Plant Virology Delivers Diverse Toolsets for Biotechnology

Trans-complementation of the viral movement protein mediates efficient expression of large target genes via a tobacco mosaic virus vector

The development of plant virus-based expression systems has expanded rapidly owing to their potential applications in gene functional and disease resistance research, and industrial production of pharmaceutical proteins. However, the low yield of certain proteins, especially high-molecular-mass proteins, restricts the production scale. In this study, we observed that the tobacco mosaic virus (TMV)-mediated expression of a foreign protein was correlated with the amount of the movement protein (MP) and developed a TMV-derived pAT-transMP vector system incorporating trans-complementation expression of MP. The system is capable of efficient expression of exogenous proteins, in particular those with a high molecular mass, and enables simultaneous expression of two target molecules. Furthermore, viral expression of competent CRISPR-Cas9 protein and construction of CRISPR-Cas9-mediated gene-editing system in a single pAT-transMP construct was achieved. The results demonstrated a novel role for TMV-MP in enhancing the accumulation of a foreign protein produced from the viral vector or a binary expression system. Further investigation of the mechanism underlying this role will be beneficial for optimization of plant viral vectors with broad applications.

Plant virology in the 21st century in China: Recent advances and future directions

Plant viruses are a group of intracellular pathogens that persistently threaten global food security. Significant advances in plant virology have been achieved by Chinese scientists over the last 20 years, including basic research and technologies for preventing and controlling plant viral diseases. Here, we review these milestones and advances, including the identification of new crop-infecting viruses, dissection of pathogenic mechanisms of multiple viruses, examination of multilayered interactions among viruses, their host plants, and virus-transmitting arthropod vectors, and in-depth interrogation of plant-encoded resistance and susceptibility determinants. Notably, various plant virus-based vectors have also been successfully developed for gene function studies and target gene expression in plants. We also recommend future plant virology studies in China.

Scanning the Horizon for Environmental Applications of Genetically Modified Viruses Reveals Challenges for Their Environmental Risk Assessment

The release of novel genetically modified (GM) virus applications into the environment for agricultural, veterinary, and nature-conservation purposes poses a number of significant challenges for risk assessors and regulatory authorities. Continuous efforts to scan the horizon for emerging applications are needed to gain an overview of new GM virus applications. In addition, appropriate approaches for risk assessment and management have to be developed. These approaches need to address pertinent challenges, in particular with regard to the environmental release of GM virus applications with a high probability for transmission and spreading, including transboundary movements and a high potential to result in adverse environmental effects. However, the current preparedness at the EU and international level to assess such GM virus application is limited. This study addresses some of the challenges associated with the current situation, firstly, by conducting a horizon scan to identify emerging GM virus applications with relevance for the environment. Secondly, outstanding issues regarding the environmental risk assessment (ERA) of GM virus applications are identified based on an evaluation of case study examples. Specifically, the limited scientific information available for the ERA of some applications and the lack of detailed and appropriate guidance for ERA are discussed. Furthermore, considerations are provided for future work that is needed to establish adequate risk assessment and management approaches.

Virus-Induced Heritable Gene Editing in Plants

Gene editing using clustered, regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) nuclease is an excellent tool for assessing gene function in plants. However, delivery of CRISPR/Cas-editing components into plant cells is still a major bottleneck and requires tissue culture-based approaches and regeneration of plants. To overcome this limitation, several plant viral vectors have recently been engineered to deliver single-guide RNA (sgRNA) targets into SpCas9-expressing plants. Here, we describe an optimized, step-by-step protocol based on the tobacco rattle virus (TRV)-based vector system to deliver sgRNAs fused to mobile tRNA sequences for efficient heritable editing in Nicotiana benthamiana and Arabidopsis thaliana model systems. The protocol described here could be adopted to study the function of any gene of interest.

