Rapid, high-level expression of glycosylated rice alpha-amylase in transfected plants by an RNA viral vector

A comprehensive review and meta-analysis of recent advances in biotechnology for plant virus research and significant accomplishments in human health and the pharmaceutical industry

Secondary metabolites made by plants and used through their metabolic routes are today's most reliable and cost-effective way to make pharmaceuticals and improve health. The concept of genetic engineering is used for molecular pharming. As more people use plants as sources of nanotechnology systems, they are adding to this. These systems are made up of viruses-like particles (VLPs) and virus nanoparticles (VNPs). Due to their superior ability to be used as plant virus expression vectors, plant viruses are becoming more popular in pharmaceuticals. This has opened the door for them to be used in research, such as the production of medicinal peptides, antibodies, and other heterologous protein complexes. This is because biotechnological approaches have been linked with new bioinformatics tools. Because of the rise of high-throughput sequencing (HTS) and next-generation sequencing (NGS) techniques, it has become easier to use metagenomic studies to look for plant virus genomes that could be used in pharmaceutical research. A look at how bioinformatics can be used in pharmaceutical research is also covered in this article. It also talks about plant viruses and how new biotechnological tools and procedures have made progress in the field.

Current Status and Perspective on the Use of Viral-Based Vectors in Eukaryotic Microalgae

During the last two decades, microalgae have attracted increasing interest, both commercially and scientifically. Commercial potential involves utilizing valuable natural compounds, including carotenoids, polysaccharides, and polyunsaturated fatty acids, which are widely applicable in food, biofuel, and pharmaceutical industries. Conversely, scientific potential focuses on bioreactors for producing recombinant proteins and developing viable technologies to significantly increase the yield and harvest periods. Here, viral-based vectors and transient expression strategies have significantly contributed to improving plant biotechnology. We present an updated outlook covering microalgal biotechnology for pharmaceutical application, transformation techniques for generating recombinant proteins, and genetic engineering tactics for viral-based vector construction. Challenges in industrial application are also discussed.

Application of Plant Viruses in Biotechnology, Medicine, and Human Health

Plant-based nanotechnology programs using virus-like particles (VLPs) and virus nanoparticles (VNPs) are emerging platforms that are increasingly used for a variety of applications in biotechnology and medicine. Tobacco mosaic virus (TMV) and potato virus X (PVX), by virtue of having high aspect ratios, make ideal platforms for drug delivery. TMV and PVX both possess rod-shaped structures and single-stranded RNA genomes encapsidated by their respective capsid proteins and have shown great promise as drug delivery systems. Cowpea mosaic virus (CPMV) has an icosahedral structure, and thus brings unique benefits as a nanoparticle. The uses of these three plant viruses as either nanostructures or expression vectors for high value pharmaceutical proteins such as vaccines and antibodies are discussed extensively in the following review. In addition, the potential uses of geminiviruses in medical biotechnology are explored. The uses of these expression vectors in plant biotechnology applications are also discussed. Finally, in this review, we project future prospects for plant viruses in the fields of medicine, human health, prophylaxis, and therapy of human diseases.

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.

Development of a new tobamovirus-based viral vector for protein expression in plants

Plants are becoming an interesting alternative system for the heterologous production of pharmaceutical proteins, providing a more scalable, cost-effective, and biologically safer option than the current expression systems. The development of plant virus expression vectors has allowed rapid and high-level transient expression of recombinant genes, and, in turn, provided an attractive plant-based production platform. Here we report the development of vectors based on the tobamovirus Pepper mild mottle virus (PMMoV) to be used in transient expression of foreign genes. In this PMMoV vector, a middle part of the viral coat protein gene was replaced by the green fluorescent protein (GFP) gene, and this recombinant genome was assembled in a binary vector suitable for plant agroinoculation. The accumulation of GFP was evaluated by observation of green fluorescent signals under UV light and by western blotting. Furthermore, by using this vector, the multiepitope gene for chikungunya virus was successfully expressed and confirmed by western blotting. This PMMoV-based vector represents an alternative system for a high-level production of heterologous protein in plants.

Protein-Based Bioproducts

Plant proteins can be used for the production of a variety of bioproducts, including films and coatings, adhesives, fibres and pharmaceuticals. Proteins derived from plant production systems have many advantages: they are safe, low-cost and rapidly deployable, allow for simple product storage and result in proteins that are properly folded, assembled and post-translationally modified. While plant-derived protein-based products are natural, renewable, biodegradable and environmentally friendly, they tend to be lower in strength and elasticity than their corresponding synthetic products. Current research in this area is focused on overcoming challenges in plant production platforms related to yield, purification, regulatory approval and customer acceptance.

