The importance of the plant root system – which is taking care of indispensable plant functions such as uptake of nutrients and water, anchorage in the substrate and interactions with symbiotic organisms – is well-recognized (1). However, to maintain primary root growth and to create lateral organs, plants require coordinated (asymmetric) anticlinal and periclinal cell divisions followed by correct cell fate specification and cell differentiation. Several proteins controlling these processes in the root were identified, including the receptor kinase ACR4, the α AURORA kinases AUR1/AUR2 and a PP2A protein phosphatase. Here, we will address the protein-protein interaction network of these key regulators in cell division plane orientation and asymmetric cell division in the Arabidopsis root. To subsequently further decode the ACR4-AUR1/2-PP2A signaling network, we will perform quantitative mass spectrometric analysis of the phosphoproteome in the relevant loss-of-function and (inducible) gain-offunction backgrounds. In addition, we will screen for small molecules that can assist in unraveling the mode of action and that can potentially be applied to crops to optimize root system architecture.