Effectiveness and stability of heterologous proteins expressed in plants by Turnip mosaic virus vector at five different insertion sites

Development of tobacco rattle virus-based platform for dual heterologous gene expression and CRISPR/Cas reagent delivery

A large number of viral delivery systems have been developed for characterizing functional genes and producing heterologous recombinant proteins in plants, and but most of them are unable to co-express two fusion-free foreign proteins in the whole plant for extended periods of time. In this study, we modified tobacco rattle virus (TRV) as a TRVe dual delivery vector, using the strategy of gene substitution. The reconstructed TRVe had the capability to simultaneously produce two fusion-free foreign proteins at the whole level of Nicotiana benthamiana, and maintained the genetic stability for the insert of double foreign genes. Moreover, TRVe allowed systemic expression of two foreign proteins with the total lengths up to ∼900 aa residues. In addition, Cas12a protein and crRNA were delivered by the TRVe expression system for site-directed editing of genomic DNA in N. benthamiana 16c line constitutively expressing green fluorescent protein (GFP). Taker together, the TRV-based delivery system will be a simple and powerful means to rapidly co-express two non-fused foreign proteins at the whole level and facilitate functional genomics studies in plants.

Modification of the N-terminal FWKG-αH1 element of potyviral HC-Pro affects its multiple functions and generates effective attenuated mutants for cross-protection

Control of plant viruses by cross-protection is limited by the availability of effective protective strains. Incorporation of an NIa-protease processing site in the extreme N-terminal region of the helper component protease (HC-Pro) of turnip mosaic virus (TuMV) resulted in a mutant virus TuHN^D I that induced highly attenuated symptoms. Recombination analysis verified that two variations, F7I mutation and amino acid 7-upstream-deletion, in HC-Pro co-determined TuHN^D I attenuation. TuHN^D I provided complete protection to Nicotiana benthamiana and Brassica campestris subsp. chinensis plants against infection by the severe parental strain. Aphid transmission tests revealed that TuHN^D I was not aphid-transmissible. An RNA silencing suppression (RSS) assay by agroinfiltration suggested the RSS-defective nature of the mutant HC-Pro. In the context (amino acids 3-17) encompassing the two variations of HC-Pro, we uncovered an FWKG-α-helix 1 (αH1) element that influenced the functions of aphid transmission and RSS, whose motifs were located far downstream. We further demonstrated that HC-Pro F7 was a critical residue on αH1 for HC-Pro functions and that reinstating αH1 in the RSS-defective HC-Pro of TuHN^D I restored the protein's RSS function. Yeast two-hybrid and bimolecular fluorescence complementation assays indicated the FWKG-αH1 element as an integral part of the HC-Pro self-interaction domain. The possibility of regulation of the mechanistically independent functions of RSS and aphid transmission by the FWKG-αH1 element is discussed. Extension of TuMV HC-Pro FWKG-αH1 variations to another potyvirus, zucchini yellow mosaic virus, also generated nonaphid-transmissible cross-protective mutant viruses. Hence, the modification of the FWKG-αH1 element can generate effective attenuated viruses for the control of potyviruses by cross-protection.

OUP accepted manuscript

Potyviridae, the largest family of known RNA viruses (realm Riboviria), belongs to the picorna-like supergroup and has important agricultural and ecological impacts. Potyvirid genomes are translated into polyproteins, which are in turn hydrolyzed to release mature products. Recent sequencing efforts revealed an unprecedented number of potyvirids with a rich variability in gene content and genomic layouts. Here, we review the heterogeneity of non-core modules that expand the structural and functional diversity of the potyvirid proteomes. We provide a family-wide classification of P1 proteinases into the functional Types A and B, and discuss pretty interesting sweet potato potyviral ORF (PISPO), putative zinc fingers, and alkylation B (AlkB)—non-core modules found within P1 cistrons. The atypical inosine triphosphate pyrophosphatase (ITPase/HAM1), as well as the pseudo tobacco mosaic virus-like coat protein (TMV-like CP) are discussed alongside homologs of unrelated virus taxa. Family-wide abundance of the multitasking helper component proteinase (HC-pro) is revised. Functional connections between non-core modules are highlighted to support host niche adaptation and immune evasion as main drivers of the Potyviridae evolutionary radiation. Potential biotechnological and synthetic biology applications of potyvirid leader proteinases and non-core modules are finally explored.

