Performance of different Bradyrhizobium strains in root nodule symbiosis under drought stress

The performance of rhizobia in soybean nodules under drought stress conditions was analyzed using five Bradyrhizobium strains selected according to the osmotic stress tolerance in liquid medium. The effect of selected rhizobial strains on root and nodule antioxidant response and symbiotic performance was evaluated in a soil pot experiment under different levels of drought stress (0, 3, 5, 7 days withholding water). Drought stress increased the guaiacol peroxidase (POX) and ionically cell wall-bound peroxidase (POD) activity, antioxidant capacity, soluble protein content in roots and nodules, reduced the shoot dry weight (SDW), and increased the nitrogen content in the roots. Under water deficit conditions the highest increase of antioxidative parameters was recorded in the nodules of strain 216, and the lowest in roots and nodules of the plants inoculated with strain 511. Inoculation with strain 511 resulted in significantly lower SDW, root dry weight (RDW) and plant nitrogen content, while application of strain 216 resulted in the highest shoot attributes. Rep-PCR characterization and 16S-23S rDNA intergenic region sequencing emphasized the differences in strains genomic organization, especially for the 216 strain showing the higher tolerance to osmotic stress. The results implicate similarity between strain performance under osmotic stress in liquid medium and in symbiotic association under drought stress. The results also suggested important contribution of rhizobial strains in enhancing antioxidative response under drought stress and in symbiotic effectiveness, indicating more sensitive and tolerant symbioses.