MicroRNAs (miRNAs) are transcribed as long primary transcripts (pri-miRNAs) by RNA polymerase II. Plant pri-miRNAs encode regulatory peptides called miPEPs, which specifically enhance the transcription of the pri-miRNA from which they originate. However, paradoxically, whereas miPEPs have been identified in different plant species, they are poorly conserved, raising the question of the mechanisms underlying their specificity. To address this point, we identify and re-annotate multiple Arabidopsis thaliana pri-miRNAs in order to identify ORF encoding miPEPs. The study of several identified miPEPs in different species show that non-conserved miPEPs are only active in their plant of origin, whereas conserved ones are active in different species. Finally, we find that miPEP activity relies on the presence of its own miORF, explaining both the lack of selection pressure on miPEP sequence and the ability for non-conserved peptides to play a similar role, i.e., to activate the expression of their corresponding miRNA.

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