Engineering cowpea mosaic virus RNA-2 into a vector to express heterologous proteins in plants

Adoption of the 2A Ribosomal Skip Principle to Track Assembled Virions of Pepper Mild Mottle Virus in Nicotiana benthamiana

The coat protein (CP) is an important structural protein that plays many functional roles during the viral cycle. In this study, the CP of pepper mild mottle virus (PMMoV) was genetically fused to GFP using the foot-and-mouth disease virus peptide 2A linker peptide and the construct (PMMoV-GFP2A) was shown to be infectious. The systemic spread of the virus was monitored by its fluorescence in infected plants. Electron microscopy and immunocolloidal gold labelling confirmed that PMMoV-GFP2A forms rod-shaped particles on which GFP is displayed. Studies of tissue ultrastructure and virion self-assembly confirmed that PMMoV-GFP2A could be used to monitor the real-time dynamic changes of CP location during virus infection. Aggregations of GFP-tagged virions appeared as fluorescent plaques in confocal laser microscopy. Altogether, PMMoV-GFP2A is a useful tool for studying the spatial and temporal changes of PMMoV CP during viral infection.

Development of Stable Infectious cDNA Clones of Tomato Black Ring Virus Tagged with Green Fluorescent Protein

Tomato black ring virus (TBRV) is a member of the Nepovirus genus in the Secoviridae family, which infects a wide range of important crop species worldwide. In this work, we constructed four cDNA infectious clones of the TBRV tagged with the green fluorescent protein (TBRV-GFP), which varied in (i) the length of the sequences flanking the GFP insert, (ii) the position of the GFP insert within the RNA2 polyprotein, and (iii) the addition of a self-cutting 2A protein. The presence of the GFP coding sequence in infected plants was verified by RT-PCR, while the infectivity and stability of the constructs were verified by mechanical inoculation of the host plants. The systemic spread of TBRV-GFP within plants was observed under UV light at a macroscopic level, monitoring GFP-derived fluorescence in leaves, and at a microscopic level using confocal microscopy. The obtained clones are a valuable tool for future studies of TBRV-host interactions, virus biology, and the long-term monitoring of its distribution in infected plants.

Complete genome sequencing and construction of full-length infectious cDNA clone of papaya ringspot virus-HYD isolate and its efficient in planta expression

Papaya ringspot virus (PRSV) is a devastating Potyvirus that causes papaya ringspot disease in Carica papaya plantations globally. In this study, the complete genome sequence of a PRSV isolate from Shankarpalli, Telangana, India, was reported and designated as PRSV-HYD (KP743981.1). The genome is a single-stranded positive-sense RNA comprising 10,341 nucleotides. Phylogenetic analysis revealed that PRSV-HYD is closely related to PRSV Pune (Aundh) isolate with 92 and 95% nucleotide and amino acid sequence identity, respectively. To develop infectious cDNA (icDNA), the complete nucleotide sequence of PRSV-HYD was cloned between the right and left borders in the binary vector pCB301 using BglII and XmaI restriction sites. Cauliflower mosaic virus (CaMV) double promoter (35S) was fused at the 5'-end and Avocado sunblotch viroid (ASBVd) ribozyme (RZ) sequence was fused to the 3' end to generate an authentic 3' viral end in the transcribed mRNAs. The icDNA generated was mobilized into the Agrobacterium tumefaciens EHA 105, and the agrobacterial cultures were infiltrated into the natural host C. papaya and a non-host Nicotiana benthamiana plants; both did not show any symptoms. In RT-PCR analysis of RNAs isolated from N. benthamiana, we could detect viral genes as early as 3 days and continued up to 28 days post infiltration. Alternatively, virion particles were purified from agroinfiltrated N. benthamiana plants and introduced into C. papaya by mechanical inoculation as well as by pinprick method. In both cases, we could see visible systemic symptoms similar to that of wild type by 40 days. Additionally, we studied the expression patterns of the genes related to plant defense, transcription factors (TFs), and developmental aspects from both C. papaya and N. benthamiana.

Citrus sudden death-associated virus as a new expression vector for rapid in planta production of heterologous proteins, chimeric virions, and virus-like particles

The more we understand the strategies used by viruses for protein expression, the more possibilities we have to exploit viruses as expression vectors for heterologous protein production. Advances in the development of virus-based expression systems have been possible due to generation of many virus infectious clones, especially those derived from plant viruses, which have the capability for rapid and high-level transient expression of proteins in plant cells, a robust and low-cost bioreactor. In this work, we generated new replicative virus expression vectors based on a previously constructed citrus sudden death-associated virus (CSDaV) infectious cDNA clone. These vectors were generated to express the reporter green fluorescent protein (GFP) in Nicotiana benthamiana leaves by taking advantage of the expression strategies used by CSDaV to produce its structural proteins. We show that higher amounts of GFP can be produced from a coat protein (CP)-independent CSDaV-based vector, compared to levels of GFP expressed from a widely used non-replicative vector (pEAQ series); or GFP can be produced in fusion with the major CSDaV CP (CPp21) to be incorporated into chimeric virions. However, GFP-recombinant CSDaV virions do not appear uniformly assembled, but more likely as mosaic particles. Cryo-electron microscopy analysis from this work revealed the structures of the wild-type and the GFP-recombinant CSDaV virions, but it was not able to reveal where exactly the GFP is displayed in the chimeric virions. We show though that the incorporation of GFP-CPp21 fusion protein into virions occurs solely due to its interaction with free/non-fused CPp21, independent of other viral proteins. Therefore, individual co-expression of GFP-CPp21 and CPp21 in the same plant cells leads to the production of chimeric virus-like particles (VLPs), while GFP-CPp21 fusion protein itself is not able to self-assemble into VLPs. The new CSDaV-based expression vectors may provide an alternative platform for use in molecular farming, either for production of heterologous proteins or as scaffold for heterologous protein display.

At3g53630 encodes a GUN1-interacting protein under norflurazon treatment

Chloroplasts are semi-autonomous organelles, with more than 95% of their proteins encoded by the nuclear genome. The chloroplast-to-nucleus retrograde signals are critical for the nucleus to coordinate its gene expression for optimizing or repairing chloroplast functions in response to changing environments. In chloroplasts, the pentatricopeptide-repeat protein GENOMES UNCOUPLED 1 (GUN1) is a master switch that senses aberrant physiological states, such as the photooxidative stress induced by norflurazon (NF) treatment, and represses the expression of photosynthesis-associated nuclear genes (PhANGs). However, it is largely unknown how the retrograde signal is transmitted beyond GUN1. In this study, a protein GUN1-INTERACTING PROTEIN 1 (GIP1), encoded by At3g53630, was identified to interact with GUN1 by different approaches. We demonstrated that GIP1 has both cytosol and chloroplast localizations, and its abundance in chloroplasts is enhanced by NF treatment with the presence of GUN1. Our results suggest that GIP1 and GUN1 may function antagonistically in the retrograde signaling pathway.

