Susceptibility to virus infection of transgenic tobacco plants expressing structural and nonstructural genes of tobacco rattle virus

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.

Transgenic resistance to Bamboo mosaic virus by expression of interfering satellite RNA

Plant genetic engineering has broadened the options for plant virus resistance and is mostly based on pathogen-derived resistance. Previously, we have shown that interfering satellite RNA (satRNA) of Bamboo mosaic virus (satBaMV) greatly reduces Bamboo mosaic virus (BaMV) accumulation and BaMV-induced symptoms in co-inoculated plants. Here, we generated a nonviral source of virus-resistant transgenic Nicotiana benthamiana and Arabidopsis thaliana by introducing interfering satBaMV. Asymptomatic transgenic N. benthamiana lines were highly resistant to BaMV virion and viral RNA infection, and the expression of the transgene BSL6 was higher in asymptomatic than mildly symptomatic lines. In addition, BaMV- and satBaMV-specific small RNAs were detectable only after BaMV challenge, and their levels were associated with genomic viral RNA or satRNA levels. By transcriptomic analysis, the salicylic acid (SA) signalling pathway was not induced in satBaMV transgenic A. thaliana in mock conditions, suggesting that two major antiviral mechanisms, RNA silencing and SA-mediated resistance, are not involved directly in transgenic satBaMV-mediated BaMV interference. In contrast, resistance is associated with the level of the interfering satBaMV transgene. We propose satBaMV-mediated BaMV interference in transgenic plants by competition for replicase with BaMV.

Resistance of transgenic tobacco plants incorporating the putative 57-kDa polymerase read-through gene of Tobacco rattle virus against rub-inoculated and nematode-transmitted virus

Nicotiana tabacum plants were transformed with the 57-kDa read-through domain of the replicase gene of Tobacco rattle virus (TRV) isolate SYM. From a total of six lines containing the viral transgene, four displayed various levels of resistance to TRV infection. Transgenic plants from line 81G were highly resistant to foliar rub-inoculation with the homologous isolate, or with isolates TRV-PpK20 and TRV-PLB, which are almost identical to TRV-SYM in RNA1 sequence. Moreover, 81G plants were moderately resistant to the serologically and genetically distinct, highly pathogenic isolate TRV-GR. Resistance characteristics of line 81G remained stable over six generations. No unambiguous correlation was established between number of transgene insertion loci and level of resistance. Transgene-specific mRNA was clearly detected in plants from susceptible lines but only at an early developmental stage in resistant plants, indicating the operation of a RNA silencing resistance mechanism. Following challenge using viruliferous vector nematodes carrying TRV-PpK20 or by rub inoculation of roots, 81G plants did not show any symptoms and virus was not detected in leaves. However, virus was detected in roots but without apparent effects on plant growth and often at low concentration. When challenged with nematodes carrying TRV-GR, symptoms in aerial parts of 81G plants were less severe and much delayed compared to non-transgenic plants, although younger plants showed less resistance than older ones. No difference was detected in transgene transcript accumulation between leaves and roots of 81G plants. This is the first work reporting a broad level of pathogen derived resistance against two geographically and genetically distinct TRV isolates transmitted directly by their nematode vectors and provides further insight into the expression of transgenic resistance against naturally transmitted soil-borne viruses.

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.

Nematode transmission of tobacco rattle virus serves as a bottleneck to clear the virus population from defective interfering RNAs

DI7 is a defective interfering RNA derived from RNA 2 of tobacco rattle tobravirus (TRV) isolate PpK20. Tobacco was transformed with DI7 cDNA fused to the CaMV 35S promoter. Upon infection of the transgenic plants with TRV isolate PpK20 or the serologically unrelated isolate PaY4, the transgenic DI7 RNA started to accumulate at high levels and strongly interfered with accumulation of wild-type (wt) RNA 2. When DI7 transgenic plants infected with isolate PpK20 were used as source plants in nematode-transmission experiments, the vector Paratrichodorus pachydermus efficiently transmitted virus to healthy bait plants. However, the nematodes transmitted only the wt virus present in the transgenic source plants, whereas virus particles containing the abundant, accumulated DI7 RNA were excluded from transmission. Evidence is presented that wt RNA 2 and DI7 RNA are encapsidated in cis by their encoded CPs, which are known to be functional and nonfunctional in transmission, respectively. This mechanism would result in defective interfering RNAs, which rapidly arise after mechanical transmission of the virus in the laboratory, being eliminated from tobraviruses under natural field conditions. Also this mechanism which acts with nematode transmitted virus isolates contrasts with that of vector-transmission of defective potyviruses and luteoviruses by wt helper viruses.