Identification and characterization of two closely related virga-like viruses latently infecting rubber trees (Hevea brasiliensis)

A novel virga-like virus, provisionally named Rubber tree latent virus 2 (RTLV2), was identified from rubber tree (Hevea brasiliensis). It is a close relative of the previously reported Rubber tree latent virus 1 (RTLV1). The complete genomes of RTLV1 and RTLV2 were sequenced and comparatively analyzed in terms of genome organization, putative gene products and phylogenetic relationship. Both RTLV1 and RTLV2 have positive-sense single-stranded RNA genomes that encode seven open reading frames (ORFs), forming a similar genomic layout. In phylogenetic analyses based on replicase and coat protein amino acid sequences, RTLV1 and RTLV2 were clustered with unclassified virga-like viruses. They are distinct from currently recognized plant virus families. RTLV1 and RTLV2 can be distinguished from members of Virgaviridae by the presence of a putative coat protein duplex and a poly(A) tail at the 3'-terminus. The authenticity of RTLV1 and RTLV2 as infectious viruses was confirmed through field investigations and transmissibility assays. In conclusion, RTLV1 and RTLV2 represent a novel plant virus group that does not readily fit into current virus families.

Development of cassava common mosaic virus-based vector for protein expression and gene editing in cassava

BACKGROUND

Plant virus vectors designed for virus-mediated protein overexpression (VOX), virus-induced gene silencing (VIGS), and genome editing (VIGE) provide rapid and cost-effective tools for functional genomics studies, biotechnology applications and genome modification in plants. We previously reported that a cassava common mosaic virus (CsCMV, genus Potexvirus)-based VIGS vector was used for rapid gene function analysis in cassava. However, there are no VOX and VIGE vectors available in cassava.

RESULTS

In this study, we developed an efficient VOX vector (CsCMV2-NC) for cassava by modifying the CsCMV-based VIGS vector. Specifically, the length of the duplicated putative subgenomic promoter (SGP1) of the CsCMV CP gene was increased to improve heterologous protein expression in cassava plants. The modified CsCMV2-NC-based VOX vector was engineered to express genes encoding green fluorescent protein (GFP), bacterial phytoene synthase (crtB), and Xanthomonas axonopodis pv. manihotis (Xam) type III effector XopAO1 for viral infection tracking, carotenoid biofortification and Xam virulence effector identification in cassava. In addition, we used CsCMV2-NC to deliver single guide RNAs (gMePDS1/2) targeting two loci of the cassava phytoene desaturase gene (MePDS) in Cas9-overexpressing transgenic cassava lines. The CsCMV-gMePDS1/2 efficiently induced deletion mutations of the targeted MePDS with the albino phenotypes in systemically infected cassava leaves.

CONCLUSIONS

Our results provide a useful tool for rapid and efficient heterologous protein expression and guide RNA delivery in cassava. This expands the potential applications of CsCMV-based vector in gene function studies, biotechnology research, and precision breeding for cassava.

Opportunities and Challenges of In Vitro Tissue Culture Systems in the Era of Crop Genome Editing

Currently, the development of genome editing (GE) tools has provided a wide platform for targeted modification of plant genomes. However, the lack of versatile DNA delivery systems for a large variety of crop species has been the main bottleneck for improving crops with beneficial traits. Currently, the generation of plants with heritable mutations induced by GE tools mostly goes through tissue culture. Unfortunately, current tissue culture systems restrict successful results to only a limited number of plant species and genotypes. In order to release the full potential of the GE tools, procedures need to be species and genotype independent. This review provides an in-depth summary and insights into the various in vitro tissue culture systems used for GE in the economically important crops barley, wheat, rice, sorghum, soybean, maize, potatoes, cassava, and millet and uncovers new opportunities and challenges of already-established tissue culture platforms for GE in the crops.