Development of Tomato bushy stunt virus-based vectors for fusion and non-fusion expression of heterologous proteins in an alternative host Nicotiana excelsiana

Plant virus-based expression systems are an alternative expression platform for the production of clinically and industrially useful recombinant proteins. Nonetheless, due to a lack of viral vector with the commercial potentials, it is urgent to design and develop new, versatile, and efficient plant virus vectors. The genome of Tomato bushy stunt virus (TBSV) offers an attractive alternative to being modified as a vector for producing heterologous proteins in plants. Here, we developed a set of novel fusion and non-fusion TBSV-CP replacement vectors, which provide more flexible and efficient tools for expressing proteins of interest in plants. An alternative tobacco plant, Nicotiana excelsiana, was used in this study as a host for newly constructed TBSV vectors because the unwanted necrotic effects were reported on the commonly used Nicotiana benthamiana host associated with expression of TBSV-encoded P19 protein. The data showed that TBSV vectors caused a symptomless infection and overexpressed reporter gene in N. excelsiana leaves, demonstrating that N. excelsiana is an ideal host plant for TBSV-mediated heterologous gene expression. Moreover, a TBSV non-fusion vector, dAUG, shows the similar accumulation level of reporter proteins to that of TMV- and PVX-based vectors in side-by-side comparison and provides more flexible aspects than the previously developed TBSV vectors. Collectively, our newly developed TBSV expression system adds a new member to the family of plant viral expression vectors and meanwhile offers a flexible and highly effective approach for producing proteins of interest in plants. KEY POINTS: • The TBSV-based transient expression system has been significantly improved. • The necrotic effects caused by viral P19 protein were avoided by the usage of N. excelsiana as a host plant. • The expression level of the non-fusion vector was similar to the most effective virus vectors reported so far.

Amino acids at the exposed C-terminus of the S coat protein of cowpea mosaic virus play different roles in particle formation and viral systemic movement

The icosahedral capsid of cowpea mosaic virus is formed by 60 copies of the large (L) and small (S) coat protein subunits. The 24-amino-acid C-terminal peptide of the S coat protein can undergo proteolytic cleavage without affecting particle stability or infectivity. Mutagenic studies have shown that this sequence is involved in particle assembly, virus movement, RNA encapsidation and suppression of gene silencing. However, it is unclear how these processes are related, and which part(s) of the sequence are involved in each process. Here, we have analysed the effect of mutations in the C-terminal region of the S protein on the assembly of empty virus-like particles and on the systemic movement of infectious virus. The results confirmed the importance of positively charged amino acids adjacent to the cleavage site for particle assembly and revealed that the C-terminal 11 amino acids are important for efficient systemic movement of the virus.

Second generation of pepino mosaic virus vectors: improved stability in tomato and a wide range of reporter genes

BACKGROUND

Vectors based on plant viruses are important tools for functional genomics, cellular biology, plant genome engineering and molecular farming. We previously reported on the construction of PepGFP2a, a viral vector based on pepino mosaic virus (PepMV) which expressed GFP efficiently and stably in plants of its experimental host Nicotiana benthamiana, but not in its natural host tomato. We have prepared a new set of PepMV-based vectors with improved stability that are able to express a wide range of reporter genes, useful for both N. benthamiana and tomato.

RESULTS

We first tested PepGFPm1 and PepGFPm2, two variants of PepGFP2a in which we progressively reduced a duplication of nucleotides encoding the N-terminal region of the coat protein. The new vectors had improved GFP expression levels and stability in N. benthamiana but not in tomato plants. Next, we replaced GFP by DsRed or mCherry in the new vectors PepDsRed and PepmCherry, respectively; while PepmCherry behaved similarly to PepGFPm2, PepDsRed expressed the reporter gene efficiently also in tomato plants. We then used PepGFPm2 and PepDsRed to study the PepMV localization in both N. benthamiana and tomato cells. Using confocal laser scanning microscopy (CLSM), we observed characteristic fluorescent bodies in PepMV-infected cells; these bodies had a cytoplasmic localization and appeared in close proximity to the cell nucleus. Already at 3 days post-agroinoculation there were fluorescent bodies in almost every cell of agroinoculated tissues of both hosts, and always one body per cell. When markers for the endoplasmic reticulum or the Golgi apparatus were co-expressed with PepGFPm2 or PepDsRed, a reorganisation of these organelles was observed, with images suggesting that both are intimately related but not the main constituents of the PepMV bodies. Altogether, this set of data suggested that the PepMV bodies are similar to the potato virus X (PVX) "X-bodies", which have been described as the PVX viral replication complexes (VRCs). To complete the set of PepMV-based vectors, we constructed a vector expressing the BAR herbicide resistance gene, useful for massive susceptibility screenings.

CONCLUSIONS

We have significantly expanded the PepMV tool box by producing a set of new vectors with improved stability and efficiency in both N. benthamiana and tomato plants. By using two of these vectors, we have described characteristic cellular bodies induced by PepMV infection; these bodies are likely the PepMV VRCs.

Manipulation of Carotenoid Metabolic Flux by Lycopene Cyclization in Ripening Red Pepper ( Capsicum annuum var. conoides) Fruits

Carotenoids are essential phytonutrients for the human body. Higher plants usually synthesize and accumulate carotenoids in their leaves, flowers, and fruits. Most carotenoids have either two β-rings on both ends or β- and ε-rings separately on two ends of their molecules and are synthesized from the acyclic lycopene as the precursor. Lycopene β- and ε-cyclases (LCYB and LCYE, respectively) catalyze the β- and ε-cyclization of lycopene, respectively, and regulate the metabolic flux from lycopene to its downstream β,β-branches (by LCYB alone) and β,ε-branches (by LCYE and LCYB). In this study, we identified and characterized genes for two LCYBs (CaLCYB1 and CaLCYB2), one LCYE (CaLCYE1), and a capsanthin/capsorubin synthase (CaCCS1) which is also able to β-cyclize lycopene from the red pepper ( Capsicum annuum var. conoides) genome. By quantifying transcript abundances of these genes and contents of different carotenoid components in ripening fruits, we observed a correlation between the induction of both CaLCYBs and the accumulation of carotenoids of the β,β-branch during ripening. Although capsanthin was accumulated in ripened fruits, our quantification demonstrated a strong induction of CaCCS1 at the breaker stage, together with the simultaneous repression of CaLCYE1 and the decrease of lutein content, suggesting the involvement of CaCCS1 in competing against CaLCYE1 for synthesizing carotenoids of the β,β-branch. Our results provide important information for future metabolic engineering studies to manipulate carotenoid biosynthesis and accumulation in fruits.

