Expression of Amino-Terminal Portions or Full-Length Viral Replicase Genes in Transgenic Plants Confers Resistance to Potato Virus X Infection

Antiviral Protection via RdRP-Mediated Stable Activation of Innate Immunity

For many emerging and re-emerging infectious diseases, definitive solutions via sterilizing adaptive immunity may require years or decades to develop, if they are even possible. The innate immune system offers alternative mechanisms that do not require antigen-specific recognition or a priori knowledge of the causative agent. However, it is unclear whether effective stable innate immune system activation can be achieved without triggering harmful autoimmunity or other chronic inflammatory sequelae. Here, we show that transgenic expression of a picornavirus RNA-dependent RNA polymerase (RdRP), in the absence of other viral proteins, can profoundly reconfigure mammalian innate antiviral immunity by exposing the normally membrane-sequestered RdRP activity to sustained innate immune detection. RdRP-transgenic mice have life-long, quantitatively dramatic upregulation of 80 interferon-stimulated genes (ISGs) and show profound resistance to normally lethal viral challenge. Multiple crosses with defined knockout mice (Rag1, Mda5, Mavs, Ifnar1, Ifngr1, and Tlr3) established that the mechanism operates via MDA5 and MAVS and is fully independent of the adaptive immune system. Human cell models recapitulated the key features with striking fidelity, with the RdRP inducing an analogous ISG network and a strict block to HIV-1 infection. This RdRP-mediated antiviral mechanism does not depend on secondary structure within the RdRP mRNA but operates at the protein level and requires RdRP catalysis. Importantly, despite lifelong massive ISG elevations, RdRP mice are entirely healthy, with normal longevity. Our data reveal that a powerfully augmented MDA5-mediated activation state can be a well-tolerated mammalian innate immune system configuration. These results provide a foundation for augmenting innate immunity to achieve broad-spectrum antiviral protection.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Genetically engineered virus-resistant plants in developing countries: current status and future prospects

Plant viruses cause severe crop losses worldwide. Conventional control strategies, such as cultural methods and biocide applications against arthropod, nematode, and plasmodiophorid vectors, have limited success at mitigating the impact of plant viruses. Planting resistant cultivars is the most effective and economical way to control plant virus diseases. Natural sources of resistance have been exploited extensively to develop virus-resistant plants by conventional breeding. Non-conventional methods have also been used successfully to confer virus resistance by transferring primarily virus-derived genes, including viral coat protein, replicase, movement protein, defective interfering RNA, non-coding RNA sequences, and protease, into susceptible plants. Non-viral genes (R genes, microRNAs, ribosome-inactivating proteins, protease inhibitors, dsRNAse, RNA modifying enzymes, and scFvs) have also been used successfully to engineer resistance to viruses in plants. Very few genetically engineered (GE) virus resistant (VR) crops have been released for cultivation and none is available yet in developing countries. However, a number of economically important GEVR crops, transformed with viral genes are of great interest in developing countries. The major issues confronting the production and deregulation of GEVR crops in developing countries are primarily socio-economic and related to intellectual property rights, biosafety regulatory frameworks, expenditure to generate GE crops and opposition by non-governmental activists. Suggestions for satisfactory resolution of these factors, presumably leading to field tests and deregulation of GEVR crops in developing countries, are given.

Analysis of potato virus X replicase and TGBp3 subcellular locations

Potato virus X (PVX) infection leads to certain cytopathological modifications of the host endomembrane system. The subcellular location of the PVX replicase was previously unknown while the PVX TGBp3 protein was previously reported to reside in the ER. Using PVX infectious clones expressing the green fluorescent protein reporter, and antisera detecting the PVX replicase and host membrane markers, we examined the subcellular distribution of the PVX replicase in relation to the TGBp3. Confocal and electron microscopic observations revealed that the replicase localizes in membrane bound structures that derive from the ER. A subset of TGBp3 resides in the ER at the same location as the replicase. Sucrose gradient fractionation showed that the PVX replicase and TGBp3 proteins co-fractionate with ER marker proteins. This localization represents a region where both proteins may be synthesized and/or function. There is no evidence to indicate that either PVX protein moves into the Golgi apparatus. Cerulenin, a drug that inhibits de novo membrane synthesis, also inhibited PVX replication. These combined data indicate that PVX replication relies on ER-derived membrane recruitment and membrane proliferation.

