In plants, developmental timing is coordinately regulated by a complex signaling network that integrates diverse intrinsic and extrinsic signals. miR172 promotes photoperiodic flowering. It also regulates adult development along with miR156, although the molecular mechanisms underlying this regulation are not fully understood. Here, we demonstrate that miR172 modulates the developmental transitions by regulating the expression of a subset of the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes, which are also regulated by miR156. The SPL3/4/5 genes were upregulated in the miR172-overproducing plants (35S:172) and its target gene mutants that exhibit early flowering. In contrast, expression of other SPL genes was not altered to a discernible level. Kinetic measurements of miR172 abundance in the transgenic plants expressing the MIR156a gene driven by a β-estradiol-inducible promoter revealed that expressions of miR172 and miR156 are not directly interrelated. Instead, the 2 miRNA signals are integrated at the SPL3/4/5 genes. Notably, analysis of developmental patterns in the 156 × 172 plants overproducing both miR172 and miR156 showed that whereas vegetative phase change was delayed as observed in the miR156-overproducing plants (35S:156), flowering initiation was accelerated as observed in the 35S:172 transgenic plants. Together, these observations indicate that although miR172 and miR156 play distinct roles in the timing of developmental phase transitions, there is a signaling crosstalk mediated by the SPL3/4/5 genes.