Plant Viruses in Plant Molecular Pharming: Toward the Use of Enveloped Viruses

Plant molecular pharming has emerged as a reliable platform for recombinant protein expression providing a safe and low-cost alternative to bacterial and mammalian cells-based systems. Simultaneously, plant viruses have evolved from pathogens to molecular tools for recombinant protein expression, chimaeric viral vaccine production, and lately, as nanoagents for drug delivery. This review summarizes the genesis of viral vectors and agroinfection, the development of non-enveloped viruses for various biotechnological applications, and the on-going research on enveloped plant viruses.

The structures of a naturally empty cowpea mosaic virus particle and its genome-containing counterpart by cryo-electron microscopy

Cowpea mosaic virus (CPMV) is a picorna-like plant virus. As well as an intrinsic interest in CPMV as a plant pathogen, CPMV is of major interest in biotechnology applications such as nanotechnology. Here, we report high resolution cryo electron microscopy (cryo-EM) maps of wild type CPMV containing RNA-2, and of naturally-formed empty CPMV capsids. The resolution of these structures is sufficient to visualise large amino acids. We have refined an atomic model for each map and identified an essential amino acid involved in genome encapsidation. This work has furthered our knowledge of Picornavirales genome encapsidation and will assist further work in the development of CPMV as a biotechnological tool.

Soil-borne wheat mosaic virus infectious clone and manipulation for gene-carrying capacity

A full-length infectious cDNA clone of soil-borne wheat mosaic virus (SBWMV; genus Furovirus; family Virgaviridae) was developed for agrobacterium delivery. The cloned virus can be agroinfiltrated to Nicotiana benthamiana for subsequent infection of wheat (Triticum aestivum, L.). The utility of the virus as a vector for gene silencing and expression was assessed through sequence insertions in multiple sites of RNA2. Virus-induced photobleaching was observed in N. benthamiana but not in wheat, despite the stability of the inserts. The SBWMV infectious clone can be used for further studies to investigate the biology of SBWMV through mutagenesis.

Development of tobacco ringspot virus-based vectors for foreign gene expression and virus-induced gene silencing in a variety of plants

We report here the development of tobacco ringspot virus (TRSV)-based vectors for the transient expression of foreign genes and for the analysis of endogenous gene function in plants using virus-induced gene silencing. The jellyfish green fluorescent protein (GFP) gene was inserted between the TRSV movement protein (MP) and coat protein (CP) regions, resulting in high in-frame expression of the RNA2-encoded viral polyprotein. GFP was released from the polyprotein via an N-terminal homologous MP-CP cleavage site and a C-terminal foot-and-mouth disease virus (FMDV) 2 A catalytic peptide in Nicotiana benthamiana. The VIGS target gene was introduced in the sense and antisense orientations into a SnaBI site, which was created by mutating the sequence following the CP stop codon. VIGS of phytoene desaturase (PDS) in N. benthamiana, Arabidopsis ecotype Col-0, cucurbits and legumes led to obvious photo-bleaching phenotypes. A significant reduction in PDS mRNA levels in silenced plants was confirmed by semi-quantitative RT-PCR.

RNA1-Independent Replication and GFP Expression from Tomato marchitez virus Isolate M Cloned cDNA

Tomato marchitez virus (ToMarV; synonymous with Tomato apex necrosis virus) is a positive-strand RNA virus in the genus Torradovirus within the family Secoviridae. ToMarV is an emergent whitefly-transmitted virus that causes important diseases in tomato (Solanum lycopersicum) in Mexico. Here, the genome sequence of the ToMarV isolate M (ToMarV-M) was determined. We engineered full-length cDNA clones of the ToMarV-M genomic RNA (RNA1 and RNA2), separately, into a binary vector. Coinfiltration of both triggered systemic infections in Nicotiana benthamiana, tomato, and tomatillo (Physalis philadelphica) plants and recapitulated the biological activity of the wild-type virus. The viral progeny generated from tomato and tomatillo plants were transmissible by the whitefly Bemisia tabaci biotype B. Also, we assessed whether these infectious clones could be used for screening tomato cultivars for resistance to ToMarV and our results allowed us to differentiate resistant and susceptible tomato lines. We demonstrated that RNA1 of ToMarV-M is required for the replication of RNA2, and it can replicate independently of RNA2. From this, ToMarV-M RNA2 was used to express the green fluorescent protein in N. benthamiana plants, which allowed us to track cell-to-cell movement. The construction of full-length infectious cDNA clones of ToMarV-M provides an excellent tool to investigate virus-host-vector interactions and elucidate the functions of torradovirus-encoded proteins or the mechanisms of replication of torradovirus genomic RNA.

Production of Hybrid Chimeric PVX Particles Using a Combination of TMV and PVX-Based Expression Vectors

We have generated hybrid chimeric potato virus X (PVX) particles by coexpression of different PVX coat protein fusions utilizing tobacco mosaic virus (TMV) and PVX-based expression vectors. Coinfection was achieved with a modified PVX overcoat vector displaying a fluorescent protein and a TMV vector expressing another PVX fluorescent overcoat fusion protein. Coexpression of the PVX-CP fusions in the same cells was confirmed by epifluorescence microscopy. Labeling with specific antibodies and transmission electron microscopy revealed chimeric particles displaying green fluorescent protein and mCherry on the surface. These data were corroborated by bimolecular fluorescence complementation. We used split-mCherry fragments as PVX coat fusions and confirmed an interaction between the split-mCherry fragments in coinfected cells. The presence of assembled split-mCherry on the surface confirmed the hybrid character of the chimeric particles.

When plant virology met Agrobacterium: the rise of the deconstructed clones

In the early days of molecular farming, Agrobacterium-mediated stable genetic transformation and the use of plant virus-based vectors were considered separate and competing technologies with complementary strengths and weaknesses. The demonstration that 'agroinfection' was the most efficient way of delivering virus-based vectors to their target plants blurred the distinction between the two technologies and permitted the development of 'deconstructed' vectors based on a number of plant viruses. The tobamoviruses, potexviruses, tobraviruses, geminiviruses and comoviruses have all been shown to be particularly well suited to the development of such vectors in dicotyledonous plants, while the development of equivalent vectors for use in monocotyledonous plants has lagged behind. Deconstructed viral vectors have proved extremely effective at the rapid, high-level production of a number of pharmaceutical proteins, some of which are currently undergoing clinical evaluation.

Development of a non-infectious encapsidated positive control RNA for molecular assays to detect foot-and-mouth disease virus

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FMDV is highly infectious and can only be handled in high-containment laboratories.

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This study has developed encapsidated control particles containing FMDV RNA.

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The construct contains target sequences for molecular assays used to detect FMDV.

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These control particles were evaluated using routine tests used for FMD diagnosis.

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These particles are non-infectious and temperature-stable.

