Patterning of distinct cell types is a fundamental process in the formation of multicellular organisms. Accordingly, a large number of researchers have focused on uncovering the intricacies of cell patterning using a variety of model organisms and experimental approaches. In plants, the patterning of the cell types in the root epidermis has been used for more than a century to study this topic because these cells are easy to observe and to manipulate. In particular, the Arabidopsis root epidermis has emerged over the last 20 years as one of the leading models for studying cell pattern formation (Grierson and Schiefelbein 2002; Ueda et al. 2005; Guimil and Dunand 2006; Schellmann et al. 2007; Schiefelbein et al. 2009).