A striking example of bacterial invasion in plants is the agriculturally important symbiosis between legumes and nitrogen-fixing Rhizobium bacteria; the major source of biologically available nitrogen. A key-step in this symbiosis is the intracellular accommodation of the bacteria, as organelle-like structures, in a newly formed organ, the root nodule. The formation of these nitrogen-fixing organelles, so-called symbiosomes, is largely unexplored and I plan to start a new research line to unravel this important process. Symbiosome formation requires a major reorganization of the host endomembrane system, which is a common theme in the intracellular accommodation of microbes in all eukaryotes. However, studying the endomembrane system in vivo is hampered because manipulation of its components is often detrimental for the host. As nodules are non-essential organs they can be manipulated without detrimental effects on normal plant development, which makes this interaction an excellent in vivo model system to study bacterial invasion of eukaryotic cells. Recent work of myself offered 3 starting points to study this process: (1) the identification of a receptor kinase that is essential for entry of the bacteria, (2) a key-component of the endomembrane system (a syntaxin) that is rapidly degraded upon entry, and (3) genome-wide transcriptome analysis of the cells where entry takes place. These starting points now offer the opportunity to identify additional key-components of the molecular mechanism controlling the endomembrane system during rhizobial entry and a toolbox of (reverse) genetic techniques is available to study these components in vivo in the model legume Medicago truncatula. The proposed research is expected to unravel key-mechanisms controlling the endomembrane system during bacterial invasion and will also give fundamental insight into endomembrane biology in general.