Resistance against cucumber mosaic virus in plants expressing the viral replicon

FTX271: A potential gene resource for plant antiviral transgenic breeding

Flammutoxin (FTX), as well as its precursor TDP, is a protein from Flammulina velutipes with antiviral activity. Transgenic tobacco with the FTX271 (gene of FTX or TDP) can not only delay the onset time of symptoms but also alleviate the symptoms caused by tobacco mosaic virus (TMV), but the mechanism is still unclear. In this study, FTX271 was introduced into Nicotiana benthamiana, and the disease resistance mechanism activated by FTX271 was speculated by transcriptomic and proteomic techniques. The results showed that TDP was detected, and some genes, proteins and pathways were significant upregulated or enriched in transgenic tobacco, including the mitogen-activated protein kinase (MAPK) cascade signal transduction pathway, the expression of hypersensitive response (HR) marker genes H1N1 and HSR203J, pathogenesis-related (PR) genes, and the key genes COI1 and lipoxygenase gene LOX2 of the jasmonic acid (JA) signaling pathway, indicating FTX271 may activate the MAPK pathway and increase the content of reactive oxygen species (ROS) and JA, which promoted the HR and inducible systemic resistance (ISR). ISR caused increased expression of peroxidase (POD) and other proteins involved in pathogen defense. In addition, transgenic tobacco may use sHSP-assisted photoreparation to alleviate the symptoms of TMV. In conclusion, JA-mediated ISR and sHSP-assisted photoreparation are activated by FTX271 to protect tobacco from TMV infection and alleviate the symptoms caused by the virus. The study provided a theoretical basis for the TMV resistance mechanism of FTX271, which may represent a potential gene resource for plant antiviral transgenic breeding.

Rapid transient protein production by the coat protein-deficient cucumber mosaic virus vector: non-packaged CMV system, NoPaCS

KEY MESSAGE

We developed a non-packaged CMV system (NoPaCS) for CMV-agroinfection with a virus-inescapable transgenic plant platform, enabling rapid, high production of a large-sequence target protein. For rapidly producing high levels of a desirable protein, many plant virus vectors have been developed. However, there is always a concern that such recombinant viruses may escape into the environment. Especially for insect-transmissible viruses, certain measures must be taken. We here developed a new cucumber mosaic virus (CMV) RNA 3-based vector that is not transmitted by aphids because we deleted the coat protein (CP) gene responsible for aphid transmission and replaced it with a foreign gene. Transgenic Nicotiana benthamiana plants expressing CMV RNA 1 (CR1Tg) were found to be the most suitable platform for producing a recombinant protein using the CMV vector. By agroinfiltrating CR1Tg plants with the RNA 2 construct and the CMV vector harboring the green fluorescence protein (GFP) gene instead of the CP gene, we achieved a high yield of GFP (e.g., ~ 750 mg/kg FW) throughout the bacteria-infiltrated tissues at 2-3 days after infiltration. Furthermore, with this CMV-agroinfection system, a large gene such as the β-glucuronidase (GUS) gene can be expressed because the viral RNAs are not necessarily encapsidated for replication. The system is designated "non-packaged CMV system (NoPaCS)".

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.

Bacterially expressed double-stranded RNAs against hot-spot sequences of tobacco mosaic virus or potato virus Y genome have different ability to protect tobacco from viral infection

Posttranscriptional gene silencing, also known as RNA interference, involves degradation of homologous mRNA sequences in organisms. In plants, posttranscriptional gene silencing is part of a defense mechanism against virus infection, and double-stranded RNA is the pivotal factor that induces gene silencing. In this paper, we got seven hairpin RNAs (hpRNAs) constructs against different hot-spot sequences of Tobacco mosaic virus (TMV) or Potato virus Y (PVY) genome. After expression in Escherichia coli HT115, we extracted the seven hpRNAs for the test in tobacco against TMV or PVY infection. The data suggest that different hpRNAs against different hot-spot sequences of TMV or PVY genome had different ability to protect tobacco plants from viral infection. The resistance to TMV conferred by the hpRNA against the TMV movement protein was stronger than other TMV hpRNAs; the resistance to PVY conferred by the hpRNA against the PVY nuclear inclusion b was better than that induced by any other PVY hpRNAs. Northern blotting of siRNA showed that the resistance was indeed an RNA-mediated virus resistance.