Positive controls are an important component of the quality-control of molecular tests used for diagnosis of livestock diseases. For high consequence agents such as foot-and-mouth disease virus (FMDV), the positive controls required to monitor template extraction, reverse transcription and amplification steps usually consist of material derived from infectious viruses. Therefore, their production is dependent upon the use of high containment facilities and their deployment carries the risks associated with inactivation of “live” FMDV. This paper describes the development of a novel non-infectious positive control that encodes FMDV RNA sequences that are encapsidated within Cowpea mosaic virus (CPMV) particles. This surrogate RNA has been engineered to contain sequences from the 5′UTR and 3D regions of FMDV targeted by many molecular assays (conventional RT-PCR, real-time RT-PCR and RT-LAMP). These sequences were inserted into a movement-deficient version of CPMV RNA-2 which is rescued from cowpea plants (Vigna unguiculota) by inoculation with RNA-1. In order to evaluate the performance of these encapsidated RNAs, nucleic acid prepared from a 10-fold dilution series was tested using a range of molecular assays. Results generated by using the molecular assays confirmed RNA-dependent amplification and the suitability of these particles for use in a range of diagnostic tests. Moreover, these CPMV particles were highly stable for periods of up to 46 days at room temperature and 37 °C. Recombinant CPMV can be used to produce high yields of encapsidated RNAs that can be used as positive and negative controls and standards in molecular assays. This approach provides a surrogate that can be potentially used outside of containment laboratories as an alternative to inactivated infectious virus for molecular diagnostic testing.

Current status of viral expression systems in plants and perspectives for oral vaccines development

During the last 25 years, the technology to produce recombinant vaccines in plant cells has evolved from modest proofs of the concept to viable technologies adopted by some companies due to significant improvements in the field. Viral-based expression strategies have importantly contributed to this success owing to high yields, short production time (which is in most cases free of tissue culture steps), and the implementation of confined processes for production under GMPs. Herein the distinct expression systems based on viral elements are analyzed. This review also presents the outlook on how these technologies have been successfully applied to the development of plant-based vaccines, some of them being in advanced stages of development. Perspectives on how viral expression systems could allow for the development of innovative oral vaccines constituted by minimally-processed plant biomass are discussed.

Development of cowpea mosaic virus-based vectors for the production of vaccines in plants

Plant viruses are emerging as an attractive alternative to stable genetic transformation for the expression of foreign proteins in plants. The main advantages of using this strategy are that viral genomes are small and easy to manipulate, infection of plants with modified viruses is simpler and quicker than the regeneration of stably transformed plants and the sequence inserted into a virus vector will be highly amplified. One use of these virus expression systems is for vaccine production. Among plant viruses, cowpea mosaic virus makes an ideal candidate for the production of such vaccines because it grows extremely well in host plants, is very stable, and the purification of virus particles, if required, is straightforward. In this article, the authors review the progress made in the development of cowpea mosaic virus-based vectors for vaccine production, making use of two main approaches: epitope presentation and polypeptide expression.

Exploring the limits of vector construction based on Citrus tristeza virus

We examined the limits of manipulation of the Citrus tristeza virus (CTV) genome for expressing foreign genes in plants. We previously created a vector with a foreign gene cassette inserted between the major and minor coat protein genes, which is position 6 from the 3' terminus. Yet, this virus has 10 3'-genes with several other potential locations for expression of foreign genes. Since genes positioned closer to the 3' terminus tend to be expressed in greater amounts, there were opportunities for producing greater amounts of foreign protein. We found that the virus tolerated insertions of an extra gene in most positions within the 3' region of the genome with substantially increased levels of gene product produced throughout citrus trees. CTV was amazingly tolerant to manipulation resulting in a suite of stable transient expression vectors, each with advantages for specific uses and sizes of foreign genes in citrus trees.

A plant derived multifunctional tool for nanobiotechnology based on Tomato bushy stunt virus

Structure, size, physicochemical properties and production strategies make many plant viruses ideal protein based nanoscaffolds, nanocontainers and nano-building blocks expected to deliver a multitude of applications in different fields such as biomedicine, pharmaceutical chemistry, separation science, catalytic chemistry, crop pest control and biomaterials science. Functionalization of viral nanoparticles through modification by design of their external and internal surfaces is essential to fully exploit the potentiality of these objects. In the present paper we describe the development of a plant derived multifunctional tool for nanobiotechnology based on Tomato bushy stunt virus. We demonstrate the ability of this system to remarkably sustain genetic modifications and in vitro chemical derivatizations of its outer surface, which resulted in the successful display of large chimeric peptides fusions and small chemical molecules, respectively. Moreover, we have defined physicochemical conditions for viral swelling and reversible viral pore gating that we have successfully employed for foreign molecules loading and retention in the inner cavity of this plant virus nanoparticles system. Finally, a production and purification strategy from Nicotiana benthamiana plants has been addressed and optimized.

A coat-independent superinfection exclusion rapidly imposed in Nicotiana benthamiana cells by tobacco mosaic virus is not prevented by depletion of the movement protein

New evidence is emerging which indicates that population variants in plant virus infections are not uniformly distributed along the plant, but structured in a mosaic-like pattern due to limitation to the superinfection imposed by resident viral clones. The mechanisms that prevent the infection of a challenge virus into a previously infected cell, a phenomenon known as superinfection exclusion (SE) or Homologous Interference, are only partially understood. By taking advantage of a deconstructed tobacco mosaic virus (TMV) system, where the capsid protein (CP) gene is replaced by fluorescent proteins, an exclusion mechanism independent of CP was unveiled. Time-course superinfection experiments provided insights into SE dynamics. Initial infection levels affecting less than 10 % of cells led to full immunization in only 48 h, and measurable immunization levels were detected as early as 6 h post-primary infection. Depletion of a functional movement protein (MP) was also seen to slow down, but not to prevent, the SE mechanism. These observations suggest a CP-independent mechanism based on competition for a host-limiting factor, which operates at very low virus concentration. The possible involvement of host factors in SE has interesting implications as it would enable the host to influence the process.

Plant virus expression vectors set the stage as production platforms for biopharmaceutical proteins

Transgenic plants present enormous potential as a cost-effective and safe platform for large-scale production of vaccines and other therapeutic proteins. A number of different technologies are under development for the production of pharmaceutical proteins from plant tissues. One method used to express high levels of protein in plants involves the employment of plant virus expression vectors. Plant virus vectors have been designed to carry vaccine epitopes as well as full therapeutic proteins such as monoclonal antibodies in plant tissue both safely and effectively. Biopharmaceuticals such as these offer enormous potential on many levels, from providing relief to those who have little access to modern medicine, to playing an active role in the battle against cancer. This review describes the current design and status of plant virus expression vectors used as production platforms for biopharmaceutical proteins.

