Interaction of DNA with the movement proteins of geminiviruses revisited

Disease Tolerance in 'Anaheim' Pepper to PepGMV-D Strain Involves Complex Interactions Between the Movement Protein Putative Promoter Region and Unknown Host Factors

Pepper golden mosaic virus (PepGMV) is a bipartite begomovirus of pepper and tomato from North America. In 'Anaheim' pepper plants PepGMV-Mo strain (Mo) causes systemic yellow foliar mosaic symptoms, while PepGMV-D strain (D) causes distortion of 1st-6th expanding leaves, and asymptomatic infection of subsequently developing leaves, like other known 'recovery' phenotypes. Infections established with DNA-A Mo and D components expressing red-shifted green fluorescent protein in place of coat protein and in situ hybridization, showed PepGMV-Mo localized to phloem and mesophyll cells, while -D was mesophyll restricted. Alignment of PepGMV-Mo and -D DNA-B components revealed three indels upstream of the BC1 gene that encodes the movement protein (MP). To determine if this non-coding region (*BC1) D-strain MP putative promoter contributed to 'recovery', plants were inoculated with chimeric DNA-B Mo/D components harboring reciprocally exchanged *BC1, and wild-type DNA-A Mo and D components. Symptoms were reminiscent but not identical to wild-type -Mo or -D infection, respectively, suggesting 'recovery' cannot be attributed solely to the *BC1. Both BC1 and D*BC1 were targeted by post-transcriptional gene silencing; however, 'recovered' leaves accumulated fewer transcripts and 21-24 nt vsiRNAs. Thus, inefficient in planta movement of PepGMV-D is associated with a non-pepper-adapted 'defective' BC1 that facilitates hyper-efficient PTGS, leading to BC1 transcript degradation that in turn limits virus spread, thereby recapitulating disease 'tolerance'.

Mutation of the conserved late element in geminivirus CP promoters abolishes Arabidopsis TCP24 transcription factor binding and decreases H3K27me3 levels on viral chromatin

In geminiviruses belonging to the genus Begomovirus, coat protein (CP) expression depends on viral AL2 protein, which derepresses and activates the CP promoter through sequence elements that lie within the viral intergenic region (IR). However, AL2 does not exhibit sequence-specific DNA binding activity but is instead directed to responsive promoters through interactions with host factors, most likely transcriptional activators and/or repressors. In this study, we describe a repressive plant-specific transcription factor, Arabidopsis thaliana TCP24 (AtTCP24), that interacts with AL2 and recognizes a class II TCP binding site in the CP promoter (GTGGTCCC). This motif corresponds to the previously identified conserved late element (CLE). We also report that histone 3 lysine 27 trimethylation (H3K27me3), an epigenetic mark associated with facultative repression, is enriched over the viral IR. H3K27me3 is deposited by Polycomb Repressive Complex 2 (PRC2), a critical regulator of gene expression and development in plants and animals. Remarkably, mutation of the TCP24 binding site (the CLE) in tomato golden mosaic virus (TGMV) and cabbage leaf curl virus (CaLCuV) CP promoters greatly diminishes H3K27me3 levels on viral chromatin and causes a dramatic delay and attenuation of disease symptoms in infected Arabidopsis and Nicotiana benthamiana plants. Symptom remission is accompanied by decreased viral DNA levels in systemically infected tissue. Nevertheless, in transient replication assays CLE mutation delays but does not limit the accumulation of viral double-stranded DNA, although single-stranded DNA and CP mRNA levels are decreased. These findings suggest that TCP24 binding to the CLE leads to CP promoter repression and H3K27me3 deposition, while TCP24-AL2 interaction may recruit AL2 to derepress and activate the promoter. Thus, a repressive host transcription factor may be repurposed to target a viral factor essential for promoter activity. The presence of the CLE in many begomoviruses suggests a common scheme for late promoter regulation.

Reassortments in single-stranded DNA multipartite viruses: Confronting expectations based on molecular constraints with field observations

Single-stranded DNA multipartite viruses, which mostly consist of members of the genus Begomovirus, family Geminiviridae, and all members of the family Nanoviridae, partly resolve the cost of genomic integrity maintenance through two remarkable capacities. They are able to systemically infect a host even when their genomic segments are not together in the same host cell, and these segments can be separately transmitted by insect vectors from host to host. These capacities potentially allow such viruses to reassort at a much larger spatial scale, since reassortants could arise from parental genotypes that do not co-infect the same cell or even the same host. To assess the limitations affecting reassortment and their implications in genome integrity maintenance, the objective of this review is to identify putative molecular constraints influencing reassorted segments throughout the infection cycle and to confront expectations based on these constraints with empirical observations. Trans-replication of the reassorted segments emerges as the major constraint, while encapsidation, viral movement, and transmission compatibilities appear more permissive. Confronting the available molecular data and the resulting predictions on reassortments to field population surveys reveals notable discrepancies, particularly a surprising rarity of interspecific natural reassortments within the Nanoviridae family. These apparent discrepancies unveil important knowledge gaps in the biology of ssDNA multipartite viruses and call for further investigation on the role of reassortment in their biology.

The Past, Present, and Future of Wheat Dwarf Virus Management-A Review

Wheat dwarf disease (WDD) is an important disease of monocotyledonous species, including economically important cereals. The causative pathogen, wheat dwarf virus (WDV), is persistently transmitted mainly by the leafhopper Psammotettix alienus and can lead to high yield losses. Due to climate change, the periods of vector activity increased, and the vectors have spread to new habitats, leading to an increased importance of WDV in large parts of Europe. In the light of integrated pest management, cultivation practices and the use of resistant/tolerant host plants are currently the only effective methods to control WDV. However, knowledge of the pathosystem and epidemiology of WDD is limited, and the few known sources of genetic tolerance indicate that further research is needed. Considering the economic importance of WDD and its likely increasing relevance in the coming decades, this study provides a comprehensive compilation of knowledge on the most important aspects with information on the causal virus, its vector, symptoms, host range, and control strategies. In addition, the current status of genetic and breeding efforts to control and manage this disease in wheat will be discussed, as this is crucial to effectively manage the disease under changing environmental conditions and minimize impending yield losses.

Geminivirus C5 proteins mediate formation of virus complexes at plasmodesmata for viral intercellular movement

Movement proteins (MPs) encoded by plant viruses deliver viral genomes to plasmodesmata (PD) to ensure intracellular and intercellular transport. However, how the MPs encoded by monopartite geminiviruses are targeted to PD is obscure. Here, we demonstrate that the C5 protein of tomato yellow leaf curl virus (TYLCV) anchors to PD during the viral infection following trafficking from the nucleus along microfilaments in Nicotiana benthamiana. C5 could move between cells and partially complement the traffic of a movement-deficient turnip mosaic virus (TuMV) mutant (TuMV-GFP-P3N-PIPO-m1) into adjacent cells. The TYLCV-C5 null mutant (TYLCV-mC5) attenuates viral pathogenicity and decreases viral DNA and protein accumulation, and ectopic overexpression of C5 enhances viral DNA accumulation. Interaction assays between TYLCV-C5 and the other eight viral proteins described in TYLCV reveal that C5 associates with C2 in the nucleus and with V2 in the cytoplasm and at PD. The V2 protein is mainly localized in the nucleus and cytoplasmic granules when expressed alone; in contrast, V2 forms small punctate granules at PD when co-expressed with C5 or in TYLCV-infected cells. The interaction of V2 and C5 also facilitates their nuclear export. Furthermore, C5-mediated PD localization of V2 is conserved in two other geminiviruses. Therefore, this study solves a long-sought-after functional connection between PD and the geminivirus movement and improves our understanding of geminivirus-encoded MPs and their potential cellular and molecular mechanisms.