Simultaneous gene expression and multi-gene silencing in Zea mays using maize dwarf mosaic virus

BACKGROUND

Maize dwarf mosaic virus (MDMV), a member of the genus Potyvirus, infects maize and is non-persistently transmitted by aphids. Several plant viruses have been developed as tools for gene expression and gene silencing in plants. The capacity of MDMV for both gene expression and gene silencing were examined.

RESULTS

Infectious clones of an Ohio isolate of MDMV, MDMV OH5, were obtained, and engineered for gene expression only, and for simultaneous marker gene expression and virus-induced gene silencing (VIGS) of three endogenous maize target genes. Single gene expression in single insertion constructs and simultaneous expression of green fluorescent protein (GFP) and silencing of three maize genes in a double insertion construct was demonstrated. Constructs with GFP inserted in the N-terminus of HCPro were more stable than those with insertion at the N-terminus of CP in our study. Unexpectedly, the construct with two insertion sites also retained insertions at a higher rate than single-insertion constructs. Engineered MDMV expression and VIGS constructs were transmissible by aphids (Rhopalosiphum padi).

CONCLUSIONS

These results demonstrate that MDMV-based vector can be used as a tool for simultaneous gene expression and multi-gene silencing in maize.

Stable Display of Artificially Long Foreign Antigens on Chimeric Bamboo mosaic virus Particles

Plant viruses can be genetically modified to generate chimeric virus particles (CVPs) carrying heterologous peptides fused on the surface of coat protein (CP) subunits as vaccine candidates. However, some factors may be especially significant in determining the properties of chimeras. In this study, peptides from various sources and of various lengths were inserted into the Bamboo mosaic virus-based (BaMV) vector CP N-terminus to examine the chimeras infecting and accumulating in plants. Interestingly, it was found that the two different strains Foot-and-mouth disease virus (FMDV) VP1 antigens with flexible linker peptides (77 or 82 amino acids) were directly expressed on the BaMV CP, and the chimeric particles self-assembled and continued to express FMDV antigens. The chimeric CP, when directly fused with a large foreign protein (117 amino acids), can self-fold into incomplete virus particles or disks. The physicochemical properties of heterologus peptides N-terminus, complex strand structures of heterologus peptides C-terminus and different flexible linker peptides, can affect the chimera accumulation. Based on these findings, using plant virus-based chimeras to express foreign proteins can increase their length limitations, and engineered plant-made CVP-based vaccines have increasing potential for further development as novel vaccines.

The N-terminus of the cauliflower mosaic virus aphid transmission protein P2 is involved in transmission body formation and microtubule interaction

Cauliflower mosaic virus (CaMV) is transmitted by aphids using the non-circulative transmission mode: when the insects feed on infected leaves, virus particles from infected cells attach rapidly to their stylets and are transmitted to a new host when the aphids change plants. Mandatory for CaMV transmission, the viral helper protein P2 mediates as a molecular linker binding of the virus particles to the aphid stylets. P2 is available in infected plant cells in a viral inclusion that is specialized for transmission and named the transmission body (TB). When puncturing an infected leaf cell, the aphid triggers an ultra-rapid viral response, necessary for virus acquisition and called transmission activation: The TB disrupts and P2 is redistributed onto cortical microtubules, together with virus particles that are simultaneously set free from virus factories and join P2 on the microtubules to form the so-called mixed networks (MNs). The MNs are the predominant structure from which CaMV is acquired by aphids. However, the P2 domains involved in microtubule interaction are not known. To identify P2 regions involved in its functions, we generated a set of P2 mutants by alanine scanning and analyzed them in the viral context for their capacity to form a TB, to interact with microtubules and to transmit CaMV. Our results show that contrary to the previously characterized P2-P2 and P2-virion binding sites in its C-terminus, the microtubule binding site is contained in the N-terminal half of P2. Further, this region is important for TB formation since some P2 mutant proteins did not accumulate in TBs but were retained in the viral factories where P2 is translated. Taken together, the N-terminus of P2 is not only involved in vector interaction as previously reported, but also in interaction with microtubules and in formation of TBs.

