Proposal of 'Candidatus Frankia californiensis', the uncultured symbiont in nitrogen-fixing root nodules of a phylogenetically broad group of hosts endemic to western North America

The genus Frankia comprises a group of nitrogen-fixing actinobacteria that form root-nodule symbioses with perennial dicotyledonous plants in the nitrogen-fixing clade. These bacteria have been characterized phylogenetically and grouped into four clusters (clusters 1-4). Cluster 2 contains mostly uncultured strains that induce nodules on species of the genera Datisca (Datiscaceae), Coriaria (Coriariaceae), Ceanothus (Rhamnaceae) and several genera in the family Rosaceae (Cercocarpus, Chamaebatia, Dryas, Purshia), all of which except members of the genus Coriaria are present within the California Floristic Province (CFP) or neighbouring areas of western North America. Those strains occurring in western North America are genetically very closely related to one another, and genetically distinct from strains characterized from other locales. We hereby propose to create a 'Candidatus Frankia californiensis' species for those cluster 2 strains of the genus Frankia with both high genetic similarity and a geographical distribution in or near the CFP.