Begomovirus-Host Interactions: Viral Proteins Orchestrating Intra and Intercellular Transport of Viral DNA While Suppressing Host Defense Mechanisms

Begomoviruses, which belong to the Geminiviridae family, are intracellular parasites transmitted by whiteflies to dicotyledonous plants thatsignificantly damage agronomically relevant crops. These nucleus-replicating DNA viruses move intracellularly from the nucleus to the cytoplasm and then, like other plant viruses, cause disease by spreading systemically throughout the plant. The transport proteins of begomoviruses play a crucial role in recruiting host components for the movement of viral DNA within and between cells, while exhibiting functions that suppress the host's immune defense. Pioneering studies on species of the Begomovirus genus have identified specific viral transport proteins involved in intracellular transport, cell-to-cell movement, and systemic spread. Recent research has primarily focused on viral movement proteins and their interactions with the cellular host transport machinery, which has significantly expanded understanding on viral infection pathways. This review focuses on three components within this context: (i) the role of viral transport proteins, specifically movement proteins (MPs) and nuclear shuttle proteins (NSPs), (ii) their ability to recruit host factors for intra- and intercellular viral movement, and (iii) the suppression of antiviral immunity, with a particular emphasis on bipartite begomoviral movement proteins.

Geminiviral betasatellites: critical viral ammunition to conquer plant immunity

Geminiviruses have mastered plant cell modulation and immune invasion to ensue prolific infection. Encoding a relatively small number of multifunctional proteins, geminiviruses rely on satellites to efficiently re-wire plant immunity, thereby fostering virulence. Among the known satellites, betasatellites have been the most extensively investigated. They contribute significantly to virulence, enhance virus accumulation, and induce disease symptoms. To date, only two betasatellite proteins, βC1, and βV1, have been shown to play a crucial role in virus infection. In this review, we offer an overview of plant responses to betasatellites and counter-defense strategies deployed by betasatellites to overcome those responses.

Virus-virus interactions alter the mechanical transmissibility and host range of begomoviruses

Introduction

Begomoviruses are mainly transmitted by whiteflies. However, a few begomoviruses can be transmitted mechanically. Mechanical transmissibility affects begomoviral distribution in the field.

Materials and methods

In this study, two mechanically transmissible begomoviruses, tomato leaf curl New Delhi virus-oriental melon isolate (ToLCNDV-OM) and tomato yellow leaf curl Thailand virus (TYLCTHV), and two nonmechanically transmissible begomoviruses, ToLCNDV-cucumber isolate (ToLCNDV-CB) and tomato leaf curl Taiwan virus (ToLCTV), were used to study the effects of virus-virus interactions on mechanical transmissibility.

Results

Nicotiana benthamiana and host plants were coinoculated through mechanical transmission with inoculants derived from plants that were mix-infected or inoculants derived from individually infected plants, and the inoculants were mixed immediately before inoculation. Our results showed that ToLCNDV-CB was mechanically transmitted with ToLCNDV-OM to N. benthamiana, cucumber, and oriental melon, whereas ToLCTV was mechanically transmitted with TYLCTHV to N. benthamiana and tomato. For crossing host range inoculation, ToLCNDV-CB was mechanically transmitted with TYLCTHV to N. benthamiana and its nonhost tomato, while ToLCTV with ToLCNDV-OM was transmitted to N. benthamiana and its nonhost oriental melon. For sequential inoculation, ToLCNDV-CB and ToLCTV were mechanically transmitted to N. benthamiana plants that were either preinfected with ToLCNDV-OM or TYLCTHV. The results of fluorescence resonance energy transfer analyses showed that the nuclear shuttle protein of ToLCNDV-CB (CBNSP) and the coat protein of ToLCTV (TWCP) localized alone to the nucleus. When coexpressed with movement proteins of ToLCNDV-OM or TYLCTHV, CBNSP and TWCP relocalized to both the nucleus and the cellular periphery and interacted with movement proteins.

Discussion

Our findings indicated that virus-virus interactions in mixed infection circumstances could complement the mechanical transmissibility of nonmechanically transmissible begomoviruses and alter their host range. These findings provide new insight into complex virus-virus interactions and will help us to understand the begomoviral distribution and to reevaluate disease management strategies in the field.

Limited role of recombination in the global diversification of begomovirus DNA-B proteins

Approximately half of the characterized begomoviruses have bipartite genomes, but the second genomic segment, the DNA-B, is understudied relative to the DNA-A, which is homologous to the entire genome of monopartite begomoviruses. We examined the evolutionary history of the two proteins encoded by the DNA-B, the genes of which make up ∼60% of the DNA-B segment, from all bipartite begomovirus species. Our dataset of 131 movement protein (MP) and nuclear shuttle protein (NSP) sequences confirmed the deep split between Old World (OW) and New World (NW) species, and showed strong support for deep, congruent branches among the OW sequences of the MP and NSP. NW sequences were much less diverse and had poor phylogenetic resolution; over half of nodes in both the NSP and MP NW clades were supported by <50% bootstrap support. This poor resolution hampered our ability to detect incongruent phylogenies between the MP and NSP datasets, and we found no statistical evidence for recombination within our MP and NSP datasets. Finally, we quantified the sequence diversity between the NW and OW proteins, showing that the NW MP has particularly low diversity, suggesting it has been subject to different evolutionary pressures than the NW NSP.

Interspecies Recombination-Led Speciation of a Novel Geminivirus in Pakistan

Recombination between isolates of different virus species has been known to be one of the sources of speciation. Weeds serve as mixing vessels for begomoviruses, infecting a wide range of economically important plants, thereby facilitating recombination. Chenopodium album is an economically important weed spread worldwide. Here, we present the molecular characterization of a novel recombinant begomovirus identified from C. album in Lahore, Pakistan. The complete DNA- A genome of the virus associated with the leaf distortion occurred in the infected C. album plants was cloned and sequenced. DNA sequence analysis showed that the nucleotide sequence of the virus shared 93% identity with those of the rose leaf curl virus and the duranta leaf curl virus. Interestingly, this newly identified virus is composed of open reading frames (ORFs) from different origins. Phylogenetic networks and complementary recombination detection methods revealed extensive recombination among the sequences. The infectious clone of the newly detected virus was found to be fully infectious in C. album and Nicotiana benthamiana as the viral DNA was successfully reconstituted from systemically infected tissues of inoculated plants, thus fulfilling Koch's postulates. Our study reveals a new speciation of an emergent ssDNA plant virus associated with C. album through recombination and therefore, proposed the tentative name 'Chenopodium leaf distortion virus' (CLDV).

