The main topics of the work described here were the cloning, characterization and localization of a 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR) from the methylerythritol phosphate pathway in isoprenoid biosynthesis in Zea mays. Previous investigations had shown an increase in DXR transcript accumulation in maize roots upon colonization by arbuscular mycorrhizal fungi, concomitant with the accumulation of apocarotenoids. After cloning of a DXR-like cDNA, the enzymatic function of the encoded protein could be demonstrated by functional heterologous expression. The recombinant protein was also used as antigen for the production of a polyclonal antiserum. The expression profile confirmed an increased transcript accumulation of ZmDXR in mycorrhizal roots and showed high amounts of transcripts also in photosynthetic leaves. Using the affinity-purified antiserum, the differential accumulation of ZmDXR in maize roots could also be verified on the protein level. To obtain a more detailed picture of DXR distribution in maize roots, immunolocalization studies were performed. in non-colonized cells, DXR is located in single plastids in the cytoplasm layer or in a plastid aggregation around the nucleus, respectively. In colonized cells, plastids multiply and are rearranged spatially. Depending on the developmental stage of the arbuscule, a plastidial network is formed that is tightly surrounding the fungal organ. The functional significance of the induction of the MEP pathway or the accumulation of the apocarotenoids in arbuscular mycorrhiza remains unclear.