Development of plant transformation technology provides novel approaches in crop improvement and promises greater efficiency and precision to plant breeding. The widest variety of transgenic strategies for introducing resistance has been attempted in viruses. Since viruses have small genomes with limited selection proteins and the vast majority of plant viruses are plus stranded RNA viruses with simple replication strategies, it has been possible to devise a number of successful methods of interfering with development of viral infections.
Breeding for disease resistance has been an area that has attracted the most attention. Resistant varieties represent one of the greatest triumphs of modern agriculture. Genetic engineering techniques are new tools in the hands of scientists with which one can bring about a permanent genetic change in plants.
The impact of transgenic virus resistance breeding in Hawaii resulted in the development of transgenic papaya variety UhsunUP, which is resistance to papaya ring spot virus and it was found to be the successful story in transgenic resistance breeding.
There are mainly two approaches for developing genetically engineered resistance depending on the source of the genes used viz., pathogen derived resistance (PDR) and Non-pathogen derived resistance. Pathogen derived resistance means the source of transgene is pathogen itself that confers resistance to plant against virus from which genome segment was derived. This method includes, coat protein mediated resistance (CPMR), replicase mediated resistance, antisense mediated resistance, satellite RNA mediated resistance and movement protein mediated resistance. In case of Non-pathogen derived resistance the source of transgenes are those utilising genes derived from either the host or any other non-pathogen sources that includes post transcriptional gene silencing (PTGS), ribozyme mediated resistance, host resistance gene and ribosome inactivating proteins (RIPs).
Transgenic crops however have not found ready acceptance among public at large and have raised concerns about the possible hazards to human being and the environment. Therefore, transgenic crops needs to be subjected for elaborative testing before releasing.
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