ABSTRACT
The discovery of phenomenon of RNA interference ( RNAi) has brought a paradigm shift in our understanding of the biological world. But it is quite interesting to know that, even before RNAi was discovered in the sense we know it today, its principles were utilized by plant virologists for protecting the plants from deadly viruses. To develop virus resistant transgenic plants, virologists relied on Pathogen-derived Resistance (PDR)—a vaccination like phenomenon where plants containing genes or sequences of a pathogen are protected from cognate or related
pathogens. This concept was pioneered in mid-eighties by John C. Sanford of Cornell University (Sanford and Johnston 1985, US Patent 5840481). The PDR approach was itself inspired from an age old agricultural practice of ‘cross-protection’, in which plants challenged with a mild strain of a virus were protected from super-infection by other related strains. Exact mechanism of PDR/cross-protection mediated resistance was obscure for quite some time (Baulcombe 1996). Initially, the dogma was that this type of resistance was protein-mediated. However, later it was shown that untranslatable transcripts of viral genes alone were suffi cient to interfere with virus replication (Lindbo and Dougherty 1992), thus providing an early clue that resistance might be RNA mediated. The phenomena of co-suppression/Post transcriptional gene silencing ( PTGS)/ RNAi were being unraveled at the same time in various model organisms and plant virologists were rather quick to point out that the PDR mediated virus resistance in plants was related to homology-dependent gene silencing (Mueller et al. 1995). So it can be said that albeit unknowingly, RNAi as a technology was used for the fi rst time even before its discovery and coining of the term ‘ RNAi’. This phase can be considered as the fi rst generation of RNAi technology.
