Content area
Full text
Several classes of small RNAs are known in plants and accumulating evidence indicates that different classes of small RNAs may function either cell autonomously or noncell autonomously to regulate gene expression. Here, a simple visual assay used to compare the biological activity of small RNAs produced via different biogenesis pathways suggests that transacting small interfering RNAs (tasiRNAs) are mobile while confirming that microRNAs (miRNAs) are not.
Small RNAs have several functions in plant cells, with small interfering RNAs (siRNAs) involved in immunity against invading nucleic acids (RNAviruses and transposons) and in silencing of repetitive DNA. In contrast,miRNAs modulate physiological and developmental gene expression, a role also played by a limited number of siRNAs produced from endogenous genes. Transgene-born or viral-induced siRNAs can move from cell to cell. Current evidence suggests that miRNAs are not mobile, while it is unknown if endogenously encoded siRNAs such as tasiRNAs act noncell autonomously. The question of small RNA mobility is biologically critical since these molecules may act as signals.
During RNA silencing, Dicer ribonuclease enzymes produce 21- to 24-nucleotide RNAs from double-stranded RNA (dsRNA) substrates (Baulcombe, 2004; Brodersen and Voinnet, 2006; Mallory and Vaucheret, 2006). The small RNAs guide RNA-induced silencing complexes (RISCs) to suppress gene expression either transcriptionally or posttranscriptionally. In Arabidopsis (Arabidopsis thaliana), miRNAs are generated by DCL1 mediated cleavage of RNA polymerase II-transcribed endogenous precursor pre-miRNAs with distinct expression patterns reflecting their roles in developmental or physiological processes. In contrast, 21-nucleotide siRNAs are derived from DCL4-mediated cleavage of dsRNA precursors of various origins, including RNAviruses and transcribed inverted repeats used in RNAi experiments. One feature of siRNAs is their induction of a cell-to-cell movement of a sequence-specific silencing signal, likely comprised of the siRNAs themselves (Dunoyer et al., 2007). Local cell-to-cell movement of siRNA-derived silencing signals requires DCL4 and an RNA-dependent RNA polymerase (RdRP), RDR2 (Dunoyer et al., 2007). The local silencing signal typically travels five to 15 cells, with movement thought to be via plasmodesmata. In plants, miRNAs act to direct RISC-mediated cleavage of a target mRNAwith nearly perfect complementarity into two fragments that are rapidly degraded in the cell (Tang et al., 2003). Due to their role in developmental processes and their molecular similarity to siRNAs, it has been proposed that miRNAs could...