Abstract
The actinobacterium Frankia alni is able to form nodules on the roots of a large spectrum of actinorhizal plants, where it converts dinitrogen to ammonia in exchange for plant photosynthates. In the present study, transcriptional analyses were performed on nitrogen-replete free-living cells and on Alnus glutinosa nodule bacteria, using whole genome microarrays. Distribution of nodule-induced genes on the genome was mostly over regions with high synteny between three Frankia genomes, while nodule-repressed genes were spread around the genome. Genes related to symbiosis, such as nif, hup2, suf and shc were highly induced. The expression of genes involved in ammonium assimilation and transport was strongly modified suggesting that bacterial ammonium assimilation was limited. Genes involved in transcriptional regulation, signaling processes, protein secretion, surface popolysaccharide biosynthesis that may play a role in symbiosis were also identified. The symbiotic transcriptome was highly similar among phylogenetically distant host plants. Finally, comparison with rhizobia transcriptome suggested that Frankia is metabolically more active in symbiosis than rhizobia.
