ROP/RAC GTPases are master regulators of cell polarity in plants, implicated in the regulation of diverse signaling cascades including cytoskeleton organization, vesicle trafficking, and Ca2+ gradients 1–8. The involvement of ROPs in differentiation processes is yet unknown. Here we show the identification of a novel ROP/RAC effector, designated interactor of constitutive active ROPs 1 (ICR1), that interacts with GTP-bound ROPs. ICR1 knockdown or silencing leads to cell deformation and loss of root stem-cell population. Ectopic expression of ICR1 phenocopies activated ROPs, inducing cell deformation of leaf-epidermis-pavement and root-hair cells 3, 5, 6, 9. ICR1 is comprised of coiled-coil domains and forms complexes with itself and the exocyst vesicle-tethering complex subunit SEC3 10–13. The ICR1-SEC3 complexes can interact with ROPs in vivo. Plants overexpressing a ROP- and SEC3-noninteracting ICR1 mutant have a wild-type phenotype. Taken together, our results show that ICR1 is a scaffold-mediating formation of protein complexes that are required for cell polarity, linking ROP/RAC GTPases with vesicle trafficking and differentiation.