Diversity of Frankia Strains, Actinobacterial Symbionts of Actinorhizal Plants


Filamentous, sporangia-forming actinobacteria of the genus Frankia are best known for their symbiotic association with phylogenetically unrelated dicotyledonous plants, called actinorhizal plants, that results in the formation of root nodules. Frankia actively fixes nitrogen within these root nodules and for this purpose develops specialized thick-walled structures, termed vesicles, that are the site of nitrogen fixation in planta and ex planta. Analysis of the molecular phylogeny of cultured and uncultured Frankia strains consistently identifies four main clusters regardless of the typing locus used. Cluster 1 includes Frankia strains which associate with Betulaceae, Myricaceae, and Casuarinaceae plants (except Gymnostoma), while cluster 2 contains the uncultured Frankia microsymbionts of species from the Coriariaceae, Datiscaceae, and Rosaceae families as well as Ceanothus of the Rhamnaceae. Frankia strains in cluster 3 form effective nodules on plants from members of the Myricaceae, Rhamnaceae, Elaeagnaceae, and Gymnostoma of the Casuarinaceae. Cluster 4 forms a broad group of atypical Frankia strains (noninfective and/or nonnitrogen-fixing) that are unable to establish or reestablish an effective association with actinorhizal plants.

While the overall phylogenetic structure is strengthened by MLSA and AFLP approaches, this relationship only marginally overlaps phenotypic-based grouping using criteria that include morphology in planta and/or in vitro, chemotaxonomic properties, host infectivity and mode of infection, physiological traits, and DNA–DNA relatedness. Besides these differences, the limited number of strains studied and the variability of these strains from study to study have hindered the advancement of these phylogenetic groupings for use in describing Frankia species. The purpose of this review is to examine the diversity of Frankia with respect to the hypothesis of genome adaptation and Frankia strain diversity evolution as these strains have evolved from ecological versatility in soil environments to symbionts with diverse degrees of host infeodation in plant root nodules.


Molecular, Cellular and Biomedical Sciences

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Journal Title

Symbiotic Endophytes



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Book Chapter


© Springer-Verlag Berlin Heidelberg 2013