Manipulating resistance to mungbean yellow mosaic virus in greengram (Vigna radiata L): Through CRISPR/Cas9 mediated editing of the viral genome

Clustered regularly interspaced short palindromic repeats (CRISPR) associated protein (CRISPR/Cas9) is an adaptive immune system of bacteria to counter the impending viral pathogen attack. With persistent improvements, CRISPR has become a versatile tool for developing molecular immunity against viruses in plants. In the current report, we utilized the Cas9 endonuclease and dual 20 bp-gRNAs targeting two different locations in single-stranded DNA-A of AC1 (rep protein) and AV1 (coat protein) of mungbean yellow mosaic virus for achieving resistance in greengram. The cotyledonary nodal explants were infected with Agrobacterium strain EHA105 harboring pMDC100-Cas9 with AC1 and AV1 gRNA cassettes and generated transgenic plants. The integration of Cas9 and gRNA cassettes in the transformed plants of greengram were confirmed by PCR and dot blot assays. Agroinfiltrated T2 transgenic lines exhibited minimal mosaic symptoms. A drastic reduction in the accumulation of AC1 and AV1 was observed in T2 transformed lines. The T7EI assay indicated that AC1 fragments were edited at a frequency of 46%, 32%, 20%, and AV1 at 38.15%, 40%, and 21.36% in MYMV infected greengram lines T2-6-2-3, T2-6-4-4, and T2-6-4-7, respectively. The manipulation of resistance to MYMV through the editing of the pathogen genome using the CRISPR/Cas9 tool can be a powerful approach to combat viruses and develop resistance in greengram.

Identification of a Novel Geminivirus in Fraxinus rhynchophylla in Korea

Fraxinus rhynchophylla, common name ash, belongs to the family Oleaceae and is found in China, Korea, North America, the Indian subcontinent, and eastern Russia. It has been used as a traditional herbal medicine in Korea and various parts of the world due to its chemical constituents. During a field survey in March 2019, mild vein thickening (almost negligible) was observed in a few ash trees. High-throughput sequencing of libraries of total DNA from ash trees, rolling-circle amplification (RCA), and polymerase chain reaction (PCR) allowed the identification of a Fraxinus symptomless virus. This virus has five confirmed open reading frames along with a possible sixth open reading frame that encodes the movement protein and is almost 2.7 kb in size, with a nonanucleotide and stem loop structure identical to begomoviruses. In terms of its size and structure, this virus strongly resembles begomoviruses, but does not show any significant sequence identity with them. To confirm movement of the virus within the trees, different parts of infected trees were examined, and viral movement was successfully observed. No satellite molecules or DNA B were identified. Two-step PCR confirmed the virion and complementary strands during replication in both freshly collected infected samples of ash tree and Nicotiana benthamiana samples agro-inoculated with infectious clones. This taxon is so distantly grouped from other known geminiviruses that it likely represents a new geminivirus genus.

Novel circular DNA virus identified in Opuntia discolor (Cactaceae) that codes for proteins with similarity to those of geminiviruses

Viral metagenomic studies have enabled the discovery of many unknown viruses and revealed that viral communities are much more diverse and ubiquitous than previously thought. Some viruses have multiple genome components that are encapsidated either in separate virions (multipartite viruses) or in the same virion (segmented viruses). In this study, we identify what is possibly a novel bipartite plant-associated circular single-stranded DNA virus in a wild prickly pear cactus, Opuntia discolor, that is endemic to the Chaco ecoregion in South America. Two ~1.8 kb virus-like circular DNA components were recovered, one encoding a replication-associated protein (Rep) and the other a capsid protein (CP). Both of the inferred protein sequences of the Rep and CP are homologous to those encoded by members of the family Geminiviridae. These two putatively cognate components each have a nonanucleotide sequence within a likely hairpin structure that is homologous to the origins of rolling-circle replication (RCR), found in diverse circular single-stranded DNA viruses. In addition, the two components share similar putative replication-associated iterative sequences (iterons), which in circular single-stranded DNA viruses are important for Rep binding during the initiation of RCR. Such molecular features provide support for the possible bipartite nature of this virus, which we named utkilio virus (common name of the Opuntia discolor in South America) components A and B. In the infectivity assays conducted in Nicotiana benthamiana plants, only the A component of utkilio virus, which encodes the Rep protein, was found to move and replicate systemically in N. benthamiana. This was not true for component B, for which we did not detect replication, which may have been due to this being a defective molecule or because of the model plants (N. benthamiana) used for the infection assays. Future experiments need to be conducted with other plants, including O. discolor, to understand more about the biology of these viral components.

Dynamic subcellular distribution of begomoviral nuclear shuttle and movement proteins

The Abutilon mosaic virus (AbMV) encodes a nuclear shuttle protein (NSP), and a movement protein (MP) which cooperatively accomplish viral DNA transport through the plant. Subcellular distribution patterns of fluorescent protein-tagged NSP and MP were tracked in Nicotiana benthamiana leaves in presence or absence of an AbMV infection using light microscopy. NSP was located within the nucleus and associated with early endosomes in the presence of MP. MP appeared at the plasma membrane, plasmodesmata and in motile vesicles, trafficking along the endoplasmic reticulum in an actin-dependent manner. MP and NSP did not co-localize and employed separate cellular pathways. Correspondingly, Förster resonance energy transfer analysis did not support physical interaction between NSP and MP. Time lapse movies illustrate the cellular dynamics of both proteins on their way around the nucleus and to the cell periphery and provide a first hint for the nuclear egress of NSP complexes.

A plant-specific syntaxin-6 protein contributes to the intracytoplasmic route for the begomovirus CabLCV

Because of limited free diffusion in the cytoplasm, viruses must use active transport mechanisms to move intracellularly. Nevertheless, how the plant single-stranded DNA begomoviruses hijack the host intracytoplasmic transport machinery to move from the nucleus to the plasmodesmata remains enigmatic. Here, we identified nuclear shuttle protein (NSP)-interacting proteins from Arabidopsis (Arabidopsis thaliana) by probing a protein microarray and demonstrated that the cabbage leaf curl virus NSP, a facilitator of the nucleocytoplasmic trafficking of viral (v)DNA, interacts in planta with an endosomal vesicle-localized, plant-specific syntaxin-6 protein, designated NSP-interacting syntaxin domain-containing protein (NISP). NISP displays a proviral function, unlike the syntaxin-6 paralog AT2G18860 that failed to interact with NSP. Consistent with these findings, nisp-1 mutant plants were less susceptible to begomovirus infection, a phenotype reversed by NISP complementation. NISP-overexpressing lines accumulated higher levels of vDNA than wild-type. Furthermore, NISP interacted with an NSP-interacting GTPase (NIG) involved in NSP-vDNA nucleocytoplasmic translocation. The NISP-NIG interaction was enhanced by NSP. We also showed that endosomal NISP associates with vDNA. NISP may function as a docking site for recruiting NIG and NSP into endosomes, providing a mechanism for the intracytoplasmic translocation of the NSP-vDNA complex toward and from the cell periphery.

