Abstract
Resources that microbial symbionts obtain from hosts may enhance fitness during free-living stages when resources are comparatively scarce. For rhizobia in legume root nodules, diverting resources from nitrogen fixation to polyhydroxybutyrate (PHB) has been discussed as a source of host-symbiont conflict. Yet, little is known about natural variation in PHB storage and its implications for rhizobial evolution. We therefore measured phenotypic variation in natural rhizobia populations and investigated how PHB might contribute to fitness in the free-living stage. We found that natural populations of rhizobia from Glycine max and Chamaecrista fasciculata had substantial, heritable variation in PHB acquisition during symbiosis. A model simulating temperature-dependent metabolic activity showed that the observed range of stored PHB per cell could support survival for a few days, for active cells, or over a century for sufficiently dormant cells. Experiments with field-isolated Bradyrhizobium in starvation culture suggest PHB is partitioned asymmetrically in dividing cells, consistent with individual-level bet-hedging previously demonstrated in E. meliloti. High-PHB isolates used more PHB over the first month, yet still retained more PHB for potential long-term survival in a dormant state. These results suggest that stored resources like PHB may support both short-term and long-term functions that contribute to fitness in the free-living stage.
| Original language | English (US) |
|---|---|
| Article number | 181124 |
| Journal | Royal Society Open Science |
| Volume | 5 |
| Issue number | 12 |
| DOIs | |
| State | Published - Dec 1 2018 |
Bibliographical note
Funding Information:Data accessibility. Supporting analysis scripts and edited datasets have been archived in the Data Repository for the University of Minnesota (DRUM), where they can be accessed from https://doi.org/10.13020/D69H6N. Authors’ contributions. K.E.M. designed the study, collected and analysed data, and drafted the manuscript. R.F.D. conceived of the ideas that the study is based on and contributed substantially to the design of the study and preparation of the manuscript. All authors gave final approval for publication. Competing interests. We have no competing interests. Funding. Both authors were funded by the Minnesota Long-Term Agricultural Research Network. K.E.M. was also funded by the Carol Pletcher Fellowship and Carolyn Crosby Grant (University of Minnesota Graduate School). Acknowledgements. We thank the researchers, students and support staff who aided our fieldwork in Minnesota Agricultural Research Stations. S. Chen, W. Gottschalk and M. Bickell helped us access their experimental plots at the Southern Research and Outreach Center. R. Faber prepared and managed our soya bean plot in the Sand Plain Research Farm. R. Pain and R. Shaw provided C. fasciculata seeds and granted access to their experimental plots in the Grey Cloud Dunes Scientific and Natural Area. G. Meyers, C. Tong, T. Teehan and E. Carson helped with fieldwork. J.M. Grossman, T. Sooksa-nguan and D. Nedveck provided advice and materials for growing and inoculating trap plants. We thank A. Dean and his laboratory members for generous access to their flow cytometer. Comments from R. Shaw, P. Kennedy, J.M. Grossman, J.G. Grossman, W. Ratcliff and two anonymous reviewers greatly improved the manuscript.
Publisher Copyright:
© 2018 The Author(s) Published by the Royal Society.
Keywords
- Dormancy
- Lifehistory
- Nitrogen fixation
- Plant-microbe interactions
- Polyhydroxyalkanoate
- Symbiosis