Trans-acting small interfering RNAs (tasiRNAs) comprise a class of endogenous small RNAs that are generated from TAS gene-derived transcripts after these are cleaved at a microRNA (miRNA) target site. Arabidopsis thaliana has four families of TAS genes: miR173 triggers tasiRNA production from TAS1 and TAS2, miR390 from TAS3 and miR828 from TAS4. The two-hit trigger model postulates that dual target sites in the same transcript are often sufficient to initiate tasiRNA production, but two hits are not always required for tasiRNA formation. Here, we characterize the function of miR173 in the formation of tasiRNAs from TAS1 transcripts, as well as the importance of the TAS1 and TAS3 transcript sequences outside the miRNA-targeting sites for tasiRNA production. We show that tasiRNAs can be produced from heterologous transcripts containing miR173 or miR390 target sites, indicating that these trigger sequences are the only cis sequences essential for tasiRNA formation.