Towards the analysis of mitochondrial physiology in nitrogen fixing root nodules

Mitochondria contribute essentially to cellular metabolism by covering the bulk of its ATP requirements. However, mitochondria also engage in many catabolic as well as some anabolic pathways, most of which are conserved among eukaryotes. Additional mitochondrial functions of plants take place in the backdrop of photosynthesis. As such, plant mitochondria have extra functions when compared to their animal counterparts, for example in supporting chloroplast nitrogen assimilation by providing 2‐oxoglutarat for the
production of glutamate. A possible nitrogen source, apart from the uptake of soil derived nitrate and ammonium, is the fixation of molecular nitrogen by symbiotic nitrogen fixation. Symbiotic nitrogen fixation takes place between several plant species of the legume family and rhizobial bacteria and is based on an exchange of metabolites between the two symbiotic partners. The bacteria provide ammonia to the plant in exchange for reduced carbon compounds. This process takes place in specialized plant organs, the root nodules. Mitochondria have some particular functions in the N2‐fixing cell. However, it is currently unclear if the interactions of mitochondria and bacteroids are of a supportive or an antagonistic nature. In this thesis, special emphasis is placed on the physiology of mitochondria in the context of symbiotic nitrogen fixation and hinges on the proteomic investigation of root nodules. Our results support the view that mitochondria are of great
importance for the fixation of atmospheric nitrogen (manuscript 1). In the course of this study a mitochondrial alanine aminotransferase has been identified, which may catalyze the transfer of the amino group from bacteroid derived alanine to 2‐oxoglutrarate, thus supplying pyruvate for ATP production and glutamate for ammonium assimilation. To investigate the role of plant mitochondria in SNF in more detail, it is necessary to conduct further studies on isolated organelles of high purity. The steps required for such a procedure, including further sub‐fractionation to improve protein coverage in proteomic
studies, are documented in manuscript 2. The workflow described in this manuscript will also allow extending the definition of plant mitochondria aside from SNF by in‐depth mass spectrometry. The current knowledge on the protein content of higher plant mitochondria is summarized by highlighting two model species, potato and Arabidopsis (publication 1).