The plant-specific RNA Polymerase IV (Pol IV) transcribes heterochromatic regions, including many transposable elements, with the well-described role of generating 24 nucleotide (nt) small interfering RNAs (siRNAs). These siRNAs target DNA methylation back to transposable elements to reinforce the boundary between heterochromatin and euchromatin. In the male gametophytic phase of the plant life cycle, pollen, Pol IV switches to generating primarily 21-22 nt siRNAs, but the biogenesis and function of these siRNAs has been enigmatic. In contrast to being pollen-specific, we identified that Pol IV generates these 21-22 nt siRNAs in sporophytic tissues, likely from the same transcripts that are processed into the more abundant 24 nt siRNAs. The 21-22 nt forms are specifically generated by the combined activities of DICER proteins DCL2/DCL4 and can participate in RNA-directed DNA methylation. These 21-22 nt siRNAs are also loaded into ARGONAUTE1, which is known to function in post-transcriptional regulation. Like other plant siRNAs and microRNAs incorporated into AGO1, we find a signature of genic mRNA cleavage at the predicted target site of these siRNAs, suggesting that Pol IVgenerated 21-22 nt siRNAs may function to regulate gene transcript abundance. Our data provides support for the existing model that in pollen Pol IV functions in gene regulation.
BackgroundTransposable elements (TEs) are mobile DNA fragments that cause mutations by inserting into genes and creating chromosomal breaks. To repress their mobility, and therefore limit the number of new mutations, eukaryotes target TE activity at the transcriptional, posttranscriptional and translational levels (reviewed in [1]). A major regulatory mechanism used to repress TEs are small RNAs, which target TE mRNAs for degradation, inhibit the translation of TE protein, and can guide de novo chromatin modification of TE loci, resulting in transcriptional silencing. In flowering plants, TE small interfering RNAs (siRNAs) are well-studied and fall into two major categories: 21-22 nucleotide (nt) siRNAs generated by RNA Polymerase II (Pol II), and 24 nt siRNAs generated by the plant specific RNA Polymerase IV (Pol IV)(reviewed in [2]).Early in plant evolution, the protein subunits of the Pol II holoenzyme duplicated and subfunctionalized into two additional RNA polymerase complexes, Pol IV and Pol V [3]. The biological function of Pol IV is to transcribe heterochromatic regions of plant genomes into nonpolyadenylated transcripts that are created for the sole purpose of siRNA generation [4]. Pol IV is guided to heterochromatic target regions of the genome by the mark of histone H3 lysine 9 2 dimethylation (H3K9me2) [5], and creates short 26-45 nt transcripts that are converted into double-stranded RNA via the RNA-DEPENDENT RNA POLYMERASE 2 protein (RDR2) [6,7]. This double-stranded RNA is then cleaved by DICER-LIKE 3 (DCL3) into predominantly 23-24 nt siRNAs [8]. Pol IV-derived 24 nt siRNAs are incorporated into the ARGONAUTE4 (AGO4) and AGO6 proteins, to guide AGO func...