Polyphasic approach for the characterization of rhizobial symbionts effective in fixing N2 with common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is a legume that has been reported as highly promiscuous in nodulating with a variety of rhizobial strains, often with low effectiveness in fixing nitrogen. The aim of this work was to assess the symbiotic efficiency of rhizobial strains isolated from common bean seeds, nodules of Arachis hypogaea, Mucuna pruriens, and soils from various Brazilian agroecosystems, followed by the characterization of elite strains identified in the first screening. Forty-five elite strains were analyzed for symbiotic properties (nodulation, plant-growth, and nitrogen-fixation parameters) under greenhouse conditions in pots containing non-sterile soil, and variation in symbiotic performance was observed. Elite strains were also characterized in relation to morpho-physiological properties, genetic profiles of rep-polymerase chain reaction (PCR; BOX), and restriction fragment length polymorphism (RFLP)-PCR of the 16S rRNA. Sequence analyses of the 16S rRNA were obtained for 17 strains representative of the main groups resulting from all previous analyses. One of the most effective strains, IPR-Pv 2604, was clustered with Rhizobium tropici, whereas strain IPR-Pv 583, showing lower effectiveness in fixing N₂, was clustered with Herbaspirillum lusitanum. Surprisingly, effective strains were clustered with unusual symbiotic genera/species, including Leifsonia xyli, Stenotrophomonas maltophilia, Burkholderia, and Enterobacter. Some strains recognized in this study were outstanding in their nitrogen-fixing capacity and therefore, show high biotechnological potential for use in commercial inoculants.