Organ development in plant meristems requires that boundary domains around the primordia remain stable despite continuous growth and cell division. In Arabidopsis, the establishment and maintenance of the boundary around organ primordia of shoot apical and floral meristems is regulated by three partially redundant transcription factors, CUC1-CUC3. On p. 4311, Laufs et al. examine the interaction between these three transcription factors and a microRNA - miR164 - during boundary regulation. They show that miR164 targets cuc1 and cuc2, but not cuc3, mRNA for degradation,explaining why miR164 overexpression results in similar patterning and floral defects as cuc1 cuc2 double mutants. They also modified cuc2mRNA to make it resistant to miR164-guided cleavage and found that this expanded the boundary domain. The authors propose that miR164 constrains the expanding boundary by clearing boundary cells of CUC1 and CUC2, thus changing their cell identity.