Bloated Rhizobia: The effect of PHB storage on Bradyrhizobium japonicum mortality during desiccation

Abstract

Soil bacteria known as rhizobia infect the roots of legumes, forming nodules, where they benefit the host by fixing atmospheric nitrogen into a usable form for the plant. In return, these rhizobia use plant-derived energy to reproduce, and many species accumulate large quantities (>50% cell dry weight) of the storage lipid poly-3-hydroxybutyrate (PHB). Since high PHB stores give rhizobia up to a threefold reproductive advantage over cells with low PHB during starvation conditions, genotypes of rhizobia that are able to escape from nodules with more PHB would be expected to increase in frequency through time. However, not all strains of rhizobia synthesize large amounts of PHB. This research investigates a potential disadvantage to storing too much PHB: rhizobia that accumulate excessive amounts of PHB may be more likely to die in a drought environment. Using soybean (Glycine max) as host plants, I extracted Bradyrhizobium japonicum from nodules, desiccated the rhizobia, and measured PHB and percent of cells killed by this treatment using flow cytrometry. The results showed a positive correlation between PHB storage and percent killed by desiccation, indicating that high PHB storage may not always be selected for in rhizobial populations, despite its reproductive advantages.