Emerging benefits of the coexistence of two microbial symbionts in pastures: Epichloidendophytes and rhizobia in a grass-legume system

GARCÍA PARISI, P. A.; GRIMOLDI, A.A.; LATTANZI, F.A.; DRUILLE M.; OMACINI M.

Buenos Aires

Simposio; 5th International Symposium of Forage Breding; 2015

Most forage plants host at least one microbial symbiont that affects their relationship with their biotic and abiotic environment. In particular, cool season grasses are naturally associated with aerial fungal endophytes, which usually increase their growth by conferring protection from herbivores. Meanwhile, legumes establish mutualistic symbioses with nitrogen-fixing bacteria. Here we identify three pathways by which the simultaneous presence of these two types of symbionts modulates the interaction between their hosts. Our hypothesis is that these private symbionts generate benefits to both the host and non-host plants, thus increasing pasture productivity and nitrogen dynamics. In a series of experiments, we manipulated the presence of the asexual endophyte Epichloëoccultans in Loliummultiflorum and rhizobia (Rhizobium leguminosarumbvtrifolii) in the legume Trifoliumrepens. First, we detected that endophytes and rhizobia interactively affected insect herbivory on their hosts since grass-endophyte symbiosis conferred associative protection to legume plants with high rhizobia-availability. Second, changes in soil induced by the grass-endophyte symbiosis increased growth and nitrogen fixation of the legume only under low availability of rhizobia. Third, each symbiont affected nitrogen acquisition by its host without impairing the functioning of the other symbioses: epichloidendophyte increases grass-N-uptake from soil, while rhizobia increases N fixation in the legume. Thus, in this experiment the presence of both symbionts generated additive effects in terms of total acquisition of nitrogen and net primary productivity. In conclusion, the benefits of grass-endophyte symbiosis on legumes, either by protecting them from insect herbivory or by changing soil conditions, depended on rhizobia availability. Meanwhile, rhizobia presence allows grass and legumes to coexist without competing for N. In consequence, the simultaneous presence of symbionts with different functional roles in different hosts induces benefits on host and non-host plants though affecting different aboveground and belowground components which, in turn, result positive for agroecosystem functioning.