Symbiotic rhizobia have one of two life histories, depending on the host legume species. In some hosts, rhizobia terminally differentiate into nitrogen-fixing bacteroids and can no longer divide, while in others, these bacteroids remain reproductive. Reproductive bacteroids accumulate resources in the form of polyhydroxybutyrate (PHB), which can be used for reproduction and survival during starvation. Nonreproductive bacteroids typically lack PHB. Accumulation of PHB reduces nitrogen fixation by consuming host resources. The legume species determines the reproductive fate of the bacteroids, but does it also determine their PHB accumulation? PHB accumulation in nonreproductive bacteroids may reduce the fitness of their undifferentiated reproductive kin in the same nodule, thereby selecting against PHB accumulation in bacteroids. Alternatively, PHB accumulation may be suppressed by host factors. Single rhizobium strains that are able to form both reproductive and nonreproductive bacteroids were compared to assess host effects on PHB levels using Nile Red staining and flow cytometry.

Results/Conclusions

PHB accumulation in bacteroids appears to be under the influence of the legume host species. Rhizobium strains that accumulate PHB as reproductive bacteroids could no longer accumulate PHB in their new host where they became terminally differentiated. Depriving bacteroids of PHB accumulation may have been a trait that evolved in some legumes as a means to increase nitrogen fixation in bacteroids. However, some rhizobia that lose reproductive viability in symbiosis have evolved ways to divert resources to their reproductive kin in the same nodule. Therefore, hoarding PHB as bacteroids may not be the only way for symbiotic rhizobia to cheat their hosts.