Strategies for antiviral resistance in transgenic plants

Genetic engineering offers a means of incorporating new virus resistance traits into existing desirable plant cultivars. The initial attempts to create transgenes conferring virus resistance were based on the pathogen-derived resistance concept. The expression of the viral coat protein gene in transgenic plants was shown to induce protective effects similar to classical cross protection, and was therefore distinguished as 'coat-protein-mediated' protection. Since then, a large variety of viral sequences encoding structural and non-structural proteins were shown to confer resistance. Subsequently, non-coding viral RNA was shown to be a potential trigger for virus resistance in transgenic plants, which led to the discovery of a novel innate resistance in plants, RNA silencing. Apart from the majority of pathogen-derived resistance strategies, alternative strategies involving virus-specific antibodies have been successfully applied. In a separate section, efforts to combat viroids in transgenic plants are highlighted. In a final summarizing section, the potential risks involved in the introduction of transgenic crops and the specifics of the approaches used will be discussed.

Breeding virus resistant potatoes (Solanum tuberosum): a review of traditional and molecular approaches

Tetraploid cultivated potato (Solanum tuberosum) is the World's fourth most important crop and has been subjected to much breeding effort, including the incorporation of resistance to viruses. Several new approaches, ideas and technologies have emerged recently that could affect the future direction of virus resistance breeding. Thus, there are new opportunities to harness molecular techniques in the form of linked molecular markers to speed up and simplify selection of host resistance genes. The practical application of pathogen-derived transgenic resistance has arrived with the first release of GM potatoes engineered for virus resistance in the USA. Recently, a cloned host virus resistance gene from potato has been shown to be effective when inserted into a potato cultivar lacking the gene. These and other developments offer great opportunities for improving virus resistance, and it is timely to consider these advances and consider the future direction of resistance breeding in potato. We review the sources of available resistance, conventional breeding methods, marker-assisted selection, somaclonal variation, pathogen-derived and other transgenic resistance, and transformation with cloned host genes. The relative merits of the different methods are discussed, and the likely direction of future developments is considered.

Molecular strategies for interrupting arthropod-borne virus transmission by mosquitoes

Arthropod-borne virus (arbovirus) infections cause a number of emerging and resurgent human and veterinary infectious diseases. Traditional means of controlling arbovirus diseases include vaccination of susceptible vertebrates and mosquito control, but in many cases these have been unavailable or ineffective, and so novel strategies for disease control are needed. One possibility is genetic manipulation of mosquito vectors to render them unable to transmit arboviruses. This review describes recent work to test the concept of pathogen-derived resistance in arthropods by expression of viral genes in mosquito cell cultures and mosquitoes. Sense and antisense genome sequences from La Crosse virus (LAC) (a member of the Bunyaviridae) and dengue viruses serotypes 1 to 4 (DEN-1 to DEN-4) (members of the Flaviviridae) were expressed in mosquito cells from double-subgenomic and replicon vectors based on Sindbis virus (a member of the Togaviridae). The cells were then challenged with homologous or related viruses. For LAC, expression of antisense sequences from the small (S) genome segment, particularly full-length antisense S RNA, effectively interfered with replication of challenge virus, whereas expression of either antisense or sense RNA from the medium (M) segment was completely ineffective in LAC inhibition. Expression of sense and antisense RNA derived from certain regions of the DEN genome also blocked homologous virus replication more effectively than did RNA from other regions. Other parameters of RNA-mediated interference have been defined, such as the time when replication is blocked and the minimum size of effector RNA. The mechanism of RNA inhibition has not been determined, although it resembles double-stranded RNA interference in other nonvertebrate systems. Prospects for application of molecular strategies to control arbovirus diseases are briefly reviewed.

