18 August 2019
A search for the secrets of successful symbiosis
Published online 19 July 2019
Insights from a genomic survey of chickpea’s symbiotic microbes could reveal strains that boost crop productivity in different environments.
Researchers have found that soil bacteria forming a symbiotic relationship with domesticated chickpea plants are more diverse than those associated with the wild variety. These microbes appear to have evolved in parallel with the domesticated crop. However, it's not clear whether these evolved microbial strains are actually better for chickpeas, in terms of boosting productivity, or just better adapted at surviving and thriving in a given environment.
Chickpea plants form lifelong partnerships with soil bacteria that dwell within nodules in their roots. These symbionts capture and convert nitrogen into essential biomolecules, such as amino acids. However, the efficiency of this process depends on interplay between environmental conditions and genetic variation in the plant and microbes, with significant implications for the yield of this important crop.
Douglas Cook of the University of California at Davis, and his collaborators, including colleagues in Morocco, used sophisticated genome sequencing technology to analyse samples of chickpeas and their symbiotic bacteria, Mesorhizobium, from around the world.
“We started at the plant’s point of origin in southeastern Turkey, characterized variation and factors like climate and soil, and then moved to other areas where chickpea cultivation has spread,” says Cook.
They found that Mesorhizobium evolved considerably in response to chickpea domestication, which began some 11,000 years ago. The bacteria homed in on a core set of genomic elements in the chickpea that enable symbiosis. The team also observed remarkable diversity across agricultural Mesorhizobium samples relative to those from wild chickpea species.
Cook is considering two possible explanations: “Are these better symbionts that have specifically adapted to various new situations, or are they compromises and trade-offs representing the best nature has to offer?” If the latter is true, then many crops may not be achieving optimal nitrogen fixation, and his team is now studying microbial strains that might prove more agriculturally advantageous.
Greenlon, A. et al. Global-level population genomics reveals differential effects of geography and phylogeny on horizontal gene transfer in soil bacteria. PNAS http://dx.doi.org/10.1073/pnas.1900056116 (2019).