FIGURE 21-21 Regulatory network in the Arabidopsis shoot meristem stem-cell niche. The transcription factor WUS (orange circles) is synthesized in the organizing-center cells and moves via plasmodesmata into stem cells, where one of its functions is to induce expression of the CLV3 hormone (green circles). Secreted CLV3 protein binds to CLV1, the CLV3 receptor protein kinase on the surface of organizing center cells; there, it activates a signal that represses WUS transcription. See E. Aichinger et al., 2014, Annu. Rev. Plant Biol. 63:615, and R. Heidstra and S. Sabatini, 2014, Nat. Rev. Mol. Cell Biol. 15:301.