Isolation and symbiotic characteristics of Mexican Frankia strains associated with Casuarina

In the absence of available symbiotic nitrogen-fixing Frankia strains assoc iated with Casuarina trees in Mexico for reforestation purposes, isolation was undertaken using root nodules from trees growing in different habitats in Mexico, from the coast of the Gulf of Mexico up to 2550 m above the sea level. A total of 24 strains were isolated and clonal cultures were obtaine d from one filament of each strain. The use of acetate as the sole carbon s ource was essential for the isolation of the endosymbiont from the nodules due to the fact that other contaminant actinomycetes utilize propionate. Cl onal cultures were obtained, and cultural and symbiotic characteristics of pure cultures were assessed. All strains grew well in stirred DPM (defined propionate medium) with no mineral nitrogen. Isolates showed hyphae, multil ocular sporangia and characteristic vesicles. The presence of the gene nifH was also demonstrated, with all strains being able to nodulate Casuarina e quisetifolia. Nitrogenase activity (acetylene reduction) of the formed root nodules varied among the different associations depending on the isolate u sed to inoculate the plants. Several of the isolates can be used as inocula nts for the propagation of Casuarina trees. (C) 2000 Elsevier Science B.V. All rights reserved.