An enzyme activity capable of endotransglycosylation of heteroxylan polysaccharides is present in plant primary cell walls

Heteroxylans in the plant cell wall have been proposed to have a role analogous to that of xyloglucans or heteromannans, forming growth-restraining networks by interlocking cellulose microfibrils. A xylan endotransglycosylase has been identified that can transglycosylate heteroxylan polysaccharides in the presence of xylan-derived oligosaccharides. High activity was detected in ripe fruit of papaya (Carica papaya), but activity was also found in a range of other fruits, imbibed seeds and rapidly growing seedlings of cereals. Xylan endotransglycosylase from ripe papaya fruit used a range of heteroxylans, such as wheat arabinoxylan, birchwood glucuronoxylan and various heteroxylans from dicotyledonous primary cell walls purified from tomato and papaya fruit, as donor molecules. As acceptor molecules, the enzyme preferentially used xylopentaitol over xylohexaitol or shorter-length acceptors. Xylan endotransglycosylase was active over a broad pH range and could perform transglycosylation reactions up to 55 °C. Xylan endotransglycosylase activity was purified from ripe papaya fruit by ultrafiltration and cation exchange chromatography. Highest endotransglycosylase activity was identified in fractions that also contained high xylan hydrolase activity and correlated with the presence of the endoxylanase CpaEXY1. Recombinant CpaEXY1 protein transiently over-expressed in Nicotiana benthamiana leaves showed both endoxylanase and xylan endotransglycosylase activities in vitro, suggesting that CpaEXY1 is a single enzyme with dual activity in planta. Purified native CpaEXY1 showed two- to fourfold higher endoxylanase than endotransglycosylase activity, suggesting that CpaEXY1 may act primarily as a hydrolase. We propose that xylan endotransglycosylase activity (like xyloglucan and mannan endotransglycosylase activities) could be involved in remodelling or re-arrangement of heteroxylans of the cellulose-non-cellulosic cell wall framework.

Plant viral vectors for delivery by Agrobacterium

Plant viral vectors delivered by Agrobacterium are the basis of several manufacturing processes that are currently in use for producing a wide range of proteins for multiple applications, including vaccine antigens, antibodies, protein nanoparticles such as virus-like particles (VLPs), and other protein and protein-RNA scaffolds. Viral vectors delivered by agrobacterial T-DNA transfer (magnifection) have also become important tools in research. In recent years, essential advances have been made both in the development of second-generation vectors designed using the 'deconstructed virus' approach, as well as in the development of upstream manufacturing processes that are robust and fully scalable. The strategy relies on Agrobacterium as a vector to deliver DNA copies of one or more viral RNA/DNA replicons; the bacteria are delivered into leaves by vacuum infiltration, and the viral machinery takes over from the point of T-DNA transfer to the plant cell nucleus, driving massive RNA and protein production and, if required, cell-to-cell spread of the replicons. Among the most often used viral backbones are those of the RNA viruses Tobacco mosaic virus (TMV), Potato virus X (PVX) and Cowpea mosaic virus (CPMV), and the DNA geminivirus Bean yellow dwarf virus. Prototypes of industrial processes that provide for high yield, rapid scale up and fast manufacturing cycles have been designed, and several GMP-compliant and GMP-certified manufacturing facilities are in place. These efforts have been successful as evidenced by the fact that several antibodies and vaccine antigens produced by magnifection are currently in clinical development.

Functional expression and characterization of sesquiterpene synthases from Artemisia annua L. using transient expression system in Nicotiana benthamiana

Artemisia annua L. produces a number of sesquiterpene synthases, which catalyze the conversion of farnesyl diphosphate to various sesquiterpenes. The cDNAs encoding amorpha-4,11-diene synthase (ADS), a key enzyme in the artemisinin biosynthesis, and epi-cedrol synthase (ECS), a complex sesquiterpene cyclization synthase, were cloned into Cowpea mosaic virus-based viral vector (pEAQ-HT) with Kozak consensus motif and C-terminal histidine tag. The plasmids were transformed into Agrobacterium LBA4404 and, agroinfiltrated into Nicotiana benthamiana leaves along with vector (pJL3:p19) containing Tomato bushy stunt virus post-transcriptional gene silencing suppressor. Quantitative PCR was carried out to measure the transcript levels at 0, 3, 6, 9, 12 and 15 days post-infiltration (dpi). The highest relative expression was observed at 9 dpi for both genes. Transiently expressed recombinant proteins of ADS and ECS were confirmed by SDS-PAGE and western blot. Recombinant proteins were extracted from 9 dpi leaves and purified by immobilized metal ion affinity chromatography using histidine tag, which produced yields of 90 and 96 mg kg⁻¹ fresh weight of leaves for ADS and ECS, respectively. Activities of the purified enzymes were assayed using gas chromatography-mass spectrometry for product identification and quantification using valencene as internal standard. The recombinant ADS and ECS converted farnesyl diphosphate into amorpha-4,11-diene (97 %) and epi-cedrol (96 %) as the major products, respectively. The purified enzymes exhibited the specific activity of 0.002 and 0.01 μmol min⁻¹ mg⁻¹ protein for ADS and ECS, respectively. The apparent k(cat) values were 2.1 × 10⁻³ s⁻¹ and 11 × 10⁻³ s⁻¹ for ADS and ECS, respectively.

