Stem cells are maintained in the niche by intercellular interactions and signaling networks. In this work, we study extracellular signals required for maintenance of the root stem cell niche in higher plants. We identify a family of functionally redundant homologous peptides that are secreted, tyrosine-sulfated, and expressed mainly in the stem cell area and the innermost layer of central columella cells. We name these peptides root meristem growth factors (RGFs). RGFs are required for maintenance of the root stem cell niche and transit amplifying cell proliferation in Arabidopsis. RGF1 defines expression levels and patterns of the stem cell transcription factor PLETHORA, mainly at the posttranscriptional level. The RGFs function independently of the auxin pathway. These peptide signals play a crucial role in postembryonic root development.