Surviving and Thriving in Terms of Symbiotic Performance of Antibiotic and Phage-Resistant Mutants of Bradyrhizobium of Soybean [Glycine max (L.) Merrill

Abstract

Rhizobial inoculation plays an important role in yielding enhancement of soybean, but it is frequently disturbed by competition with bacterial population present in the soil. Identification of potential indigenous rhizobia as competitive inoculants for efficient nodulation and N(2)-fixation of soybean was assessed under laboratory and field conditions. Two indigenous bradyrhizobial isolates (MPSR033 and MPSR220) and its derived different antibiotic (streptomycin and gentamicin) and phage (RT5 and RT6)-resistant mutant strains were used for competition study. Nodulation occupancy between parent and mutant strains was compared on soybean cultivar JS335 under exotic condition. Strain MPSR033 Sm(r)V(r) was found highly competitive for nodule occupancy in all treatment combinations. On the basis of laboratory experiments four indigenous strains (MPSR033, MPSR033 Sm(r), MPSR033 Sm(r)V(r), MPSR220) were selected for their symbiotic performance along with two exotic strains (USDA123 and USDA94) on two soybean cultivars under field conditions. A significant symbiotic interaction between Bradyrhizobium strains and soybean cultivar was observed. Strain MPSR033 Sm(r)V(r) was found superior among the rhizobial treatments in seed yield production with both cultivars. The 16S rRNA region sequence analysis of the indigenous strains showed close relationship with Bradyrhizobium yuanmingense strain. These findings widen out the usefulness of antibiotic-resistance marked phage-resistant bradyrhizobial strains in interactive mode for studying their symbiotic effectiveness with host plant, and open the way to study the mechanism of contact-dependent growth inhibition in rhizobia.