Engineering aphid transmission of foxtail mosaic virus in the presence of potyvirus helper component proteinase through coat protein modifications

Biotechnologies that use plant viruses as plant enhancement tools have shown great potential to flexibly engineer crop traits, but field applications of these technologies are still limited by efficient dissemination methods. Potyviruses can be rapidly inoculated into plants by aphid vectors due to the presence of the potyviral helper component proteinase (HC-Pro), which binds to the DAG motif of the coat protein (CP) of the virion. Previously it was determined that a naturally occurring DAG motif in the non-aphid-transmissible potexvirus, potato aucuba mosaic virus (PAMV), is functional when a potyviral HC-Pro is provided to aphids in plants. The DAG motif of PAMV was successfully transferred to the CP of another non-aphid-transmissible potexvirus, potato virus X, to convey aphid transmission capabilities in the presence of HC-Pro. Here, we demonstrate that DAG-containing segments of the CP from two different potyviruses (sugarcane mosaic virus and turnip mosaic virus), and from the previously used potexvirus, PAMV, can make the potexvirus, foxtail mosaic virus (FoMV), aphid-transmissible when fused with the FoMV CP. We show that DAG-containing FoMVs are transmissible by aphids that have prior access to HC-Pro through potyvirus-infected plants or ectopic expression of HC-Pro. The transmission efficiency of the DAG-containing FoMVs varied from less than 10 % to over 70 % depending on the length and composition of the surrounding amino acid sequences of the DAG-containing segment, as well as due to the recipient plant species. Finally, we show that the engineered aphid-transmissible FoMV is still functional as a plant enhancement resource, as endogenous host target genes were silenced in FoMV-infected plants after aphid transmission. These results suggest that aphid transmission could be engineered into non-aphid-transmissible plant enhancement viral resources to facilitate their field applications.

Virus-Induced Gene Silencing (VIGS): A Powerful Tool for Crop Improvement and Its Advancement towards Epigenetics

Virus-induced gene silencing (VIGS) is an RNA-mediated reverse genetics technology that has evolved into an indispensable approach for analyzing the function of genes. It downregulates endogenous genes by utilizing the posttranscriptional gene silencing (PTGS) machinery of plants to prevent systemic viral infections. Based on recent advances, VIGS can now be used as a high-throughput tool that induces heritable epigenetic modifications in plants through the viral genome by transiently knocking down targeted gene expression. As a result of the progression of DNA methylation induced by VIGS, new stable genotypes with desired traits are being developed in plants. In plants, RNA-directed DNA methylation (RdDM) is a mechanism where epigenetic modifiers are guided to target loci by small RNAs, which play a major role in the silencing of the target gene. In this review, we described the molecular mechanisms of DNA and RNA-based viral vectors and the knowledge obtained through altering the genes in the studied plants that are not usually accessible to transgenic techniques. We showed how VIGS-induced gene silencing can be used to characterize transgenerational gene function(s) and altered epigenetic marks, which can improve future plant breeding programs.

Critical points for the design and application of RNA silencing constructs for plant virus resistance

Control of plant virus diseases is a big challenge in agriculture as is resistance in plant lines to infection by viruses. Recent progress using advanced technologies has provided fast and durable alternatives. One of the most promising techniques against plant viruses that is cost-effective and environmentally safe is RNA silencing or RNA interference (RNAi), a technology that could be used alone or along with other control methods. To achieve the goals of fast and durable resistance, the expressed and target RNAs have been examined in many studies, with regard to the variability in silencing efficiency, which is regulated by various factors such as target sequences, target accessibility, RNA secondary structures, sequence variation in matching positions, and other intrinsic characteristics of various small RNAs. Developing a comprehensive and applicable toolbox for the prediction and construction of RNAi helps researchers to achieve the acceptable performance level of silencing elements. Although the attainment of complete prediction of RNAi robustness is not possible, as it also depends on the cellular genetic background and the nature of the target sequences, some important critical points have been discerned. Thus, the efficiency and robustness of RNA silencing against viruses can be improved by considering the various parameters of the target sequence and the construct design. In this review, we provide a comprehensive treatise regarding past, present and future prospective developments toward designing and applying RNAi constructs for resistance to plant viruses.

