Nitrogen addition does not influence pre-infection partner choice in the legume-rhizobium symbiosis.

Abstract

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