Complete and Reduced Life Cycles in Rhizobium

SUMMARY: Strains of Rhizobium from a wide variety of host plants were found to possess either a complete or a reduced life cycle. In the former, which was characteristic of strains from certain wild legumes and garden flowers, the bacteroids within the nodule were branched and septate; the free-living stages included small Bacterium-like forms which were responsible for infection of the host plant, and large, occasionally Gram-positive forms resembling Bacillus, which produced both specialized, coceoid swarmers and resistant endospores. The reduced cycle was commonly found in Rhizobium from cultivated field crops. The bacteroids were single cells of irregular outline; the free-living stages were reduced to the Bacteriumlike stage alone, sometimes so actively motile as to perform the function of swarmers. This condition is regarded as degenerate and due to a parasitic habit. The mode of formation of the endospores, which differs in detail from that of Bacillus, is described. The morphology of various stages in the life cycle and a slight tendency to Gram-positivity render it apparent that Rhizobium is a specialized genus of Bacillaceae. probably related to the plant parasitic B. polymyxa group, with which it also has biochemical affinities.