Despite host sanctions, cheating rhizobia may prosper

Improving legume-rhizobia symbiosis is one key to reducing our need for nitrogen fertilizer, with its economic and environmental costs.  Research on rhizobial symbioses with wild legumes may provide some useful ideas.

Kelsey Gano-Cohen and colleagues in the lab of Joel Sachs have just published a paper in Ecology Letters, showing that wild rhizobia strains that are apparently less beneficial to their legume host tend to be more abundant in the soil.  I write “apparently” because it may be risky to extrapolate from growth of host plants in a greenhouse, inoculated with a single strain of rhizobia, and the effects of that strain on plants in the field that host many strains per plant.  Also, their negative correlation between strain abundance and apparent benefit is fairly weak, though statistically significant and not inconsistent with variability typical of field studies.

Their results certainly don’t show a positive relationship between strain abundance and benefits to host.  We would expect a positive relationship if there is “widespread fitness alignment” between rhizobia and the host.  Two ways that such alignment could perhaps sometimes occur (without violating conservation of matter in rhizobial allocation of resources between N2 fixation and their own current or future reproduction) include 1) host sanctions that consistently swamp any benefits rhizobia get from diverting resources from N2 fixation, or 2) selection mainly on seedlings so small that they host only one rhizobial strain, linking fitness benefits of host and rhizobia.

Porter and Simms have previously reported “selection for cheating” in a different legume-rhizobia symbiosis, using nodule size as a (weakly- correlated) proxy for rhizobial fitness benefits from symbiosis.  We count rhizobia per nodule and often measure a key resource rhizobia acquire during symbiosis, but Gano-Cohen’s measurements of relative strain abundance in the soil is arguably an even better measure of rhizobial fitness.  I mention these methodological issues because, even though the legume-rhizobia symbiosis is relatively easy to work with, it’s still tricky to reliably measure the benefits each species obtains.


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