A Begomovirus Nuclear Shuttle Protein-Interacting Immune Hub: Hijacking Host Transport Activities and Suppressing Incompatible Functions

Begomoviruses (Geminiviridae family) represent a severe constraint to agriculture worldwide. As ssDNA viruses that replicate in the nuclei of infected cells, the nascent viral DNA has to move to the cytoplasm and then to the adjacent cell to cause disease. The begomovirus nuclear shuttle protein (NSP) assists the intracellular transport of viral DNA from the nucleus to the cytoplasm and cooperates with the movement protein (MP) for the cell-to-cell translocation of viral DNA to uninfected cells. As a facilitator of intra- and intercellular transport of viral DNA, NSP is predicted to associate with host proteins from the nuclear export machinery, the intracytoplasmic active transport system, and the cell-to-cell transport complex. Furthermore, NSP functions as a virulence factor that suppresses antiviral immunity against begomoviruses. In this review, we focus on the protein-protein network that converges on NSP with a high degree of centrality and forms an immune hub against begomoviruses. We also describe the compatible host functions hijacked by NSP to promote the nucleocytoplasmic and intracytoplasmic movement of viral DNA. Finally, we discuss the NSP virulence function as a suppressor of the recently described NSP-interacting kinase 1 (NIK1)-mediated antiviral immunity. Understanding the NSP-host protein-protein interaction (PPI) network will probably pave the way for strategies to generate more durable resistance against begomoviruses.

Fine-tuning expression of begomoviral movement and nuclear shuttle proteins confers cell-to-cell movement to mastreviral replicons in Nicotiana benthamiana leaves

Geminiviruses are a group of small plant viruses responsible for devastating crop damage worldwide. The emergence of agricultural diseases caused by geminiviruses is attributed in part to their high rates of recombination, leading to complementary function between viral components across species and genera. We have developed a mastreviral reporter system based on bean yellow dwarf virus (BeYDV) that replicates to high levels in the plant nucleus, expressing very high levels of GFP. To investigate the potential for complementation of movement function by other geminivirus genera, the movement protein (MP) and nuclear shuttle protein (NSP) from the bipartite begomovirus Bean dwarf mosaic virus (BDMV) were produced and characterized in Nicotiana benthamiana leaves. While overexpression of MP and NSP strongly inhibited GFP expression from the mastreviral reporter and caused adverse plant symptoms, optimizing the expression levels of MP and NSP allowed functional cell-to-cell movement. Hybrid virus vectors were created that express BDMV MP and NSP from mastreviral replicons, allowing efficient cell-to-cell movement comparable to native BDMV replicons. We find that the expression levels of MP and NSP must be fine-tuned to provide sufficient MP/NSP for movement without eliciting the plant hypersensitive response or adversely impacting gene expression from viral replicons. The ability to confer cell-to-cell movement to mastrevirus replicons depended strongly on replicon size: 2.1-2.7 kb replicons were efficiently moved, while 3 kb replicons were inhibited, and 3.9 kb replicons were very strongly inhibited. Optimized expression of MP/NSP from the normally phloem-limited Abutilon mosaic virus (AbMV) allows efficient movement in non-phloem cells.

Different forms of African cassava mosaic virus capsid protein within plants and virions

One geminiviral gene encodes the capsid protein (CP), which can appear as several bands after electrophoresis depending on virus and plant. African cassava mosaic virus-Nigeria CP in Nicotiana benthamiana, however, yielded one band (~ 30 kDa) in total protein extracts and purified virions, although its expression in yeast yielded two bands (~ 30, 32 kDa). Mass spectrometry of the complete protein and its tryptic fragments from virions is consistent with a cleaved start M1, acetylated S2, and partial phosphorylation at T12, S25 and S62. Mutants for additional potentially modified sites (N223A; C235A) were fully infectious and formed geminiparticles. Separation in triton acetic acid urea gels confirmed charge changes of the CP between plants and yeast indicating differential phosphorylation. If the CP gene alone was expressed in plants, multiple bands were observed like in yeast. A high turnover rate indicates that post-translational modifications promote CP decay probably via the ubiquitin-triggered proteasomal pathway.

Begomoviral Movement Protein Effects in Human and Plant Cells: Towards New Potential Interaction Partners

Geminiviral single-stranded circular DNA genomes replicate in nuclei so that the progeny DNA has to cross both the nuclear envelope and the plasmodesmata for systemic spread within plant tissues. For intra- and intercellular transport, two proteins are required: a nuclear shuttle protein (NSP) and a movement protein (MP). New characteristics of ectopically produced Abutilon mosaic virus (AbMV) MP (MPAbMV), either authentically expressed or fused to a yellow fluorescent protein or epitope tags, respectively, were determined by localization studies in mammalian cell lines in comparison to plant cells. Wild-type MPAbMV and the distinct MPAbMV: reporter protein fusions appeared as curled threads throughout mammalian cells. Co-staining with cytoskeleton markers for actin, intermediate filaments, or microtubules identified these threads as re-organized microtubules. These were, however, not stabilized by the viral MP, as demonstrated by nocodazole treatment. The MP of a related bipartite New World begomovirus, Cleome leaf crumple virus (ClLCrV), resulted in the same intensified microtubule bundling, whereas that of a nanovirus did not. The C-terminal section of MPAbMV, i.e., the protein's oligomerization domain, was dispensable for the effect. However, MP expression in plant cells did not affect the microtubules network. Since plant epidermal cells are quiescent whilst mammalian cells are proliferating, the replication-associated protein RepAbMV protein was then co-expressed with MPAbMV to induce cell progression into S-phase, thereby inducing distinct microtubule bundling without MP recruitment to the newly formed threads. Co-immunoprecipitation of MPAbMV in the presence of RepAbMV, followed by mass spectrometry identified potential novel MPAbMV-host interaction partners: the peptidyl-prolyl cis-trans isomerase NIMA-interacting 4 (Pin4) and stomatal cytokinesis defective 2 (SCD2) proteins. Possible roles of these putative interaction partners in the begomoviral life cycle and cytoskeletal association modes are discussed.

