Competition between Rhizobium strain NGR234 and Bradyrhizobium strain CP283 for nodulation in siratro investigated with the GUS reporter gene

The use of the GUS reporter gene was found to be very suitable for studying the competitiveness of the Rhizobium strain NGR234 and Bradyrhizobium stra in CP283 for nodulation in siratro, However, the expression from the transp oson mTn5SSgusA20 declined in the nodules of old plants, particularly the n odules at 35 d after inoculation and onward. Siratro inoculated with both g usA-marked NGR234 and CP283 strains showed a similar nodulation and acetyle ne reduction activity (ARA) to those of their parental strains. No major ch anges in nodulation and symbiotic properties in these marked strains were o bserved. When the Rhizobium strain NGR234 and Bradyrhizobium strain CP283 w ere inoculated separately to siratro, both of them were found equally effec tive for nodulation in the plant. But when the Rhizobium strain NGR234, and gusA-marked Bradyrhizobium strain CP283 were co-inoculated to the plants i n a ratio of 1:1, the strain NGR234 occupied 75% of the nodules, while, the strain CP283 occupied only 25%, irrespective of the growth stages and inoc ulum concentrations. Similar results were also observed in the plants in wh ich the Bradyrhizobium strain CP283 and gusA-marked Rhizobium strain NGR234 had been co-inoculated, Thus, the Rhizobium strain NGR234 was more competi tive for nodulation in siratro.