Construction of Full-Length Infectious cDNA Clones of Citrus Mosaic Virus RNA1 and RNA2 and Infection of Citrus Seedlings by Agrobacterium-Mediated Vacuum-Infiltration

The development of full-length infectious cDNA clones for plant RNA viruses is important for studying their molecular biological characteristics, functional genomics, pathogenesis, and vectorization applications. Citrus mosaic virus (CiMV), a member of the genus Sadwavirus, is of economic importance to the citrus industry and comprises a bipartite, positive-sense, single-stranded RNA genome encapsidated in icosahedral virions. In the present study, full-length cDNA clones of CiMV RNA1 and RNA2 were constructed based on a ternary yeast-Escherichia coli-Agrobacterium tumefaciens shuttle vector, pTY, using transformation-associated recombination (TAR) strategy. Infectivity of cDNA clones of CiMV RNA1 and RNA2 was examined in multiple citrus varieties via Agrobacterium-mediated vacuum-infiltration (AVI) through symptom observation, RT-PCR, and virion detection with an electron microscope. Furthermore, the genome-sized RT-PCR fragments of RNA1 and RNA2 were obtained from symptomatic Jinchengyou (Citrus grandis) plants infected by the cloned virus (CiMV211). In addition, CiMV211 produced typical symptoms of wild-type CiMV in cowpea (Vigna angularis) plants inoculated by Agrobacterium-mediated injection. This is the first report of infectious cDNA clones of CiMV, which may lay the foundation for research on the pathogenesis and vectorization of the virus.

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.

Expression of an extremophilic xylanase in Nicotiana benthamiana and its use for the production of prebiotic xylooligosaccharides

A gene construct encoding a xylanase, which is active in extreme conditions of temperature and alkaline pH (90 °C, pH 10.5), has been transitorily expressed with high efficiency in Nicotiana benthamiana using a viral vector. The enzyme, targeted to the apoplast, accumulates in large amounts in plant tissues in as little as 7 days after inoculation, without detrimental effects on plant growth. The properties of the protein produced by the plant, in terms of resistance to temperature, pH, and enzymatic activity, are equivalent to those observed when Escherichia coli is used as a host. Purification of the plant-produced recombinant xylanase is facilitated by exporting the protein to the apoplastic space. The production of this xylanase by N. benthamiana, which avoids the hindrances derived from the use of E. coli, namely, intracellular production requiring subsequent purification, represents an important step for potential applications in the food industry in which more sustainable and green products are continuously demanded. As an example, the use of the enzyme producing prebiotic xylooligosdaccharides from xylan is here reported.

Virus-Induced Gene Editing and Its Applications in Plants

CRISPR/Cas-based genome editing technologies, which allow the precise manipulation of plant genomes, have revolutionized plant science and enabled the creation of germplasms with beneficial traits. In order to apply these technologies, CRISPR/Cas reagents must be delivered into plant cells; however, this is limited by tissue culture challenges. Recently, viral vectors have been used to deliver CRISPR/Cas reagents into plant cells. Virus-induced genome editing (VIGE) has emerged as a powerful method with several advantages, including high editing efficiency and a simplified process for generating gene-edited DNA-free plants. Here, we briefly describe CRISPR/Cas-based genome editing. We then focus on VIGE systems and the types of viruses used currently for CRISPR/Cas9 cassette delivery and genome editing. We also highlight recent applications of and advances in VIGE in plants. Finally, we discuss the challenges and potential for VIGE in plants.

