MicroRNAs are endogenous, small non-coding RNAs that regulate gene expression at the post-transcriptional level by cleaving target mRNAs. Mature microRNAs are products of the processing of their primary transcripts (pri-miRNAs). Now, it has been discovered that the products of the translation of some plant pri-miRNAs are peptide molecules (miPEP). These peptides have the capacity to physically interact with their open reading frames (ORFs) in the transcribed pri-miRNAs and, thus, positively regulate the accumulation of these RNAs and the corresponding mature microRNAs. Most conserved microRNAs play an important role in plants development and their response to stress. In this work, we obtained transgenic Physcomitrium patens moss plants containing Brassica oleracea miPEP156a ORF in the genome under the control of a strong 35S cauliflower mosaic virus promoter and analyzed the effect of the exogenous peptide on the transcription of this ORF in the protonemata of two transgenic moss lines. It turned out that the chemically synthesized peptide miPEP156a increases the accumulation of its own mRNA during moss culture growth, as was previously shown in studies by foreign researchers and in our own work for a number of peptides in monocotyledonous and dicotyledonous plants. These findings confirm that pri-miRNA regions that are located outside the coding region of the peptide are not required for transcriptional activation. Moreover, we have also succeeded in showing that the presence of a specific promoter of the microRNA gene does not affect the phenomenon of transcription activation; this phenomenon per se is not species-specific and is observed in transgenic plants, regardless of the origin of the miPEP.