A historical overview of RNAi in plants.

RNA interference, or RNAi, is arguably one of the most significant discoveries in biology in the last several decades. First recognized in plants (where it was called post-transcriptional gene silencing, PTGS) RNAi is a gene down-regulation mechanism since demonstrated to exist in all eukaryotes. In RNAi, small RNAs (of about 21-24 nucleotides) function to guide specific effector proteins (members of the Argonaute protein family) to a target nucleotide sequence by complementary base pairing. The effector protein complex then down-regulates the expression of the targeted RNA or DNA. Small RNA-directed gene regulation systems were independently discovered (and named) in plants, fungi, worms, flies, and mammalian cells. Collectively, PTGS, RNA silencing, and co-suppression (in plants); quelling (in fungi and algae); and RNAi (in Caenorhabditis elegans, Drosophila, and mammalian cells) are all examples of small RNA-based gene regulation systems. From the very beginning, plant research has had a major impact on our understanding of RNAi. The purpose of this chapter is to provide an historical perspective and overview on the discovery, characterization, and applications of RNAi in plants.