Transgenic potato plants expressing mammalian 2'-5' oligoadenylate synthetase are protected from potato virus X infection under field conditions

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.

Pivoting the plant immune system from dissection to deployment

Diverse and rapidly evolving pathogens cause plant diseases and epidemics that threaten crop yield and food security around the world. Research over the last 25 years has led to an increasingly clear conceptual understanding of the molecular components of the plant immune system. Combined with ever-cheaper DNA-sequencing technology and the rich diversity of germ plasm manipulated for over a century by plant breeders, we now have the means to begin development of durable (long-lasting) disease resistance beyond the limits imposed by conventional breeding and in a manner that will replace costly and unsustainable chemical controls.

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.

Resistance mechanisms to plant viruses: an overview

To obtain virus-resistant host plants, a range of operational strategies can be followed nowadays. While for decades plant breeders have been able to introduce natural resistance genes in susceptible genotypes without knowing precisely what these resistance traits were, currently a growing number of (mostly) dominant resistance genes have been cloned and analyzed. This has led not only to a better understanding of the plant's natural defence systems, but also opened the way to use these genes beyond species borders. Besides using natural resistance traits, also several novel, "engineered" forms of virus resistance have been developed over the past 15 years. The first successes were obtained embarking from the principle of pathogen-derived resistance (PDR) by transforming host plants with viral genes or sequences with the purpose to block a specific step during virus multiplication in the plant. As an unforeseen spin-off of these investments, the phenomenon of post-translational gene silencing (PTGS) was discovered, which to date is by far the most successful way to engineer resistance. It is generally believed that PTGS reflects a natural defence system of the plant, and part of the hypothesized components required for PTGS have been identified. As counteracting strategy, and confirming PTGS to be a natural phenomenon, a considerable number of viruses have acquired gene functions by which they can suppress PTGS. In addition to PDR and PTGS, further strategies for engineered virus resistance have been explored, including the use of pokeweed antiviral protein (PAP), 2',5'-oligoadenylate synthetase and "plantibodies". This paper will give a brief overview of the major strategies that have become operational during the past 10 years.

Constitutive expression of interferon-induced human MxA protein in transgenic tobacco plants does not confer resistance to a variety of RNA viruses

MxA is a key component in the interferon-induced antiviral defense in humans. After viral infections, MxA is rapidly induced and accumulates in the cytoplasm. The multiplication of many RNA viruses, including all bunyaviruses tested so far, is inhibited by MxA. These findings prompted us to express MxA in plants in an attempt to create resistance to tospoviruses. Here, we report the generation of transgenic tobacco plants that constitutively express MxA under the control of the 35 S cauliflower mosaic virus promotor. Northern and western blot analysis confirmed the expression of MxA in several transgenic plant lines. MxA expression had no obvious detrimental effects on plant growth and fertility. However, challenge experiments with tomato spotted wilt virus, tomato chlorotic spot virus, and groundnut ringspot virus revealed no increased resistance of MxA-transgenic tobacco plants to tospovirus infections. Neither was the multiplication of tobacco mosaic virus, cucumber mosaic virus and potato virus Y inhibited in MxA-transgenic plants. The results indicate that the expression of human MxA alone does not enhance virus resistance in planta.

Expansion and molecular evolution of the interferon-induced 2'-5' oligoadenylate synthetase gene family

The mammalian 2'-5' oligoadenylate synthetases (2'-5'OASs) are enzymes that are crucial in the interferon-induced antiviral response. They catalyze the polymerization of ATP into 2'-5'-linked oligoadenylates which activate a constitutively expressed latent endonuclease, RNaseL, to block viral replication at the level of mRNA degradation. A molecular evolutionary analysis of available OAS sequences suggests that the vertebrate genes are members of a multigene family with its roots in the early history of tetrapods. The modern mammalian 2'-5'OAS genes underwent successive gene duplication events resulting in three size classes of enzymes, containing one, two, or three homologous domains. Expansion of the OAS gene family occurred by whole-gene duplications to increase gene content and by domain couplings to produce the multidomain genes. Evolutionary analyses show that the 2'-5'OAS genes in rodents underwent gene duplications as recently as 11 MYA and predict the existence of additional undiscovered OAS genes in mammals.