KEY MESSAGE

Agroinfiltration of leaves of Nicotiana bentamiana can be used to produce recombinant biosynthetic enzymes as exemplified by two sesquiterpene synthases from Artemisia annua in relatively high yields.

Efficient production of human acidic fibroblast growth factor in pea (Pisum sativum L.) plants by agroinfection of germinated seeds

BACKGROUND

For efficient and large scale production of recombinant proteins in plants transient expression by agroinfection has a number of advantages over stable transformation. Simple manipulation, rapid analysis and high expression efficiency are possible. In pea, Pisum sativum, a Virus Induced Gene Silencing System using the pea early browning virus has been converted into an efficient agroinfection system by converting the two RNA genomes of the virus into binary expression vectors for Agrobacterium transformation.

RESULTS

By vacuum infiltration (0.08 Mpa, 1 min) of germinating pea seeds with 2-3 cm roots with Agrobacteria carrying the binary vectors, expression of the gene for Green Fluorescent Protein as marker and the gene for the human acidic fibroblast growth factor (aFGF) was obtained in 80% of the infiltrated developing seedlings. Maximal production of the recombinant proteins was achieved 12-15 days after infiltration.

CONCLUSIONS

Compared to the leaf injection method vacuum infiltration of germinated seeds is highly efficient allowing large scale production of plants transiently expressing recombinant proteins. The production cycle of plants for harvesting the recombinant protein was shortened from 30 days for leaf injection to 15 days by applying vacuum infiltration. The synthesized aFGF was purified by heparin-affinity chromatography and its mitogenic activity on NIH 3T3 cells confirmed to be similar to a commercial product.

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.

Development of electronic barcodes for use in plant pathology and functional genomics

We have developed a novel 'electronic barcode' system that uses radio frequency identification (RFID) tags, cell phones, and portable computers to link phenotypic, environmental, and genomic data. We describe a secure, inexpensive system to record and retrieve data from plant samples. It utilizes RFID tags, computers, PDAs, and cell phones to link, record, and retrieve positional, and functional genomic data. Our results suggest that RFID tags can be used in functional genomic screens to record information that is involved in plant development or disease.

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.

Engineering viral expression vectors for plants: the 'full virus' and the 'deconstructed virus' strategies

Plant viral vectors are being successfully developed and exploited for the industrial-scale expression of heterologous proteins and as a research tool for studies of gene expression. The initial engineering strategy (the 'full virus' vector strategy) aimed to design a vector that was essentially a wildtype virus, which was modified to carry and express a heterologous sequence that encoded a gene of interest. The new emerging trend (the 'deconstructed virus' vector strategy) reflects an ideology that recognises the inherent limitations of the viral process. It attempts to 'deconstruct' the virus, by eliminating functions that are limiting or undesired, and to rebuild it, either by delegating the missing necessary functions to the host (which is genetically modified to provide those functions) or by replacing them with analogous functions that are not derived from a virus.

'Molecular farming' of antibodies in plants

'Molecular farming' is the production of valuable recombinant proteins in transgenic organisms on an agricultural scale. While plants have long been used as a source of medicinal compounds, molecular farming represents a novel source of molecular medicines, such as plasma proteins, enzymes, growth factors, vaccines and recombinant antibodies, whose medical benefits are understood at a molecular level. Until recently, the broad use of molecular medicines was limited because of the difficulty in producing these proteins outside animals or animal cell culture. The application of molecular biology and plant biotechnology in the 1990s showed that many molecular medicines or vaccines could be synthesised in plants and this technology is termed 'molecular farming'. It results in pharmaceuticals that are safer, easier to produce and less expensive than those produced in animals or microbial culture. An advantage of molecular farming lies in the ability to perform protein production on a massive scale using hectares of cultivated plants. These plants can then be harvested and transported using the agricultural infrastructure. Thus, molecular farming allows rapid progress from genetic engineering to crop production, and new cash crops producing recombinant proteins are already being commercially exploited. We speculate that as functional genomics teaches us more about the nature of disease, molecular farming will produce many of the protein therapeutics that can remedy it.

