All organisms, especially multicellular ones, require formative cell divisions – which are mainly asymmetric (stem) cell divisions – to produce tissues and organs (Scheres and Benfey, 1999). Therefore, a tight control of such divisions and preventing over-proliferation is crucial for the postembryonic growth and development of plants. For instance, overexpression of the cell cycle regulators E2FA/DPA leads to proliferation and non-formative cell divisions resulting in plants with more cells but with a shorter main root, fewer lateral roots, and smaller leaves (De Veylder et al., 2002; De Smet et al., in prep). This control of formative cell divisions is not only critical in plants, to orchestrate the formation of lateral roots (De Smet et al., 2008; De Smet et al., in prep) and to coordinate stem cell divisions in the root tip (Wildwater et al., 2005) or patterning divisions in the embryo (Grunewald et al., in prep), but also in animals to prevent irregular divisions or cancers (Clevers, 2005). In plants, there are a number of pathways and developmental processes that are used to study this delicate balance, such as auxin-mediated cell cycle activation during lateral root initiation (Vanneste et al., 2005; De Smet et al, in prep), stomata development (Bergmann and Sack, 2007; Lampard et al., 2008, Lai et al., 2005), or stem cell divisions in the main root tip (De Smet et al., 2008; Wildwater et al., 2005). In animals, a typical example of a control mechanism for formative cell divisions is the Wnt/Wg-Frizzled ligand-receptor signaling cascade - with Wnt/Wg acting as a morphogen - affecting the plane of cell division and cell identity (Ulloa and Briscoe, 2007). We recently identified the membrane associated receptor-like kinase ACR4 as a key regulator of formative cell divisions in the root (De Smet et al., 2008). Notwithstanding ACR4 and its homologues/orthologues have been studied during the development of epidermis and aleuron (Watanabe et al., 2004; Gifford et al., 2003; Becraft, 1996), very little is known about the direct upstream regulation or the downstream targets. Here, I propose to further characterize this homeostatic ligand/receptor signaling mechanism that can integrate mobile signaling molecules to control formative cell divisions during organogenesis and, as such, provides an excellent tool to study short range, cell-to-cell communication during growth and development.