Viruses and gene silencing in plants

Genetic engineering of virus resistance in plants may be conferred by transgenes based on sequences from the viral genome. In many instances the underlying mechanism involves the transgenically expressed proteins. However there are other examples in which the mechanism is based on RNA. It appears that this mechanism is related to post transcriptional gene silencing in transgenic plants. This gene silencing is likely to involve antisense RNA produced by the action of a host-encoded RNA dependent RNA polymerase. The natural role of this mechanism is as a genetic immune system conferring protection against viruses. There may also be a genomic role of the process reflected in RNA directed methylation of transgenes. Further understanding of this mechanism has obvious implications for virus resistance in plants. In addition the gene silencing can be used as a component of a new technology with application in functional genomics.

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.

Gene transfer with subsequent removal of the selection gene from the host genome

A general method of gene transfer that does not leave behind a selectable marker in the host genome is described. A luciferase gene was introduced into the tobacco genome by using the hygromycin phosphotransferase gene (hpt) as a linked selectable marker. Flanked by recombination sites from the bacteriophage P1 Cre/lox recombination system, the hpt gene was subsequently excised from the plant genome by the Cre recombinase. The Cre-catalyzed excision event in the plant genome was precise and conservative--i.e., without loss or alteration of nucleotides in the recombinant site. After removal of the Cre-encoding locus by genetic segregation, plants were obtained that had incorporated only the desired transgene. Gene transfer without the incorporation of antibiotic-resistance markers in the host genome should ease public concerns over the field release of transgenic organisms expressing such traits. Moreover, it would obviate the need for different selectable markers in subsequent rounds of gene transfer into the same host.

Plants transformed with a tobacco mosaic virus nonstructural gene sequence are resistant to the virus

Nicotiana tabacum cv. Xanthi nn plants were transformed with nucleotides 3472-4916 of tobacco mosaic virus (TMV) strain U1. This sequence contains all but the three 3 terminal nucleotides of the TMV 54-kDa gene, which encodes a putative component of the replicase complex. These plants were resistant to infection when challenged with either TMV U1 virions or TMV U1 RNA at concentrations of up to 500 micrograms/ml or 300 micrograms/ml, respectively, the highest concentrations tested. Resistance was also exhibited when plants were inoculated at 100 micrograms/ml with the closely related TMV mutant YSI/1 but was not shown in plants challenged at the same concentrations with the more distantly related TMV strains U2 or L or cucumber mosaic virus. Although the copy number of the 54-kDa gene sequence varied in individual transformants from 1 to approximately 5, the level of resistance in plants was not dependent on the number of copies of the 54-kDa gene sequence integrated. The transformed plants accumulated a 54-kDa gene sequence-specific RNA transcript of the expected size, but no protein product was detected.

Genetic engineering of plants for virus resistance

Historically, control of plant virus disease has involved numerous strategies which have often been combined to provide effective durable resistance in the field. In recent years, the dramatic advances obtained in plant molecular virology have enhanced our understanding of viral genome organizations and gene functions. Moreover, genetic engineering of plants for virus resistance has recently provided promising additional strategies for control of virus disease. At present, the most promising of these has been the expression of coat-protein coding sequences in plants transformed with a coat protein gene. Other potential methods include the expression of anti-sense viral transcripts in transgenic plants, the application of artificial anti-sense mediated gene regulation to viral systems, and the expression of viral satellite RNAs, RNAs with endoribonuclease activity, antiviral antibody genes, or human interferon genes in plants.