Phenotypic characteristics and genetic diversity of rhizobia nodulating soybean in Egyptian soils

Twenty rhizobial strains isolated from the root nodules of soybean (Glycine max L.) were collected from nine governorates representing different agro-climatic and soil conditions in Egypt. The strains were characterized using a polyphasic approach, including nodulation pattern, phenotypic characterization, 16S rDNA sequencing, nifH and nodA symbiotic genes sequencing, and rep-PCR fingerprinting. Symbiotic properties assay revealed that all local rhizobial strains showed a wide spectrum of prolific nodulation and a marked increase in plant growth parameters compared to the un-inoculated control. Complete sequencing of 16S rRNA demonstrated that, native soybean nodulating rhizobia are phylogenetically related to Bradyrhizobium, Ensifer and Rhizobium (syn. Agrobacterium) genera. Study of tolerance ability to environmental stresses revealed that local strains survived in a wide pH ranges (pH 5-11) and a few of them tolerated high acidic conditions (pH 4). Agrobacterium strains were identified as the highest salt-tolerant and were survived under 6% NaCl, however Ensifer strains were the uppermost heat-tolerant and can grow at 42°C. Agrobacterium strains have been shown to harbor nifH and nodA genes similar to those in other fast growing soybean symbionts and were largely distinct from symbiotic genes of slow growing bradyrhizobia. The symbiotic effectiveness stability of Agrobacterium strains to nodulate soybean roots was confirmed using plant nodulation assay.