Dynamic Virus-Dependent Subnuclear Localization of the Capsid Protein from a Geminivirus

Viruses are intracellular parasites with a nucleic acid genome and a proteinaceous capsid. Viral capsids are formed of at least one virus-encoded capsid protein (CP), which is often multifunctional, playing additional non-structural roles during the infection cycle. In animal viruses, there are examples of differential localization of CPs associated to the progression of the infection and/or enabled by other viral proteins; these changes in the distribution of CPs may ultimately regulate the involvement of these proteins in different viral functions. In this work, we analyze the subcellular localization of a GFP- or RFP-fused CP from the plant virus Tomato yellow leaf curl virus (TYLCV; Fam. Geminiviridae) in the presence or absence of the virus upon transient expression in the host plants Nicotiana benthamiana and tomato. Our findings show that, in agreement with previous reports, when the CP is expressed alone it localizes mainly in the nucleolus and weakly in the nucleoplasm. Interestingly, the presence of the virus causes the sequential re-localization of the CP outside of the nucleolus and into discrete nuclear foci and, eventually, into an uneven distribution in the nucleoplasm. Expression of the viral replication-associated protein, Rep, is sufficient to exclude the CP from the nucleolus, but the localization of the CP in the characteristic patterns induced by the virus cannot be recapitulated by co-expression with any individual viral protein. Our results demonstrate that the subcellular distribution of the CP is a dynamic process, temporally regulated throughout the progression of the infection. The regulation of the localization of the CP is determined by the presence of other viral components or changes in the cellular environment induced by the virus, and is likely to contribute to the multifunctionality of this protein. Bearing in mind these observations, we suggest that viral proteins should be studied in the context of the infection and considering the temporal dimension in order to comprehensively understand their roles and effects in the interaction between virus and host.

Small RNA and methylation responses in susceptible and tolerant landraces of cassava infected with South African cassava mosaic virus

Endogenous small RNAs (sRNAs) associated with gene regulatory mechanisms respond to virus infection, and virus-derived small RNAs (vsRNAs) have been implicated in recovery or symptom remission in some geminivirus-host interactions. Transcriptional gene silencing (TGS) (24 nt vsRNAs) and post transcriptional gene silencing (PTGS) (21-23 nt vsRNAs) have been associated with geminivirus intergenic (IR) and coding regions, respectively. In this Illumina deep sequencing study, we compared for the first time, the small RNA response to South African cassava mosaic virus (SACMV) of cassava landrace TME3 which shows a recovery and tolerant phenotype, and T200, a highly susceptible landrace. Interestingly, different patterns in the percentage of SACMV-induced normalized total endogenous sRNA reads were observed between T200 and TME3. Notably in virus-infected T200 there was an increase in 21 nt sRNAs during the early pre-symptomatic response (12dpi) compared to mock, while in TME3, the 22 nt sRNA size class was predominant at 32days post infection with SACMV. While vsRNAs of 21-24 nt size classes mapped to the entire SACMV DNA-A and DNA-B genome components in T200 and TME3, vsRNA population counts were lower at 32 (symptomatic stage) and 67 dpi (recovery stage) in tolerant TME3 compared with T200 (non-recovery). It is suggested that the high accumulation of primary vsRNAs, which correlated with high virus titers and severe symptoms in susceptible T200, may be due to failure to target SACMV-derived mRNA. Likewise, in contrast, in TME3 low vsRNA counts may represent efficient PTGS of viral mRNA, leading to a depletion/sequestration of vsRNA populations, supporting a role for PTGS in tolerance/recovery in TME3. Notably, in TME3 at recovery (67 dpi) the percentage (expressed as a percentage of total vsRNA counts) of redundant and non-redundant (unique) 24 nt vsRNAs increased dramatically. Since methylation of the SACMV genome was not detected by bisulfite sequencing, and vsRNA counts targeting the intergenic region (where the promoters reside) were very low in both the tolerant or susceptible landraces, we could not provide conclusive evidence that 24 nt vsRNA-mediated RNA directed genome methylation plays a central role in disease phenotype in these landraces, notwithstanding recognition for a possible role in histone modification in TME3. This work represents an important step toward understanding variable roles of sRNAs in different cassava genotype-geminivirus interactions.

Properties of African Cassava Mosaic Virus Capsid Protein Expressed in Fission Yeast

The capsid proteins (CPs) of geminiviruses combine multiple functions for packaging the single-stranded viral genome, insect transmission and shuttling between the nucleus and the cytoplasm. African cassava mosaic virus (ACMV) CP was expressed in fission yeast, and purified by SDS gel electrophoresis. After tryptic digestion of this protein, mass spectrometry covered 85% of the amino acid sequence and detected three N-terminal phosphorylation sites (threonine 12, serines 25 and 62). Differential centrifugation of cell extracts separated the CP into two fractions, the supernatant and pellet. Upon isopycnic centrifugation of the supernatant, most of the CP accumulated at densities typical for free proteins, whereas the CP in the pellet fraction showed a partial binding to nucleic acids. Size-exclusion chromatography of the supernatant CP indicated high order complexes. In DNA binding assays, supernatant CP accelerated the migration of ssDNA in agarose gels, which is a first hint for particle formation. Correspondingly, CP shifted ssDNA to the expected densities of virus particles upon isopycnic centrifugation. Nevertheless, electron microscopy did not reveal any twin particles, which are characteristic for geminiviruses.

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.

Only minimal regions of tomato yellow leaf curl virus (TYLCV) are required for replication, expression and movement

The IL-60 platform, consisting of a disarmed form of tomato yellow leaf curl virus (TYLCV) and auxiliary components, was previously developed as a nontransgenic universal vector system for gene expression and silencing that can express an entire operon in plants. IL-60 does not allow rolling-circle replication; hence, production of viral single-stranded (ss) DNA progeny is prevented. We used this double-stranded (ds) DNA-restricted platform (uncoupled from the dsDNA→ssDNA replication phase of progeny viral DNA) for functional genomics studies of TYLCV. We report that the noncoding 314-bp intergenic region (IR) is the only viral element required for viral dsDNA replication. None of the viral genes are required, suggesting recruitment of host factors that recognize the IR. We further show that IR-carrying reporter genes are also capable of replication but remain confined to the cells into which they were introduced. Only two sense-oriented viral genes (V1 and V2) need to be added to the IR-carrying construct for expression and movement. Hence, any IR-dsDNA construct supplemented with V1 and V2 becomes a replication-competent, mobile and expressing plant plasmid. All viral functions (replication, expression and movement) are determined by the IR and the sense-oriented genes. The complementary-oriented viral genes have auxiliary roles in the late phase of the virus “life cycle”. The previously reported involvement of some viral genes in expression and movement is therefore revised.

Geminivirus protein structure and function

Geminiviruses are a family of plant viruses that cause economically important plant diseases worldwide. These viruses have circular single-stranded DNA genomes and four to eight genes that are expressed from both strands of the double-stranded DNA replicative intermediate. The transcription of these genes occurs under the control of two bidirectional promoters and one monodirectional promoter. The viral proteins function to facilitate virus replication, virus movement, the assembly of virus-specific nucleoprotein particles, vector transmission and to counteract plant host defence responses. Recent research findings have provided new insights into the structure and function of these proteins and have identified numerous host interacting partners. Most of the viral proteins have been shown to be multifunctional, participating in multiple events during the infection cycle and have, indeed, evolved coordinated interactions with host proteins to ensure a successful infection. Here, an up-to-date review of viral protein structure and function is presented, and some areas requiring further research are identified.