Application of developmental regulators to improve in planta or in vitro transformation in plants

Plant genetic transformation is a crucial step for applying biotechnology such as genome editing to basic and applied plant science research. Its success primarily relies on the efficiency of gene delivery into plant cells and the ability to regenerate transgenic plants. In this study, we have examined the effect of several developmental regulators (DRs), including PLETHORA (PLT5), WOUND INDUCED DEDIFFERENTIATION 1 (WIND1), ENHANCED SHOOT REGENERATION (ESR1), WUSHEL (WUS) and a fusion of WUS and BABY-BOOM (WUS-P2A-BBM), on in planta transformation through injection of Agrobacterium tumefaciens in snapdragons (Antirrhinum majus). The results showed that PLT5, WIND1 and WUS promoted in planta transformation of snapdragons. An additional test of these three DRs on tomato (Solanum lycopersicum) further demonstrated that the highest in planta transformation efficiency was observed from PLT5. PLT5 promoted calli formation and regeneration of transformed shoots at the wound positions of aerial stems, and the transgene was stably inherited to the next generation in snapdragons. Additionally, PLT5 significantly improved the shoot regeneration and transformation in two Brassica cabbage varieties (Brassica rapa) and promoted the formation of transgenic calli and somatic embryos in sweet pepper (Capsicum annum) through in vitro tissue culture. Despite some morphological alternations, viable seeds were produced from the transgenic Bok choy and snapdragons. Our results have demonstrated that manipulation of PLT5 could be an effective approach for improving in planta and in vitro transformation efficiency, and such a transformation system could be used to facilitate the application of genome editing or other plant biotechnology application in modern agriculture.

VIGS Goes Viral: How VIGS Transforms Our Understanding of Plant Science

Virus-induced gene silencing (VIGS) has developed into an indispensable approach to gene function analysis in a wide array of species, many of which are not amenable to stable genetic transformation. VIGS utilizes the posttranscriptional gene silencing (PTGS) machinery of plants to restrain viral infections systemically and is used to downregulate the plant's endogenous genes. Here, we review the molecular mechanisms of DNA- and RNA-virus-based VIGS, its inherent connection to PTGS, and what is known about the systemic spread of silencing. Recently, VIGS-based technologies have been expanded to enable not only gene silencing but also overexpression [virus-induced overexpression (VOX)], genome editing [virus-induced genome editing (VIGE)], and host-induced gene silencing (HIGS). These techniques expand the genetic toolbox for nonmodel organisms even more. Further, we illustrate the versatility of VIGS and the methods derived from it in elucidating molecular mechanisms, using tomato fruit ripening and programmed cell death as examples. Finally, we discuss challenges of and future perspectives on the use of VIGS to advance gene function analysis in nonmodel plants in the postgenomic era.

Transient expression systems to rewire plant carotenoid metabolism

Enrichment of foodstuffs with health-promoting metabolites such as carotenoids is a powerful tool to fight against unhealthy eating habits. Dietary carotenoids are vitamin A precursors and reduce risk of several chronical diseases. Additionally, carotenoids and their cleavage products (apocarotenoids) are used as natural pigments and flavors by the agrofood industry. In the last few years, major advances have been made in our understanding of how plants make and store carotenoids in their natural compartments, the plastids. In part, this knowledge has been acquired by using transient expression systems, notably agroinfiltration and viral vectors. These techniques allow profound changes in the carotenoid profile of plant tissues at the desired developmental stage, hence preventing interference with normal plant growth and development. Here we review how transient expression approaches have contributed to learn about the structure and regulation of plant carotenoid biosynthesis and to rewire carotenoid metabolism and storage for efficient biofortification of plant tissues.

Production of Potyvirus-Derived Nanoparticles Decorated with a Nanobody in Biofactory Plants

