RESEARCH HIGHLIGHTS Nature Reviews Molecular Cell Biology | AOP, published online 3 April 2014; doi:10.1038/nrm3788
RNA INTERFERENCE
microRNAs suppress transposons in response to epigenetic activation, miRNAs mediate a PTGS mechanism that is responsible for transposon silencing
The efficiency of RNAi can be increased through the amplification of secondary siRNAs by RNA-dependent RNA polymerases (RDRs; also known as RdRPs). In plants, microRNAs (miRNAs) trigger RDR6 to produce 21‑nucleotide-long secondary siRNAs during post-transcriptional gene silencing (PTGS). Creasey et al. now report that, in Arabidopsis thaliana, miRNAs mediate the silencing of reactivated transposable elements by triggering the RDR6‑dependent biogenesis of siRNAs, which the same group previously termed epigenetically activated siRNAs (easiRNAs). Previously, easiRNAs were found to accumulate in an RDR6‑dependent manner in wild-type pollen and in the
NPG
inflorescence tissue of DNA methylation mutants such as DECREASED DNA METHYLATION 1 (DDM1), where transposable elements are activated by loss of DNA methylation. As easiRNAs genetically resemble other 21‑nucleotide-long secondary siRNAs, the authors reasoned that the accumulation of transposon-derived easiRNAs might be determined by miRNAs. To test this, they introduced a mutation in DICER-LIKE 1 (DCL1) into DDM1‑mutant plants to disrupt miRNA biogenesis and found a reduction in easiRNA levels. As RNA cleavage is an intermediary step in the amplification of secondary siRNAs by RDRs, they sequenced potential miRNA-directed transposon cleavage sites in the inflorescence tissue of DDM1 mutants and found that >1,200 easiRNA-producing transposable elements that were predicted to be miRNA targets are indeed cleaved. To further test the requirement for miRNAs in easiRNA biogenesis, the authors produced transgenic lines that express a reporter gene fused to part of the ATGP1 transposon that harbours the predicted target site of the pollen-specific miRNA, miR845b. Sequencing of small RNA from pollen revealed the existence of novel 21‑nucleotide-long easiRNAs that surround the miR845b target site, which were not found in pollen with constructs that lack the miRNA target site. This demonstrates that easiRNA formation depends on the targeting of transposable elements by miRNA. The expression of transposable elements can also be suppressed by transcriptional gene silencing (TGS), which is mediated by RDR2 and
NATURE REVIEWS | MOLECULAR CELL BIOLOGY
by 24‑nucleotide-long heterochromatic siRNAs (hetsiRNAs) that guide cytosine methylation to transposon sequences. Creasey et al. detected an overall loss of hetsiRNAs from transposable elements in DDM1 mutants and the recovery of many of them in DDM1 RDR6- and DDM1 DCL1‑mutant plants. Furthermore, they found that hundreds of transposable elements that did not produce hetsiRNAs generated them in DDM1 RDR6 plants. Thus, miRNA- and RDR6‑mediated easiRNA biogenesis seems to inhibit RDR2‑dependent hetsiRNA production from transposable elements. The results demonstrate that, in response to epigenetic activation, miRNAs mediate a PTGS mechanism that is responsible for transposon silencing. Such a mechanism could have evolved to suppress transposon-mediated genomic instability during the sensitive stage of epigenetic reprogramming in the germ line, when TGS is less active. The exclusion of RDR2 and the TGS machinery from this process enables transposable elements to evade heritable DNA methylation and silencing by TGS, and it could explain why transposable elements retained potentially harmful miRNA-binding sites. Eytan Zlotorynski ORIGINAL RESEARCH PAPER Creasey, K. M. et al. miRNAs trigger widespread epigenetically activated siRNAs from transposons in Arabidopsis. Nature http://dx.doi.org/10.1038/nature13069 (2014) FURTHER READING Haag, J. R. & Pikaard, C. S. Multisubunit RNA polymerases IV and V: purveyors of non-coding RNA for plant gene silencing. Nature Rev. Mol. Cell Biol. 12, 483–492 (2011)