Twenty years of transgenic plants resistant to Cucumber mosaic virus

Plant genetic engineering has promised researchers improved speed and flexibility with regard to the introduction of new traits into cultivated crops. A variety of approaches have been applied to produce virus-resistant transgenic plants, some of which have proven to be remarkably successful. Studies on transgenic resistance to Cucumber mosaic virus probably have been the most intense of any plant virus. Several effective strategies based on pathogen-derived resistance have been identified; namely, resistance mediated by the viral coat protein, the viral replicase, and post-transcriptional gene silencing. Techniques using non-pathogen-derived resistance strategies, some of which could offer broader resistance, generally have proven to be much less effective. Not only do the results obtained so far provide a useful guide to help focus on future strategies, but they also suggest that there are a number of possible mechanisms involved in conferring these resistances. Further detailed studies on the interplay between viral transgene-derived molecules and their host are needed in order to elucidate the mechanisms of resistance and pathogenicity.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

RNAs 1 and 2 of Alfalfa mosaic virus, expressed in transgenic plants, start to replicate only after infection of the plants with RNA 3

RNAs 1 and 2 of the tripartite genome of Alfalfa mosaic virus (AMV) encode the two viral replicase subunits. Full-length DNA copies of RNAs 1 and 2 were used to transform tobacco plants (R12 lines). None of the transgenic lines showed resistance to AMV infection. In healthy R12 plants, the transcripts of the viral cDNAs were copied by the transgenic viral replicase into minus-strand RNAs but subsequent steps in replication were blocked. When the R12 plants were inoculated with AMV RNA 3, this block was lifted and the transgenic RNAs 1 and 2 were amplified by the transgenic replicase together with RNA 3. The transgenic expression of RNAs 1 and 2 largely circumvented the role of coat protein (CP) in the inoculum that is required for infection of nontransgenic plants. The results for the first time demonstrate the role of CP in AMV plus-strand RNA synthesis at the whole plant level.

Transgenically expressed cucumber mosaic virus RNA 1 simultaneously complements replication of cucumber mosaic virus RNAs 2 and 3 and confers resistance to systemic infection

Tobacco plants transformed with a cDNA copy of RNA 1 of the Fny strain of cucumber mosaic virus (CMV) promoted the asymptomatic accumulation of inoculated viral RNAs 2 and 3, which could be detected in noninoculated leaves, suggesting that the transgene also permitted viral long-distance movement. Typical symptoms of infection appeared later and correlated with the appearance of viral RNA 1 regenerated from the transgenic mRNA. Although all R0-generation plants were susceptible to Fny-CMV, one line displaying variable susceptibility to the virus in R1-and R2-generations led to selected R3-generation lines with systemic resistance to Fny-CMV. In the inoculated leaves of resistant plants, a dramatic decrease in the accumulation of viral RNA 1 was observed, relative to susceptible plants. No viral RNAs were detected in noninoculated leaves of the resistant plants, but such leaves were susceptible to infection. Furthermore, these leaves could sustain replication of inoculated CMV RNAs 2 and 3, indicating that a complete transgene-silencing had not been induced. Although a transgene-mediated, CMV RNA 1-suppression occurred in the inoculated leaf of resistant plants, the absence of a complete systemically acquired silencing suggests the existence of additional interferences with viral infection that prevented systemic infection by viral RNAs 2 and 3.

Consistent gene silencing in transgenic plants expressing a replicating potato virus X RNA

Tobacco plants were transformed with constructs in which the transgene was a cDNA of replicating potato virus X (PVX) RNA. The constructs, referred to here as amplicons, were the intact genome of PVX and PVX constructs modified to carry the beta-glucuronidase (GUS) reporter gene either as an additional gene or as a replacement for the coat protein gene (PVX/GUS/CP and PVX/GUS respectively). Transformed plants carrying these constructs displayed several phenotypes that we attribute to post-transcriptional gene silencing. These phenotypes include the absence of viral symptoms, low accumulation of transgene-derived RNA, extreme strain-specific resistance against PVX, low and non-uniform GUS expression (in the PVX/GUS and PVX/GUS/CP plants) and suppression of transiently expressed RNA sharing homology with the transgene. Importantly, the amplicon-mediated gene silencing was exhibited in all lines tested. There was no evidence of gene silencing in seven lines expressing a PVX RNA that was unable to replicate. From these data we conclude that the replicating viral RNA is a potent trigger of gene silencing. Moreover, amplicon-mediated gene silencing provides an important new strategy for the consistent activation of gene silencing in transgenic plants.