Plant gene co-suppression; basis of the molecular machinery of interfering RNA

RNA interference (RNAi), also known as post-transcriptional gene silencing (PTGS) co-suppression, is considered one of the most significant discoveries in molecular biology during the last several years. First recognized in plants, the starting point for its historical overview begins in the late 1980s and early 1990s when researchers used genetic engineering to alter flower color. RNAi is considered a gene down-regulation mechanism demonstrated to exist in all eukaryotes, where small RNAs (of approximately 21-24 nucleotides in size) function to guide specific effector proteins (Argonaute protein family) to a target nucleotide sequence by complementary base pairing. Subsequently, the effector protein complex down-regulates the expression of a RNA or DNA target. Although the small RNAi-directed gene regulation system was independently discovered in plants, fungi, worms and mammalian cells, scientific attention has been focused mainly on the regulation of development, biotic and abiotic stress responses and genome stability through controlling plant gene expression. On the other hand, the small interfering (si) RNA-mediated RNA silencing also functions as a neutral antiviral defense mechanism. The purpose of this review is to provide an overview of the discovery and molecular characterization of RNAi in plants.