence of a TYLCV infectious clone encoding a dysfunctional C5. Collectively, this study reveals that TYLCV C5 is a pathogenicity determinant and RNA silencing suppressor, hence expanding our knowledge of the functional repertoire of the TYLCV proteome.

Insights into the multifunctional roles of geminivirus-encoded proteins in pathogenesis

Geminiviruses are a major threat to agriculture in tropical and subtropical regions of the world. Geminiviruses have small genome with limited coding capacity. Despite this limitation, these viruses have mastered hijacking the host cellular metabolism for their survival. To compensate for the small size of their genome, geminiviruses encode multifunctional proteins. In addition, geminiviruses associate themselves with satellite DNA molecules which also encode proteins that support the virus in establishing successful infection. Geminiviral proteins recruit multiple host factors, suppress the host defense, and manipulate host metabolism to establish infection. We have updated the knowledge accumulated about the proteins of geminiviruses and their satellites in the context of pathogenesis in a single review. We also discuss their interactions with host factors to provide a mechanistic understanding of the infection process.

The C5 protein encoded by Ageratum leaf curl Sichuan virus is a virulence factor and contributes to the virus infection

Earlier reports have indicated that begomoviruses encode four proteins (AC1/C1, AC2/C2, AC3/C3, and AC4/C4 proteins) using complementary-sense DNA as the template. In recent years, several reports have shown that some begomoviruses also encode an AC5/C5 protein from the complementary DNA strand, and these AC5/C5 proteins play different roles in virus infections. Here, we provide evidence showing that Ageratum leaf curl Sichuan virus (ALCScV), a monopartite begomovirus, also encodes a C5 protein that is important for disease symptom formation and can affect viral replication. Infection of Nicotiana benthamiana plants with a potato virus X (PVX)-based vector carrying the ALCScV C5 gene resulted in more severe disease symptoms and higher virus accumulation levels. ALCScV C5 protein can be found in the cytoplasm and the nucleus. Furthermore, this protein is also a suppressor of posttranscriptional gene silencing. Mutational analysis showed that knockout of C5 gene expression significantly reduced ALCScV-induced disease symptoms and virus accumulation, while expression of the C5 gene using the PVX-based vector enhanced ALCScV accumulation in coinfected N. benthamiana plants.

Targets and Mechanisms of Geminivirus Silencing Suppressor Protein AC2

Geminiviruses are plant DNA viruses that infect a wide range of plant species and cause significant losses to economically important food and fiber crops. The single-stranded geminiviral genome encodes a small number of proteins which act in an orchestrated manner to infect the host. The fewer proteins encoded by the virus are multifunctional, a mechanism uniquely evolved by the viruses to balance the genome-constraint. The host-mediated resistance against incoming virus includes post-transcriptional gene silencing, transcriptional gene silencing, and expression of defense responsive genes and other cellular regulatory genes. The pathogenicity property of a geminiviral protein is linked to its ability to suppress the host-mediated defense mechanism. This review discusses what is currently known about the targets and mechanism of the viral suppressor AC2/AL2/transcriptional activator protein (TrAP) and explore the biotechnological applications of AC2.

Functional analysis of a novel βV1 gene identified in a geminivirus betasatellite

Betasatellites (DNA β) are circular ssDNA molecules that are associated with monopartite geminiviruses and exert a positive effect on the viral infection. Betasatellites encode one protein, named βC1, on the complementary strand; βC1 functions as a pathogenicity factor and RNA silencing suppressor. In this report, we describe the identification of another betasatellite-encoded protein, βVl, which also contributes to symptom development. The βVl open reading frame can be found on the viral strand of approximately 40% of reported betasatellite sequences, and is conserved in position and sequence. The presence of the βVl transcript was observed in plants infected with Tomato yellow leaf curl China virus (TYTCCNV) along with its associated betasatellite Tomato yellow leaf curl China betasatellite (TYTCCNB). Mutant viruses unable to produce βVl showed reduced virulence and decreased viral load. Ectopic expression of the TYTCCNB-PV1 gene in Nicotiana benthamiana plants from a PVX-based vector resulted in leaf mosaic and chlorosis. We further demonstrated that the βVl protein could elicit hypersensitive response (HR)-type cell death in N. benthamiana leaves. Our results uncover a novel betasatellite-encoded protein that contributes to the virus infection, and this discover gives us a more complete view of the plant-geminivirus interaction landscape.

Analysis of Pathogenicity and Virulence Factors of Ageratum leaf curl Sichuan virus

Ageratum leaf curl Sichuan virus (ALCScV) is a novel monopartite begomovirus, which was identified from Ageratum conyzoides plants in Sichuan Province, China. In this study, we showed that ALCScV can induce typical dwarf and downward leaf-curling symptoms in Ageratum conyzoides, Helianthus annuus, and Nicotiana benthamiana plants and that the noncognate betasatellite can enhance disease symptoms and increase viral accumulation. Expression of the ALCScV-encoded V2, C1, and C4 proteins through a Potato virus X (PVX) vector caused severe symptoms in N. benthamiana. Further study revealed no symptoms in N. benthamiana plants inoculated with infectious ALCScV clones lacking the C4 protein and that the relative viral DNA accumulation levels significantly decreased when compared with ALCScV-inoculated plants. Thus, our mutational analyses demonstrated that C4 is a pathogenicity determinant that plays key roles in symptom formation and virus accumulation. Furthermore, we also demonstrated that the second glycine of C4 was critical for ALCScV pathogenicity.

Maintenance of Cotton Leaf Curl Multan Betasatellite by Tomato Leaf Curl New Delhi Virus-Analysis by Mutation

Viruses of the genus Begomovirus (family Geminiviridae) are economically important phytopathogens that are transmitted plant-to-plant by the whitefly Bemisia tabaci. Most Old World (OW) begomoviruses are monopartite and many of these interact with symptoms and host range determining betasatellites. Tomato leaf curl New Delhi virus (ToLCNDV) is one of only a few OW begomoviruses with a bipartite genome (components known as DNA A and DNA B). Four genes [AV2, coat protein (CP), transcriptional-activator protein (TrAP), and AC4] of ToLCNDV were mutated and the effects of the mutations on infectivity, symptoms and the ability to maintain Cotton leaf curl Multan betasatellite (CLCuMuB) were investigated. Infectivity and virus/betasatellite DNA titer were assessed by Southern blot hybridization, PCR, and quantitative PCR. The results showed TrAP of ToLCNDV to be essential for maintenance of CLCuMuB and AV2 to be important only in the presence of the DNA B. AC4 was found to be important for the maintenance of CLCuMuB in the presence of, but indispensable in the absence of, the DNA B. Rather than being required for maintenance, the CP was shown to possibly interfere with maintenance of the betasatellite. The findings show that the interaction between a bipartite begomovirus and a betasatellite is more complex than just trans-replication. Clearly, multiple levels of interactions are present and such associations can cause additional significant losses to crops although the interaction may not be stable.

