Genomic approaches toward understanding the actinorhizal symbiosis: an update on the status of the Frankia genomes

The actinorhizal symbiosis is a mutualistic relationship between an actinobacterium from the genus Frankia and a wide variety of dicotyledonous plants representing 8 different families of angiosperms. Molecular phylogenetic approaches have identified four major Frankia lineages that have distinctive plant host ranges. Since the first published three Frankia genomes, an effort was undertaken to provide full genomic databases covering all four well established Frankia lineages and to provide depth of the number of strains covered. Here, we report on the updated status of these sequencing efforts. At present, there are 25 complete or draft Frankia genomes that have been sequenced and annotated, and several others are now in the pipeline being sequenced. An overview of the Frankia genomes will be presented focusing on their general genomic properties including size of the pan- and core-gene pool, size relationship and genome plasticity. Furthermore, a description of biosynthetic potential and a discussion about genes (nitrogenase, hopanoid biosynthesis, truncated hemoglobin, hydrogenase uptake gene clusters) involved in the symbiosis will be discussed. The absence of common nod genes within these Frankia genomes provides clues about the host-microbe recognition process for the actinorhizal symbiosis