Viral nanoparticles (VNPs) have recently attracted attention for their use as building blocks for novel materials to support a range of functions of potential interest in nanotechnology and medicine. Viral capsids are ideal for presenting small epitopes by inserting them at an appropriate site on the selected coat protein (CP). VNPs presenting antibodies on their surfaces are considered highly promising tools for therapeutic and diagnostic purposes. Due to their size, nanobodies are an interesting alternative to classic antibodies for surface presentation. Nanobodies are the variable domains of heavy-chain (VHH) antibodies from animals belonging to the family Camelidae, which have several properties that make them attractive therapeutic molecules, such as their small size, simple structure, and high affinity and specificity. In this work, we have produced genetically encoded VNPs derived from two different potyviruses—the largest group of RNA viruses that infect plants—decorated with nanobodies. We have created a VNP derived from zucchini yellow mosaic virus (ZYMV) decorated with a nanobody against the green fluorescent protein (GFP) in zucchini (Cucurbita pepo) plants. As reported for other viruses, the expression of ZYMV-derived VNPs decorated with this nanobody was only made possible by including a picornavirus 2A splicing peptide between the fused proteins, which resulted in a mixed population of unmodified and decorated CPs. We have also produced tobacco etch virus (TEV)-derived VNPs in Nicotiana benthamiana plants decorated with the same nanobody against GFP. Strikingly, in this case, VNPs could be assembled by direct fusion of the nanobody to the viral CP with no 2A splicing involved, likely resulting in fully decorated VNPs. For both expression systems, correct assembly and purification of the recombinant VNPs was confirmed by transmission electron microscope; the functionality of the CP-fused nanobody was assessed by western blot and binding assays. In sum, here we report the production of genetically encoded plant-derived VNPs decorated with a nanobody. This system may be an attractive alternative for the sustainable production in plants of nanobody-containing nanomaterials for diagnostic and therapeutic purposes.

VIGE: virus-induced genome editing for improving abiotic and biotic stress traits in plants

Agricultural production is hampered by disease, pests, and environmental stresses. To minimize yield loss, it is important to develop crop cultivars with resistance or tolerance to their respective biotic and abiotic constraints. Transformation techniques are not optimized for many species and desirable cultivars may not be amenable to genetic transformation, necessitating inferior cultivar usage and time-consuming introgression through backcrossing to the preferred variety. Overcoming these limitations will greatly facilitate the development of disease, insect, and abiotic stress tolerant crops. One such avenue for rapid crop improvement is the development of viral systems to deliver CRISPR/Cas-based genome editing technology to plants to generate targeted beneficial mutations. Viral delivery of genomic editing constructs can theoretically be applied to span the entire host range of the virus utilized, circumventing the challenges associated with traditional transformation and breeding techniques. Here we explore the types of viruses that have been optimized for CRISPR/Cas9 delivery, the phenotypic outcomes achieved in recent studies, and discuss the future potential of this rapidly advancing technology.

Genetic Transformation of Apomictic Grasses: Progress and Constraints

The available methods for plant transformation and expansion beyond its limits remain especially critical for crop improvement. For grass species, this is even more critical, mainly due to drawbacks in in vitro regeneration. Despite the existence of many protocols in grasses to achieve genetic transformation through Agrobacterium or biolistic gene delivery, their efficiencies are genotype-dependent and still very low due to the recalcitrance of these species to in vitro regeneration. Many plant transformation facilities for cereals and other important crops may be found around the world in universities and enterprises, but this is not the case for apomictic species, many of which are C4 grasses. Moreover, apomixis (asexual reproduction by seeds) represents an additional constraint for breeding. However, the transformation of an apomictic clone is an attractive strategy, as the transgene is immediately fixed in a highly adapted genetic background, capable of large-scale clonal propagation. With the exception of some species like Brachiaria brizantha which is planted in approximately 100 M ha in Brazil, apomixis is almost non-present in economically important crops. However, as it is sometimes present in their wild relatives, the main goal is to transfer this trait to crops to fix heterosis. Until now this has been a difficult task, mainly because many aspects of apomixis are unknown. Over the last few years, many candidate genes have been identified and attempts have been made to characterize them functionally in Arabidopsis and rice. However, functional analysis in true apomictic species lags far behind, mainly due to the complexity of its genomes, of the trait itself, and the lack of efficient genetic transformation protocols. In this study, we review the current status of the in vitro culture and genetic transformation methods focusing on apomictic grasses, and the prospects for the application of new tools assayed in other related species, with two aims: to pave the way for discovering the molecular pathways involved in apomixis and to develop new capacities for breeding purposes because many of these grasses are important forage or biofuel resources.

