Regulatory peptide encoded by the primary transcript (pri-miR171b) of the microRNA miR171b that enhances the accumulation of its corresponding mature miRNA. Acts probably as a transcriptional activator of its corresponding pri-miRNA. Has no effect on the accumulation of other miRNAs. Addition of synthetic miPEP171b increases the abundance of miR171b, with consequent reduction of lateral root formation. Lateral root initiations.