Rhizobium etli and Rhizobium gallicum nodulate Phaseolus vulgaris in Egyptian soils and display cultivar-dependent symbiotic efficiency

Twelve rhizobial isolates were recovered from nodules of common bean (Phaseolus vulgaris) grown in two different locations of Egyptian soils. The most effective strains for nodule formation and nitrogen fixation were selected. Strain specificity with the bean cultivars Saxa, Canoca and Giza 6 from Germany, Colombia and Egypt were studied. The strains were characterized by amplified rDNA restriction analysis of 16S and 23S rDNA (ARDRA), plasmid DNA content and 16S rDNA sequencing. A high degree of genetic diversity was observed among the strains used. The strains were separated into three genotype groups. Genotype A was displayed by seven isolates classified as Rhizobium etli, while genotype B was displayed by a single isolate, classified as R. gallicum. Genotype C included four isolates which were unable to re-nodulate Phaseolus vulgaris, which were related to Agrobacterium tumefaciens. Single strains were further characterized by specific physiological tests and measurements such as acetylene reduction activity, nodule/root biomass ratio, shoot and root fresh weight.