ABSTRACT
MicroRNAs (miRNAs) are endogenous, noncoding single-stranded RNA molecules of ~22 nucleotides (nt) in length, which regulate gene expression in both plants and animals by targeting mRNAs through cleavage or translational repression. The first miRNA to be discovered was lin4, known to control the timing of larval development in Caenorhabditis elegans (Lee et al. 1993). As part of the regulatory pathway, lin4 RNAs are paired to the 3ʹ untranslated region (UTR) of target mRNA, leading to its translational repression, which, in turn, activates the transition from cell divisions of the first stage to the second stage of larval development (Lee et al. 1993; Wightman et al. 1993). In Arabidopsis, 16 differentially expressed plant miRNAs were first identified during development, and the plant miRNAs are generally 20-24 nt long, transcribed from independent genes, and evolutionarily conserved (Reinhart et al. 2002).