A cucumber mosaic virus based expression system for the production of porcine circovirus specific vaccines

Potential porcine circovirus type 2 (PCV2) capsid protein epitopes, suitable for expression on the surface of cucumber mosaic virus (CMV) particles were determined by a thorough analysis of the predicted PCV capsid protein structure. The ab initio protein structure prediction was carried out with fold recognition and threading methods. The putative PCV epitopes were selected on the basis of PCV virion models and integrated into the plant virus coat protein, after amino acid position 131. The recombinants were tested for infectivity and stability on different Nicotiana species and stable recombinant virus particles were purified. The particles were tested for their ability to bind to PCV induced porcine antibodies and used for specific antibody induction in mice and pigs. The results showed that PCV epitopes expressed on the CMV surface were recognized by the porcine antibodies and they were also able to induce PCV specific antibody response. Challenge experiment with PCV2 carried out in immunized pigs showed partial protection against the infection. Based on these results it was concluded that specific antiviral vaccine production for the given pathogen was feasible, offering an inexpensive way for the mass production of such vaccines.

An effective virus-based gene silencing method for functional genomics studies in common bean

BACKGROUND

Common bean (Phaseolus vulgaris L.) is a crop of economic and nutritious importance in many parts of the world. The lack of genomic resources have impeded the advancement of common bean genomics and thereby crop improvement. Although concerted efforts from the "Phaseomics" consortium have resulted in the development of several genomic resources, functional studies have continued to lag due to the recalcitrance of this crop for genetic transformation.

RESULTS

Here we describe the use of a bean pod mottle virus (BPMV)-based vector for silencing of endogenous genes in common bean as well as for protein expression. This BPMV-based vector was originally developed for use in soybean. It has been successfully employed for both protein expression and gene silencing in this species. We tested this vector for applications in common bean by targeting common bean genes encoding nodulin 22 and stearoyl-acyl carrier protein desaturase for silencing. Our results indicate that the BPMV vector can indeed be employed for reverse genetics studies of diverse biological processes in common bean. We also used the BPMV-based vector for expressing the green fluorescent protein (GFP) in common bean and demonstrate stable GFP expression in all common bean tissues where BPMV was detected.

CONCLUSIONS

The availability of this vector is an important advance for the common bean research community not only because it provides a rapid means for functional studies in common bean, but also because it does so without generating genetically modified plants. Here we describe the detailed methodology and provide essential guidelines for the use of this vector for both gene silencing and protein expression in common bean. The entire VIGS procedure can be completed in 4-5 weeks.

Development of expression vectors based on pepino mosaic virus

BACKGROUND

Plant viruses are useful expression vectors because they can mount systemic infections allowing large amounts of recombinant protein to be produced rapidly in differentiated plant tissues. Pepino mosaic virus (PepMV) (genus Potexvirus, family Flexiviridae), a widespread plant virus, is a promising candidate expression vector for plants because of its high level of accumulation in its hosts and the absence of severe infection symptoms. We report here the construction of a stable and efficient expression vector for plants based on PepMV.

RESULTS

Agroinfectious clones were produced from two different PepMV genotypes (European and Chilean), and these were able to initiate typical PepMV infections. We explored several strategies for vector development including coat protein (CP) replacement, duplication of the CP subgenomic promoter (SGP) and the creation of a fusion protein using the foot-and-mouth disease virus (FMDV) 2A catalytic peptide. We found that CP replacement vectors were unable to move systemically and that vectors with duplicated SGPs (even heterologous SGPs) suffered from significant transgene instability. The fusion protein incorporating the FMDV 2A catalytic peptide gave by far the best results, maintaining stability through serial passages and allowing the accumulation of GFP to 0.2-0.4 g per kg of leaf tissue. The possible use of PepMV as a virus-induced gene silencing vector to study gene function was also demonstrated. Protocols for the use of this vector are described.

CONCLUSIONS

A stable PepMV vector was generated by expressing the transgene as a CP fusion using the sequence encoding the foot-and-mouth disease virus (FMDV) 2A catalytic peptide to separate them. We have generated a novel tool for the expression of recombinant proteins in plants and for the functional analysis of virus and plant genes. Our experiments have also highlighted virus requirements for replication in single cells as well as intercellular and long-distance movement.

Efficient and stable expression of GFP through Wheat streak mosaic virus-based vectors in cereal hosts using a range of cleavage sites: formation of dense fluorescent aggregates for sensitive virus tracking

A series of Wheat streak mosaic virus (WSMV)-based expression vectors were developed by engineering a cycle 3 GFP (GFP) cistron between P1 and HC-Pro cistrons with several catalytic/cleavage peptides at the C-terminus of GFP. WSMV-GFP vectors with the Foot-and-mouth disease virus 1D/2A or 2A catalytic peptides cleaved GFP from HC-Pro but expressed GFP inefficiently. WSMV-GFP vectors with homologous NIa-Pro heptapeptide cleavage sites did not release GFP from HC-Pro, but efficiently expressed GFP as dense fluorescent aggregates. However, insertion of one or two spacer amino acids on either side of NIb/CP heptapeptide cleavage site or deletion in HC-Pro cistron improved processing by NIa-Pro. WSMV-GFP vectors were remarkably stable in wheat for seven serial passages and for 120 days postinoculation. Mite transmission efficiencies of WSMV-GFP vectors correlated with the amount of free GFP produced. WSMV-GFP vectors infected the same range of cereal hosts as wild-type virus, and GFP fluorescence was detected in most wheat tissues.

Transient expression systems for plant-derived biopharmaceuticals

In the molecular farming area, transient expression approaches for pharmaceutical proteins production, mainly recombinant monoclonal antibodies and vaccines, were developed almost two decades ago and, to date, these systems basically depend on Agrobacterium-mediated delivery and virus expression machinery. We survey here the current state-of-the-art of this research field. Several vectors have been designed on the basis of DNA- and RNA-based plant virus genomes and viral vectors are used both as single- and multicomponent expression systems in different combinations depending on the protein of interest. The obvious advantages of these systems are ease of manipulation, speed, low cost and high yield of proteins. In addition, Agrobacterium-mediated expression also allows the production in plants of complex proteins assembled from subunits. Currently, the transient expression methods are preferential over any other transgenic system for the exploitation of large and unrestricted numbers of plants in a contained environment. By designing optimal constructs and related means of delivery into plant cells, the overall technology plan considers scenarios that envisage high yield of bioproducts and ease in monitoring the whole spectrum of upstream production, before entering good manufacturing practice facilities. In this way, plant-derived bioproducts show promise of high competitiveness towards classical eukaryotic cell factory systems.

