College of Arts and Sciences
Plant Molecular Biology; Role of Small RNAs in RNA Silencing, Host Defense and Development
Small RNAs have emerged as important regulators of plant defense and development. Small interfering RNAs (siRNAs) are ~21-24 nucleotides in length and mediate RNA silencing. Double-stranded RNA generated during viral replication or derived from aberrant transgenes is digested into siRNAs that direct sequence specific degradation of the corresponding viral or transgene RNA. RNA silencing functions, therefore, to protect the host from viral infection and the expression of aberrant transgenes. SiRNAs or siRNA-like RNAs have also been implicated in transcriptional gene silencing and chromosome elimination.
MicroRNAs (miRNAs) constitute another class of small RNAs. These small RNAs are encoded in the genomes of plants and animals and are processed from highly structured precursors. Although hundreds of these miRNAs have been identified in the last year, the functions of only three have been elucidated. Interestingly, these miRNAs mediate the expression of important developmental regulators. Two miRNA regulate gene expression at the level of translation, and one miRNA appears to alter mRNA stability. Considering the large number of miRNA genes in diverse species, it is likely that some miRNAs regulate gene expression at other levels, such as transcription, mRNA localization, or mRNA processing.
The biogenesis of miRNA and siRNAs share some features but also differ in certain respects. Both are cleaved from highly structured or double-stranded RNA precursors by an RNase III known as Dicer, and members of the agonaunte gene family have been implicated in the formation and/or function of both siRNAs and miRNAs. However, the siRNA and miRNA biogenesis pathways are distinct in that the viral suppressor of silencing, helper component proteinase (Hc-Pro), eliminates siRNAs but enhances miRNA accumulation. Furthermore, there are different genetic requirements for the biogenesis of siRNAs and miRNAs.
Future experimentation will employ viral suppressors of silencing such as Hc-Pro and mutational analysis to dissect the mechanisms controlling the biogenesis and function of siRNA and miRNAs.
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