Introduction

Posttranscriptional gene silencing (PTGS) is a natural, universal mechanism that degrades both cellular and viral mRNA in a homology-dependant manner in diverse eukaryotes, and has now become a major area of research and development in both plants and animals. It was first discovered in plants (Napoli et al., 1990), while a mechanistically similar phenomenon is known to occur in a wide range of organisms, including Caenorhabditis elegans, Drosophila melanogaster and mammals termed RNA-interference (RNAi) (Fire et al., 1998, Hammond et al., 2000) and in Neurospora crassa termed quelling (Cogoni and Macino, 1997). Transgenes and viruses can induce PTGS in plants, and it is now recognized as a natural defense mechanism against virus infection (Hamilton and Baulcombe, 1999). Recent studies at the molecular level revealed that all of these phenomena are considered as manifestations of a general RNA-targeting pathway (Vance and Vaucheret, 2001). The mechanism by which a virus infection triggers PTGS in plants is not fully understood, but it is evident that dsRNA is a strong inducer of PTGS (Waterhouse et al., 2001). Such dsRNA molecules are produced during RNA virus replication using their own RNA-dependent RNA polymerase (RdRP), or alternatively host RdRPs convert any “aberrant” ssRNA in the cell, from viral origin or cell origin, into dsRNA (Dalmay et al., 2000; Ahlquist, 2002). The biology and biochemistry of RNAi was discussed in detail in Section I of this book.