Ischemia-induced changes in 2'-5'-oligoadenylate synthethase mRNA levels in rat brain: comparison with changes produced by perturbations of endoplasmic reticulum calcium homeostasis in neuronal cell cultures

2'-5' Oligoadenylate synthetase (OAS) expression is induced by interferon or viral infection of cells. To better understand ischemia-induced changes in gene expression and to elucidate the possible underlying mechanisms, changes in OAS mRNA levels were evaluated after 30 min four-vessel occlusion and 2, 4, 8 or 24 h recovery and compared to the temporal profile of changes in mRNA levels induced by a transient depletion of endoplasmic reticulum (ER) calcium stores in primary neuronal cell cultures. OAS mRNA levels dropped during early recovery both in vivo and in vitro. After 6 h recovery from ER calcium pool depletion, OAS mRNA levels increased to about 350% of controls and returned to control levels after 24 h of recovery. After 24 h recovery from ischemia, OAS mRNA levels rose to about 390% of controls in the hippocampus and striatum and to 210% of the control value in the cortex. It is concluded that transient ischemia place cells into an antiviral state, most pronounced in the hippocampus and striatum, and that disturbances of ER calcium homeostasis may contribute to this process.

The 2-5A system: modulation of viral and cellular processes through acceleration of RNA degradation

The 2-5A system is an RNA degradation pathway that can be induced by the interferons (IFNs). Treatment of cells with IFN activates genes encoding several double-stranded RNA (dsRNA)-dependent synthetases. These enzymes generate 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A) from ATP. The effects of 2-5A in cells are transient since 2-5A is unstable in cells due to the activities of phosphodiesterase and phosphatase. 2-5A activates the endoribonuclease 2-5A-dependent RNase L, causing degradation of single-stranded RNA with moderate specificity. The human 2-5A-dependent RNase is an 83.5 kDa polypeptide that has little, if any, RNase activity, unless 2-5A is present. 2-5A binding to RNase L switches the enzyme from its off-state to its on-state. At least three 2',5'-linked oligoadenylates and a single 5'-phosphoryl group are required for maximal activation of the RNase. Even though the constitutive presence of 2-5A-dependent RNase is observed in nearly all mammalian cell types, cellular amounts of 2-5A-dependent mRNA and activity can increase after IFN treatment. One well-established role of the 2-5A system is as a host defense against some types of viruses. Since virus infection of cells results in the production and secretion of IFNs, and since dsRNA is both a frequent product of virus infection and an activator of 2-5A synthesis, the replication of encephalomyocarditis virus, which produces dsRNA during its life cycle, is greatly suppressed in IFN-treated cells as a direct result of RNA decay by the activated 2-5A-dependent RNase. This review covers the organic chemistry, enzymology, and molecular biology of 2-5A and its associated enzymes. Additional possible biological roles of the 2-5A system, such as in cell growth and differentiation, human immunodeficiency virus replication, heat shock, atherosclerotic plaque, pathogenesis of Type I diabetes, and apoptosis, are presented.

Cellular responses to interferon-gamma

Interferons are cytokines that play a complex and central role in the resistance of mammalian hosts to pathogens. Type I interferon (IFN-alpha and IFN-beta) is secreted by virus-infected cells. Immune, type II, or gamma-interferon (IFN-gamma) is secreted by thymus-derived (T) cells under certain conditions of activation and by natural killer (NK) cells. Although originally defined as an agent with direct antiviral activity, the properties of IFN-gamma include regulation of several aspects of the immune response, stimulation of bactericidal activity of phagocytes, stimulation of antigen presentation through class I and class II major histocompatibility complex (MHC) molecules, orchestration of leukocyte-endothelium interactions, effects on cell proliferation and apoptosis, as well as the stimulation and repression of a variety of genes whose functional significance remains obscure. The implementation of such a variety of effects by a single cytokine is achieved by complex patterns of cell-specific gene regulation: Several IFN-gamma-regulated genes are themselves components of transcription factors. The IFN-gamma response is itself regulated by interaction with responses to other cytokines including IFN-alpha/beta, TNF-alpha, and IL-4. Over 200 genes are now known to be regulated by IFN-gamma and they are listed in a World Wide Web document that accompanies this review. However, much of the cellular response to IFN-gamma can be described in terms of a set of integrated molecular programs underlying well-defined physiological systems, for example the induction of efficient antigen processing for MHC-mediated antigen presentation, which play clearly defined roles in pathogen resistance. A promising approach to the complexity of the IFN-gamma response is to extend the analysis of the less understood IFN-gamma-regulated genes in terms of molecular programs functional in pathogen resistance.

