Frankia

Frankia is a genus of filamentous, Gram-positive actinobacteria in the family Frankiaceae, order Frankiales. Members form nitrogen-fixing symbioses with a group of woody plants known as actinorhizal plants, enabling them to colonize nitrogen-poor soils. In the roots of these hosts, Frankia induces the formation of specialized nodules within which the bacteria differentiate into vesicles and fix atmospheric nitrogen, supplying ammonia to the plant in exchange for carbon and a protective environment from the host.

Frankia species are adapted to soil environments and can exist as saprotrophs or as symbionts. They are typically non-motile and form branched hyphae. The symbiosis is generally host-specific, with different Frankia strains nodulating particular actinorhizal hosts.

Actinorhizal hosts include alder (Alnus spp.), bayberry (Myrica spp.), casuarinas (Casuarina spp.), Elaeagnus spp., Ceanothus spp., and related woody plants. The nitrogen fixed by Frankia contributes significantly to soil fertility, especially in barren or degraded lands. Because of this property, Frankia-based inoculants are used in ecological restoration and forestry projects to improve seedling establishment and growth.

Taxonomy and genomics: Frankia belongs to the Actinobacteria and shows considerable genetic diversity among strains; genomes are relatively large and may carry plasmids. Frankia is generally considered non-pathogenic and mutualistic with its plant hosts.