Molecular strategies for interrupting arthropod-borne virus transmission by mosquitoes

Arthropod-borne virus (arbovirus) infections cause a number of emerging and resurgent human and veterinary infectious diseases. Traditional means of controlling arbovirus diseases include vaccination of susceptible vertebrates and mosquito control, but in many cases these have been unavailable or ineffective, and so novel strategies for disease control are needed. One possibility is genetic manipulation of mosquito vectors to render them unable to transmit arboviruses. This review describes recent work to test the concept of pathogen-derived resistance in arthropods by expression of viral genes in mosquito cell cultures and mosquitoes. Sense and antisense genome sequences from La Crosse virus (LAC) (a member of the Bunyaviridae) and dengue viruses serotypes 1 to 4 (DEN-1 to DEN-4) (members of the Flaviviridae) were expressed in mosquito cells from double-subgenomic and replicon vectors based on Sindbis virus (a member of the Togaviridae). The cells were then challenged with homologous or related viruses. For LAC, expression of antisense sequences from the small (S) genome segment, particularly full-length antisense S RNA, effectively interfered with replication of challenge virus, whereas expression of either antisense or sense RNA from the medium (M) segment was completely ineffective in LAC inhibition. Expression of sense and antisense RNA derived from certain regions of the DEN genome also blocked homologous virus replication more effectively than did RNA from other regions. Other parameters of RNA-mediated interference have been defined, such as the time when replication is blocked and the minimum size of effector RNA. The mechanism of RNA inhibition has not been determined, although it resembles double-stranded RNA interference in other nonvertebrate systems. Prospects for application of molecular strategies to control arbovirus diseases are briefly reviewed.

RNA-mediated virus resistance in transgenic plants

In recent years the concept of pathogen-derived resistance (PDR) has been successfully exploited for conferring resistance against viruses in many crop plants. Starting with coat protein-mediated resistance, the range has been broadened to the use of other viral genes as a source of PDR. However, in the course of the efforts, often no clear correlation could be made between expression levels of the transgenes and observed virus resistance levels. Several reports mentioned high resistance levels using genes incapable of producing protein, but in these cases, even plants accumulating high amounts of transgene RNA were not most resistant. To accommodate these unexplained observations, a resistance mechanism involving specific breakdown of viral RNAs has been proposed. Recent progress towards understanding the RNA-mediated resistance mechanism and similarities with the co-suppression phenomenon will be discussed.

Viral RNA as a potential target for two independent mechanisms of replicase-mediated resistance against cucumber mosaic virus

Transgenic tobacco showing replicase-mediated resistance against cucumber mosaic virus (CMV) can be infected by the strain K-CMV. By use of chimeric constructs between full-length cDNA clones of RNA2 of strains Fny-CMV and K-CMV, the existence of two independent mechanisms of replicase-mediated resistance against viral replication and movement of Fny-CMV was demonstrated in these plants. The data indicate that viral RNA may serve as the target for both mechanisms of resistance. A positive correlation was observed between the amount of K-CMV RNA2 sequence present in the chimeric constructs and the ability to overcome the inhibition of replication, whereas a sequence domain was delimited in K-CMV RNA2 responsible for the ability of this strain to break resistance against virus movement.

Analysis of transgenic tobacco plants expressing a truncated form of a potyvirus coat protein nucleotide sequence

Transgenic Nicotiana tabacum cv. Burley 49 plants were generated which expressed a tobacco etch virus (TEV) coat protein (CP) gene construct containing a stop codon positioned at codon 147. This gene construct was expected to produce a TEV CP lacking the carboxy-terminal 118 amino acids of the full-length 264 amino acid CP. TEV CP gene transcripts of the expected size could be detected in transgenic plants but the expected truncated CP could not be detected. Ten independent transgenic lines expressing this form of the TEV CP gene were examined in detail. Two transgenic plant lines were resistant to aphid- or mechanically transmitted TEV and one line was highly resistant. Protoplasts derived from the highly resistant plant line did not support virus replication. The data suggested that the expression of this mutated form of the TEV CP gene could interfere with TEV replication and displayed features associated with RNA-mediated virus resistance.