A mammalian 2-5A system functions as an antiviral pathway in transgenic plants

Resistance to virus infections in higher vertebrates is mediated in part through catalysis of RNA decay by the, interferon-regulated 2-5A system. A functional 2-5A system requires two enzymes, a 2-5A synthetase that produces 5'-phosphorylated, 2',5'-linked oligoadenylates (2-5A) in response to double-stranded RNA, and the 2-5A-dependent RNase L. We have coexpressed these human enzymes in transgenic tobacco plants by using a single plasmid containing the cDNAs for both human RNase L and a low molecular weight form of human 2-5A synthetase under control of different, constitutive promoters. Expression of the human cDNAs in the transgenic plants was demonstrated from Northern blots, by specific enzyme assays, and by immunodetection (for RNase L). Infection of leaves, detached or in planta, of the coexpressing transgenic plants by tobacco mosaic virus, alfalfa [correction of alfafa] mosaic virus, or tobacco etch virus resulted in necrotic lesions. In contrast, leaves expressing 2-5A synthetase or RNase L alone and leaves containing the plasmid vector alone produced typical systemic infections. While alfalfa mosaic virus produced lesions only in the inoculated leaves regardless of the concentration of virus in the inoculum, high, but not low, levels of tobacco etch virus inoculum resulted in escape of virus to uninoculated leaves. Nevertheless, there was a substantial reduction of tobacco etch virus yield as measured by ELISA assay in the coexpressing transgenic plants. These results indicate that expression of a mammalian 2-5A system in plants provides resistance to virus infections.

Disruption of virus movement confers broad-spectrum resistance against systemic infection by plant viruses with a triple gene block

White clover mosaic virus strain O (WClMV-O), species of the Potexvirus genus, contains a set of three partially overlapping genes (the triple gene block) that encodes nonvirion proteins of 26 kDa, 13 kDa, and 7 kDa. These proteins are necessary for cell-to-cell movement in plants but not for replication. The WClMV-O 13-kDa gene was mutated (to 13*) in a region of the gene that is conserved in all viruses known to possess triple-gene-block proteins. All 10 13* transgenic lines of Nicotiana benthamiana designed to express the mutated movement protein were shown to be resistant to systemic infection by WClMV-O at 1 microgram of WClMV virions per ml, whereas all plants from susceptible control lines became systemically infected. Of the 13* transgenic lines, 3 selected for their abundant seed supply were shown to be resistant to systemic infection when challenged by inoculation with three different WClMV strains (O, M, and J) or with WClMV-O RNA at 10 micrograms/ml. Most plants were also resistant to systemic infection at inoculum concentrations up to 250 micrograms of WClMV virions per ml. In addition, the three 13* transgenic plant lines were found to be resistant to systemic infection with two other members of the Potexvirus group, potato virus X and narcissus mosaic virus, and the Carlavirus potato virus S but not to be resistant to tobacco mosaic virus of the Tobamovirus group. These results indicate that virus resistance can be engineered into transgenic plants by expression of dominant negative mutant forms of triple-gene-block movement proteins.

Principles and background for the construction of transgenic plants displaying multiple virus resistance

We investigated the possibility of reconstructing the 2'-5' oligoadenylate (2-5A) pathway into the plant kingdom to achieve multiple virus resistance. Differently phosphorylated 2-5A trimers and tetramers inhibited TMV RNA translation in cell-free systems. In wheat germ extracts the most potent inhibitors were nonphosphorylated forms of 2-5A. Triphosphorylated forms of 2-5A were deposphorylated and hydrolysed in plant extracts. Since we could not detect homologous DNA to mammalian 2-5A synthetase cDNA in tobacco or potato, we cloned rat 2-5A synthetase cDNA and transformed it by the Agrobacterium-mediated mechanism into tobacco and potato. Transformed tobacco plants were resistant to PVS infection and propagation of PVX was reduced. In transgenic potatoes tolerance to PVX and, in one transgenic clone, also to PVY was observed.