H Smith Plastics LTD

Exogenous Transposable Elements Circumvent Identity-Based Silencing Permitting the Dissection of Expression Dependent Silencing

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Courtesy of H Smith Plastics LTD

The propagation of epigenetic marks has received a great deal of attention, yet the initiation of epigenetic silencing of a new transgene, virus, or transposable element (TE) remains enigmatic. The overlapping and simultaneous function of multiple silencing mechanisms has obscured this area of investigation. Here, we have revealed two broad mechanisms that can initiate silencing independently: identity-based and expression-dependent silencing. We found that identity-based silencing is targeted by 21-22 or 24 nucleotide small interfering RNAs (siRNAs) generated from previously silenced regions of the genome. By transforming exogenous TEs into Arabidopsis thaliana, we circumvented identity-based silencing allowing us to isolate and investigate the molecular mechanism of expression-dependent silencing. We found that several siRNA-generating mechanisms all trigger de novo expression-dependent RNA-directed DNA methylation (RdDM) through RNA Polymerase V. In addition, while full-length TEs quickly progress beyond RdDM to heterochromatin formation and the final maintenance methylation state, TE fragments stall at the RdDM phase. Lastly, we found that transformation into a mutant genotype followed by introgression into wild type does not result in the same level of silencing as direct transformation into wild type. This demonstrates that the plant genotype during a narrow window of time during TE insertion (or transgene transformation) is key for establishing the transgenerational extent of epigenetic silencing.

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