A single miR390 targeting event is sufficient for triggering TAS3-tasiRNA biogenesis in Arabidopsis

In plants, tasiRNAs form a class of endogenous secondary siRNAs produced through the action of RNA-DEPENDENT-RNA-POLYMERASE-6 (RDR6) upon microRNA-mediated cleavage of non-coding TAS RNAs. In Arabidopsis thaliana, TAS1, TAS2 and TAS4 tasiRNA production proceeds via a single cleavage event mediated by 22nt-long or/and asymmetric miRNAs in an ARGONAUTE-1 (AGO1)-dependent manner. By contrast, tasiRNA production from TAS3 seems to follow the so-called ‘two-hit’ process, where dual targeting of TAS3, specifically mediated by the 21nt-long, symmetric miR390, initiates AGO7-dependent tasiRNA production. Interestingly, features for TAS3 tasiRNA production differ in other plant species and we show here that such features also enable TAS3 tasiRNA biogenesis in Arabidopsis, and that a single miR390 targeting event is, in fact, sufficient for this process, suggesting that the ‘one-hit’ model underpins all the necessary rudiments of secondary siRNA biogenesis from plant TAS transcripts. Further results suggest that the two-hit configuration likely enhances the fidelity of tasiRNA production and, hence, the accuracy of downstream gene regulation. Finally, we show that a ‘non-cleavable one-hit’ process allows tasiRNA production from both TAS1 and TAS3 transcripts, indicating that RDR6 recruitment does not require miRNA cleavage, nor does the recruitment, as we further show, of SUPRRESSOR-OF-GENE-SILENCING-3, indispensable for tasiRNA generation.