A tobamovirus expression vector for agroinfection of legumes and Nicotiana

The highest recombinant protein expression levels in plants have been achieved using tobacco mosaic virus (TMV) vectors via agroinoculation of the tobacco, Nicotiana benthamiana. These vectors have been utilized for pharmaceutical protein production and also can serve as rapid gene expression screens for proteonomics. We have constructed a similar vector based on the legume-infecting tobamovirus, sunn hemp mosaic virus (SHMV), by deleting the coat protein gene (SHMV eliminate coat protein gene or SHEC). SHEC/GFP co-agroinoculated with a 35S/p19 binary yielded 600 microg GFP/gfw (25% TSP) in N. benthamiana. In the absence of p19, SHEC/GFP expression was nearly eliminated. SHEC also yielded strong GUS production in agroinoculated Medicago trunculata, Pinto bean, cowpea, pea and lentil even without the aid of systemic infection. A full-length version (SHAC, SHMV alternate coat protein) was created by adding to SHEC the coat protein subgenomic promoter and ORF from the tobamovirus, tobacco mild green mottle virus (TMGMV). SHAC induced a slowly developing, symptomless infection of N. benthamiana and may be of use as a virus induced gene silencing (VIGS) vector.

The development of an efficient multipurpose bean pod mottle virus viral vector set for foreign gene expression and RNA silencing

Plant viral vectors are valuable tools for heterologous gene expression, and because of virus-induced gene silencing (VIGS), they also have important applications as reverse genetics tools for gene function studies. Viral vectors are especially useful for plants such as soybean (Glycine max) that are recalcitrant to transformation. Previously, two generations of bean pod mottle virus (BPMV; genus Comovirus) vectors have been developed for overexpressing and silencing genes in soybean. However, the design of the previous vectors imposes constraints that limit their utility. For example, VIGS target sequences must be expressed as fusion proteins in the same reading frame as the viral polyprotein. This requirement limits the design of VIGS target sequences to open reading frames. Furthermore, expression of multiple genes or simultaneous silencing of one gene and expression of another was not possible. To overcome these and other issues, a new BPMV-based vector system was developed to facilitate a variety of applications for gene function studies in soybean as well as in common bean (Phaseolus vulgaris). These vectors are designed for simultaneous expression of multiple foreign genes, insertion of noncoding/antisense sequences, and simultaneous expression and silencing. The simultaneous expression of green fluorescent protein and silencing of phytoene desaturase shows that marker gene-assisted silencing is feasible. These results demonstrate the utility of this BPMV vector set for a wide range of applications in soybean and common bean, and they have implications for improvement of other plant virus-based vector systems.

Cowpea mosaic virus: the plant virus-based biotechnology workhorse

In the 50 years since it was first described, Cowpea mosaic virus (CPMV) has become one of the most intensely studied plant viruses. Research in the past 15 to 20 years has shifted from studying the underlying genetics and structure of the virus to focusing on ways in which it can be exploited in biotechnology. This work led first to the use of virus particles to present peptides, then to the creation of a variety of replicating virus vectors and finally to the development of a highly efficient protein expression system that does not require viral replication. The circle has been completed by the use of the latter system to create empty particles for peptide presentation and other novel uses. The history of CPMV in biotechnology can be likened to an Ouroborus, an ancient symbol depicting a snake or dragon swallowing its own tail, thus forming a circle.

Cowpea mosaic virus-based systems for the expression of antigens and antibodies in plants

This chapter describes the use of Cowpea mosaic virus-based vectors for the production of foreign proteins such as antigens and antibodies in plants. The systems include vectors based on both full-length and deleted versions of RNA-2. In both cases, the modified RNA-2 is replicated by coinoculation with RNA-1. The constructs based on full-length RNA-2 retain the ability to spread systemically throughout an inoculated plant and the infection can be passaged. The vector based on a deleted version of RNA-2 can stably incorporate larger inserts but lacks the ability to move systemically. However, it has the added advantage of biocontainment. In both cases, vector constructs modified to contain a foreign gene of interest can be delivered by agroinfiltration to obtain transient expression of the foreign protein. If required, the same constructs can also be used for stable nuclear transformation. Both types of vector have proved effective for the production in plants of a diverse range of proteins including antigens and antibodies.

Towards engineering glucosinolates into non-cruciferous plants

Glucosinolates are amino acid-derived secondary metabolites present in cruciferous plants. Glucosinolates and their hydrolysis products are involved in defence against insects and pathogens, but are also known for their characteristic flavor and their cancer-preventive and antibacterial properties. This wide range of bioactivities has prompted a desire to engineer glucosinolates into non-cruciferous plants. We report the one-step transfer of the last three steps of the benzylglucosinolate pathway (comprising the C-S lyase, glycosyltransferase and sulfotransferase) from Arabidopsis to tobacco. This was achieved using an expression construct consisting of a single 2A polycistronic open reading frame, which allowed the expression of the three coding-sequences from a single promoter. When compared to wildtype plants, transgenic tobacco lines showed increased ability to convert the intermediate phenylacetothiohydroxamate to benzylglucosinolate upon in vivo feeding. Enzymatic assays using plant extracts demonstrated that the individual activities required for this conversion were enhanced in the transgenic plants. The relatively high conversion by wildtype plants in feeding assays supports the hypothesis that the last part of the glucosinolate pathway was recruited from existing detoxification reactions. Immunoblots confirmed that individual proteins were being successfully produced from the 2A polycistronic open reading frame, albeit fusion proteins could also be detected. In summary, we transferred the last three steps of the benzylglucosinolate pathway to tobacco as a first step towards engineering glucosinolates into non-cruciferous plants.

Expression of multiple proteins using full-length and deleted versions of cowpea mosaic virus RNA-2

The use of multiple copies of vectors based on either full-length or deleted versions of cowpea mosaic virus RNA-2 for the production of heteromeric proteins in plants was investigated. Co-infiltration of two full-length RNA-2 constructs containing different marker genes into Nicotiana benthamiana in the presence of RNA-1 showed that the two foreign proteins were efficiently expressed within the same cell in inoculated tissue. Furthermore, the proteins were co-localized to the same subcellular compartments, an essential prerequisite for heteromer formation. However, segregation of two separate RNA-2 molecules, and therefore expression of the two proteins, was observed on systemic spread of the recombinant viruses. Thus, efficient assembly of heteromeric proteins is likely to occur only in inoculated tissue. To determine the optimum approach for expression in inoculated tissue, the heavy and light chains of the blood group-typing immunoglobulin G (IgG) C5-1 were inserted into full-length and deleted versions of RNA-2, and the constructs were agroinfiltrated in the presence of RNA-1. The results obtained showed that full-size IgG molecules accumulated using both approaches, but that the levels were significantly higher when deleted RNA-2 vectors were used. The levels were also greatly enhanced by the inclusion of an endoplasmic reticulum retention signal at the C-terminus of the heavy chain. As the potential benefit of using full-length RNA-2 constructs, the ability to spread systemically, appears to be irrelevant to the production of heteromeric proteins, the use of deleted versions of RNA-2 is clearly advantageous, particularly as they offer the benefit of biocontainment.