Highly efficient heritable genome editing in wheat using an RNA virus and bypassing tissue culture

Genome editing provides novel strategies for improving plant traits but mostly relies on conventional plant genetic transformation and regeneration procedures, which can be inefficient. In this study, we have engineered a Barley stripe mosaic virus-based sgRNA delivery vector (BSMV-sg) that is effective in performing heritable genome editing in Cas9-transgenic wheat plants. Mutated progenies were present in the next generation at frequencies ranging from 12.9% to 100% in three different wheat varieties, and 53.8%-100% of mutants were virus free. We also achieved multiplex mutagenesis in progeny using a pool of BSMV-sg vectors harboring different sgRNAs. Furthermore, we devised a virus-induced transgene-free editing procedure to generate Cas9-free wheat mutants by crossing BSMV-infected Cas9-transgenic wheat pollen with wild-type wheat. Our study provides a robust, convenient, and tissue culture-free approach for genome editing in wheat through virus infection.

An Overview of the Application of Viruses to Biotechnology

Viruses may cause devastating diseases in several organisms; however, they are simple systems that can be manipulated to be beneficial and useful for many purposes in different areas. In medicine, viruses have been used for a long time in vaccines and are now being used as vectors to carry materials for the treatment of diseases, such as cancer, being able to target specific cells. In agriculture, viruses are being studied to introduce desirable characteristics in plants or render resistance to biotic and abiotic stresses. Viruses have been exploited in nanotechnology for the deposition of specific metals and have been shown to be of great benefit to nanomaterial production. They can also be used for different applications in pharmacology, cosmetics, electronics, and other industries. Thus, viruses are no longer only seen as enemies. They have shown enormous potential, covering several important areas in our lives, and they are making our lives easier and better. Although viruses have already proven their potential, there is still a long road ahead. This prompt us to propose this theme in the Special Issue "The application of viruses to biotechnology". We believe that the articles gathered here highlight recent significant advances in the use of viruses in several fields, contributing to the current knowledge on virus applications.

Modulation of Antigen Display on PapMV Nanoparticles Influences Its Immunogenicity

Background: The papaya mosaic virus (PapMV) vaccine platform is a rod-shaped nanoparticle made of the recombinant PapMV coat protein (CP) self-assembled around a noncoding single-stranded RNA (ssRNA) template. The PapMV nanoparticle induces innate immunity through stimulation of the Toll-like receptors (TLR) 7 and 8. The display of the vaccine antigen at the surface of the nanoparticle, associated with the co-stimulation signal via TLR7/8, ensures a strong stimulation of the immune response, which is ideal for the development of candidate vaccines. In this study, we assess the impact of where the peptide antigen is fused, whether at the surface or at the extremities of the nanoparticles, on the immune response directed to that antigen. Methods: Two different peptides from influenza A virus were used as model antigens. The conserved M2e peptide, derived from the matrix protein 2 was chosen as the B-cell epitope, and a peptide derived from the nucleocapsid was chosen as the cytotoxic T lymphocytes (CTL) epitope. These peptides were coupled at two different positions on the PapMV CP, the N- (PapMV-N) or the C-terminus (PapMV-C), using the transpeptidase activity of Sortase A (SrtA). The immune responses, both humoral and CD8+ T-cell-mediated, directed to the peptide antigens in the two different fusion contexts were analyzed and compared. The impact of coupling density at the surface of the nanoparticle was also investigated. Conclusions: The results demonstrate that coupling of the peptide antigens at the N-terminus (PapMV-N) of the PapMV CP led to an enhanced immune response to the coupled peptide antigens as compared to coupling to the C-terminus. The difference between the two vaccine platforms is linked to the enhanced capacity of the PapMV-N vaccine platform to stimulate TLR7/8. We also demonstrated that the strength of the immune response increases with the density of coupling at the surface of the nanoparticles.