Salt Tolerance in Astragalus cicer Microsymbionts: The Role of Glycine Betaine in Osmoprotection

In this study, we have investigated intrinsic salt tolerance of Astragalus cicer microsymbionts (USDA3350, ACMP18) and the role of exogenous glycine betaine in osmoprotection in these bacteria. Salt stress was imposed by NaCl concentrations ranging from 0.5 to 2 %. A. cicer mesorhizobia were capable of tolerating up to 2 % sodium chloride with a population count that was inversely proportional to the salt content. When the extracellular concentration of NaCl was raised to 2 %, the generation time of the UDSA3350 strain in the mid-exponential phase of growth was 3.9-times greater than that in the no-salt control medium, whereas the ACMP18 strain survived under the same conditions but did not multiply. Application of 1 mM glycine betaine into the salt-stressed rhizobium cultures increased the number of culturable bacteria, pointing out that this molecule was involved in restoration of osmotic balance. The decline in A. cicer symbiont viability in the medium with sodium chloride and the osmoprotective role of glycine betaine for these bacteria was confirmed in the experiment using the live/dead Bac Light Bacterial Vibility Kit. Data presented in this study showed the presence of proU-like genes in the genomes of A. cicer rhizobia with high sequence similarity to the genes of the ProU-like system in Sinorhizobium meliloti and the proU operon of Escherichia coli.