Construction of stable infectious full-length and eGFP-tagged cDNA clones of Mirabilis crinkle mosaic virus via In-Fusion cloning

Mirabilis crinkle mosaic virus (MiCMV) was tentatively classified as a new member of the genus Potyvirus in the family Potyviridae in 2019. However, it was considered to be basella rugose mosaic virus based on the sequence similarity of the coat protein (CP) region. In this study, infectious MiCMV cDNA clones under the control of the 35S promoter were constructed with an In-Fusion cloning method. Systemically infected leaves of Mirabilis jalapa and Nicotiana benthamiana plants inoculated with pMiCMV and pMiCMV-NIb/eGFP had mosaic symptoms by 5 dpi. Infections were confirmed by a western blot analysis, electron microscopy, RT-PCR, and the inoculation of N. benthamiana seedlings with progeny virions. Systemic infections were not observed after Nicotiana glutinosa leaves were similarly inoculated, with eGFP fluorescence detected only in the inoculated leaves. Interestingly, the symptoms induced by pMiCMV and pMiCMV-NIb/eGFP were not similar to those caused by the wild-type MiCMV in Basella rubra plants. Moreover, RT-PCR analyses of B. rubra plants with virus-specific primers (MicpF and MicpR) indicated that a non-target fragment corresponding to the MiCMV CP coding region was amplified. This is the first report of the construction of a biologically active, full-length cDNA copy of the MiCMV RNA genome.

Application of Reverse Genetics in Functional Genomics of Potyvirus

Numerous potyvirus studies, including virus biology, transmission, viral protein function, as well as virus–host interaction, have greatly benefited from the utilization of reverse genetic techniques. Reverse genetics of RNA viruses refers to the manipulation of viral genomes, transfection of the modified cDNAs into cells, and the production of live infectious progenies, either wild-type or mutated. Reverse genetic technology provides an opportunity of developing potyviruses into vectors for improving agronomic traits in plants, as a reporter system for tracking virus infection in hosts or a production system for target proteins. Therefore, this review provides an overview on the breakthroughs achieved in potyvirus research through the implementation of reverse genetic systems.

A Conserved Helix in the C-Terminal Region of Watermelon Silver Mottle Virus Nonstructural Protein S Is Imperative For Protein Stability Affecting Self-Interaction, RNA Silencing Suppression, and Pathogenicity

In orthotospovirus, the nonstructural protein S (NSs) is the RNA-silencing suppressor (RSS) and pathogenicity determinant. Here, we demonstrate that a putative α-helix, designated H8, spanning amino acids 338 to 369 of the C-terminal region of the NSs protein, is crucial for self-interaction of watermelon silver mottle virus NSs protein and that the H8 affects RSS function. Co-immunoprecipitation, yeast two-hybrid, and bimolecular fluorescence complementation analyses revealed that the triple point mutation (TPM) of H8 amino acids Y338A, H350A, and F353A resulted in NSs protein self-interaction dysfunction. Transient expression of H8-deleted (ΔH8) and TPM NSs proteins in Nicotiana benthamiana plants by agroinfitration indicated that these proteins have weaker RSS activity and are far less stable than wild-type (WT) NSs. However, an electrophoretic mobility assay revealed that small interfering RNA (siRNA) binding ability of TPM NSs protein is not compromised. The pathogenicity assay of WT NSs protein expressed by the attenuated turnip mosaic virus vector restored severe symptoms in recombinant-infected N. benthamiana plants but not for ΔH8 or TPM proteins. Taken together, we conclude that the H8 helix in the C-terminal region of NSs protein is crucial for stabilizing NSs protein through self-interaction to maintain normal functions of RSS and pathogenicity, but not for NSs-siRNA binding activity.

