ABSTRACT
The actinobacterial genus Frankia encompasses sporulating filamentous bacteria (actinomycetes) that fix N2; they are defined by their ability to induce N2-fixing root nodules on a broad range of ‘actinorhizal plants’. Actinorhizal plants, in turn, are defined by their ability to form root nodules when in symbiosis with Frankia. Within the root nodule, Frankia fixes nitrogen that is transported to the host plant in amounts sufficient to supply most of the plant's nitrogen requirements. This symbiosis allows actinorhizal plants to invade and proliferate in soils that are low in combined nitrogen. Although similar in outcome, the symbiosis differs markedly from the rhizobium-legume symbiosis. The overall nodule architecture more closely resembles a foreshortened lateral root rather than a unique plant organ, and the plants have evolved a variety of mechanisms to modulate the levels of free O2 that would otherwise inactivate nitrogenase (Benson and Silvester, 1993). In common with legumes, however, the plants belong to the ‘nitrogen-fixing Clade’ within the Rosid I lineage initially described by Soltis et al. (1995).
