Responses of Azorhizobium caulinodans to cadmium stress

The symbiosis of Azorhizobium caulinodans and an annul legume Sesbania rostrata was recently found to be tolerant to cadmium pollution by an unknown mechanism. In this study, A. caulinodans ORS571 and ZY-20 showed much stronger tolerance to cadmium than a mutant ORS571-X15 and a common Rhizobium sp., with minimum inhibitory concentration values as high as 4 and 5 mM (versus 1 and 0.1 mM) on yeast extract mannitol agar medium, respectively. Although Cd uptake by all three strains of A. caulinodans were mostly from absorption rather than binding (both loosely or tightly) on cell surface, in resistant strains a higher portion of extractable Cd was bound on the cell surface vs. absorbed (about 1:2.5 ratio) compared to the sensitive mutant (about 1:35.1 ratio). These results suggest that certain level of metal exclusion by a permeability barrier was involved in the mechanism of resistance to Cd by A. caulinodans ORS571 and ZY-20. Over the 12-h period of cultivation in yeast extract mannitol agar medium with Cd addition, the Cd concentrations in the outer membrane and periplasm and spheroplast were the highest at the first 3 h, and declined steadily over time. The fact that Cd concentrations in spheroplast of all three strains were many folds higher than those in outer membrane and periplasm, suggests that extracellular sequestration was not the only mechanism of Cd tolerance in A. caulinodans. The decline of Cd concentrations was significantly faster and started earlier in strains ORS571 and ZY-20 than in ORS571-X15. This suggests a second, probably more substantial, mechanism involves active transport of the metal from the cell, e.g., some efflux system for maintaining homeostasis under cadmium stress.