Most organisms interact simultaneously with many mutualistic species (e.g., plants with microbial symbionts, ants, and pollinators), however mutualism studies have traditionally focused on bipartite interactions between a single partner and host. Bipartite studies can underestimate the effects of these mutualism on ecological and evolutionary dynamics if synergism occurs among functionally different partners, and overestimate it if partners are in conflict. This multispecies context can alter the quantity or quality of resources obtained from a partner and how these resources are allocated. Using the tripartite mutualism between nitrogen-fixing bacteria, arbuscular mycorrhizal fungi, and the model legume Medicago truncatula, we examined the consequences of multiple microbial mutualists for the fitness of host plants and its resource accumulation and allocation. Specifically, we grew >200 plant genotypes in a factorial experiment manipulating the bacteria and the fungi. We then collected data to determine the effect of this multispecies mutualisms on (1) fitness of host plant, (2) fitness alignment between the plant and the N-fixing bacteria, (3) amount of biological nitrogen provided by the bacteria, and (4) the allocation of biological nitrogen investment in seeds.
Results/Conclusions
Interactions with multiple microbes had important consequences for host plant fitness and allocation. For example, fitness alignment between N-fixing bacteria and the plant only occurred in the presence of mycorrhizal fungi (fungi’s presence*bacteria’s fitness: F1,342=23.3, P<0.0001). Without the fungi present, there was no relationship between bacterial fitness and plant fitness (t1,168 =0.8, P=0.41), but their fitness became positively aligned when fungi was present (t1,174 = 6.3, P<0.0001). Further, plant investment in roots versus shoots was strongly impacted by the multiple mutualist context (fungi’s presence*bacteria’s presence: P=0.0045). Compared to the control treatment (no microbes), plants grown in the presence of mycorrhizal fungi alone invested more in roots, and plants grown with bacteria alone invested more in shoots. When grown with both partners together they invest even more in shoots relative to roots, a non-additive microbial effect on investment. We are currently assessing inorganic and biologically-fixed nitrogen using stable isotopes and predict that the presence of mycorrhizal fungi will alter accumulation and allocation of fixed nitrogen in seeds. We expect the direction and strength of fungal effects on N-fixation to depend on the plant genotype. Overall, our result demonstrate that multispecies mutualisms can have important consequences for fitness and allocation in plants.