Cells in multicellular organisms organise along body and tissue axes. Cellular processes, such as division plane orientation, must be aligned with these polarity axes to generate functional 3-dimensional morphology, particularly in plants, where cell walls prevent cell migration. While some polarly localized plant proteins are known, molecular mechanisms of polarity establishment or its translation to division orientation are elusive, in part because regulators in animals and fungi appear to be missing from plant genomes. Cell polarity is first established in the embryo, but this has long been an intractable experimental model. My team has developed the genetic, cell biological and biochemical tools that now render the early Arabidopsis embryo an exquisite model for studying cell polarity and oriented division. Recent efforts already led to the unexpected identification of a novel family of deeply conserved polar plant proteins that share a structural domain with key animal polarity regulators. In the DIRNDL project, we will capitalize upon our unique position and foundational results, and use complementary approaches to discover the plant cell polarity and division orientation system. Firstly, we will address the function of the newly identified conserved polarity proteins, and determine mechanistic convergence of polarity regulators across multicellular kingdoms. Furthermore, we will use proteomic approaches to systematically identify polar proteins, and a genetic approach to identify regulators of polarity and division orientation, essential for embryogenesis. We will functionally analyse polar proteins and regulators both in Arabidopsis and the liverwort Marchantia to help prioritize conserved components, and to facilitate genetic analysis of protein function. Finally, we will use a cell-based system for engineering polarity de novo using the regulators identified in the project, and thus reveal the mechanisms that provide direction in plant development.