|An alder root nodule.|
Frankia is a genus of nitrogen-fixing, filamentous bacteria that live in symbiosis with actinorhizal plants, similar to the Rhizobium bacteria found in the root nodules of legumes in the family Fabaceae. Bacteria of this genus also form root nodules.
This genus was originally named by Jørgen Brunchorst in 1886 to honor the German biologist, Albert Bernhard Frank. Brunchorst considered the organism he had identified to be a filamentous fungus. Becking redefined the genus in 1970 as containing prokaryotic actinomycetes and created the family Frankiaceae within the Actinomycetales. He retained the original name of Frankia for the genus.
Frankia alni is the only named species in this genus, but a great many strains are specific to different plant species. The bacteria are filamentous and convert atmospheric nitrogen into ammonia via the enzyme nitrogenase, a process known as nitrogen fixation. They do this while living in root nodules on actinorhizal plants. The bacteria can supply most or all of the nitrogen requirements of the host plant. As a result, actinorhizal plants colonise and often thrive in soils that are low in plant nutrients.
Several Frankia genomes are now available which may help clarify how the symbiosis between prokaryote and plant evolved, how the environmental and geographical adaptations occurred, the metabolic diversity, and the horizontal gene flow among the symbiotic prokaryotes.
- All species in the genus Alnus in the family Betulaceae
- Some species in all four genera in the family Casuarinaceae
- Certain species in the genus Coriaria in the family Coriariaceae
- Datisca cannabina and Datisca glomerata in the family Datiscaceae
- All species in the three genera in the family Elaeagnaceae, Elaeagnus, Shepherdia, and Hippophae
- All species in the genera Myrica, Morella, and Comptonia in the family Myricaceae.
- All species in six genera in the family Rhamnaceae, Ceanothus, Colletia, Discaria, Trevoa, and possibly Adolphia
- Some species in the family Rosaceae including all the species in the genera Cercocarpus, Cowania, Purshia, Chamaebatia, and some species of Dryas