Replicase-mediated resistance: a novel type of virus resistance in transgenic plants?

Genetically engineered plants expressing either intact or mutant forms of the virus-encoded replicase subunit are resistant to infection by the virus from which the transgene was obtained. In many instances, the resistance is very effective and will be useful in the field. However, some unexpected features of the resistance in these transgenic plants indicate that simple models for the mechanism of replicase-mediated resistance do not apply.

Broad-spectrum virus resistance in transgenic plants expressing pokeweed antiviral protein

Exogenous application of pokeweed antiviral protein (PAP), a ribosome-inhibiting protein found in the cell walls of Phytolacca americana (pokeweed), protects heterologous plants from viral infection. A cDNA clone for PAP was isolated and introduced into tobacco and potato plants by transformation with Agrobacterium tumefaciens. Transgenic plants that expressed either PAP or a double mutant derivative of PAP showed resistance to infection by different viruses. Resistance was effective against both mechanical and aphid transmission. Analysis of the vacuum infiltrate of leaves expressing PAP showed that it is enriched in the intercellular fluid. Analysis of resistance in transgenic plants suggests that PAP confers viral resistance by inhibiting an early event in infection. Previous methods for creating virus-resistant plants have been specific for a particular virus or closely related viruses. To protect plants against more than one virus, multiple genes must be introduced and expressed in a single transgenic line. Expression of PAP in transgenic plants offers the possibility of developing resistance to a broad spectrum of plant viruses by expression of a single gene.

Different mechanisms protect transgenic tobacco against tomato spotted wilt and impatiens necrotic spot Tospoviruses

We generated transgenic tobacco plants expressing the sense or antisense untranslatable N coding sequence of the lettuce isolate of tomato spotted wilt virus (TSWV-BL) as well as transgenic plants containing the promoterless N gene of the virus. Both sense and antisense untranslatable N gene RNAs provided protection against homologous and closely related isolates but not against distantly related Tospoviruses. These RNA-mediated protections were most effective in plants that synthesized low levels of the respective RNA species and appears to be achieved through the inhibition of viral replication. Unlike the sense RNA-mediated protection, the level of the antisense RNA-mediated protection depended on the concentration of the inoculum and the size of the test plants. Comparisons with previous results in transgenic plants expressing the intact N gene suggest that resistance to homologous and closely related TSWV isolates in plants that express low levels of the translatable N gene is due to the presence of the N gene transcript and not the N protein. In contrast, resistance to distantly related Tospoviruses is due to accumulation of high levels of the N protein and not due to the presence of the N gene transcript.

Plants that express a potyvirus proteinase gene are resistant to virus infection

Transgenic tobacco plants that express the genome-linked protein/proteinase-coding region of the potyvirus tobacco vein mottling virus (TVMV) were produced and tested for their reaction to inoculation with TVMV and two other potyviruses. These plants did not develop disease symptoms after being inoculated with large doses of TVMV but were as susceptible to infection by the other potyviruses as were control plants. Lines of tobacco that express the coat protein- or the nonstructural cylindrical inclusion protein-coding regions were also produced. The coat protein transgenic plants were protected against all three potyviruses, and the cylindrical inclusion transgenic plants were susceptible to all three potyviruses. These results indicate that some, but not all, TVMV genes can be used to confer protection against potyviruses in plants. The results also suggest that combinations of viral genes in transgenic plants might improve protection against potyviruses.

Genetic engineering of grain and pasture legumes for improved nutritive value

This review describes work aimed at the improvement of the nutritive value of grain and forage legumes using gene transfer techniques. Two traits which are amenable to manipulation by genetic engineering have been identified. These are plant protein quality and lignin content. In order to increase the quality of protein provided by the legume grains peas and lupins, we are attempting to introduce into these species chimeric genes encoding a sunflower seed protein rich in the sulphur-containing amino acids methionine and cysteine. These genes are designed to be expressed only in developing seeds of transgenic host plants. Chimeric genes incorporating a similar protein-coding region, but different transcriptional controls, are being introduced into the forage legumes lucerne and subterranean clover. In this case the genes are highly expressed in the leaves of transformed plants, and modifications have been made to the sunflower seed protein-coding sequences in order to increase the stability of the resultant protein in leaf tissue. Another approach to increasing plant nutritive value is represented by attempts to reduce the content of indigestible lignin in lucerne.

