Coexistence of N2-fixing and non-fixing rhizobia in the legume-Rhizobium symbiosis: explanations from modelling and experiments.

DIANA E. MARCO

León

Congreso; XV CONGRESS OF THE SPANISH SOCIETY OF NITROGEN FIXATION, AND THE IV PORTUGUESE-SPANISHCONGRESS ON NITROGEN FIXATION; 2015

SEFIN- Univ. de León

Mutualistic and symbiotic interactions impose challenging ecological and evolutionary problems. Their origin and persistence in nature is difficult to explain since the existence of exploitative, 'cheating' partners that could erode the interaction is common. Host (plant) sanctions against non N2 fixing, cheating symbionts have been proposed as a force stabilizing mutualism in legume-Rhizobium symbiosis. Penalizations would include decreased nodular rhizobial viability and/or early nodule senescence in nodules occupied by cheating rhizobia. I analyze the ecological and evolutionary stability of Rhizobium-legume symbiosis when "cheating" strains are present, using a combination of experiments and mathematical modelling. In different experiments, soybean plants were inoculated with two rhizobial strains, a cooperative, normal N2 fixing strain and an isogenic non-fixing, ?perfect? cheating mutant derivative that lacks nitrogenase activity but has the same nodulation abilities. Based on these experiments, a population dynamic model with and without the inclusion of plant host sanctions was developed. No experimental evidence of functioning plant host sanctions to cheater rhizobia was found. Plant populations persist in spite of the presence of cheating rhizobia without the need of incorporating any sanction against the cheater populations in the model, under the realistic assumption that plants can at least get some amount of fixed N2 from the effectively mutualistic rhizobia occupying some nodules. Inclusion of plant sanctions merely reduces the time needed for reaching plant population equilibrium and leads to the unrealistic effect of ultimate extinction of cheater strains in soil. Different factors were added to the model to resemble realistic field conditions. For example, competition for nodulation and co-occupation of the same nodule by strains with different fixation abilities are important sources of concern in cultivated legumes. The effect of plant stress was also included in the model. The potential consequences for agricultural practices like artificial inoculations when strains with different fixation abilities are present are discussed.