Polycomb group (PcG) proteins play important roles in regulating developmental phase transitions in plants; however, little is known about the role of the PcG machinery in regulating the transition from juvenile to adult phase. Here, we show that Arabidopsis (Arabidopsis thaliana) B lymphoma Moloney murine leukemia virus insertion region1 homolog (BMI1) POLYCOMB REPRESSIVE COMPLEX1 (PRC1) components participate in the repression of microRNA156 (miR156). Loss of AtBMI1 function leads to the up-regulation of the primary transcript of MIR156A and MIR156C at the time the levels of miR156 should decline, resulting in an extended juvenile phase and delayed flowering. Conversely, the PRC1 component EMBRYONIC FLOWER (EMF1) participates in the regulation of SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE and MIR172 genes. Accordingly, plants impaired in EMF1 function displayed misexpression of these genes early in development, which contributes to a CONSTANSindependent up-regulation of FLOWERING LOCUS T (FT) leading to the earliest flowering phenotype described in Arabidopsis. Our findings show how the different regulatory roles of two functional PRC1 variants coordinate the acquisition of flowering competence and help to reach the threshold of FT necessary to flower. Furthermore, we show how two central regulatory mechanisms, such as PcG and microRNA, assemble to achieve a developmental outcome.Polycomb group (PcG) proteins are conserved epigenetic regulators that mediate gene repression through the incorporation of histone-modifying marks (Calonje, 2014). As far as it is known, PcG proteins associate in two multiprotein complexes in Arabidopsis (Arabidopsis thaliana): POLYCOMB REPRES-SIVE COMPLEX1 (PRC1) and PRC2. The combined activity of the two complexes is required for stable repression of the target genes.The major function of PRC2 is to perform histone H3 lysine-27 trimethylation (H3K27me3) through the methyltransferase activity of CURLY LEAF (CLF) and SWINGER (SWN) during sporophyte development or of MEDEA in the endosperm (Chanvivattana et al., 2004). Other PRC2 components are the Drosophila melanogaster suppressor of zeste12 homologs VERNALIZATION2 (VRN2), EMBRYONIC FLOWER2 (EMF2), and FERTILIZATION-INDEPENDENT SEED2, which confer specificity to the resulting PRC2s even though they have some overlapping functions (Chanvivattana et al., 2004), and finally MULTICOPY SUPPRESSOR OF INHIBITORY REGULATOR OF THE RAS-CYCLIC AMP PATHWAY and FERTILIZATION-INDEPENDENT ENDOSPERM, which are common subunits for the different PRC2s (Derkacheva and Hennig, 2014). On the other hand, the identity of Arabidopsis PRC1 is not defined yet. PRC1-mediated function can be histone 2A monoubiquitination (H2Aub) dependent, through the E3 ubiquitin ligase activity of the PRC1 RING finger proteins Arabidopsis B lymphoma Moloney murine leukemia virus insertion region1 homolog 1A (AtBMI1A)/B/C and AtRING1A/B, or H2Aub independent, which requires the activity of the PRC1 component EMF1 (Bratzel et al., 2010(Bratzel et al., , 2012Yang et al., 2013a;Calo...