A sugarcane mosaic virus vector for gene expression in maize

Zea mays L. ssp. mays (maize) is an important crop plant as well as model system for genetics and plant biology. The ability to select among different virus-based platforms for transient gene silencing or protein expression experiments is expected to facilitate studies of gene function in maize and complement experiments with stable transgenes. Here, we describe the development of a sugarcane mosaic virus (SCMV) vector for the purpose of protein expression in maize. An infectious SCMV cDNA clone was constructed, and heterologous genetic elements were placed between the protein 1 (P1) and helper component-proteinase (HC-Pro) cistrons in the SCMV genome. Recombinant SCMV clones engineered to express green fluorescent protein (GFP), β-glucuronidase (GUS), or bialaphos resistance (BAR) protein were introduced into sweet corn (Golden × Bantam) plants. Documentation of developmental time courses spanning maize growth from seedling to tasseling showed that the SCMV genome tolerates insertion of foreign sequences of at least 1,809 nucleotides at the P1/HC-Pro junction. Analysis of insert stability showed that the integrity of GFP and BAR coding sequences was maintained longer than that of the much larger GUS coding sequence. The SCMV isolate from which the expression vector is derived is able to infect several important maize inbred lines, suggesting that this SCMV vector has potential to be a valuable tool for gene functional analysis in a broad range of experimentally important maize genotypes.

Transient expression of anti-VEFGR2 nanobody in Nicotiana tabacum and N. benthamiana

In human, the interaction between vascular endothelial growth factor (VEGF) and its receptor (VEGFR2) is critical for tumor angiogenesis. This is a vital process for cancer tumor growth and metastasis. Blocking VEGF/VEGFR2 conjugation by antibodies inhibits the neovascularization and tumor metastasis. This investigation designed to use a transient expression platform for production of recombinant anti-VEGFR2 nanobody in tobacco plants. At first, anti-VEGFR2-specific nanobody gene was cloned in a Turnip mosaic virus (TuMV)-based vector, and then, it was expressed in Nicotiana benthamiana and Nicotiana tabacum cv. Xanthi transiently. The expression of nanobody in tobacco plants were confirmed by reverse transcription-polymerase chain reaction (RT-PCR), dot blot, enzyme-linked immunosorbent assays (ELISA), and Western blot analysis. It was shown that tobacco plants could accumulate nanobody up to level 0.45% of total soluble protein (8.3 µg/100 mg of fresh leaf). This is the first report of the successful expression of the camelied anti-VEFGR2 nanobody gene in tobacco plants using a plant viral vector. This system provides a fast solution for production of pharmaceutical and commercial proteins such as anti-cancer nanobodies in tobacco plants.

Oral edible plant vaccine containing hypoallergen of American cockroach major allergen Per a 2 prevents roach-allergic asthma in a murine model

Background

American cockroaches (Periplaneta americana) are an important indoor allergen source and a major risk factor for exacerbations and poor control of asthma. We previously reported that allergen components from American cockroaches exhibit varying levels of pathogenicity. Sensitization to major American cockroach allergen, Per a 2, correlated with more severe clinical phenotypes among patients with allergic airway diseases.

Materials and methods

In this study, we examined whether oral plant vaccine-encoding full-length Per a 2 clone-996 or its hypoallergenic clone-372 could exert a prophylactic role in Per a 2-sensitized mice. The cDNAs coding Per a 2–996 and Per a 2–372 were inserted into TuMV vector and expressed in Chinese cabbage. Adult female BALB/c mice were fed with the cabbage extracts for 21 days and subsequently underwent two-step sensitization with recombinant Per a 2.

Results

Per a 2-specific IgE measured by in-house ELISA in the sera of Per a 2-372-treated groups were significantly lower than in the control groups after allergen challenge but not the Per a 2-996-treated group. Moreover, Per a 2–372 vaccine markedly decreased airway hyper-responsiveness and infiltration of inflammatory cells into the lungs, as well as reduced mRNA expression of IL-4 and IL-13 in comparison with the control mice.