The C2 protein of tomato leaf curl Taiwan virus is a pathogenicity determinant that interferes with expression of host genes encoding chromomethylases

Tomato (Solanum lycopersicum) is one of the most important crops worldwide and is severely affected by geminiviruses. Tomato leaf curl Taiwan virus (ToLCTWV), belonging to the geminiviruses, was isolated in Taiwan and causes tremendous crop loss. The geminivirus-encoded C2 proteins are crucial for a successful interaction between the virus and host plants. However, the exact functions of the viral C2 protein of ToLCTWV have not been investigated. We analyzed the molecular function(s) of the C2 protein by transient or stable expression in tomato cv. Micro-Tom and Nicotiana benthamiana. Severe stunting of tomato and N. benthamiana plants infected with ToLCTWV was observed. Expression of ToLCTWV C2-green fluorescent protein (GFP) fusion protein was predominately located in the nucleus and contributed to activation of a coat protein promoter. Notably, the C2-GFP fluorescence was distributed in nuclear aggregates. Tomato and N. benthamiana plants inoculated with potato virus X (PVX)-C2 displayed chlorotic lesions and stunted growth. PVX-C2 elicited hypersensitive responses accompanied by production of reactive oxygen species in N. benthamiana plants, which suggests that the viral C2 was a potential recognition target to induce host-defense responses. In tomato and N. benthamiana, ToLCTWV C2 was found to interfere with expression of genes encoding chromomethylases. N. benthamiana plants with suppressed NbCMT3-2 expression were more susceptible to ToLCTWV infection. Transgenic N. benthamiana plants expressing the C2 protein showed decreased expression of the NbCMT3-2 gene and pNbCMT3-2::GUS (β-glucuronidase) promoter activity. C2 protein is an important pathogenicity determinant of ToLCTWV and interferes with host components involved in DNA methylation.

The C2 Protein from the Geminivirus Tomato Yellow Leaf Curl Sardinia Virus Decreases Sensitivity to Jasmonates and Suppresses Jasmonate-Mediated Defences

An increasing body of evidence points at a role of the plant hormones jasmonates (JAs) in determining the outcome of plant-virus interactions. Geminiviruses, small DNA viruses infecting a wide range of plant species worldwide, encode a multifunctional protein, C2, which is essential for full pathogenicity. The C2 protein has been shown to suppress the JA response, although the current view on the extent of this effect and the underlying molecular mechanisms is incomplete. In this work, we use a combination of exogenous hormone treatments, microarray analysis, and pathogen infections to analyze, in detail, the suppression of the JA response exerted by C2. Our results indicate that C2 specifically affects certain JA-induced responses, namely defence and secondary metabolism, and show that plants expressing C2 are more susceptible to pathogen attack. We propose a model in which C2 might interfere with the JA response at several levels.

Mungbean yellow mosaic Indian virus encoded AC2 protein suppresses RNA silencing by inhibiting Arabidopsis RDR6 and AGO1 activities

RNA silencing refers to a conserved RNA-directed gene regulatory mechanism in a wide range of eukaryotes. It plays an important role in many processes including growth, development, genome stability, and antiviral defense in the plants. Geminivirus encoded AC2 is identified as an RNA silencing suppressor protein, however, the mechanism of action has not been characterized. In this paper, we elucidate another mechanism of AC2-mediated suppression activity of Mungbean Yellow Mosaic India Virus (MYMIV). The AC2 protein, unlike many other suppressors, does not bind to siRNA or dsRNA species and its suppression activity is mediated through interaction with key components of the RNA silencing pathway, viz., RDR6 and AGO1. AC2 interaction inhibits the RDR6 activity, an essential component of siRNA and tasi-RNA biogenesis and AGO1, the major slicing factor of RISC. Thus the study identifies dual sites of MYMIV-AC2 interference and probably accounts for its strong RNA silencing suppression activity.

Early function of the Abutilon mosaic virus AC2 gene as a replication brake

The C2/AC2 genes of monopartite/bipartite geminiviruses of the genera Begomovirus and Curtovirus encode important pathogenicity factors with multiple functions described so far. A novel function of Abutilon mosaic virus (AbMV) AC2 as a replication brake is described, utilizing transgenic plants with dimeric inserts of DNA B or with a reporter construct to express green fluorescent protein (GFP). Their replicational release upon AbMV superinfection or the individual and combined expression of epitope-tagged AbMV AC1, AC2, and AC3 was studied. In addition, the effects were compared in the presence and in the absence of an unrelated tombusvirus suppressor of silencing (P19). The results show that AC2 suppresses replication reproducibly in all assays and that AC3 counteracts this effect. Examination of the topoisomer distribution of supercoiled DNA, which indicates changes in the viral minichromosome structure, did not support any influence of AC2 on transcriptional gene silencing and DNA methylation. The geminiviral AC2 protein has been detected here for the first time in plants. The experiments revealed an extremely low level of AC2, which was slightly increased if constructs with an intron and a hemagglutinin (HA) tag in addition to P19 expression were used. AbMV AC2 properties are discussed with reference to those of other geminiviruses with respect to charge, modification, and size in order to delimit possible reasons for the different behaviors.

IMPORTANCE

The (A)C2 genes encode a key pathogenicity factor of begomoviruses and curtoviruses in the plant virus family Geminiviridae. This factor has been implicated in the resistance breaking observed in agricultural cotton production. AC2 is a multifunctional protein involved in transcriptional control, gene silencing, and regulation of basal biosynthesis. Here, a new function of Abutilon mosaic virus AC2 in replication control is added as a feature of this protein in viral multiplication, providing a novel finding on geminiviral molecular biology.

Altered expression of Arabidopsis genes in response to a multifunctional geminivirus pathogenicity protein

BACKGROUND

Geminivirus AC2 is a multifunctional protein that acts as a pathogenicity factor. Transcriptional regulation by AC2 appears to be mediated through interaction with a plant specific DNA binding protein, PEAPOD2 (PPD2), that specifically binds to sequences known to mediate activation of the CP promoter of Cabbage leaf curl virus (CaLCuV) and Tomato golden mosaic virus (TGMV). Suppression of both basal and innate immune responses by AC2 in plants is mediated through inactivation of SnRK1.2, an Arabidopsis SNF1 related protein kinase, and adenosine kinase (ADK). An indirect promoter targeting strategy, via AC2-host dsDNA binding protein interactions, and inactivation of SnRK1.2-mediated defense responses could provide the opportunity for geminiviruses to alter host gene expression and in turn, reprogram the host to support virus infection. The goal of this study was to identify changes in the transcriptome of Arabidopsis induced by the transcription activation function of AC2 and the inactivation of SnRK1.2.