Replication modes of Maize streak virus mutants lacking RepA or the RepA-pRBR interaction motif

The plant-infecting mastreviruses (family Geminiviridae) express two distinct replication-initiator proteins, Rep and RepA. Although RepA is essential for systemic infectivity, little is known about its precise function. We therefore investigated its role in replication using 2D-gel electrophoresis to discriminate the replicative forms of Maize streak virus (MSV) mutants that either fail to express RepA (RepA(-)), or express RepA that is unable to bind the plant retinoblastoma related protein, pRBR. Whereas amounts of viral DNA were reduced in two pRBR-binding deficient RepA mutants, their repertoires of replicative forms changed only slightly. While a complete lack of RepA expression was also associated with reduced viral DNA titres, the only traces of replicative intermediates of RepA(-) viruses were those indicative of recombination-dependent replication. We conclude that in MSV, RepA, but not RepA-pRBR binding, is necessary for single-stranded DNA production and efficient rolling circle replication.

The induction of stromule formation by a plant DNA-virus in epidermal leaf tissues suggests a novel intra- and intercellular macromolecular trafficking route

Stromules are dynamic thin protrusions of membrane envelope from plant cell plastids. Despite considerable progress in understanding the importance of certain cytoskeleton elements and motor proteins for stromule maintenance, their function within the cell has yet to be unraveled. Several viruses cause a remodulation of plastid structures and stromule biogenesis within their host plants. For RNA-viruses these interactions were demonstrated to be relevant to the infection process. An involvement of plastids and stromules is assumed in the DNA-virus life cycle as well, but their functional role needs to be determined. Recent findings support a participation of heat shock cognate 70 kDa protein (cpHSC70-1)-containing stromules induced by a DNA-virus infection (Abutilon mosaic virus, AbMV, Geminiviridae) in intra- and intercellular molecule exchange. The chaperone cpHSC70-1 was shown to interact with the AbMV movement protein (MP). Bimolecular fluorescence complementation confirmed the interaction of cpHSC70-1 and MP, and showed a homo-oligomerization of either protein in planta. The complexes were detected at the cellular margin and co-localized with plastids. In healthy plant tissues cpHSC70-1-oligomers occurred in distinct spots at chloroplasts and in small filaments extending from plastids to the cell periphery. AbMV-infection induced a cpHSC70-1-containing stromule network that exhibits elliptical dilations and transverses whole cells. Silencing of the cpHSC70 gene revealed an impact of cpHSC70 on chloroplast stability and restricted AbMV movement, but not viral DNA accumulation. Based on these data, a model is suggested in which these stromules function in molecule exchange between plastids and other organelles and perhaps other cells. AbMV may utilize cpHSC70-1 for trafficking along plastids and stromules into a neighboring cell or from plastids into the nucleus. Experimental approaches to investigate this hypothesis are discussed.

Progressive aggregation of Tomato yellow leaf curl virus coat protein in systemically infected tomato plants, susceptible and resistant to the virus

Tomato yellow leaf curl virus (TYLCV) coat protein (CP) accumulated in tomato leaves during infection. The CP was immuno-detected in the phloem associated cells. At the early stages of infection, punctate signals were detected in the cytoplasm, while in the later stages aggregates of increasing size were localized in cytoplasm and nuclei. Sedimentation of protein extracts through sucrose gradients confirmed that progress of infection was accompanied by the formation of CP aggregates of increasing size. Genomic ssDNA was found in the cytoplasm and in the nucleus, while the dsDNA replicative form was exclusively associated with the nucleus. CP-DNA complexes were detected by immuno-capture PCR in nuclear and cytoplasmic large aggregates. Nuclear aggregates contained infectious particles transmissible to test plants by whiteflies. In contrast to susceptible tomatoes, the formation of large CP aggregates in resistant plants was delayed. By experimentally changing the level of resistance/susceptibility of plants, we showed that maintenance of midsized CP aggregates was associated with resistance, while large aggregates where characteristic of susceptibility. We propose that sequestering of virus CP into midsized aggregates and retarding the formation of large insoluble aggregates containing infectious particles is part of the response of resistant plants to TYLCV.

Antibegomoviral activity of the agrobacterial virulence protein VirE2

Mungbean yellow mosaic geminivirus (MYMV) causes severe yellow mosaic disease in blackgram, mungbean, Frenchbean, pigeonpea, soybean and mothbean. We attempted to induce resistance against this virus using the transcriptional activator protein gene deleted in the C-terminal activation domain (TrAP-∆AD) and Agrobacterium tumefaciens virE2. MYMV is known to replicate in agroinoculated tobacco leaf discs. Three transgenic tobacco plants which harboured a truncated MYMV transcriptional activator protein gene and two tobacco plants transformed with the octopine type A. tumefaciens virE2 gene were agroinoculated with an A. tumefaciens strain which harboured the partial dimers of both DNA A and DNA B of MYMV. The level of viral DNA accumulation in leaf discs of transgenic plants correlated inversely to the level of the MYMV TrAP-∆AD transcript. Two VirE2-transgenic plants, which complemented tumorigenesis of a virE2 mutant A. tumefaciens strain, effectively reduced MYMV DNA accumulation in the leaf disc agroinoculation assay.

Histone H3 interacts and colocalizes with the nuclear shuttle protein and the movement protein of a geminivirus

Geminiviruses are plant-infecting viruses with small circular single-stranded DNA genomes. These viruses utilize nuclear shuttle proteins (NSPs) and movement proteins (MPs) for trafficking of infectious DNA through the nuclear pore complex and plasmodesmata, respectively. Here, a biochemical approach was used to identify host factors interacting with the NSP and MP of the geminivirus Bean dwarf mosaic virus (BDMV). Based on these studies, we identified and characterized a host nucleoprotein, histone H3, which interacts with both the NSP and MP. The specific nature of the interaction of histone H3 with these viral proteins was established by gel overlay and in vitro and in vivo coimmunoprecipitation (co-IP) assays. The NSP and MP interaction domains were mapped to the N-terminal region of histone H3. These experiments also revealed a direct interaction between the BDMV NSP and MP, as well as interactions between histone H3 and the capsid proteins of various geminiviruses. Transient-expression assays revealed the colocalization of histone H3 and NSP in the nucleus and nucleolus and of histone H3 and MP in the cell periphery and plasmodesmata. Finally, using in vivo co-IP assays with a Myc-tagged histone H3, a complex composed of histone H3, NSP, MP, and viral DNA was recovered. Taken together, these findings implicate the host factor histone H3 in the process by which an infectious geminiviral DNA complex forms within the nucleus for export to the cell periphery and cell-to-cell movement through plasmodesmata.

