Strain identification of highly-competitive bean rhizobia isolated from root nodules: Use of fluorescent antibodies, plasmid profiles and gene probes

Kim 5 and Viking 1 are highly-competitive N-fixing bean rhizobia (Rhizobium leguminosarum biovar phaseoli). Leonard jar studies were used to determine which of the two isolates was the most competitive. A mixed inoculant of both strains was added to bean seed (Phaseolus vulgar is cultivar Tendergreen) in sterile sand. Single-strain inoculants were used for seed grown in soil containing indigenous rhizobia. Nodule occupancy was determined after harvest. Plasmid profiles of nodule isolates from bean seed inoculated with equal numbers of both strains showed that Kim 5 occupied 72% of the nodules. Plasmid profiles for Kim 5 revealed three distinctive bands as compared to two bands from Viking 1. One band appeared to be common to both strains, whereas Kim 5 had two unique bands. A ³²P-labelled gene probe to one of the unique Kim 5 plasmids was used to-detect Kim 5 isolated from nodules by colony hybridization. This technique showed that Kim 5 was found in 76.2% of the nodules. Fluorescent antibodies were also used to distinguish between isolates. When single-strain inoculants were used in non-sterile soil, fluorescent antibodies showed that Kim 5 was recovered in 67.5% of the nodule isolates compared to 17.6% for Viking 1, while the other isolates were indigenous rhizobia. These data demonstrate that Kim 5 is more competitive than Viking 1.