RESULTS

Using full-length and truncated AC2 proteins, microarray analyses identified 834 genes differentially expressed in response to the transcriptional regulatory function of the AC2 protein at one and two days post treatment. We also identified 499 genes differentially expressed in response to inactivation of SnRK1.2 by the AC2 protein at one and two days post treatment. Network analysis of these two sets of differentially regulated genes identified several networks consisting of between four and eight highly connected genes. Quantitative real-time PCR analysis validated the microarray expression results for 10 out of 11 genes tested.

CONCLUSIONS

It is becoming increasingly apparent that geminiviruses manipulate the host in several ways to facilitate an environment conducive to infection, predominantly through the use of multifunctional proteins. Our approach of identifying networks of highly connected genes that are potentially co-regulated by geminiviruses during infection will allow us to identify novel pathways of co-regulated genes that are stimulated in response to pathogen infection in general, and virus infection in particular.

Interaction between the transcription factor AtTIFY4B and begomovirus AL2 protein impacts pathogenicity

The begomovirus AL2 protein is a transcriptional activator, a silencing suppressor, and inhibitor of basal defense. AL2 forms a complex at the CP promoter, through interaction with a plant-specific DNA-binding protein, Arabidopsis PEAPOD2 (also known as TIFY4B). AtTIFY4B has three domains (PPD, TIFY and CCT_2) conserved between homologs from different plant species. We confirmed that the AL2 protein from Tomato golden mosaic virus and Cabbage leaf curl virus interacts with TIFY4B from Arabidopsis, tomato and Nicotiana benthamiana in the nucleus of plant cells. Bimolecular Fluorescence Complementation demonstrated that the interaction involves both the TIFY and CCT_2 domains. Surprisingly, amino acids 84-150 can prevent AtTIFY4B from localizing to the nucleus, and interaction with AL2 results in some of the protein re-entering the nucleus. When AtTIFY4B is over-expressed, we observe an increase in mean latent period, where systemic symptoms are detected on average, 4 days later than in mock treated plants. This appears to be a consequence of reduced viral DNA titers, possibly related to the role of TIFY4B in cell cycle arrest. Our results point to a potential role for TIFY4B in host defense against geminiviruses. Expression of TIFY4B in N. benthamiana increases in response to geminivirus infection, which would result in suppression of proliferation, reducing viral replication. Geminiviruses may counter this defense response through an interaction between AL2 and TIFY4B, which would inhibit TIY4B function. The consequence of this inhibition would be failure to arrest the cell cycle, providing an environment conducive for geminivirus replication.

Geminivirus AL2 protein induces expression of, and interacts with, a calmodulin-like gene, an endogenous regulator of gene silencing

RNA silencing is an innate cellular response involved in antiviral defense. Arabidopsis calmodulin-like protein 39 (At-rgsCaM) is related to known regulators of RNA silencing in tomato and Nicotiana tabacum. Geminivirus AL2 protein functions to suppress post-transcriptional and transcriptional gene silencing, possibly through induction of an endogenous regulator. In support of this, the At-rgsCaM promoter responds to Tomato golden mosaic virus (TGMV) AL2 in protoplasts and geminivirus infection increases rgsCaM expression in Arabidopsis and Nicotiana benthamiana. Further, over-expression of rgsCaM leads to increased susceptibility to infection, as a consequence of increased viral DNA loads. It has been shown that rgsCaM may target silencing suppressors of RNA viruses for degradation via the autophagy pathway. This interaction occurs within the cytoplasm, but AL2 interacts with rgsCaM in the nucleus. It is tempting to speculate that AL2 may act to sequester rgsCaM in the nucleus to prevent targeting of AL2 for degradation.

Replication of Plant Viruses

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses co-infecting cells.

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses coinfecting cells.

Effects of genetic changes to the begomovirus/betasatellite complex causing cotton leaf curl disease in South Asia post-resistance breaking

Cotton leaf curl disease (CLCuD) has been a problem for cotton production across Pakistan and north-eastern India since the early 1990s. The appearance of the disease has been attributed to the introduction, and near monoculture of highly susceptible cotton varieties. During the intervening period the genetic make-up of the virus(es) causing the disease has changed dramatically. The most prominent of these changes has been in response to the introduction of CLCuD-resistant cotton varieties in the late 1990s, which provided a brief respite from the losses due to the disease. During the 1990s the disease was shown to be caused by multiple begomoviruses and a single, disease-specific betasatellite. Post-resistance breaking the complex encompassed only a single begomovirus, Cotton leaf curl Burewala virus (CLCuBuV), and a recombinant version of the betasatellite. Surprisingly CLCuBuV lacks an intact transcriptional-activator protein (TrAP) gene. The TrAP gene is found in all begomoviruses and encodes a product of ∼134 amino acids that is important in virus-host interactions; being a suppressor of post-transcriptional gene silencing (host defence) and a transcription factor that modulates host gene expression, including microRNA genes. Recent studies have highlighted the differences between CLCuBuV and the earlier viruses that are part of on-going efforts to define the molecular basis for resistance breaking in cotton.

Differential pathogenicity among Tomato leaf curl Gujarat virus isolates from India

Tomato leaf curl Gujarat virus (ToLCGV) has been identified as one of the most destructive pathogens causing tomato leaf curl disease (ToLCD) in India. In the tomato growing regions of Dhanbad and Ramgarh, plants bearing severe symptoms of ToLCD such as leaf curling, leaf crinkling, yellowing and leaf rolling was observed in the farmer fields. The association of begomovirus in these samples was confirmed by PCR and the causal viruses were identified as the isolates of ToLCGV. However, association of cognate DNA B component could not be ascertained from these samples. Indeed, like other Old World begomoviruses, the present ToLCGV isolates were found to be associated with a particular betasatellite, Tomato yellow leaf curl Thailand betasatellite (TYLCTHB). Although DNA A of both ToLCGV isolates could alone infect tomato inducing systemic symptoms, the difference in virulence was observed. Co-inoculation of TYLCTHB reduced the incubation period without influencing the accumulation of helper virus DNA and hence, differential pathogenesis among ToLCGV isolates was governed by the helper component rather than betasatellite. ToLCGV infection with DNA B increases the accumulation of DNA A component of Dhanbad isolate but not of Ramgarh isolate. Results indicated that the begomovirus identified from Ramgarh sample was a mild strain of ToLCGV.

