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
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Summary. Genetic engineering of virus resistance in plants may be conferred by trans genes 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.
Introduction Viruses are important in their own right as agents of disease in plants and animals. However they are also important as probes of biological systems. Many fundamental processes have been revealed using viruses and virus-based enabling technologies have contributed to discoveries in biology. In this paper I describe an emerging story in plant virology that fits this pattern in several respects.
Genetic engineering of virus resistance The first approach to genetic engineering of virus resistance in plants involved transgenic expression of the tobacco mosaic virus (TMV) coat protein [1]. This first approach was remarkably successful although, more than ten years later, the precise mechanism is not fully understood. One possibility is that the transgenic coat protein inhibits disassembly of TMV particles in the initially infected cell [36]. However, from the available information, it is also possible that the transgenic protein blocks virion receptor sites or is an elicitor of host defense [15J. Spurred on by the initial success with TMV there were numerous attempts to generate coat protein mediated protection against other viruses [4,9,20,48]. There have also attempts to genetically engineer virus resistance by transgenic
C. H. Calisher et al. (eds.), 100 Years of Virology © Springer-Verlag Wien 1999
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D. Baulcombe
expression of viral genes other than the coat protein gene [3, 7, 17, 32]. It was reasoned, for example, that if transgenic expression of the coat protein could confer resistance by affecting the virions then expression of the replication enzyme might affect virus replication. In the same way, expression of the movement protein might affect viral movement. Many of these exercises were successful in that they produced virus-resistant transgenic plants. However the detailed analysis in some of the lines revealed two significant features that were not easily explained by a protein-based mechanism [5]. First there could be resistance with transgenes specifying a non translatable RNA. The second anomaly was the finding that the level of resistance did not correl
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