Understanding the structure and function of endomembrane compartments is central to a mechanistic understanding of eukaryotic cell behavior. Multi-cellular organisms show an increased complexity and specialization in their endomembrane trafficking pathways. Higher plants have independently developed multi-cellularity and show a differently structured, but highly complex endomembrane system that regulates numerous, fundamental processes, such as cell wall composition, plant nutrition or immune responses. However, the specificities of plant endomembrane trafficking are only insufficiently addressed by homology-based approaches, which are inherently biased and limited to modules and pathways that are conserved between animals/yeast and plants. I propose to address this by undertaking forward genetic approaches for regulators of endocytic trafficking in Arabidopis with newly developed tools. In addition, I will establish the root endodermis as a model to address the mechanism of epithelial polarity establishment in plants. Epithelia are a fundamental feature of multi-cellular organisms and have independently evolved in plants and animals. The root endodermis is a tissue of central importance for plant nutrition. It is accessible to analysis and displays all the defining features of an epithelium. Studying the endodermis will allow me to investigate how independent or conserved the mechanisms of epithelial polarity are. Apart from the immediate interest for a number of plant developmental and adaptive responses, I contend that both parts of my proposal are also of general, fundamental interest. Current comparisons between yeast and animals do not give us any reliable and coherent idea about what is truly fundamental or derived in eukaryotic membrane organization. Unbiased research on plant membrane trafficking will provide insight into an additional, divergent type of eukaryotic cell and allow a better appreciation of the evolution of eukaryotic membrane organization.