BIOLOGY OF FRANKIA STRAINS, ACTINOMYCETE SYMBIONTS OF ACTINORHIZAL PLANTS

Frankia strains are N2-fixing actinomycetes whose isolation and cultiv ation were first reported in 1978. They induce N2-fixing root nodules on diverse nonleguminous (actinorhizal) plants that are important in e cological successions and in land reclamation and remediation. The gen us Frankia encompasses a diverse group of soil actinomycetes that have in common the formation of multilocular sporangia, filamentous growth , and nitrogenase-containing vesicles enveloped in multilaminated lipi d envelopes. The relatively constant morphology of vesicles in culture s is modified by plant interactions in symbiosis to give a diverse arr ay of vesicle shapes. Recent studies of the genetics and molecular gen etics of these organisms have begun to provide a new insights into hig her-plant-bacterium interactions that lead to productive N2-fixing sym bioses. Sufficient information about the relationship of Frankia strai ns to other bacteria, and to each other, is now available to warrant t he creation of some species based on phenotypic and genetic criteria.