Trans-acting short-interfering RNAs (tasiRNAs) are conserved in the plant kingdom. The miR390-derived TAS3 pathway, in particular, regulates auxin signalling however not all components of the pathway in higher angiosperms are present in the Marchantia polymorpha genome. Here, Hung et al. determined that MpDCL4 processes MpTAS3 double-stranded RNA (dsRNA) to generate tasiRNAs, including tasi78A – a novel tasiRNA, which together with miR11700 regulates the cytochrome P450 gene, MpCYP78A, to alter auxin-signaling pathways and cell-size development of M. polymorpha gemmae.