title: Isolation and symbiotic characteristics of two Tn5-derived phage-resistant Bradyrhizobium japonicum strains that nodulate soybean.
abstract: Using transponson Tn5 mutagenesis, two transconjugants of Bradyrhizobium japonicum with the properties of both phage resistance and ability to induce nodulation were isolated at the frequency of 0.02%. These transconjugants were tested for their symbiotic performance on soybean cv. JS335 under greenhouse and field conditions. Both phage-resistant mutants induced nodules (nod (+)), but the transconjugant B. japonicum E13 was ineffective in nitrogen fixation (fix (-)). Rhizobiophage presence in the inoculum of phage-resistant mutants did not influence the symbiotic effectiveness. The mixture of wild strain and phage in the inoculum caused reduced symbiotic performance under controlled conditions, while under a field environment phage (100 and 500 mul of approximately 10(8) particles ml(-1)) presence did not have any recognizable effect on increased nodule dry weight, nitrogenase activity, or foliar N(2) content. On the basis of restriction fragment length polymorphism analysis, phage-sensitive, less effective, homologous bradyrhizobia belonging to B. japonicum were detected in root nodules of both inoculated and uninoculated plants. Inoculation of a higher concentration of phage in the inoculum significantly reduced the symbiotic performance, while the lower concentration of phage did not show any effect on phage-susceptible, less effective, homologous bradyrhizobia or, thus, symbiotic efficiency under field conditions. The phage-resistant mutant B. japonicum A49 showed effective symbiosis as efficient as that of the wild strain. Inoculation of phage-resistant mutants with lytic phage may reduce the occupancy of phage-susceptible, ineffective/less effective/mediocre homologous bradyrhizobia strains under natural complex soil conditions.
authors.affiliation: Microbial Genetics Laboratory, Department of Genetics and Plant Breeding, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi, India. cappunu@yahoo.com
authors.forename: C
authors.initials: C
authors.lastname: Appunu
authors.forename: B
authors.initials: B
authors.lastname: Dhar
mesh_terms: D001435:Bacteriophages / Q000502:physiology*; D020369:Bradyrhizobium / Q000235:genetics* / Q000302:isolation & purification / Q000821:virology; D004251:DNA Transposable Elements; D009154:Mutation; D009586:Nitrogen Fixation; D012988:Soil Microbiology; D013025:Glycine max / Q000235:genetics / Q000382:microbiology / Q000502:physiology*; D013559:Symbiosis
chemical_list: D004251:DNA Transposable Elements