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Research into the process of nodulation in the model legume Medicago truncatula has yielded major advances in our understanding of the symbiotic signaling pathway and of the molecular basis of plant root interactions with Rhizobium. Whether this knowledge can be extrapolated to the still poorly known actinorhizal symbioses has been under investigation for more than 20 years. Thanks to the development of genomics, significant data have been obtained in both Frankia and actinorhizal plants, together with the possibility to perform certain functional analyses of plant symbiotic genes in several actinorhizal plants. Like in legumes, flavonoids play an important role during the early stages of actinorhizal nodulation, but signaling molecules synthesized by Frankia in response to plant host recognition have not yet been characterized. Some recent data suggest that the structure of the signals secreted by Frankia differs from that of the lipochitooligosaccharides produced by most rhizobial strains. A major recent breakthrough was the discovery that successful root colonization by nitrogen-fixing microorganisms and arbuscular mycorrhiza rely on a common symbiosis pathway in actinorhizal plants and legumes. The evolutionary pattern emerging from these data reinforce the hypothesis of a common genetic ancestor of the Fabid (Eurosid I) nodulating clade with a genetic predisposition for nodulation partially recruited from a more ancient endomycorrhization process.