Development of a novel recombinant encapsidated RNA particle: Evaluation as an internal control for diagnostic RT-PCR

This report describes the generation of novel encapsidated RNA particles and their evaluation as in-tube internal controls in diagnostic real-time reverse-transcription PCR (rRT-PCR) assays for the detection of RNA viruses. A cassette containing sequences of 2 diagnostic primer sets for foot-and-mouth disease virus (FMDV) and a set for swine vesicular disease virus (SVDV) was engineered into a full-length cDNA clone containing the RNA-2 segment of Cowpea Mosaic Virus (CPMV). After co-inoculation with a plasmid that expressed CPMV RNA-1, recombinant virus particles were rescued from cowpea plants (Vigna unguiculata). RNA contained in these particles was amplified in diagnostic rRT-PCR assays used for detection of FMDV and SVDV. Amplification of these internal controls was used to confirm that rRT-PCR inhibitors were absent from clinical samples, thereby verifying negative assay results. The recombinant CPMVs did not reduce the analytical sensitivity of the rRT-PCRs when amplification of the insert was performed in the same tube as the diagnostic target. This system provides an attractive solution to the production of internal controls for rRT-PCR assays since CPMV grows to high yields in plants, the particles are thermostable, RNase resistant and simple purification of RNA-2 containing capsids yields a preparation which is non-infectious.

Expression of human acidic fibroblast growth factor in Nicotiana benthamiana with a potato-virus-X-based binary vector

The aFGF (acidic fibroblast growth factor) plays an important role in morphogenesis, angiogenesis and wound healing and is therefore of potential medical interest. A DNA fragment encoding haFGF (human aFGF) has been cloned into the PVX (potato virus X)-based binary vector (pgR107) and transiently expressed in Nicotiana benthamiana (a wild Australian tobacco) by agroinfection. Approx. 1 week after agroinfection, the recombinant haFGF accumulated in the agroinfected plants reached up to 1% of the total soluble protein. haFGF was then purified on heparin-Sepharose CL-6B. The purified haFGF could stimulate the growth of NIH 3T3 cells, suggesting that the recombinant haFGF expressed via PVX viral vector in N. benthamiana was active biologically.

Replication-independent long-distance trafficking by viral RNAs in Nicotiana benthamiana

Viruses with separately encapsidated genomes could have their genomes introduced into different leaves of a plant, thus necessitating long-distance trafficking of the viral RNAs for successful infection. To examine this possibility, individual or combinations of genome segments from the tripartite Brome mosaic virus (BMV) were transiently expressed in leaves of Nicotiana benthamiana plants using engineered Agrobacterium tumefaciens. BMV RNA3 was found to traffic from the initial site of expression to other leaves of the plant, as detected by RNA gel blot analyses and also by the expression of an endoplasmic reticulum-targeted green fluorescent protein. When RNA3 trafficked into leaves containing the BMV replication enzymes, RNA replication, transcription, and virion production were observed. RNA3 trafficking occurred even when it did not encode the movement or capsid proteins. However, coexpression of the movement protein increased the trafficking of BMV RNAs. BMV RNA1 and RNA2 could also traffic throughout the plant, but less efficiently than RNA3. All three BMV RNAs trafficked bidirectionally to sink leaves near the apical meristem as well as to the source leaves at the bottom of the stem, suggesting that trafficking used the phloem. These results demonstrate that BMV RNAs can use a replication-independent mechanism to traffic in N. benthamiana.

Stability of Potato virus X expression vectors is related to insert size: implications for replication models and risk assessment

We investigated the stability of expression constructs based on Potato virus X (PVX) as a function of insert length. Five different inserts ranging in length from 261 to 1,758 bp (human proinsulin, murine interleukin-10, HIV-1 nef, petunia expansin-1 and human gad65) were expressed using a PVX vector in Nicotiana benthamiana plants for three sequential passages. Using a competitive RT-PCR approach we demonstrated that insert-deletion could occur in the first infection cycle for all inserts, but that this was much more likely to be the case for longer ones. This suggested a negative correlation between insert length and vector stability. Sequence analysis of the deleted constructs suggested that recombination usually occurred at sites close to the duplicated sub-genomic promoter, but in a smaller number of cases the foreign gene itself was probably involved, resulting in partially deleted constructs containing transgene fragments. The implications of these results in the context of recombinant protein expression and its risks are discussed.

Identification of Common Epitopes on a Conserved Region of NSs Proteins Among Tospoviruses of Watermelon silver mottle virus Serogroup

ABSTRACT The NSs protein of Watermelon silver mottle virus (WSMoV) was expressed by a Zucchini yellow mosaic virus (ZYMV) vector in squash. The expressed NSs protein with a histidine tag and an additional NIa protease cleavage sequence was isolated by Ni(2+)-NTA resins as a free-form protein and further eluted after sodium dodecyl sulfate-polyacrylamide gel electrophoresis for production of rabbit antiserum and mouse monoclonal antibodies (MAbs). The rabbit antiserum strongly reacted with the NSs crude antigen of WSMoV and weakly reacted with that of a high-temperature-recovered gloxinia isolate (HT-1) of Capsicum chlorosis virus (CaCV), but not with that of Calla lily chlorotic spot virus (CCSV). In contrast, the MAbs reacted strongly with all crude NSs antigens of WSMoV, CaCV, and CCSV. Various deletions of the NSs open reading frame were constructed and expressed by ZYMV vector. Results indicate that all three MAbs target the 89- to 125-amino-acid (aa) region of WSMoV NSs protein. Two indispensable residues of cysteine and lysine were essential for MAbs recognition. Sequence comparison of the deduced MAbs-recognized region with the reported tospoviral NSs proteins revealed the presence of a consensus sequence VRKPGVKNTGCKFTMHNQIFNPN (denoted WNSscon), at the 98- to 120-aa position of NSs proteins, sharing 86 to 100% identities among those of WSMoV, CaCV, CCSV, and Peanut bud necrosis virus. A synthetic WNSscon peptide reacted with the MAbs and verified that the epitopes are present in the 98- to 120-aa region of WSMoV NSs protein. The WSMoV sero-group-specific NSs MAbs provide a means for reliable identification of tospoviruses in this large serogroup.

