Nitrogen addition does not influence pre-infection partner choice in the legume-rhizobium symbiosis.
OBJECTIVE
Resource mutualisms such as the symbiosis between legumes and nitrogen-fixing rhizobia are context dependent and are sensitive to various aspects of the environment, including nitrogen (N) addition. Mutualist hosts such as legumes are also thought to use mechanisms such as partner choice to discriminate among potential symbionts that vary in partner quality (fitness benefits conferred to hosts) and thus impose selection on rhizobium populations. Together, context dependency and partner choice might help explain why the legume-rhizobium mutualism responds evolutionarily to N addition, since plant-mediated selection that shifts in response to N might be expected to favor different rhizobium strains in different N environments.


METHODS
We test for the influence of context dependency on partner choice in the model legume, Medicago truncatula, using a factorial experiments with three plant families across three N levels with a mixed inoculation of three rhizobia strains.


RESULTS
Neither the relative frequencies of rhizobium strains occupying host nodules, nor the size of those nodules, differed in response to N level.


CONCLUSIONS
Despite the lack of context dependence, plant genotypes respond very differently to mixed populations of rhizobia, suggesting that these traits are genetically variable and thus could evolve in response to longer-term increases in N.
University of Illinois, Department of Plant Biology, Urbana, Illinois 61801, USA mikegrillo@gmail.com.
Michael A
MA
Grillo
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada M5S 3B2.
John R
JR
Stinchcombe
University of Illinois, Department of Plant Biology, Urbana, Illinois 61801, USA.
Katy D
KD
Heath
D005838:Genotype; D046913:Medicago truncatula / Q000235:genetics / Q000382:microbiology / Q000502:physiology*; D009586:Nitrogen Fixation; D055170:Plant Root Nodulation; D020579:Sinorhizobium / Q000235:genetics / Q000502:physiology*; D013559:Symbiosis
Fabaceae; Medicago truncatula; context-dependency; legume–rhizobium; mutualism; nitrogen deposition; nodulation; partner choice