Conformation-selective methylation of geminivirus DNA

Geminiviruses with small circular single-stranded DNA genomes replicate in plant cell nuclei by using various double-stranded DNA (dsDNA) intermediates: distinct open circular and covalently closed circular as well as heterogeneous linear DNA. Their DNA may be methylated partially at cytosine residues, as detected previously by bisulfite sequencing and subsequent PCR. In order to determine the methylation patterns of the circular molecules, the DNAs of tomato yellow leaf curl Sardinia virus (TYLCSV) and Abutilon mosaic virus were investigated utilizing bisulfite treatment followed by rolling circle amplification. Shotgun sequencing of the products yielded a randomly distributed 50% rate of C maintenance after the bisulfite reaction for both viruses. However, controls with unmethylated single-stranded bacteriophage DNA resulted in the same level of C maintenance. Only one short DNA stretch within the C2/C3 promoter of TYLCSV showed hyperprotection of C, with the protection rate exceeding the threshold of the mean value plus 1 standard deviation. Similarly, the use of methylation-sensitive restriction enzymes suggested that geminiviruses escape silencing by methylation very efficiently, by either a rolling circle or recombination-dependent replication mode. In contrast, attempts to detect methylated bases positively by using methylcytosine-specific antibodies detected methylated DNA only in heterogeneous linear dsDNA, and methylation-dependent restriction enzymes revealed that the viral heterogeneous linear dsDNA was methylated preferentially.

Analysis of a new strain of Euphorbia mosaic virus with distinct replication specificity unveils a lineage of begomoviruses with short Rep sequences in the DNA-B intergenic region

BACKGROUND

Euphorbia mosaic virus (EuMV) is a member of the SLCV clade, a lineage of New World begomoviruses that display distinctive features in their replication-associated protein (Rep) and virion-strand replication origin. The first entirely characterized EuMV isolate is native from Yucatan Peninsula, Mexico; subsequently, EuMV was detected in weeds and pepper plants from another region of Mexico, and partial DNA-A sequences revealed significant differences in their putative replication specificity determinants with respect to EuMV-YP. This study was aimed to investigate the replication compatibility between two EuMV isolates from the same country.

RESULTS

A new isolate of EuMV was obtained from pepper plants collected at Jalisco, Mexico. Full-length clones of both genomic components of EuMV-Jal were biolistically inoculated into plants of three different species, which developed symptoms indistinguishable from those induced by EuMV-YP. Pseudorecombination experiments with EuMV-Jal and EuMV-YP genomic components demonstrated that these viruses do not form infectious reassortants in Nicotiana benthamiana, presumably because of Rep-iteron incompatibility. Sequence analysis of the EuMV-Jal DNA-B intergenic region (IR) led to the unexpected discovery of a 35-nt-long sequence that is identical to a segment of the rep gene in the cognate viral DNA-A. Similar short rep sequences ranging from 35- to 51-nt in length were identified in all EuMV isolates and in three distinct viruses from South America related to EuMV. These short rep sequences in the DNA-B IR are positioned downstream to a ~160-nt non-coding domain highly similar to the CP promoter of begomoviruses belonging to the SLCV clade.

CONCLUSIONS

EuMV strains are not compatible in replication, indicating that this begomovirus species probably is not a replicating lineage in nature. The genomic analysis of EuMV-Jal led to the discovery of a subgroup of SLCV clade viruses that contain in the non-coding region of their DNA-B component, short rep gene sequences located downstream to a CP-promoter-like domain. This assemblage of DNA-A-related sequences within the DNA-B IR is reminiscent of polyomavirus microRNAs and could be involved in the posttranscriptional regulation of the cognate viral rep gene, an intriguing possibility that should be experimentally explored.

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.

Involvement of C4 protein of beet severe curly top virus (family Geminiviridae) in virus movement

BACKGROUND

Beet severe curly top virus (BSCTV) is a leafhopper transmitted geminivirus with a monopartite genome. C4 proteins encoded by geminivirus play an important role in virus/plant interaction.

METHODS AND FINDINGS

To understand the function of C4 encoded by BSCTV, two BSCTV mutants were constructed by introducing termination codons in ORF C4 without affecting the amino acids encoded by overlapping ORF Rep. BSCTV mutants containing disrupted ORF C4 retained the ability to replicate in Arabidopsis protoplasts and in the agro-inoculated leaf discs of N. benthamiana, suggesting C4 is not required for virus DNA replication. However, both mutants did not accumulate viral DNA in newly emerged leaves of inoculated N. benthamiana and Arabidopsis, and the inoculated plants were asymptomatic. We also showed that C4 expression in plant could help C4 deficient BSCTV mutants to move systemically. C4 was localized in the cytosol and the nucleus in both Arabidopsis protoplasts and N. benthamiana leaves and the protein appeared to bind viral DNA and ds/ssDNA nonspecifically, displaying novel DNA binding properties.

CONCLUSIONS

Our results suggest that C4 protein in BSCTV is involved in symptom production and may facilitate virus movement instead of virus replication.

A plastid-targeted heat shock cognate 70kDa protein interacts with the Abutilon mosaic virus movement protein

The movement protein (MP) of bipartite geminiviruses facilitates cell-to-cell as well as long-distance transport within plants and influences viral pathogenicity. Yeast two-hybrid assays identified a chaperone, the nuclear-encoded and plastid-targeted heat shock cognate 70kDa protein (cpHSC70-1) of Arabidopsis thaliana, as a potential binding partner for the Abutilon mosaic virus (AbMV) MP. In planta, bimolecular fluorescence complementation (BiFC) analysis showed cpHSC70-1/MP complexes and MP homooligomers at the cell periphery and co-localized with chloroplasts. BiFC revealed cpHSC70-1 oligomers associated with chloroplasts, but also distributed at the cellular margin and in filaments arising from plastids reminiscent of stromules. Silencing the cpHSC70 gene of Nicotiana benthamiana using an AbMV DNA A-derived gene silencing vector induced minute white leaf areas, which indicate an effect on chloroplast stability. Although AbMV DNA accumulated within chlorotic spots, a spatial restriction of these occurred, suggesting a functional relevance of the MP-chaperone interaction for viral transport and symptom induction.

Caulimoviridae tubule-guided transport is dictated by movement protein properties

Plant viruses move through plasmodesmata (PD) either as nucleoprotein complexes (NPCs) or as tubule-guided encapsidated particles with the help of movement proteins (MPs). To explore how and why MPs specialize in one mechanism or the other, we tested the exchangeability of MPs encoded by DNA and RNA virus genomes by means of an engineered alfalfa mosaic virus (AMV) system. We show that Caulimoviridae (DNA genome virus) MPs are competent for RNA virus particle transport but are unable to mediate NPC movement, and we discuss this restriction in terms of the evolution of DNA virus MPs as a means of mediating DNA viral genome entry into the RNA-trafficking PD pathway.

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.

