The symbiosis between the nitrogen fixing acinobacterium Frankia and its actinorhizal host plant is very old and their co-evolution has shaped their niche in the environment. Nitrogen is most often the limiting element in soil, and symbiotic plants can, with the help of their micrsymbionts, compete in an efficient way. Actinorhizal plants are found all over the world and the nitrogen fixing capability of Frankia contributes greatly to the global nitrogen cycle as well as to the microclimate around these particular plants. The symbiosis between the two is carried out in a modified lateral root known as a nodule, where the bacteria are hosted by the plant. The nodule is a specialized organ in which Frankia is provided with energy in exchange for fixed nitrogen. In this thesis the focus has been to examine the two actinorhizal plants Alnus glutinosa and Datisca glomerata and their two different Frankia symbionts, Frankia alni ACN14a and F. datiscae Dg1.

In order to compare the symbiosis of D. glomerata to other actinorhizal symbioses, the transcriptomes of roots and nodules were compared. F. datiscae Dg1, an unculturable bacterium, was isolated from D. glomerata nodules, and its genome was sequenced and analyzed. In this genome genes were found that are novel in the actinorhizal symbiosis and potentially involved in signaling to the plant host. These genes, known as common nod genes, have previously been described in the microsymbionts of legumes, rhizobia, where their products are involved in initiating nodulation and determining host specificity. Moreover we were able to show that the nod genes in actinobacteria could be the evolutionary origin of the nod genes in rhizobial species. To get a more comprehensive understanding of the comparative nitrogen and carbon metabolism in roots and nodules of D. glomerata and A. glutinosa, the nodule transcriptomes from both symbiotic bacteria were compared.