Lysogeny in Bradyrhizobium japonicum and Its Effect on Soybean Nodulation

Rhizobiophage V, isolated from soil in the vicinity of soybean roots, was strongly lytic on Bradyrhizobium japonicum 123B (USDA 123) but only mildly lytic on strain L4-4, a chemically induced small-colony mutant of 123. Numerous bacteriophage-resistant variants were isolated from L4-4 infected with phage V; two were studied in detail and shown to be lysogenic. The two, L4-4 (V5) and L4-4 (V12), are the first reported examples of temperate-phage infection in B. japonicum. Phage V and its derivative phages V5 and V12 were closely related on the basis of common sensitivity to 0.01 M sodium citrate and phage V antiserum, phage immunity tests, and apparently identical morphology when examined by electron microscopy. However, the three phages differed in host range and in virulence. Lysogens L4-4 (V5) and L4-4 (V12) were immune to infection by phages V and V5 but not to infection by V12. Southern hybridization analysis confirmed the incorporation of phage V into the genomes of strains L4-4(V5) and L4-4(V12) and also demonstrated the incorporation of phage V into the genome of a phage V-resistant derivative of USDA 123 designated 123 (V2). None of the three lysogens, L4-4(V5), L4-4(V12), or 123B(V2), was able to nodulate soybean plants. However, Southern hybridization profile data indicated that phage V had not incorporated into any of the known B. japonicum nodulation genes.