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

GARCIA PARISI, PA; GRIMOLDI, A; LATTANZI, F; DRUILLE, M; OMACINI, M.

CABA

Congreso; International Symposium of Forage Breeding; 2015

Mostforage plants host at least one microbial symbiont that affects theirrelationship with their biotic and abiotic environment. In particular, coolseason grasses are naturally associated with aerial fungal endophytes, whichusually increase their growth by conferring protection from herbivores.Meanwhile, legumes establish mutualistic symbioses with nitrogen-fixingbacteria. Here we identifythree pathways by which the simultaneous presence of these two types ofsymbionts modulates the interaction between their hosts. Our hypothesis is that these privatesymbionts generate benefits to both the host and non-host plants, thusincreasing pasture productivity and nitrogen dynamics. In a series ofexperiments, we manipulated the presence of the asexual endophyte Epichloëoccultans in Lolium multiflorum and rhizobia (Rhizobiumleguminosarum bv trifolii) in the legume Trifolium repens.  First, we detected that endophytes andrhizobia interactively affected insect herbivory on their hosts sincegrass-endophyte symbiosis conferred associative protection to legume plants with high rhizobia-availability.Second, changes in soil induced by the grass-endophyte symbiosis increasedgrowth and nitrogen fixation of the legume only under low availability ofrhizobia. Third, each symbiontaffected nitrogen acquisition by its host without impairing the functioning ofthe other symbioses: epichloid endophyte increasesgrass-N-uptake from soil, while rhizobia increases N fixation in thelegume. Thus, in this experiment the presence of both symbionts generatedadditive effects in terms of total acquisition of nitrogen and net primaryproductivity. In conclusion,the benefits of grass-endophyte symbiosis on legumes, either by protectingthem from insect herbivory or by changing soil conditions, dependedon rhizobia availability.  Meanwhile, rhizobia presence allowsgrass and legumes to coexist without competing for N. In consequence, thesimultaneous presence of symbionts with different functional roles in differenthosts induces benefits on host and non-host plants through affectingdifferent aboveground and belowground components which, in turn, result positive for  agroecosystem functioning.