Functional equivalence of HMGA- and histone H1-like domains in a bacterial transcriptional factor

Histone H1 and high-mobility group A (HMGA) proteins compete dynamically to modulate chromatin structure and regulate DNA transactions in eukaryotes. In prokaryotes, HMGA-like domains are known only in Myxococcus xanthus CarD and its Stigmatella aurantiaca ortholog. These have an N-terminal module absent in HMGA that interacts with CarG (a zinc-associated factor that does not bind DNA) to form a stable complex essential in regulating multicellular development, light-induced carotenogenesis, and other cellular processes. An analogous pair, CarD(Ad) and CarG(Ad), exists in another myxobacterium, Anaeromyxobacter dehalogenans. Intriguingly, the CarD(Ad) C terminus lacks the hallmark HMGA DNA-binding AT-hooks and instead resembles the C-terminal region (CTR) of histone H1. We find that CarD(Ad) alone could not replace CarD in M. xanthus. By contrast, when introduced with CarG(Ad), CarD(Ad) functionally replaced CarD in regulating not just 1 but 3 distinct processes in M. xanthus, despite the lower DNA-binding affinity of CarD(Ad) versus CarD in vitro. The ability of the cognate CarD(Ad)-CarG(Ad) pair to interact, but not the noncognate CarD(Ad)-CarG, rationalizes these data. Thus, in chimeras that conserve CarD-CarG interactions, the H1-like CTR of CarD(Ad) could replace the CarD HMGA AT-hooks with no loss of function in vivo. More tellingly, even chimeras with the CarD AT-hook region substituted by human histone H1 CTR or full-length H1 functioned in M. xanthus. Our domain-swap analyses showing functional equivalence of HMGA AT-hooks and H1 CTR in prokaryotic transcriptional regulation provide molecular insights into possible modes of action underlying their biological roles.

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.

Intragenomic rearrangement in TT viruses: a possible role in the pathogenesis of disease

A role for the ubiquitous Torque teno (TT) viruses in the pathogenesis of disease has not been resolved. In vivo and in vitro intragenomic rearrangement of TT virus genomes has been demonstrated. Replication in cell culture of a subviral molecule (411 bp) occurs through oligomerisation of RNA transcripts. Although the functions of the respective TT viral genes, as well as the newly formed genes in the rearranged subviral molecules, are largely unknown, certain similarities to genes of plant viruses of the family Geminiviridae will be described. A degree of similarity to certain cellular genes poses the question as to a role of molecular mimicry in the pathogenesis of autoimmune disease and diabetes.

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.

Viable chimaeric viruses confirm the biological importance of sequence specific maize streak virus movement protein and coat protein interactions

Background

A variety of interactions between up to three different movement proteins (MPs), the coat protein (CP) and genomic DNA mediate the inter- and intra-cellular movement of geminiviruses in the genus Begomovirus. Although movement of viruses in the genus Mastrevirus is less well characterized, direct interactions between a single MP and the CP of these viruses is also clearly involved in both intra- and intercellular trafficking of virus genomic DNA. However, it is currently unknown how specific these MP-CP interactions are, nor how disruption of these interactions might impact on virus viability.

Results

Using chimaeric genomes of two strains of Maize streak virus (MSV) we adopted a genetic approach to investigate the gross biological effects of interfering with interactions between virus MP and CP homologues derived from genetically distinct MSV isolates. MP and CP genes were reciprocally exchanged, individually and in pairs, between maize (MSV-Kom)- and Setaria sp. (MSV-Set)-adapted isolates sharing 78% genome-wide sequence identity. All chimaeras were infectious in Zea mays c.v. Jubilee and were characterized in terms of symptomatology and infection efficiency. Compared with their parental viruses, all the chimaeras were attenuated in symptom severity, infection efficiency, and the rate at which symptoms appeared. The exchange of individual MP and CP genes resulted in lower infection efficiency and reduced symptom severity in comparison with exchanges of matched MP-CP pairs.

Conclusion

Specific interactions between the mastrevirus MP and CP genes themselves and/or their expression products are important determinants of infection efficiency, rate of symptom development and symptom severity.

DNA recognition properties of the cell-to-cell movement protein (MP) of soybean isolate of Mungbean yellow mosaic India virus (MYMIV-Sb)

Bipartite geminiviruses possess two movement proteins (NSP and MP), which mediate the intra- and intercellular movement. In order to accomplish the transport process the movement proteins interact with viral nucleic acids in a sequence non-specific manner. To investigate the nucleic acid recognition properties of MP of MYMIV-Sb, the protein was expressed in Escherichia coli as a fusion protein with maltose-binding protein (MBP) and purified in native condition. Gel mobility shift assay was employed for analyzing the DNA recognition properties of purified MBP-MP fusion protein. The analyses demonstrated the sequence non-specific binding of MYMIV-Sb MP to both ds and ssDNA and its high affinity for ssDNA. MP of MYMIV-Sb did not show any specificity towards various forms of plasmid DNA but displayed size selection towards linear dsDNA.

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.

Characterization of DNA intermediates of an arising geminivirus

Weeds of the genus Sida collected in Brazil have harbored several geminiviruses persistently over decades of vegetative propagation. They serve as cradles for new geminiviruses originating from pseudorecombination (reassortment) or molecular recombination, as has been exemplified by Sida micrantha mosaic-associated viruses (SimMV). One of such viruses has developed recently and naturally by recombination between a DNA A and a DNA B of different ancestors. We used two-dimensional gel electrophoresis and hybridization to visualize viral DNA intermediates in mixed infections as well as after transfer of single viruses into test plants. DNA intermediates which indicate multitasking in replication (rolling circle and recombination-dependent replication) were readily detected in all cases. A conspicuous increase in multimerization of circular single-stranded (ss) DNA could be attributed to the recently recombined geminivirus, suggesting poor adaptation to the host and/or inefficient gene regulation. Consequences of the accumulation of multimeric ssDNA were analyzed using nucleoprotein particle purification and electron microscopy. SimMV nucleoprotein exhibited pleomorphic structures in addition to the typical twin particles. This report provides the first analysis of DNA intermediates of an arising geminivirus.

A universal expression/silencing vector in plants

A universal vector (IL-60 and auxiliary constructs), expressing or silencing genes in every plant tested to date, is described. Plants that have been successfully manipulated by the IL-60 system include hard-to-manipulate species such as wheat (Triticum duram), pepper (Capsicum annuum), grapevine (Vitis vinifera), citrus, and olive (Olea europaea). Expression or silencing develops within a few days in tomato (Solanum lycopersicum), wheat, and most herbaceous plants and in up to 3 weeks in woody trees. Expression, as tested in tomato, is durable and persists throughout the life span of the plant. The vector is, in fact, a disarmed form of Tomato yellow leaf curl virus, which is applied as a double-stranded DNA and replicates as such. However, the disarmed virus does not support rolling-circle replication, and therefore viral progeny single-stranded DNA is not produced. IL-60 does not integrate into the plant's genome, and the construct, including the expressed gene, is not heritable. IL-60 is not transmitted by the Tomato yellow leaf curl virus's natural insect vector. In addition, artificial satellites were constructed that require a helper virus for replication, movement, and expression. With IL-60 as the disarmed helper "virus," transactivation occurs, resulting in an inducible expressing/silencing system. The system's potential is demonstrated by IL-60-derived suppression of a viral-silencing suppressor of Grapevine virus A, resulting in Grapevine virus A-resistant/tolerant 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.