Extracellular signal peptide that regulates cell fate. Represses root apical meristem maintenance. Regulates the transition of protophloem cells from proliferation to differentiation, thus impinging on postembryonic growth capacity of the root meristem; this signaling pathway requires CRN and CLV2 (PubMed:28607033). Mostly expressed in leaves and apex, and, to a lower extent, in seedlings, flowers, stems and siliques. The O-glycosylation (arabinosylation) of the hydroxyproline Pro-100 enhances binding affinity of the CLE21p peptide for its receptor. Belongs to the CLV3/ESR signal peptide family.