By CLSM analysis a strong proliferation of plastids, mitochondria and endoplasmic reticulum in the vicinity of intact arbuscules was shown. Proliferation of organells probabely reflects an increase in the metabolic activity of colonized cells in the root cortex. Irrespective of the colonisation state the quantity of plastids and mitochondria was much higher in the stele compared to the cortex. These studies were performed with composite plants of M. truncatula after root transformation (plastids, mitochondria) and transgene ertGFP-Nicotiana benthaminana plants (ER) respectively. The sequence of Green Fluorescent Protein (GFP) was linked to an appropriate signal sequence for translocation of the protein into the organelles, which could then visualized as green fluorescent structures. Immunolocalisation studies of GFP and DXR, a plastid enzyme of the MEP pathway in mycorrhizal ptGFP transformed roots of M. truncatula showed different developmental states. In the vicinity of young arbuscules a strong proliferation of elliptical plastids with no detectable or very low DXR levels took place. The shape of plastids surrounding older arbuscules was drastically changed to extended pleiomorphic structures containing high DXR contents, while near to degenerating arbuscules few DXR rich indented plastids were observed. For the investigation of the plastid division the plastid division protein FtsZ1 from M. truncatula was cloned and characterized. The soluble recombinant protein without the predicted plastid target sequence showed selfassembly into long protofilaments and a high GTPase activity, properties similar to FtsZ proteins of prokaryontes. Affinity purified antibodies against recombinant FtsZ1 protein of M. truncatula detected a lot of FtsZ1 signals near the branches of young arbuscules. This corresponds to a high division activity of plastids in the early developmental stage of arbuscules. But several FtsZ rings were also shown in the vicinity of older arbuscules colocalized with extended DXR rich plastids.