Frankia

Frankia is a genus of filamentous, nitrogen-fixing bacteria that form root nodules with certain non-legume plants. These symbioses let host plants obtain biologically usable nitrogen from the atmosphere.

Actinorhizal symbiosis: A nitrogen-fixing partnership between Frankia bacteria and roots of plants in several non-legume families. This is the root-nodule partnership Frankia forms with its plant hosts.
Alnus: A genus of trees and shrubs, commonly called alders, that forms nitrogen-fixing nodules with Frankia. Alders are prominent hosts used to study and apply Frankia-mediated nitrogen inputs.
Actinobacteria: A major bacterial phylum whose members include soil organisms, decomposers, and antibiotic producers. Frankia belongs to this broad bacterial lineage.
Rhizobium: A genus of bacteria that forms nitrogen-fixing root-nodule symbioses with legumes. Rhizobium performs a comparable service, but chiefly with leguminous hosts.
Frankia vesicle: A specialized Frankia cell structure that encloses nitrogenase during nitrogen fixation. Its multilayered envelope limits oxygen exposure to nitrogenase inside nodules.
Casuarina: A genus of trees and shrubs that forms nitrogen-fixing root nodules with Frankia. Casuarina species are planted in some regions where Frankia symbiosis can aid establishment on poor soils.
Filamentous bacteria: Bacteria that grow as elongated chains or branching filaments rather than only as isolated cells. Frankia’s branching filaments give its colonies a fungus-like appearance.
Legume: A plant in the family Fabaceae, including peas, beans, and clovers. Legumes typically partner with rhizobia, unlike Frankia’s non-legume hosts.
Nitrogenase: An enzyme complex that converts atmospheric nitrogen into ammonia using substantial energy. Frankia uses this oxygen-sensitive enzyme to make atmospheric nitrogen available to its host.
Elaeagnus: A genus of shrubs and small trees whose roots can form nitrogen-fixing symbioses with Frankia. Its species extend Frankia symbiosis beyond the best-known alder hosts.

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