Elimination of L-A double-stranded RNA virus of Saccharomyces cerevisiae by expression of gag and gag-pol from an L-A cDNA clone

We report that expression of a nearly full-length cDNA clone of the L-A double-stranded RNA virus causes virus loss in a wild-type strain of Saccharomyces cerevisiae. We show that in this system exclusion of the L-A virus is independent of the presence of the packaging site or of cis sites for replication and transcription and completely dependent on expression of functional recombinant gag and gag-pol fusion protein. Thus, this exclusion is not explained in terms of overexpression of packaging signals. Mutation of the chromosomal SKI2 gene, known to repress the copy number of double-stranded RNA cytoplasmic replicons of S. cerevisiae, nearly eliminates the exclusion. We suggest that exclusion is due to competition by proteins expressed from the plasmid for a possibly limiting cellular factor. Our hypotheses on exclusion of L-A proteins may also apply to resistance to plant viruses produced by expression of viral replicases in transgenic plants.

Strategies to protect crop plants against viruses: pathogen-derived resistance blossoms

Since 1986, the ability to confer resistance against an otherwise devastating virus by introducing a single pathogen-derived or virus-targeted sequence into the DNA of a potential host plant has had a marked influence on much of the research effort, focus, and short-term objectives of plant virologists throughout the world. The vast literature on coat protein-mediated protection, for example, attests to our fascination for unraveling fundamental molecular mechanism(s), our (vain) search for a unifying hypothesis, our pragmatic interest in commercially exploitable opportunities for crop protection, and our ingenuity in manipulating transgene constructions to broaden their utility and reduce real or perceived environmental risk issues. Other single dominant, pathogen-derived plant resistance genes have recently been discovered from a wide variety of viruses and are operative in an ever-increasing range of plant species. Additional candidates seem limited only by the effort invested in experimentation and by our ingenuity and imagination. This review attempts to consider, in a critical way, the current state of the art, some exceptions, and some proposed rules. The final impression, from all the case evidence considered, is that normal virus replication requires a subtle blend of host- and virus-coded proteins, present in critical relative concentrations and at specific times and places. Any unregulated superimposition of interfering protein or nucleic acid species can, therefore, result in an apparently virus-resistant plant phenotype.

Extreme resistance to potato virus X infection in plants expressing a modified component of the putative viral replicase

Three types of mutation were introduced into the sequence encoding the GDD motif of the putative replicase component of potato virus X (PVX). All three mutations rendered the viral genome completely noninfectious when inoculated into Nicotiana clevelandii or into protoplasts of Nicotiana tabacum (cv. Samsun NN). In order to test whether these negative mutations could inactivate the viral genome in trans, the mutant genes were expressed in transformed N.tabacum (cv. Samsun NN) under control of the 35S RNA promoter of cauliflower mosaic virus and the transformed lines were inoculated with PVX. In 10 lines tested in which the GDD motif was expressed as GAD or GED there was no effect on susceptibility to PVX. In two of four lines transformed to express the ADD form of the conserved motif, the F1 and F2 progeny plants were highly resistant to infection by PVX, although only to strains closely related to the source of the transgene. The resistance was associated with suppression of PVX accumulation in the inoculated and systemic leaves and in protoplasts of the transformed plants, although some low level viral RNA production was observed in the inoculated but not the systemic leaves when the inoculum was as high as 100 or 250 micrograms/ml PVX RNA. These results suggest for a plant virus, as reported previously for Q beta phage, that virus resistance may be engineered by expression of dominant negative mutant forms of viral genes in transformed cells.