Conclusion

Our data suggest that oral administration of edible plant vaccine encoding Per a 2 hypo-allergen may be used as a prophylactic strategy against the development of cockroach allergy.

Seed tolerance to deterioration in arabidopsis is affected by virus infection

Seed longevity is the period during which the plant seed is able to germinate. This property is strongly influenced by environment conditions experienced by seeds during their formation and storage. In the present study we have analyzed how the biotic stress derived from the infection of Cauliflower mosaic virus (CaMV), Turnip mosaic virus (TuMV), Cucumber mosaic virus (CMV) and Alfalfa mosaic virus (AMV) affects seed tolerance to deterioration measuring germination rates after an accelerated aging treatment. Arabidopsis wild type plants infected with AMV and CMV rendered seeds with improved tolerance to deterioration when compared to the non-inoculated plants. On the other hand, CaMV infection generated seeds more sensitive to deterioration. No seeds were obtained from TuMV infected plants. Similar pattern of viral effects was observed in the double mutant athb22 athb25, which is more sensitive to accelerated seed aging than wild type. However, we observed a significant reduction of the seed germination for CMV (65% vs 55%) and healthy (50% vs 30%) plants in these mutants. The seed quality differences were overcomed using the A. thaliana athb25-1D dominant mutant, which over accumulated gibberellic acid (GA), except for TuMV which generated some siliques with low seed tolerance to deterioration. For AMV and TuMV (in athb25-1D), the seed quality correlated with the accumulation of the messengers of the gibberellin 3-oxidase family, the mucilage of the seed and the GA1. For CMV and CaMV it was not a good correlation suggesting that other factors are affecting seed viability.

Production of Japanese Encephalitis Virus Antigens in Plants Using Bamboo Mosaic Virus-Based Vector

Japanese encephalitis virus (JEV) is among the major threats to public health in Asia. For disease control and prevention, the efficient production of safe and effective vaccines against JEV is in urgent need. In this study, we produced a plant-made JEV vaccine candidate using a chimeric virus particle (CVP) strategy based on bamboo mosaic virus (BaMV) for epitope presentation. The chimeric virus, designated BJ2A, was constructed by fusing JEV envelope protein domain III (EDIII) at the N-terminus of BaMV coat protein, with an insertion of the foot-and-mouth disease virus 2A peptide to facilitate the production of both unfused and epitope-presenting for efficient assembly of the CVP vaccine candidate. The strategy allowed stable maintenance of the fusion construct over long-term serial passages in plants. Immuno-electron microscopy examination and immunization assays revealed that BJ2A is able to present the EDIII epitope on the surface of the CVPs, which stimulated effective neutralizing antibodies against JEV infection in mice. This study demonstrates the efficient production of an effective CVP vaccine candidate against JEV in plants by the BaMV-based epitope presentation system.

Fluorescently Tagged Potato virus Y: A Versatile Tool for Functional Analysis of Plant-Virus Interactions

Potato virus Y (PVY) is an economically important plant virus that infects Solanaceous crops such as tobacco and potato. To date, studies into the localization and movement of PVY in plants have been limited to detection of viral RNA or proteins ex vivo. Here, a PVY N605 isolate was tagged with green fluorescent protein (GFP), characterized and used for in vivo tracking. In Nicotiana tabacum cv. Xanthi, PVY N605-GFP was biologically comparable to nontagged PVY N605, stable through three plant-to-plant passages and persisted for four months in infected plants. GFP was detected before symptoms and fluorescence intensity correlated with PVY RNA concentrations. PVY N605-GFP provided in vivo tracking of long-distance movement, allowing estimation of the cell-to-cell movement rate of PVY in N. tabacum cv. Xanthi (7.1 ± 1.5 cells per hour). PVY N605-GFP was adequately stable in Solanum tuberosum cvs. Désirée and NahG-Désirée and able to infect S. tuberosum cvs. Bintje and Bea, Nicotiana benthamiana, and wild potato relatives. PVY N605-GFP is therefore a powerful tool for future studies of PVY-host interactions, such as functional analysis of viral and plant genes involved in viral movement.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

