Isolation of infective and effective Frankia strains from root nodules of Alnus acuminata (Betulaceae)

Two Frankia strains were isolated from root nodules of Alnus acuminata coll ected in the Tucumano-oranense forest, Argentina. Monosporal cultures were obtained by plating a spore suspension of each strain and isolating a singl e colony. The strains (named AacI and AacIII) showed branched mycelia with polymorphic sporangia and NIR-vesicles. They differed in their ability to u se carbon sources: the AacI strain grew well on pyruvate, while the AacIII strain grew on mineral medium supplemented with glucose or, alternatively, with sucrose. The two strains were sensitive to oleandomycin, erythromycin, kanamycin, penicillin G, streptomycin and chloramphenicol at 5 mug/ml. The AcIII strain exhibited a moderate resistance to rifampicin, ampicillin and vancomycin. The nitrogenase activity in vitro of the strains was significa ntly higher in basal medium without nitrogen than that determined in the pr esence of ammonium chloride. Both strains were infective on seedlings of Al nus glutinosa, inducing an approximately similar percentage of nodulated pl ants (80%), although strain AacIII produced a higher number of nodules per plant (less than or equal to 15) than strain AacI (less than or equal to6). They were also effective for nitrogen fixation in planta, determined by th e acetylene reduction assay.