Silencing as a Tool for Transgenic Crop Improvement

Abstract

RNA silencing, also known as post-transcriptional gene silencing (PTGS) or RNA interference (RNAi), is a form of RNA degradation believed to be an important defense against foreign nucleic acids (Waterhouse et al. 2001). It was initially discovered in plants and was thought to function as part of a defense mechanism against viruses (Ratcliff et al. 1997). Subsequently, it was shown to be a common gene-silencing mechanism occurring in all eukaryotes, including plants and animals. The term RNAi was coined for the phenomenon when it was observed in the nematode Caenorhabditis elegans (Fire et al. 1998). However, this phenomenon of RNAi (PTGS) had actually been reported previously in transgenic petunia but was referred to as cosuppression, because transformation with a sense chalcone synthase transgene suppressed the expression of both the transgene and the endogenous gene (Napoli et al. 1990). It is now widely accepted that dsRNA is the effective trigger of PTGS/RNAi in plants and that this process operates by sequence-specific degradation (Kusaba 2004). Several milestones related to RNAi-based silencing are summarized in Table 6.1. In plants, cosuppression, PTGS, and virus-induced gene silencing (VIGS), all describe a homology-dependent gene-silencing phenomenon that involves what is more broadly known as RNAi. The science of RNAi broadly includes a few different and diverse RNA-silencing pathways that alter the expression levels of specific genes in plants, mediate the amplification and mobile signal mechanisms in RNAi pathways, and yield RNA-mediated DNA methylation (Baulcombe 2004; Lippman and Martienssen 2004).