Identification of host proteins modulated by the virulence factor AC2 of Tomato chlorotic mottle virus in Nicotiana benthamiana

Tomato, one of the most important crops cultivated worldwide, has been severely affected by begomoviruses such as the Tomato chlorotic mottle virus (ToCMoV). Virulence factor AC2 is considered crucial for a successful virus-plant interaction and is known to act as a transcriptional activator and in some begomoviruses to function as an RNA silencing suppressor factor. However, the exact functions of the AC2 protein of the begomovirus ToCMoV are not yet established. The aim of the present study was to identify differentially expressed proteins of the model plant Nicotiana benthamiana in response to the expression of the AC2 gene, isolated from ToCMoV. N. benthamiana plants were inoculated with Agrobacterium tumefaciens containing the viral vector Potato virus X (PVX) and with the PVX-AC2 construction. 2DE was performed and proteins were identified by MS. The results showed that the expression of ToCMoV AC2 alters the levels of several host proteins, which are important for normal plant development, causing an imbalance in cellular homeostasis. This study highlights the effect of AC2 in the modulation of plant defense processes by increasing the expression of several oxidative stress-related and pathogenesis-related proteins, as well as its role in modulating the proteome of the photosynthesis and energy production systems.

Identification of sequences required for AL2-mediated activation of the tomato golden mosaic virus-yellow vein BR1 promoter

A 108 bp sequence has been identified in the tomato golden mosaic virus-yellow vein (yvTGMV) B component that is necessary and sufficient for AL2-mediated activation of the BR1 promoter. The sequence appears to have a bipartite arrangement, with elements located between -144 to -77 and -59 to -36 from the transcription start site, with both being required for activation by AL2. These sequences are located upstream of a TATA box and bind nuclear proteins from spinach, tomato and Arabidopsis. These sequences are also capable of binding Arabidopsis PPD2, which has been shown previously to interact with the yvTGMV coat protein (CP) promoter. We have identified two putative transcription factor-binding sites (CCAAT and GTGANTG10) that are conserved in sequences necessary for activation of the yvTGMV BR1, as well as the yvTGMV and cabbage leaf curl virus (CabLCV) CP promoters, which are all activated by AL2. The yvTGMV BR1 promoter exhibits AL2-independent expression in vascular tissue, similar to the yvTGMV, CabLCV and spinach curly top virus CP promoters. Together, this further confirms a common regulatory mechanism for AL2-mediated activation of bipartite begomovirus promoters.

Sequences within the Spinach curly top virus virion sense promoter are necessary for vascular-specific expression of virion sense genes

Sequences necessary for activity of the Spinach curly top virus virion sense promoter have been identified within an 84 bp region upstream of two transcription start sites located at nt 252 and 292. RNAs initiating at these sites are expressed at equivalent levels in SCTV-infected Arabidopsis and from promoter-reporter constructs. The promoter is capable of directing expression of all three virion sense genes, although not to the same degree. While CP and V3 expression are similar, expression of V2 is elevated. The promoter is active in transient leaf infusion assays in the absence of C2. In Nicotiana benthamiana plants the promoter is active in vascular tissue and under no conditions did we detect promoter activity in the mesophyll. This is in contrast to begomoviruses where the virion sense promoter is dependent on AL2, a positional homolog of C2, and the promoter is functional in both vascular and mesophyll tissue.

Functional analysis of gene-silencing suppressors from tomato yellow leaf curl disease viruses

Tomato yellow leaf curl disease (TYLCD) is caused by a complex of phylogenetically related Begomovirus spp. that produce similar symptoms when they infect tomato plants but have different host ranges. In this work, we have evaluated the gene-silencing-suppression activity of C2, C4, and V2 viral proteins isolated from the four main TYLCD-causing strains in Spain in Nicotiana benthamiana. We observed varying degrees of local silencing suppression for each viral protein tested, with V2 proteins from all four viruses exhibiting the strongest suppression activity. None of the suppressors were able to avoid the spread of the systemic silencing, although most produced a delay. In order to test the silencing-suppression activity of Tomato yellow leaf curl virus (TYLCV) and Tomato yellow leaf curl Sardinia virus (TYLCSV) proteins in a shared (tomato) and nonshared (bean) host, we established novel patch assays. Using these tools, we found that viral proteins from TYLCV were able to suppress silencing in both hosts, whereas TYLCSV proteins were only effective in tomato. This is the first time that viral suppressors from a complex of disease-causing geminiviruses have been subject to a comprehensive analysis using two economically important crop hosts, as well as the established N. benthamiana plant model.

Effects of the mutation of selected genes of cotton leaf curl Kokhran virus on infectivity, symptoms and the maintenance of cotton leaf curl Multan betasatellite

Cotton leaf curl Kokhran virus (CLCuKoV) is a cotton-infecting monopartite begomovirus (family Geminiviridae). The effects of mutation of the coat protein (CP), V2, C2 and C4 genes of CLCuKoV on infectivity and symptoms in Nicotiana benthamiana were investigated. Each mutation introduced a premature stop codon which would lead to premature termination of translation of the gene. Mutation of the CP gene abolished infectivity. However, transient expression of the CLCuKoV CP gene under the control of the Cauliflower mosaic virus 35S promoter (35S-Ko(CP)), at the point of inoculation, led to a small number of plants in which viral DNA could be detected by PCR in tissues distal to the inoculation site. Mutations of the V2, C2 and C4 genes reduced infectivity. The V2 and C2 mutants did not induce symptoms, whereas the C4 mutation was associated with attenuated symptoms. Infections of plants with the C4 mutant were associated with viral DNA levels equivalent to the wild-type virus, whereas viral DNA levels for the V2 mutant were low, detectable by Southern blot hybridisation, and for the C2 mutant were detectable only by PCR. Significantly, transient expression of the CLCuKoV C2 gene at the point of inoculation, raised virus DNA levels in tissues distal to the inoculation site such that they could be detected by Southern hybridisation, although they remained at well below the levels seen for the wild-type virus, but reduced the infectivity of the virus. These findings are consistent with earlier mutation studies of monopartite begomoviruses and our present knowledge concerning the functions of the four genes suggesting that the CP is essential for long distance spread of the virus in plants, the C4 is involved in modulating symptoms, the C2 interferes with host defence and the V2 is involved in virus movement. The results also suggest that the V2, C2 and C4 may be pathogenicity determinants. Additionally the effects of the mutations of CLCuKoV genes on infections of the virus in the presence of its cognate betasatellite, Cotton leaf curl Multan betasatellite (CLCuMuB), were investigated. Mutation of the C4 gene had no effect on maintenance of the betasatellite, although the betasatellite enhanced symptoms. Inoculation of the C2 mutant with CLCuMuB raised the infectivity of the virus to near wild-type levels, although the numbers of plants in which the betasatellite was maintained was reduced, in comparison to wild-type virus infections with CLCuMuB, and viral DNA could not be detected by Southern hybridisation. Transient expression of the C2 gene at the point of inoculation raised virus DNA levels in tissues distal to the inoculation site but also reduced the infectivity of the virus and the numbers of plants in which the betasatellite was maintained. CLCuMuB restored the infectivity of the V2 mutant to wild-type levels but only in a small number of plants was the satellite maintained and infections were non-symptomatic. Although inoculation of the CP mutant with CLCuMuB did not restore infectivity, co-inoculation with 35S-Ko(CP) increased the number of plants in which the virus could be detected, in comparison to plants inoculated with the mutant and 35S-Ko(CP), and also resulted in two plants (out of 15 inoculated) in which the betasatellite could be detected by PCR. This indicates that the V2, C2 and almost certainly the CP are important for the maintenance of betasatellites by monopartite begomoviruses. The significance of these findings is discussed.

