Prolonged host-plant absence selects for rhizobia strains that divert twice as much energy from N fixation to polyhydroxybutyrate during symbiosis.

Authors: Muller, Katherine; DENISON, FORD - University Of Minnesota

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2024
Publication Date: 5/17/2024
Citation: Muller, K.E., Denison, F.R. 2024. Prolonged host-plant absence selects for rhizobia strains that divert twice as much energy from N fixation to polyhydroxybutyrate during symbiosis.Agronomy Journal. 116(4):1882-1893. https://doi.org/10.1002/agj2.21578.
DOI: https://doi.org/10.1002/agj2.21578

Interpretive Summary: Problem statement: Legume crops obtain most or all of their nitrogen from symbiotic N-fixing bacteria in their roots, known as rhizobia. Legumes provide rhizobia with carbon, which they use to support N-fixation and other functions. Past research shows that beneficial rhizobia strains applied as inoculum can support higher yields than resident rhizobia strains. However, inoculation often provides little-to-no benefit because resident rhizobia outcompete the inoculum strain. The research presented in this paper addresses fundamental questions about how the environment shapes resident rhizobia populations, in order to understand what limits their benefits to legume crops. Accomplishment: We provide the first field-based evidence for conflicts-of-interest between legumes and symbiotic rhizobia over how rhizobia use the carbon they receive from host plants. Using a long-term soybean-maize cropping experiment, we found that evidence that rhizobia able to hoard large amounts of host-supplied carbon in a storage compound called polyhydroxybutyrate (PHB) had an advantage in persisting over years without a legume host crop. We also found evidence that host crops can reduce the relative abundance of hoarder rhizobia. Contribution: Our results are significant to scientific research on symbiotic nitrogen fixation and mutualisms in general. We found real-world evidence for long-standing hypotheses that had only been investigated in the laboratory, under artificial conditions. We also raise novel questions that other scientists may investigate in the future.

Technical Abstract: Premise: The mutualism between legume plants and symbiotic N-fixing rhizobia involves conflicts over how rhizobia allocate host-supplied carbon. Rhizobia could increase their fitness by hoarding carbon in a storage compound called polyhydroxybutyrate (PHB), but host sanctions could limit the fitness of rhizobia that divert too much carbon away from N-fixation. While these hypotheses are supported by lab experiments and modeling, it is unclear how they contribute to evolutionary dynamics in natural rhizobia populations. We used resident rhizobia in a long-term cropping experiment to investigate the hypothesis that free-living and symbiotic environments create opposing selection pressures for PHB hoarding in rhizobia populations. Methods: A common-garden experiment compared PHB accumulation among rhizobia populations differing in the history of host availability due to long-term cropping treatments. We inoculated soybean plants with field soil, extracted rhizobia from nodules, used flow cytometry to measure PHB per rhizobial cell within each nodule. Key results: Rhizobia populations showed a pattern of increasing PHB accumulation over years between hosts. Average PHB accumulation was twice as high for rhizobia sampled after five or thirty years of a non-host crop (corn) than for rhizobia sampled after one, five, or thirty years of a host crop (soybean). A single year of soybean following five years of corn reduced the frequency of high-PHB rhizobia to similar levels as five- or thirty-year soybean monocultures. Conclusions: Together, these results suggest that selection for carbon-hoarding in rhizobia increases with host scarcity and decreases due to host sanctions when host are present.