Isolation and characterization of new efficient and competitive bean (Phaseolus vulgaris L.) rhizobia from Brazil

The common bean (Phaseolus vulgaris L.) is widely cultivated in South and Central America and Africa, but inoculation with rhizobia often does not lead to a response in field experiments. A selection program was started in the State of Paraná, Brazil, in which three promising strains, PRF 35, PRF 54 and PRF 81, showing high rates of N2 fixation, were competitive and tolerated high temperatures. The performance of the strains was also verified in four field experiments, where inoculation with PRF 81 allowed yield increases of up to 906 kg ha−1, compared with the non-inoculated (control) with a high population of native bean rhizobia. The high performance of PRF 81 was confirmed in several other field trials carried out in Brazil, leading to its recommendation for use in commercial Brazilian inoculants. PRF 34, PRF 54 and PRF 81 were further characterized and compared with four strains, representative of bean rhizobia species in an effort to define variables which could aid future selection programs. The Brazilian strains showed unique profiles of protein, lipopolysaccharide and PCR using specific (ERIC and REP) or arbitrary short primers. The DNA fingerprints obtained with specific or arbitrary primers showed that strains PRF 35 and PRF 54 were genetically very close, nevertheless, there were substantial differences between the strains in nodulation and N2 fixation rates, as well as in the synthesis of Nod factors after induction with naringenin. The Brazilian strains showed Nod factor profiles similar to those of R. tropici type IIA CFN 299 and IIB CIAT 899 strains, and mixed characteristics of both types. That is, they were unable to grow in LB and PY minus Ca, as with type IIA, but were tolerant to high temperature, acidity, and had the same PCR product with Y1 and Y2 primers, as type IIB strain. The Brazilian strains showed mixed host range spectra between strain types IIA and IIB and, by the analysis of 17 fatty acids, strains PRF 35 and PRF 54 were grouped with CFN 299 and PRF 81 with CIAT 899. The performance of strain PRF 81 in field experiments indicates future potential for identification of new competitive and efficient R. tropici strains for tropical and subtropical areas.