The plasma membrane (PM) is a physical barrier that defines the boundaries of a cell. It not only isolates the cell interior from the environment, but also enables cell communication and a selective exchange of solutes. To serve those contrasting functions, the PM has a dynamic structure consisting of a lipid double layer and proteins embedded in it.

The main focus of this thesis is a small glycosylphosphatidylinositol-anchored protein called an Early Nodulin-Like 9 protein (ENODL9). ENODL9 localizes specifically to sieve elements (SE) which are highly specialized cells, responsible for long distance transport of nutrients in plants.

By using advanced fluorescence microscopy techniques, it is shown here that ENODL9 forms clusters in the PM, under hyperosmotic conditions. The clustering is driven upon binding to ATP, which is present in the extracellular space and seems to be a part of osmotic stress signaling in plants. Furthermore, it was established that ENODL9 clustering affects the organization of the PM and distribution of other PM proteins. Analysis of the phenotype of mutant lines revealed that ENODL9 has an important role for plant development and the adaptation to osmotic stress. This resulted in the hypothesis that ENODL9 acts as a SE specific receptor for extracellular ATP.