Prolonged soybean absence in the field selects for rhizobia that accumulate more polyhydroxybutyrate during symbiosis

Abstract

During symbiosis, C that rhizobia respire to power N fixation can be stored as polyhydroxybutyrate (PHB), shown to support rhizobia survival under laboratory starvation. We collected soil in 2015 from four replicate plots per treatment in two long‐term experiments at Waseca, MN. Treatments differed in the intervals between soybean [Glycine max (L.) Merr.] hosts. We measured PHB accumulation in eight nodules per plant from four soybean (cv. MN0095) trap plants per soil sample. Trap plants were arranged in a greenhouse, common‐garden experiment, and PHB accumulation was measured using flow cytometry. Treatments sampled after long intervals without soybean (greater than 2 years) showed a greater relative abundance of high‐PHB strains. Treatments sampled after the first year of soybean following 5 years of a non‐host crop showed a decreased relative abundance of high‐PHB strains, compared to treatments sampled after long intervals without soybean. The latter result is consistent with the hypothesis (not tested directly here) that some high‐PHB strains were “sanctioned” by plants as less beneficial. Our results suggest that rhizobia strains with the tendency to allocate more C to N fixation at the expense of PHB accumulation may be less likely to persist where soybean is grown infrequently or where soil conditions make PHB particularly valuable. However, with typical 2‐year rotations in Minnesota, differences in PHB storage are unlikely to have a major effect on the relative survival of strains.