Relocation of the NIb gene in the tobacco etch potyvirus genome

Potyviruses express most of their proteins from a long open reading frame that is translated into a large polyprotein processed by three viral proteases. To understand the constraints on potyvirus genome organization, we relocated the viral RNA-dependent RNA polymerase (NIb) cistron to all possible intercistronic positions of the Tobacco etch virus (TEV) polyprotein. Only viruses with NIb at the amino terminus of the polyprotein or in between P1 and HC-Pro were viable in tobacco plants.

Fate of artificial microRNA-mediated resistance to plant viruses in mixed infections

Artificial microRNAs (amiRNAs) are the expression products of engineered microRNA (miRNA) genes that efficiently and specifically downregulate RNAs that contain complementary sequences. Transgenic plants expressing high levels of one or more amiRNAs targeting particular sequences in the genomes of some RNA viruses have shown specific resistance to the corresponding virus. This is the case of the Arabidopsis thaliana transgenic line 12-4 expressing a high level of the amiR159-HC-Pro targeting 21 nucleotides in the Turnip mosaic virus (TuMV) (family Potyviridae) cistron coding for the viral RNA-silencing suppressor HC-Pro that is highly resistant to TuMV infection. In this study, we explored the fate of this resistance when the A. thaliana 12-4 plants are challenged with a second virus in addition to TuMV. The A. thaliana 12-4 plants maintained the resistance to TuMV when this virus was co-inoculated with Tobacco mosaic virus, Tobacco rattle virus (TRV), Cucumber mosaic virus (CMV), Turnip yellow mosaic virus, Cauliflower mosaic virus (CaMV), Lettuce mosaic virus, or Plum pox virus. However, when the plants were preinfected with these viruses, TuMV was able to co-infect 12-4 plants preinfected with TRV, CaMV, and, particularly, CMV. Therefore, preinfection by another virus jeopardizes the amiRNA-mediated resistance to TuMV.

Induction of protective immunity in chickens immunized with plant-made chimeric Bamboo mosaic virus particles expressing very virulent Infectious bursal disease virus antigen

Very virulent Infectious bursal disease virus (vvIBDV) causes a highly contagious disease in young chickens and leads to significant economic loss in the poultry industry. Effective new vaccines are urgently needed. Autonomously replicating plant virus-based vector provides attractive means for producing chimeric virus particles (CVPs) in plants that can be developed into vaccines. In this study, we demonstrate the potential for vaccine development of Bamboo mosaic virus (BaMV) epitope-presentation system, where the antigen from vvIBDV VP2 was fused to the N-terminus of BaMV coat protein. Accordingly, an infections plasmid, pBIBD2, was constructed. Inoculation of the recombinant BaMV clone pBIBD2 enabled the generation of chimeric virus, BIBD2, and stable expression of IBDV VP2 antigen on its coat protein. After intramuscular immunization with BIBD2 CVPs, chickens produced antibodies against IBDV and were protected from vvIBDV (V263/TW strain) challenges. These results corroborate the feasibility of BaMV-based CVP platform in plants for the development and production of vaccines against IBDV.

Dynamics of the multiplicity of cellular infection in a plant virus

Recombination, complementation and competition profoundly influence virus evolution and epidemiology. Since viruses are intracellular parasites, the basic parameter determining the potential for such interactions is the multiplicity of cellular infection (cellular MOI), i.e. the number of viral genome units that effectively infect a cell. The cellular MOI values that prevail in host organisms have rarely been investigated, and whether they remain constant or change widely during host invasion is totally unknown. Here, we fill this experimental gap by presenting the first detailed analysis of the dynamics of the cellular MOI during colonization of a host plant by a virus. Our results reveal ample variations between different leaf levels during the course of infection, with values starting close to 2 and increasing up to 13 before decreasing to initial levels in the latest infection stages. By revealing wide dynamic changes throughout a single infection, we here illustrate the existence of complex scenarios where the opportunity for recombination, complementation and competition among viral genomes changes greatly at different infection phases and at different locations within a multi-cellular host.

