The role of hopanoids in fortifying rhizobia against a changing climate
Tookmanian, E. M.; Belin, B. J.; Sáenz, J. P.; Newman, D. K. (2021) Environmental Microbiology. DOI: 10.1111/1462-2920.15594
Summary
This review article discusses how Hopanoids in the outer membrane of rhizobia may underpin their resilience to osmotic stress exacerbated by climate change. Rhizobia must survive both the soil environment and the symbiosome space within root nodules, each presenting distinct physicochemical challenges. The review synthesizes evidence that structural remodeling of lipid A and hopanoids are instrumental in stress acclimation, with hopanoid biosynthesis overrepresented in the Bradyrhizobium clade, and proposes that hopanoid-fortified membranes could inform development of climate-resilient biofertilizer strains.
Key Findings
- Hopanoid biosynthesis genes are overrepresented in bradyrhizobia, the most abundant soil bacteria globally
- Changes in soil water potential are predicted to be the single greatest climate-related stressor affecting plant-microbe interactions
- Hopanoids interact with lipid A in the outer membrane, with some bradyrhizobia attaching hopanoids directly to lipid A (HoLA)
- The functional analogy between hopanoids and sterols extends to their role in organizing outer membrane lipids under osmotic stress
- Rhizobia experience osmotic challenges in two distinct life phases: bulk soil and the plant symbiosome space
- The review highlights hopanoid-mediated lipid ordering as a mechanism for maintaining outer membrane integrity under desiccation
Our Contribution
Saenz contributed expertise on hopanoid biophysics and the Hopanoid-Sterol Analogy, drawing on prior work demonstrating that hopanoids order membranes analogously to cholesterol.
Significance
This review connects fundamental membrane biophysics of Hopanoids to an applied agricultural context, framing hopanoid-mediated stress tolerance as relevant to sustainable nitrogen fixation under climate change. It highlights the potential for membrane engineering approaches to improve rhizobial biofertilizer efficacy.