To coordinate growth in multicellular organisms, cells rely on extracellular signaling pathways. In plants these often involve LRR-RLKs (LEUCINE-RICH REPEAT RECEPTOR-LIKE KINASEs). The CLE (CLAVATA3/ENDOSPERM SURROUNDING REGION-related) family peptide ligands bind LRR-RLKs including A. thaliana CLAVATA1 and the closely related three BAM (BARELY ANY MERISTEM) receptors. bam1/2/3 triple mutants show formative division defects in root ground tissue, and in fact are necessary for the activation of these formative divisions by a subset of CLE peptides. This novel CLE/BAM signaling module activates expression of a key part of the cell cycle machinery, CYCLIND6;1, and thus represents an extracellular signaling pathway controlling asymmetric cell divisions in roots.