Laying the groundwork to breed for mutualisms : Can alfalfa increase populations of rhizobia that provide them more Nitrogen?

Developing crop varieties that can select and reward helpful microbes from agricultural fields
represents an alternative to developing beneficial inoculum that often fails to establish and
colonize in agricultural fields. We previously showed that the model annual legume,
Medicago truncatula, can reward the nitrogen-fixing bacteria Sinorhizobium meliloti that live
in specialized chimeric root organs called nodules. Here we take the next step by asking if a
close perennial relative Medicago sativa, which is also a key forage crop, also has this ability.
In a randomized, replicated nitrogen-free greenhouse experiment, we inoculated three alfalfa
varieties with individual and mixed inoculations of a collection of 117 Sinorhizobium
meliloti strains initially isolated from these three varieties. We inoculated each host variety
with each strain individually to measure nitrogen-fixing benefits after 10 weeks of growth
using host biomass as a proxy. We also inoculated each host with a mixture of all 117 strains
and whole genome sequenced nodule pools from each plant to infer relative strain fitness in
each host variety. By examining the correlation between rhizobial and host benefits, we found
that all three host varieties can weakly enrich beneficial rhizobia. However, the strength of
the relationship differs. Our study supports the continued exploration of selectively breeding
alfalfa to maintain a helpful, resilient population of nitrogen-fixing bacteria and thus ensure
long-term mutualism success.