Symbiotic nitrogen-fixation has been enthusiastically studied over the last century. Despite rhizobia-legume symbioses has been the main object of these studies, in the last two decades increasing attention was devoted to the nitrogen-fixing root nodules induced by a group of soil actinomycetes, belonging to the genus Frankia sp., in a taxonomically broad spectrum of dicotyledonous plants. In the present review we emphasize the recent advances in actinorhizal research from the early molecular events of actinorhizal infection and the recent gene expression studies in actinorhizas, to the proposed feed back regulation mechanisms of nodule formation and the phylogenetic view of how actinorhizal symbioses have evolved. In addition, to complement an overview on Frankia diversity, soil ecology and the present difficulties faced when studying these bacteria as a laboratory model, the latest hypotheses about the infectious role played by these microsymbionts are discussed.

The present possibility to genetically modify actinorhizal species belonging to the Casuarinaceae family and the recent reported successful experiments in transforming a Frankia strain, create a strong stimulus to develop in a near future manipulated actinorhizal symbiotic models for research.