How do less-expensive nitrogen alternatives affect legume sanctions on rhizobia?

The evolutionary stability of mutualistic interactions involving multiple partners requires "sanctioning"-the ability to influence the fitness of each partner based on its respective contribution. Sanctions must be sensitive to even small differences if even slightly less-beneficial partners could gain a fitness advantage by diverting resources away from the mutualistic service toward their own reproductive fitness. Here, we test whether legume hosts sanction even mediocre N-fixing rhizobial strains by influencing either nodule growth (which limits rhizobial cell numbers) or carbon accumulation (polyhydroxybutryate or PHB) per rhizobial cell.

Poly-3-hydroxybutyrate (PHB) supports survival and reproduction in starving rhizobia.

The carbon that rhizobia in root nodules receive from their host powers both N(2) fixation, which mainly benefits the host, and rhizobium reproduction. Rhizobia also store energy in the lipid poly-3-hydroxybutyrate (PHB), which may enhance rhizobium survival when they are carbon limited, either in nodules or in the soil between hosts. There can be a conflict of interest between rhizobia and legumes over the rate of PHB accumulation, due to a metabolic tradeoff between N(2) fixation and PHB accumulation.