Viruses are fast evolving organisms for which changes in fitness and virulence are driven by interactions between genomes such as recombination, functional complementation, and competition. Viruses being intra-cellular parasites, one basic parameter determines the potential for such interactions: the cellular multiplicity of infection (cellular MOI), defined as the number of genome units actually penetrating and co-replicating within individual cells of the host. Despite its importance for virus evolution, this trait has scarcely been investigated. For example, there are only three point estimates for eukaryote-infecting viruses while the possibility that the cellular MOI may vary during the infection or across organs of a given host individual has never been conclusively addressed. By monitoring the cellular MOI in plants infected by the Cauliflower mosaic virus we found remarkably ample variations during the development of the infection process in successive leaf levels. Our results reveal that the opportunities for recombination, complementation and competition among viral genomes can greatly change at different infection phases and at different locations within a multi-cellular host.

Development of Tobacco necrosis virus A as a vector for efficient and stable expression of FMDV VP1 peptides

Plant virus-based expression systems provide attractive alternatives for production of animal virus-originated antigenic peptides. In the present study, an infectious cDNA clone of Tobacco necrosis virus A Chinese isolate (TNV-A(C)) was used for expression of different peptides derived from Foot and mouth disease virus (FMDV) serotype O VP1 fused downstream of the coat protein (CP) open reading frame (ORF). Chenopodium amaranticolor inoculated with in vitro transcripts of the chimaeras developed symptoms similar to those caused by wild-type TNV-A(C). Western blot and RT-PCR detection of the infected leaves demonstrated that the chimaeras were infective, and a large number of self-assembled virions could be purified and observed under electron microscopy. Immunogold labelling revealed that highly expressed FMDV VP1 peptides could be displayed on the surfaces of virus particles. Additional immunoblotting and DNA sequence analyses showed that most of the chimaeras contained unmodified foreign peptides even after six successive passages in C. amaranticolor and three passages in Nicotiana benthamiana. Our results also suggest that the amino acid sequence and peptide length have a substantial influence on viral morphogenesis and systemic infections. Finally, animal experiments showed that purified chimaeric virus particles (CVPs) could induce a strong immune response against FMDV structural protein VP1 via an intramuscular route. And when inoculated nasally, CVPs could induce systemic and mucosal immune responses in mice.

Three heterologous proteins simultaneously expressed from a chimeric potyvirus: infectivity, stability and the correlation of genome and virion lengths

Three heterologous proteins were simultaneously expressed from a chimeric potyvirus Potato virus A (PVA) in Nicotiana benthamiana. The genes for green fluorescent protein of Aequoria victoriae ("G"; 714 nucleotides, nt), luciferase of Renilla reniformis ("L", 933 nt) and beta-glucuronidase of Escherichia coli ("U", 1806 nt) were inserted into the engineered cloning sites at the N-terminus of the P1 domain, the junction of P1 and helper component protein (HC-Pro), and the junction of the viral replicase (NIb) and coat protein (CP), respectively, in an infectious PVA cDNA. The proteins were expressed as part of the viral polyprotein and subsequently released by cleavage at the flanking proteolytic cleavage sites by P1 (one site) or the NIa-Pro proteinase (other sites). The engineered viral genome (pGLU, 13311 nt) was 39.2% larger than wild-type PVA (9565 nt) and infected plants of N. benthamiana systemically. pGLU was stable and expressed all three heterologous proteins, also following the second infection cycle initiated by sap-inoculation of new plants with the progeny viruses. The gene for GUS showed some inherent instabilities, as also reported in other studies. Accumulation of pGLU in infected leaves was lower by a magnitude as compared to the vector viruses pG0U and p0LU used to express two heterologous proteins. Hence, pGLU may have reached the maximum genome size that can still function and complete the PVA infection cycle. Examination of virions by electron microscopy indicated that the virion lengths of PVA chimera with various numbers of inserts were directly proportional to their genome lengths.