Hopanoids confer robustness to physicochemical variability in Bradyrhizobium diazoefficiens

Tookmanian, E.; Junghans, L.; Kulkarni, G.; Ledermann, R.; Sáenz, J.; Newman, D. K.; Becker, A. (2022) Journal of Bacteriology, 204(7), e00442-21. DOI: 10.1128/jb.00442-21

Summary

This study investigated why extended-hopanoid-deficient mutants of Bradyrhizobium diazoefficiens show severe growth defects in culture but near-normal symbiotic performance in root nodules. The work demonstrated that growth defects of the DhpnH mutant can be partially compensated by optimal osmotic and divalent cation concentrations in the medium. Biophysical measurements of lipid packing and membrane permeability showed that extended Hopanoids confer robustness to environmental variability, explaining the discrepancy: the controlled nodule environment buffers the loss of hopanoids, while the variable soil environment does not.

Key Findings

• Hopanoids are conditionally essential in B. diazoefficiens: a conditional SHC expression strain could not grow on one medium but grew on another with different osmolarity and cation composition
• Growth defects of DhpnH mutants were rescued by increasing divalent cation concentration (Ca2+ and Mg2+) or adjusting osmolarity
• Extended hopanoids reduced membrane permeability and increased lipid packing order, as measured by laurdan generalized polarization
• The physicochemical stability of the root nodule environment explains why hopanoid-deficient mutants perform near-normally in symbiosis
• Hopanoids provide their greatest fitness advantage in the variable soil environment rather than the controlled symbiosome space

Our Contribution

Saenz and Junghans at the B CUBE Center (TU Dresden) contributed biophysical membrane measurements, including lipid packing and permeability assays that demonstrated the mechanism by which hopanoids confer lipid ordering and environmental robustness.

Significance

This work resolved a longstanding paradox in hopanoid biology by showing that the fitness benefit of Hopanoids is context-dependent, manifesting primarily under physicochemical variability. The findings indicate that for improving legume-rhizobium symbioses through strain selection or engineering, the full rhizobial life cycle from soil to nodule must be considered.