VOLUME 15 | MAY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
RNA INTERFERENCE
microRNAs suppress transposons in response to epigenetic activation, miRNAs mediate a PTGS mechanism that is responsible for transposon silencing
The efficiency of RNAi can be increased through the amplification of secondary siRNAs by RNA-dependent RNA polymerases (RDRs; also known as RdRPs). In plants, microRNAs (miRNAs) trigger RDR6 to produce 21‑nucleotide-long secondary siRNAs during post-transcriptional gene silencing (PTGS). Creasey et al. now report that, in Arabidopsis thaliana, miRNAs mediate the silencing of reactivated transposable elements by triggering the RDR6‑dependent biogenesis of siRNAs, which the same group previously termed epigenetically activated siRNAs (easiRNAs). Previously, easiRNAs were found to accumulate in an RDR6‑dependent manner in wild-type pollen and in the
NPG
inflorescence tissue of DNA methylation mutants such as DECREASED DNA METHYLATION 1 (DDM1), where transposable elements are activated by loss of DNA methylation. As easiRNAs genetically resemble other 21‑nucleotide-long secondary siRNAs, the authors reasoned that the accumulation of transposon-derived easiRNAs might be determined by miRNAs. To test this, they introduced a mutation in DICER-LIKE 1 (DCL1) into DDM1‑mutant plants to disrupt miRNA biogenesis and found a reduction in easiRNA levels. As RNA cleavage is an intermediary step in the amplification of secondary siRNAs by RDRs, they sequenced potential miRNA-directed transposon cleavage sites in the inflorescence tissue of DDM1 mutants and found that >1,200 easiRNA-producing transposable elements that were predicted to be miRNA targets are indeed cleaved. To further test the requirement for miRNAs in easiRNA biogenesis, the authors produced transgenic lines that express a reporter gene fused to part of the ATGP1 transposon that harbours the predicted target site of the pollen-specific miRNA, miR845b. Sequencing of small RNA from pollen revealed the existence of novel 21‑nucleotide-long easiRNAs that surround the miR845b target site, which were not found in pollen with constructs that lack the miRNA target site. This demonstrates that easiRNA formation depends on the targeting of transposable elements by miRNA. The expression of transposable elements can also be suppressed by transcriptional gene silencing (TGS), which is mediated by RDR2 and
NATURE REVIEWS | MOLECULAR CELL BIOLOGY
by 24‑nucleotide-long heterochromatic siRNAs (hetsiRNAs) that guide cytosine methylation to transposon sequences. Creasey et al. detected an overall loss of hetsiRNAs from transposable elements in DDM1 mutants and the recovery of many of them in DDM1 RDR6- and DDM1 DCL1‑mutant plants. Furthermore, they found that hundreds of transposable elements that did not produce hetsiRNAs generated them in DDM1 RDR6 plants. Thus, miRNA- and RDR6‑mediated easiRNA biogenesis seems to inhibit RDR2‑dependent hetsiRNA production from transposable elements. The results demonstrate that, in response to epigenetic activation, miRNAs mediate a PTGS mechanism that is responsible for transposon silencing. Such a mechanism could have evolved to suppress transposon-mediated genomic instability during the sensitive stage of epigenetic reprogramming in the germ line, when TGS is less active. The exclusion of RDR2 and the TGS machinery from this process enables transposable elements to evade heritable DNA methylation and silencing by TGS, and it could explain why transposable elements retained potentially harmful miRNA-binding sites. Eytan Zlotorynski ORIGINAL RESEARCH PAPER Creasey, K. M. et al. miRNAs trigger widespread epigenetically activated siRNAs from transposons in Arabidopsis. Nature http://dx.doi.org/10.1038/nature13069 (2014) FURTHER READING Haag, J. R. & Pikaard, C. S. Multisubunit RNA polymerases IV and V: purveyors of non-coding RNA for plant gene silencing. Nature Rev. Mol. Cell Biol. 12, 483–492 (2011)
VOLUME 15 | MAY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
