Frankia is a genus of nitrogen fixing, filamentous bacteria that live in symbiosis with actinorhizal plants, similar to the Rhizobia bacteria that are found in the root nodules of legumes in the Fabaceae family. Bacteria of this genus also form root nodules.
The genus Frankia was originally named by Jørgen Brunchorst in 1886 to honor the Swiss microbiologist, A. B. Frank. Brunchorst considered the organism that 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 there are a great many strains specific to different plant species. The bacteria are filamentous and convert atmospheric nitrogen into ammonia, 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 Betulaceae family.
- Some species in all four genera in the Casuarinaceae family.
- Certain species in the genus Coriaria in the Coriariaceae family.
- Datisca cannabina and Datisca glomerata in the Datiscaceae family.
- All species in the three genera in the Elaeagnaceae family. These are Elaeagnus, Shepherdia and Hippophae.
- All species in the genera Myrica, Morella and Comptonia in the family Myricaceae.
- All species in six genera in the Rhamnaceae family. These are Ceanothus, Colletia, Discaria, Kentrothamnus, Retanilla and Trevoa , and possibly Adolphia.
- Some species in the Roseacea family including all the species in the genera Cercocarpus, Cowania and Purshia and some species of Dryas..