The notion that the introduction of alien RNA into an organism can cause the silencing of endogenous genes and transgenes came to light in plants during the last decades of the 20th century. It was based on revealing virus-induced gene silencing (VIGS) and on the protection against pathogenic viruses by pre-infection with less pathogenic plant viruses or components of such viruses as well as on co-suppression phenomena. The breakthrough in RNA silencing research was the discovery of Mello, Fire and associates that double-stranded RNAs (dsRNAs) can silence specifically homologous genes in the nematode Caenorhabditis elegans. The discovery in C. elegans, published in 1998, immediately initiated studies in protozoa, metazoa, fungi, and plants, and similar RNA silencing mechanisms, albeit with some notable differences, were subsequently revealed in almost all eukaryotic organisms in which they were looked for. Investigators dealing with the different organisms were well aware of each others' results and a very active field of study emerged within a few years. Investigators of plant RNA silencing benefited from the findings in other organisms, especially in C. elegans, Drosophila, and mammals, where the protein complexes involved in RNA silencing, such as the Dicer complex and the RNA-induced silencing complex (RISC), were studied intensively. The study of RNA silencing in plants followed two avenues. In one avenue the process of initiation of endogenous dsRNA was followed, also the fate and the impact of dsRNA that was introduced into plant cells was investigated. It was found how this dsRNA is cut into ∼21 nt fragments and the derived ssRNA of ∼21 nt may guide the RISC to cleave specific mRNA sequences. In the other avenue the formation of ‘hairpin,’ or ‘stem loop’ RNA sequences, from transcripts of genomic sequences, was investigated. The ‘maturation’ of these RNA structures into mature microRNA was studied and the possible roles of endogenously formed and introduced microRNAs in the regulation of expression of plant genes were gradually revealed. This review will update the findings in these two avenues.
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Vol. 41 • No. 2