MicroRNA-mediated establishment of transcription factor gradients controlling developmental phase transitions.

The juvenile-to-adult phase transition is an important and critical step during plant development to ensure maximum reproductivity. This transition is regulated by different pathways, in some of which microRNAs are considered to be essential key components. In seed plants, miR156 and miR172 act sequentially in well characterized pathways to induce the vegetative phase change and floral formation by the establishment of spatiotemporal gradients of their cognate target transcripts that encode master regulators of development. Recently, we reported on an unrelated, moss-specific miRNA that acts similarly in the control of the juvenile-to-adult phase transition in Physcomitrella patens. Physcomitrella miR534a defines the spatial expression of two transcripts encoding BLADE-ON-PETIOLE (BOP) transcriptional coactivators in a cytokinin-dependent manner. We propose that this miRNA-mediated control is a major mechanism underlying the cytokinin-induced formation of the gametophore meristem in Physcomitrella. Furthermore, it suggests a convergent evolution of miRNA-controlled pathways regulating phase transitions in seed and non-seed plants.