Introduction: microRNAs meet plant development
The conspicuous group of 20–21 nt small RNAs designated as microRNAs (miRNAs) was first discovered in Caenorhabditis elegans. In 1993, lin-4, the first identified miRNA, was shown to recognize several partially complementing sites in the 3′ UTR of the lin-14 mRNA and to inhibit its translation (Lee et al., 1993). At that time it was thought, however, that miRNAs were an oddity restricted to worms. Their participation in biological pathways in other animals was vastly underestimated and any role in plant development was therefore even more unexpected.
The situation changed with the arrival of the new millennium when it was recognized that a second worm miRNA, let-7 (Reinhart et al., 2000), was conserved in many animals with bilateral symmetry (Pasquinelli et al., 2000). Several dozens of new miRNAs were then readily identified through cloning efforts in Drosophila, Caenorhabditis and humans (Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001). By 2002 there were already four small RNA libraries made from Arabidopsis thaliana allowing a first systematic scan for miRNAs in the fully sequenced model plant (Llave et al., 2002a; Mette et al., 2002; Park et al., 2002; Reinhart et al., 2002).
The knowledge on miRNA biogenesis and action burst when a connection with an apparently unrelated process called cosuppression in plants or RNA interference in animals was discovered (Zamore and Haley, 2005).