A highly specific microRNA-mediated mechanism silences LTR retrotransposons of strawberry

Small RNAs are involved in a plethora of functions in plant genomes. In general, transcriptional gene silencing is mediated by 24-nucleotide siRNAs and is required for keeping transposable elements in a silenced state while microRNAs are not commonly associated with transposon silencing. In this study we performed small RNA transcriptome and degradome analyses of the Rosaceae model plant Fragaria vesca (the woodland strawberry) at the genome-wide level and identified miRNA families and their targets. We report a highly specific mechanism of LTR retrotransposon silencing mediated by an abundant, ubiquitously expressed fve-miR1511 generated from a single locus. This miRNA specifically targets LTR retroelements, silencing them post-transcriptionally by perfectly pairing to the highly conserved primer binding site (PBS) for methionyl-initiator tRNA essential for reverse transcription. We investigated possible origins of this miRNA and we present evidence that the pre-miR1511 hairpin structure likely derived from a locus coding for tRNA(iM) (et) through a single microinversion event. Our study shows that miRNA can target retrotransposons specifically and constitutively and contribute to features such as genome stability, size and architecture in a far more direct way than previously considered.