Frankia is a filamentous actinomycete that forms nitrogen-fixing root nodules (actinorhizal nodules) in non-leguminous plants like Alnus (alder), Casuarina, and Ceanothus. Rhizobium and Bradyrhizobium nodulate legumes; Azorhizobium is stem-nodulating; Clostridium is a free-living anaerobic nitrogen fixer.