MYMIV-AC2, a geminiviral RNAi suppressor protein, has potential to increase the transgene expression

Gene silencing is one of the limiting factors for transgene expression in plants. But the plant viruses have learnt to suppress gene silencing by encoding the protein(s), called RNA silencing suppressor(s) (RSS). Hence, these proteins could be used to overcome the limitation for transgene expression. The RNAi suppressors, namely HC-Pro and P19, have been shown to enhance the transgene expression but other RSS proteins have not been screened for similar role. Moreover, none of RSSs from the DNA viruses are known for enhancing the expression of transgenes. The Mungbean Yellow Mosaic India Virus (MYMIV) belonging to the genus Begomovirus within the family of Geminiviridae encodes an RSS called the AC2 protein. Here, we used AC2 to elevate the expression of the transgenes. Upon introduction of MYMIV-AC2 in the silenced GFP transgenic tobacco lines, by either genetic hybridisation or transgenesis, the GFP expression was enhanced several fold in F1 and T0 lines. The GFP-siRNA levels were much reduced in F1 and T0 lines compared with those of the initial parental silenced lines. The enhanced GFP expression was also observed at the cellular level. This approach was also successful in enhancing the expression of another transgene, namely topoisomeraseII.

Molecular interaction between two cassava geminiviruses exhibiting cross-protection

There are increasing reports of geminivirus mixed infections of field plant hosts. These mixed infections have been suggested to result in recombinations, emergence of new viruses and new disease epidemics. We previously reported the occurrence of mixed infection between African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV) resulting in severe symptoms in cassava fields in Cameroon. Here, we show that reassortment of DNA-A and DNA-B components of ACMV and EACMCV does not form viable recombinants. However, in the presence of both components of either virus, the DNA-A component of the other virus replicated and spread in the absence of its DNA-B component. This result suggests that failure of ACMV and EACMCV to form viable recombinants is due to the inability of each DNA-A component to trans-replicate the heterologous DNA-B component. This study also shows that ACMV DNA-A induces a resistance to ACMV and EACMCV as indicated by absence or late symptom development. Moreover, this resistance enabled plants to recover from severe symptoms caused by EACMCV in Nicotiana benthamiana, suggesting that the resistance induced is not specific to ACMV and is consistent with the phenomenon of cross-protection between related viruses.

Selection of target sequences as well as sequence identity determine the outcome of RNAi approach for resistance against cotton leaf curl geminivirus complex

Cotton leaf curl disease is caused by a geminivirus complex that involves multiple distinct begomoviruses and a disease-specific DNA satellite, cotton leaf curl Multan betasatellite (CLCuMB), which is essential to induce disease symptoms. Here we have investigated the use of RNA interference (RNAi) for obtaining resistance against one of the viruses, Cotton leaf curl Multan virus (CLCuMV), associated with the disease. Three hairpin RNAi constructs were produced containing either complementary-sense genes essential for replication/pathogenicity or non-coding regulatory sequences of CLCuMV. In transient assays all three RNAi constructs significantly reduced the replication of the virus in inoculated tissues. However, only one of the constructs, that targeting the overlapping genes involved in virus replication and pathogenicity (the replication-associated protein (Rep), the transcriptional activator protein and the replication enhancer protein) was able to prevent systemic movement of the virus, although the other constructs significantly reduced the levels of virus in systemic tissues. In the presence of CLCuMB, however, a small number of plants co-inoculated with even the most efficient RNAi construct developed symptoms of virus infection, suggesting that the betasatellite may compromise resistance. Further analyses, using Rep gene sequences of distinct begomoviruses expressed from a PVX vector as the target, are consistent with the idea that the success of the RNAi approach depends on sequence identity to the target virus. The results show that selection of both the target sequence, as well as the levels of identity between the construct and target sequence, determine the outcome of RNAi-based resistance against geminivirus complexes.

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.

In silico analysis reveals that several tomato microRNA/microRNA* sequences exhibit propensity to bind to tomato leaf curl virus (ToLCV) associated genomes and most of their encoded open reading frames (ORFs)

Tomato leaf curl virus (ToLCV) is a member of family geminiviridae that constitute rapidly emerging group of phytopathogens posing threat to a large number of vegetable crops worldwide. Three different genomes are found to be associated with ToLCV viz., DNA-A, DNA-B and beta satellite DNA. MicroRNAs (miRs) are known to govern several fundamental processes in eukaryotes, including basal defense mechanisms. In animals, it has been demonstrated that certain host miRs prevent viral establishment by directly interfering with pathogen replication or by binding to viral transcripts. However, in spite of the existence of huge families of phytopathogenic viruses, no such mechanism has been observed in plants. In the present study, we performed in silico analysis to investigate whether tomato encoded miR/miR* sequences possess any potential to bind to viral genome and/or encoded ORFs. We observed that different sequences can bind to ToLCNDV DNA-A, ToLCNDV DNA-B and ToLCNDV associated DNA beta genomes and most of the encoded ORFs. Interestingly, our analysis revealed that several miR* species could similarly target genome and ORFs of ToLCNDV suggesting novel role of miR* in host defense response. This observation holds much importance as miR* molecules are presently thought to follow degradation pathway and are not assigned with any function. Moreover, we could predict targets for these miR* sequences that are generally involved in plant metabolism. Overall, these results shed light on new paradigm of intricate host-pathogen interactions via miRNA pathway.

The interaction between geminivirus pathogenicity proteins and adenosine kinase leads to increased expression of primary cytokinin-responsive genes

Pathogenicity proteins (AL2/C2) of begomo- and curtoviruses suppress silencing through inhibition of the methyl cycle, as a consequence of inhibiting adenosine kinase (ADK). ADK phosphorylates cytokinin nucleosides, helping maintain a pool of bioactive cytokinins through interconversion of free-bases, nucleosides and nucleotides. We provide evidence that inhibiting ADK affects expression of primary cytokinin-responsive genes. Specifically, we demonstrate increased activity of a primary cytokinin-responsive promoter in adk mutant Arabidopsis plants, and in response to silencing ADK expression or inhibiting ADK activity in transient assays. Similar changes in expression are observed in geminivirus infected tissue and when AL2/C2 are over-expressed. Increased cytokinin-responsive promoter activity may therefore be a consequence of an ADK/AL2/C2 interaction. Application of exogenous cytokinin increases susceptibility to geminivirus infection, characterized by a reduced mean latent period and enhanced viral replication. Thus, ADK appears to be a high value target of geminiviruses that includes increasing expression of primary cytokinin-responsive genes.

