The arbuscular mycorrhizal (AM) symbiosis is a mutualistic association between soil-borne fungi and the roots of most plant species. Involving the bilateral exchange of nutrients, the symbiosis is connected to drastic changes in plant cell organelle morphology and physiology. Root plastids, in particular, are forming extensive, network-like structures covering the main symbiotic interface, i.e., intracellular, highly branched haustorium-like fungal structures called arbuscules. These plastid networks are highly dynamic and are formed and degraded concomitantly with the formation and degradation of arbuscules. By producing basic metabolic building blocks like fatty acids and animo acids as well as a number of secondary metabolites, plastids are the main biosynthetic organelles of plant cells. Accordingly, they are responsible for a number of biochemical tasks during arbuscule formation and degradation. The increase in fatty acid and amino acid biosyntheses connected to root colonization, e.g., is reflected by respective change of metabolite and transcript levels on the whole root level. The main plastid secondary metabolites formed specifically in AM roots are a group of various apocarotenoids. To investigate plastid functions in the colonization process, we are setting up subcellular proteomic analyses for the roots of Medicago truncatula. To achieve this, root plastids are purified and the enrichment of plastid preparations verified by western-blot analyses and contrast light microscopy. Then, comparative protein profiling of the non inoculated and Glomus intraradices-inoculated plastid preparations will be realized by both gel and gel-free strategies. Proteins will be characterized subsequently by LC-MS/MS identification. This way, we expect to identify soluble and membrane-bound proteins specifically involved in plastid functions during the colonization by AM fungi, which will help us to better understand the essential role(s) of theses organelles in the symbiotic process.