Engineering the genome of Grapevine virus A into a vector for expression of proteins in herbaceous plants

Grapevine virus A (GVA), a species of the genus Vitivirus, consists of a approximately 7.4 kb single-stranded RNA genome of positive polarity, organized into five open reading frames (ORFs). In addition to grape varieties, GVA infects Nicotiana benthamiana plants and protoplasts. We engineered the genome of GVA as a vector that includes duplication of homologous sequences that contain the promoter of the movement protein (MP) sgRNA, supplemented by enzymatic restriction sites to be used as a convenient tool for transient expression of foreign genes from an individual sgRNA. The resulting vector was able to infect and to move in N. benthamiana plants in a manner similar to the wild-type GVA, but it was not stable and the inserted sequence was lost from the genome. Replacing the duplicated promoter with a GVA-MP promoter derived from a distantly related isolate of GVA improved the stability of the inserted sequence. The resulting vector was successfully used to express the reporter gene beta-glucuronidase (GUS) and the coat protein gene of Citrus tristeza virus in inoculated N. benthamiana plants. Development of a useful GVA vector is expected to find a use as a biotechnological tool for improvement of grapevines and it may enable vine breeders to bypass obstacles involved in genetic manipulation of perennial and fruiting plants.

A bipartite system for the constitutive and inducible expression of high levels of foreign proteins in plants

We have developed combined transgene/virus vector systems for the expression of heterologous proteins in plants. The systems are based on the bipartite RNA plant virus, cowpea mosaic virus (CPMV), and involve the amplification of integrated copies of either full-length or deleted versions of RNA-2 carrying a foreign gene. In the case of plants transgenic for full-length versions of RNA-2 carrying the green fluorescent protein (GFP), amplification can be achieved by supplying RNA-1 either exogenously or by crossing. This allows either inducible or constitutive expression of the foreign gene and results in an infection that can be passaged to further plants. Replication of deleted versions of RNA-2 harbouring GFP requires the presence of both RNA-1 and a suppressor of gene silencing, a function which we show can be supplied by HcPro from potato virus Y. Replication of the deleted versions of RNA-2 can be achieved by supplying the suppressor and RNA-1 either exogenously or by crossing, showing that this system can also be used in an inducible and constitutive format. The use of deleted forms of RNA-2 has the advantage that no infectious virus is produced, providing an effective method of biocontainment. The CPMV-based systems have advantages over existing plant expression systems in terms of the expression levels obtainable and the simplicity and flexibility of use, and should be of great practical benefit in the development of plants as bioreactors.

Development of Bean pod mottle virus-based vectors for stable protein expression and sequence-specific virus-induced gene silencing in soybean

Plant virus-based vectors provide valuable tools for expression of foreign proteins in plants and for gene function studies. None of the presently available virus vectors is suitable for use in soybean. In the present study, we produced Bean pod mottle virus (BPMV)-based vectors that are appropriate for gene expression and virus-induced gene silencing (VIGS) in soybean. The genes of interest were inserted into the RNA2-encoded polyprotein open reading frame between the movement protein (MP) and the large coat protein (L-CP) coding regions. Additional proteinase cleavage sites were created to flank the foreign protein by duplicating the MP/L-CP cleavage site. To minimize the chances of homologous recombination and thus insert instability, we took advantage of the genetic code degeneracy and altered the nucleotide sequence of the duplicated regions without affecting amino acid sequences. The recombinant BPMV constructs were stable following several serial passages in soybean and relatively high levels of protein expression were attained. Successful expression of several proteins with different biological activities was demonstrated from the BPMV vector. These included the reporter proteins GFP and DsRed, phosphinothricin acetyltransferase (encoded by the herbicide resistance bar gene), and the RNA silencing suppressors encoded by Tomato bushy stunt virus, Turnip crinkle virus, Tobacco etch virus, and Soybean mosaic virus. The possible use of BPMV as a VIGS vector to study gene function in soybean was also demonstrated with the phytoene desaturase gene. Our results suggest that the BPMV-based vectors are suitable for expression of foreign proteins in soybean and for functional genomics applications.

The Potential of Plant Virus Vectors for Vaccine Production

Plants viruses are versatile vectors that allow the rapid and convenient production of recombinant proteins in plants. Compared with production systems based on transgenic plants, viral vectors are easier to manipulate and recombinant proteins can be produced more quickly and in greater yields. Over the last few years, there has been much interest in the development of plant viruses as vectors for the production of vaccines, either as whole polypeptides or epitopes displayed on the surface of chimeric viral particles. Several viruses have been extensively developed for vaccine production, including tobacco mosaic virus, potato virus X and cowpea mosaic virus. Vaccine candidates have been produced against a range of human and animal diseases, and in many cases have shown immunogenic activity and protection in the face of disease challenge. In this review, we discuss the advantages of plant virus vectors, the development of different viruses as vector systems, and the immunological experiments that have demonstrated the principle of plant virus-derived vaccines.

Purification and serological analyses of tospoviral nucleocapsid proteins expressed by Zucchini yellow mosaic virus vector in squash

A plant viral vector engineered from an in vivo infectious clone of Zucchini yellow mosaic virus (ZYMV) was used to express the nucleocapsid proteins (NPs) of tospoviruses in planta. The open reading frames (ORFs) of NPs of different serogroups of tospoviruses, including Tomato spotted wilt virus, Impatiens necrotic spot virus, Watermelon silver mottle virus, Peanut bud necrosis virus, and Watermelon bud necrosis virus (WBNV), were in frame inserted in between the P1 and HC-Pro genes of the ZYMV vector. Six histidine residues and an NIa protease cleavage site were added at the C-terminal region of the inserts to facilitate purification and process of free form of the expressed NPs, respectively. Approximately 1.2-2.5 mg/NPs 100 g tissues were purified from leaf extracts of zucchini squash. The expressed WBNV NP was used as an immunogen for the production of highly specific polyclonal antisera and monoclonal antibodies. The procedure provides a convenient and fast way for production of large quantities of pure NPs of tospoviruses in planta. The system also has a potential for production of any proteins of interest in cucurbits.

Use of viral vectors for vaccine production in plants

The small size of plant viral genomes, the ease with which they can be manipulated, and the simplicity of the infection process is making the viral vectors an attractive alternative to the transgenic systems for the expression of foreign proteins in plants. One use of these virus expression systems is for vaccine production. There are two basic types of viral system that have been developed for the production of immunogenic peptides and proteins in plants: epitope presentation and polypeptide expression systems. In this review, we discuss advances made in this field.

The use of viral vectors to produce hepatitis B virus core particles in plants

The expression and assembly of the hepatitis B virus (HBV) nucleocapsid protein (HBcAg) were investigated in plants using viral vectors. Constructs based on either Potato virus X (PVX) or Cowpea mosaic virus (CPMV) containing the sequence of HBcAg were able to infect the appropriate host plants and remained genetically stable during infection. Analysis of HBcAg expression revealed that the protein can self-assemble into core-like particles and that the assembled material could be partially purified by differential centrifugation. Thus, the use of viral vectors can be considered a practical method for rapid production of assembled HBcAg particles in plants. This approach provides a means whereby a variety of chimaeric particles can be assessed quickly and cheaply for various diagnostic and vaccine applications.