Identification of the virulence factors and suppressors of posttranscriptional gene silencing encoded by Ageratum yellow vein virus, a monopartite begomovirus

Ageratum yellow vein disease (AYVD) is caused by the association of a Tomato leaf curl Java betasatellite [Indonesia:Indonesia 1:2003] (ToLCJB-[ID:ID1:03]) with a begomovirus component. Our previous results demonstrated that ToLCJB-[ID:ID:03] is essential for induction of leaf curl symptoms in plants and transgene expression of its betaC1 gene in Nicotiana benthamiana plants induces virus-like symptoms. Here we show that Ageratum yellow vein virus-Indonesia [Indonesia: Tomato] (AYVV-ID[ID:Tom]) alone could systemically infect the plants and induced upward leaf curl symptoms. ToLCJB-[ID:ID1:03] was required, in addition to AYVV-ID[ID:Tom], for induction of severe downward leaf curl disease in N. benthamiana plants. However, DNAbeta01fsbetaC1, which encompasses a frameshift mutation, did not induce severe symptoms in N. benthamiana when co-inoculated with AYVV-ID[ID:Tom]. The infectivity analysis of AYVV-ID[ID:Tom] and its associated betasatellite encoded genes using Potato virus X (PVX) vector were carried out in N. benthamiana, indicate that the V2 and betaC1 genes are symptom determinants. We have identified the DNA encoded V2 and its betasatellite, ToLCJB-[ID:ID1:03], encoded betaC1 proteins as efficient silencing suppressors of posttranscriptional gene silencing (PTGS) by using an Agrobacterium co-infiltration or heterologous PVX vector assays. However, the results also showed weak suppression of gene silencing activities for C2 and C4 induced by GFP and mRNA associated with GFP was detected. Furthermore, confocal imaging analysis of ToLCJB-[ID:ID1:03] betaC1 in the epidermal cells of N. benthamiana shows that this protein is accumulated towards the periphery of the cell and around the nucleus, however, V2 accumulated in the cell cytoplasm, C4 associated with plasma membrane and C2 exclusively targeted into nucleus. In this study, we identified as many as four distinct suppressors of RNA silencing encoded by AYVV-ID[ID:Tom] and its cognate betasatellite in the family Geminiviridae, counteracting innate antiviral response.

The Arabidopsis PEAPOD2 transcription factor interacts with geminivirus AL2 protein and the coat protein promoter

The TrAP protein of bipartite begomoviruses activates the coat protein (CP) promoter in mesophyll and derepresses the promoter in vascular tissue through two sequences, located 60-125 bp and 1.2-1.5 kbp respectively, upstream of the CP gene. TrAP does not, however, exhibit specific binding to either sequence directly. We have identified a plant-specific DNA-binding protein, Arabidopsis PEAPOD2 (PPD2), that specifically binds sequences mediating activation of the CP promoter of Tomato golden mosaic virus (TGMV) and Cabbage leaf curl virus in mesophyll. This protein does not however, bind sequences required for TrAP-mediated derepression in phloem. TGMV TrAP interacts with the PPD2/CP promoter complex in electrophoretic mobility shift assays. PPD2 is associated with the nucleus, as expected for a transcription factor, but is not capable of activating transcription directly. Thus, geminivirus TrAP is likely targeted to the CP promoter through interaction with PPD2, leading to activation of CP gene expression.

Regulation of Tomato golden mosaic virus AL2 and AL3 gene expression by a conserved upstream open reading frame

A translational regulatory mechanism for Tomato golden mosaic virus (TGMV) complementary-sense gene expression has been characterized. TGMV transcribes two mRNAs, AL-1935 and AL-1629 transcripts, both of which contain the AL2 and AL3 open reading frames. However, AL2 is only expressed from AL-1629 whereas AL3 is expressed from both. Three AUG translation initiation codons are located upstream of both the AL2 and AL3 coding regions, within the 5'-untranslated region (UTR) of the AL-1935 transcript. Translation can initiate at the first AUG, specifying the C-terminal 122 amino acids of the AL1 protein (cAL1). Initiation of translation at this AUG is inhibitory for the downstream expression of both AL2 and AL3. This is most likely due to the terminator codon of cAL1 being positioned after the AUG initiation codon for the AL2 ORF. The mechanism by which AL3 is expressed from AL-1935 is currently unknown but a gap between the cAL1 termination codon and the start of AL3 suggests that it may involve reinitiation and/or internal initiation. In contrast, expression of AL3 from AL-1629 most likely occurs via leaky ribosome scanning since the AL3 initiation codon occurs before the terminator codon of AL2. Mutation of the AUG encoding cAL1 in the curtovirus, Spinach curly top virus, leads to increased infectivity as measured by a shorter latent period. Together this suggests that geminiviruses use a post-translational regulatory mechanism to regulate the synthesis of viral proteins important for replication and suppression of host defenses.

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.

Identification of sequence elements regulating promoter activity and replication of a monopartite begomovirus-associated DNA beta satellite

DNA beta is a circular single-stranded satellite DNA associated with certain monopartite begomoviruses (family Geminiviridae) which causes economically important diseases such as cotton leaf curl disease. DNA beta contains a single gene, betaC1, which encodes a pathogenicity protein responsible for symptom production. Transient expression studies in Nicotiana tabacum using the beta-glucuronidase reporter gene driven by a betaC1 promoter-deletion series of the DNA beta associated with cotton leaf curl Multan virus identified a 68 nt region (between -139 and -207) which is important for betaC1 transcription. This 68 nt region contains a G-box (CACGTG) located 143 nt upstream of the betaC1 start codon. Mutation of the G-box resulted in a significant reduction in betaC1 promoter activity and DNA beta replication efficiency. In addition, the G-box motif was found to bind specifically to a protein(s) in nuclear extracts prepared from tobacco leaf tissues. Our results indicate that interaction of the G-box motif with host nuclear factors is important for efficient gene expression and replication of DNA beta.

Functional analysis of bipartite begomovirus coat protein promoter sequences

We demonstrate that the AL2 gene of Cabbage leaf curl virus (CaLCuV) activates the CP promoter in mesophyll and acts to derepress the promoter in vascular tissue, similar to that observed for Tomato golden mosaic virus (TGMV). Binding studies indicate that sequences mediating repression and activation of the TGMV and CaLCuV CP promoter specifically bind different nuclear factors common to Nicotiana benthamiana, spinach and tomato. However, chromatin immunoprecipitation demonstrates that TGMV AL2 can interact with both sequences independently. Binding of nuclear protein(s) from different crop species to viral sequences conserved in both bipartite and monopartite begomoviruses, including TGMV, CaLCuV, Pepper golden mosaic virus and Tomato yellow leaf curl virus suggests that bipartite begomoviruses bind common host factors to regulate the CP promoter. This is consistent with a model in which AL2 interacts with different components of the cellular transcription machinery that bind viral sequences important for repression and activation of begomovirus CP promoters.