Expression and purification of a neuropeptide nocistatin using two related plant viral vectors

Both odontoglossum ringspot virus (ORSV) and tobacco mosaic virus (TMV) were investigated as expression viral vectors for the expression of a neuropeptide nocistatin. Chimeras of ORSV and TMV were constructed by fusion of 17 amino acids of mouse nocistatin (mNST) to the C-terminal of the coat protein (CP) gene via a Factor Xa cleavage linker to yield ORSV-mNST and TMV-mNST. Expression of the mNST peptide was demonstrated by immuno-transmission electron microscopy, western blot, mass spectrometry and radioimmunoassay. Serial passaging of the chimeric viruses revealed loss of mNST from TMV-mNST by the fifth passage. The mNST was maintained in ORSV-mNST throughout six passages. The mNST peptide could be effectively cleaved and purified from chimeric ORSV CP. To our knowledge, this is the first successful attempt in obtaining a complete peptide with no additional amino acid sequence after expression and purification through the use of either ORSV or TMV as vectors.

Epigenetic modification of plants with systemic RNA viruses

Knowledge of gene function is critical to the development of new plant traits for improved agricultural and industrial applications. Viral expression vectors offer a rapid and proven method to provide epigenetic expression of foreign sequences throughout infected plants. Expression of these sequences from viral vectors can lead to gain- or loss-of-function phenotypes, allowing gene function to be determined by phenotypic or biochemical effects in the infected plant. Tobacco mosaic virus and barley stripe mosaic virus expression vectors have been developed to express foreign gene sequences in dicotyledonous and monocotyledonous hosts, respectively. Large-scale application of both viral vector systems for gene function discovery in Nicotiana and barley hosts resulted in high infection rates and produced distinctive visual phenotypes in approximately 5% of transfected plants. Novel genes expressing potential herbicide target proteins in addition to genes promoting stem elongation, leaf development and apical dominance were identified in the large-scale screening. This report illustrates the adaptability of viral vectors for gene function discovery in higher plants.

Engineering zucchini yellow mosaic potyvirus as a non-pathogenic vector for expression of heterologous proteins in cucurbits

Plant virus vectors provide an attractive biotechnological tool for the transient expression of foreign genes in whole plants. As yet there has been no use of recombinant viruses for the improvement of commercial crops. This is mainly because the viruses used to create vectors usually cause significant yield loss and can be transmitted in the field. A novel attenuated zucchini yellow mosaic potyvirus (AG) was used for the development of an environmentally safe non-pathogenic virus vector. The suitability of AG as an expression vector in plants was tested by analysis of two infectious viral constructs, each containing a distinct gene insertion site. Introduction of a foreign viral coat protein gene into AG genome between the P1 and HC-Pro genes, resulted in no expression in planta. In contrast, the same gene was stably expressed when inserted between NIb and CP genes, suggesting that this site is more suitable for a gene vector. Virus-mediated expression of reporter genes was observed in squash and cucumber leaves, stems, roots and edible fruit. Furthermore, AG stably expressed human interferon-alpha 2, an important human anti-viral drug, without affecting plant development and yield. Interferon biological activity was measured in cucumber and squash fruit. Together, these data corroborate a biotechnological utility of AG as a non-pathogenic vector for the expression of a foreign gene, as a benefit trait, in cucurbits and their edible fruit.

Molecular farming of pharmaceutical proteins

Molecular farming is the production of pharmaceutically important and commercially valuable proteins in plants. Its purpose is to provide a safe and inexpensive means for the mass production of recombinant pharmaceutical proteins. Complex mammalian proteins can be produced in transformed plants or transformed plant suspension cells. Plants are suitable for the production of pharmaceutical proteins on a field scale because the expressed proteins are functional and almost indistinguishable from their mammalian counterparts. The breadth of therapeutic proteins produced by plants range from interleukins to recombinant antibodies. Molecular farming in plants has the potential to provide virtually unlimited quantities of recombinant proteins for use as diagnostic and therapeutic tools in health care and the life sciences. Plants produce a large amount of biomass and protein production can be increased using plant suspension cell culture in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can also produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the molecular farming of recombinant therapeutics, including vaccines, diagnostics, such as recombinant antibodies, plasma proteins, cytokines and growth factors.