Supervirulent pseudorecombination and asymmetric synergism between genomic components of two distinct species of begomovirus associated with severe tomato leaf curl disease in India

Isolates of two distinct begomovirus species, the severe strain of the species Tomato leaf curl New Delhi virus (tomato leaf curl New Delhi virus-[India:New Delhi:Severe:1992]; ToLCNDV-[IN:ND:Svr:92], bipartite) and the Varanasi strain of the species Tomato leaf curl Gujarat virus (tomato leaf curl Gujarat virus-[India:Varanasi:2001]; ToLCGV-[IN:Var:01], mono/bipartite) infect tomato (Lycopersicon esculentum) and cause severe yield losses in northern India. This study investigated the infectivity properties of genomic components of these two species. Both pseudorecombinants were infectious in Nicotiana benthamiana, Nicotiana tabacum and L. esculentum. Enhanced pathogenicity was observed when DNA-A of ToLCNDV-[IN:ND:Svr:92] was trans-complemented with ToLCGV-[IN:Var:01] DNA-B, and was consistently associated with an increase in accumulation of ToLCGV-[IN:Var:01] DNA-B. Mixed infection of ToLCNDV-[IN:ND:Svr:92] and ToLCGV-[IN:Var:01] always showed extremely severe symptoms, suggesting a synergistic interaction between these two viruses. Southern blot analysis of viral DNAs from infected plants showed a significantly higher level of accumulation of both ToLCNDV components and DNA-B of ToLCGV-[IN:Var:01] with no alteration to levels of DNA-A of ToLCGV-[IN:Var:01]. Symptom development and/or higher infectivity of the supervirulent pseudorecombinants correlated with the increased levels of DNA-B accumulation. Protoplast replication assays revealed that enhanced infectivity by the pseudorecombinant occurred at the level of replication, as DNA-A of ToLCNDV-[IN:ND:Svr:92] enhanced ToLCGV-[IN:Var:01] DNA-B replication, whose accumulation was in turn increased by ToLCGV-[IN:Var:01] DNA-A. This is the first report demonstrating a virulent pseudorecombinant between two distinct species of begomoviruses that infect tomato, and is the second report on synergism between begomoviruses. The results revealed that ToLCGV-[IN:Var:01] DNA-B is capable of associating with different DNA-A components, despite having different iteron sequences.

Early and late gene expression in pepper huasteco yellow vein virus

Viral infections usually take place in an orderly manner and can be divided into at least two phases: an early and a late stage. In geminiviruses, plant viruses with a circular, single-stranded DNA genome, expression of viral genes involves complex regulation strategies that suggest the existence of a pattern of temporal gene expression. In this work, the transcription of pepper huasteco yellow vein virus (PHYVV) genes was studied. Green fluorescent protein replacements and RT-PCR analyses were used to monitor PHYVV gene expression chronologically in suspension cells and plant tissue. A model is proposed to describe the order of geminivirus gene expression, where the genes that encode Rep, TrAP and REn are expressed during an early stage of infection. The genes that encode the coat protein and the nuclear shuttle protein are expressed during the late stage of infection.

The hypersensitive response to tomato leaf curl New Delhi virus nuclear shuttle protein is inhibited by transcriptional activator protein

The hypersensitive response (HR) is a common feature of plant disease resistance reactions and a type of programmed cell death (PCD). Many pathogens are able to modulate pathways involved in cell death. In contrast to animal viruses, inhibitors of PCD activity have not been identified for plant-infecting viruses. Previously, we have reported that the nuclear shuttle protein (NSP) of Tomato leaf curl New Delhi virus (ToLCNDV) induces an HR in Nicotiana tabacum and Lycopersicon esculentum plants when expressed under the control of the Cauliflower mosaic virus 35S promoter. However, HR is not evident in plants infected with ToLCNDV, suggesting that the virus encodes a factor (or factors) that counters this response. Analysis of all ToLCNDV-encoded genes pinpointed the transcriptional activator protein (TrAP) as the factor mediating the anti-HR effect. Deletion mutagenesis showed the central region of TrAP, containing a zinc finger domain and nuclear localization signal, to be important in inhibiting the HR. These results demonstrate that TrAP counters HR-induced cell death, the first such activity identified for a plant-infecting virus.

The mungbean yellow mosaic begomovirus transcriptional activator protein transactivates the viral promoter-driven transgene and causes toxicity in transgenic tobacco plants

The Begomovirus transcriptional activator protein (TrAP/AC2/C2) is a multifunctional protein which activates the viral late gene promoters, suppresses gene silencing, and determines pathogenicity. To study TrAP-mediated transactivation of a stably integrated gene, we generated transgenic tobacco plants with a Mungbean yellow mosaic virus (MYMV) AV1 late gene promoter-driven reporter gene and supertransformed them with the MYMV TrAP gene driven by a strong 35S promoter. We obtained a single supertransformed plant with an intact 35S-TrAP gene that activated the reporter gene 2.5-fold. However, 10 of the 11 supertransformed plants did not have the TrAP region of the T-DNA, suggesting the likely toxicity of TrAP in plants. Upon transformation of wild-type tobacco plants with the TrAP gene, six of the seven transgenic plants obtained had truncated T-DNAs which lacked TrAP. One plant, which had the intact TrAP gene, did not express TrAP. The apparent toxic effect of the TrAP transgene was abolished by mutations in its nuclear-localization signal or zinc-finger domain and by deletion of its activation domain. Therefore, all three domains of TrAP, which are required for transactivation and suppression of gene silencing, also are needed for its toxic effect.

A conserved binding site within the Tomato golden mosaic virus AL-1629 promoter is necessary for expression of viral genes important for pathogenesis

We have identified a nine base pair sequence in Tomato golden mosaic virus that is required for binding of nuclear proteins from tobacco and Arabidopsis to viral DNA. The sequence is located within the promoter for a 0.7 kb complementary sense mRNA (AL-1629). Mutation of the binding site results in a two- to six-fold reduction in the accumulation of AL-1629 mRNA, leading to reduced AL2 and AL3 gene expression. Viral sequences located immediately adjacent to the core binding site appear to influence AL2 and AL3 expression, but retain some binding affinity to a soluble host protein(s). The ability of a nuclear protein(s) to bind sequences within the AL-1629 promoter correlates with efficient viral DNA replication, as mutation of these sequences results in reduced viral DNA levels. Analysis of begomo- and curtoviruses indicates extensive conservation of this binding site, which suggests a common mechanism regulating expression of two viral genes involved in replication and suppression of host defense responses.

Functional modulation of the geminivirus AL2 transcription factor and silencing suppressor by self-interaction

The DNA genomes of geminiviruses have a limited coding capacity that is compensated for by the production of small multifunctional proteins. The AL2 protein encoded by members of the genus Begomovirus (e.g., Tomato golden mosaic virus) is a transcriptional activator, a silencing suppressor, and a suppressor of a basal defense. The related L2 protein of Beet curly top virus (genus Curtovirus) shares the pathogenicity functions of AL2 but lacks transcriptional activation activity. It is known that AL2 and L2 can suppress local silencing by interacting with adenosine kinase (ADK) and can suppress basal defense by interacting with SNF1 kinase. However, how the activities of these viral proteins are regulated remains an unanswered question. Here, we provide some answers by demonstrating that AL2, but not L2, interacts with itself. The zinc finger-like motif (CCHC) is required but is not sufficient for AL2 self-interaction. Alanine substitutions for the invariant cysteine residues that comprise the motif abolish self-interaction or cause aberrant subnuclear localization but do not abolish interaction with ADK and SNF1. Using bimolecular fluorescence complementation, we show that AL2:AL2 complexes accumulate primarily in the nucleus, whereas AL2:ADK and L2:ADK complexes accumulate mainly in the cytoplasm. Further, the cysteine residue mutations impair the ability of AL2 to activate the coat protein promoter but do not affect local silencing suppression. Thus, AL2 self-interaction correlates with nuclear localization and efficient activation of transcription, whereas AL2 and L2 monomers can suppress local silencing by interacting with ADK in the cytoplasm.

AL1-dependent repression of transcription enhances expression of Tomato golden mosaic virus AL2 and AL3

Studies using Nicotiana benthamiana protoplasts have determined that repression of upstream transcription by AL1 protein enhances AL2 and AL3 expression in Tomato golden mosaic virus (TGMV). Mutations resulting in the inability of TGMV AL1 protein to associate with its cognate binding site, result in a decrease in both AL2 and AL3 expression. Reduced expression correlates with an increase in transcription from the AL62 start site, and decreased transcription from downstream initiation sites (AL1935 and AL1629) present within the AL1 coding region. The results demonstrate that, in a tobacco protoplast system, repression of AL62 transcription, regulated through binding of AL1 protein to sequences in the origin of replication, is required prior to AL2 and AL3 gene expression from the AL1935 and AL1629 viral transcripts. This provides a mechanism to regulate expression of AL2, which is involved in suppression of host defense responses and is required for late gene expression.

Transcriptional analysis of complementary sense genes in Spinach curly top virus and functional role of C2 in pathogenesis

Spinach curly top virus (SCTV), the fifth characterized Curtovirus species belonging to the family Geminiviridae, is an agriculturally significant plant pathogen representing an emerging disease threat in the southern United States. The SCTV genome comprises a single DNA chromosome of approximately 3.0 kb, with the potential to code for seven proteins larger than 10 kDa but which relies extensively on the host for replication and transcription of its genome. In this study, we have identified viral and complementary sense transcripts in SCTV-infected plants, confirming a bidirectional transcription strategy for SCTV. The most abundant RNA maps to the virion sense (1.1-kb transcript) and is comparable in size and location to that observed in Beet curly top virus (BCTV). Two complementary sense transcripts (1.7 and 0.7 kb) were identified in SCTV-infected plants. The large, 1.7-kb transcript is comparable in size and position to that identified in BCTV and several begomoviruses and most likely encodes the C1 protein. Both complementary sense RNAs could potentially direct expression of C2 and C3 from polycistronic mRNAs. A mutation in the C2 gene of SCTV results in expression of a truncated protein of 38 amino acids that is capable of interacting with two cellular kinases, AKIN11 and ADK2, and the resulting mutant virus remains highly infectious. A second mutant virus can only express the first three amino acids of the C2 protein and is unable to interact with the same kinases. However, this mutant virus still remains infectious, although a reduction in infectivity and symptom severity was seen in both Arabidopsis and spinach. A possible relationship between the interaction of C2 with AKIN11 and ADK2 and disease severity is presented.

Distinct viral sequence elements are necessary for expression of Tomato golden mosaic virus complementary sense transcripts that direct AL2 and AL3 gene expression

Transient expression studies using Nicotiana benthamiana protoplasts and plants have identified sequences important for transcription of complementary sense RNAs derived from Tomato golden mosaic virus (TGMV) DNA component A that direct expression of AL2 and AL3. Transcription of two complementary sense RNAs, initiating at nucleotides 1,935 (AL1935) and 1,629 (AL1629), is directed by unique sequences located upstream of each transcription initiation site. One element is located between 28 and 124 nucleotides (nt) upstream of the AL1935 transcription start site, which differs from a second element located 150 nt downstream, between 129 and 184 nt upstream of the AL1629 transcription start site. Transcription initiation at nucleotide 1,935 is lower than that at nucleotide 1,629 as determined by run-on transcription assays, and the resulting transcript is only capable of expressing AL3. The transcript initiating at nucleotide 1,629 is capable of directing expression of both AL2 and AL3, although expression of AL3 is up to fourfold greater than that for AL2. Nuclear factors purified from tobacco suspension cells bind to sequences upstream of both AL1935 and AL1629, correlating with the ability of these sequences to direct gene expression. Thus, in tobacco, regulatory sequences direct transcription of two unique TGMV messenger RNAs that differentially express AL2 and AL3.

Promoters, transcripts, and regulatory proteins of Mungbean yellow mosaic geminivirus

Geminiviruses package circular single-stranded DNA and replicate in the nucleus via a double-stranded intermediate. This intermediate also serves as a template for bidirectional transcription by polymerase II. Here, we map promoters and transcripts and characterize regulatory proteins of Mungbean yellow mosaic virus-Vigna (MYMV), a bipartite geminivirus in the genus Begomovirus. The following new features, which might also apply to other begomoviruses, were revealed in MYMV. The leftward and rightward promoters on DNA-B share the transcription activator AC2-responsive region, which does not overlap the common region that is nearly identical in the two DNA components. The transcription unit for BC1 (movement protein) includes a conserved, leader-based intron. Besides negative-feedback regulation of its own leftward promoter on DNA-A, the replication protein AC1, in cooperation with AC2, synergistically transactivates the rightward promoter, which drives a dicistronic transcription unit for the coat protein AV1. AC2 and the replication enhancer AC3 are expressed from one dicistronic transcript driven by a strong promoter mapped within the upstream AC1 gene. Early and constitutive expression of AC2 is consistent with its essential dual function as an activator of viral transcription and a suppressor of silencing.