What have we learned from our studies of co-occurring endophyte, rhizobia and mycorrhizal symbioses? The friends of my enemy can be my friends too

GARCIA PARISI, PA; MINAS, A; OMACINI, M

Simposio; 10th International Symposium on Fungal endophytes of grasses; 2018

Nitrogen-fixingbacteria (rhizobia) and arbuscular mycorrhizal fungi (AMF) are common rootsymbionts that play a key role in grasslands and pastures as they regulate nutrient andcarbon cycles and influence ecosystem production and multifunctionality. Despite theincreasing awareness that grass‑endophyte symbiosis may affect wholeneighborhoods, little is known about its impacts on interactions betweennon-endophytic plants and their symbionts. Here, we summarize recent findingson the significance of a safe grass-endophyte symbiosis for the functionalcapacity of rhizobia and AMF which can form symbioses with both the host grassand a forage legume. We present examples of how grass-endophyte symbiosismodifies belowground conditions and, in turn, influences soil symbioticpotential, legume performance and nitrogen acquisition by the host grass. Inour experiments, we used Loliummultiflorum seeds from a naturalized Pampean population with high or lowinfection level with E. occultans(E+: >95% and E-:< 5%, respectively) and Trifolium repens L. seeds inoculated with Rhizobium leguminosarum bv.trifolii, and inoculated (or not) the soils with three AMF species (Funneliformis mossaeae, Simiglomus hoi,Rhizophagus intraradices). Firstly, we detected that soil conditioning byE+ plants can reduce between 25% and 40 % the number of AMF spores, of rhizobialnodules and of established legume seedlings. In this experiment, these negativeeffects of endophytes had no effect on the functioning of legume-rhizobiasymbiosis (e.g., atmospheric nitrogen fixation per plant) in mycorrhizal andnon-mycorrhizal soils. Secondly, a recent mesocosms experiment designed toevaluate the ecological mechanisms showed that root litter deposited by E+plants reduced more than three times legume emergence and establishment.Finally, when legumes and grass plants grew together with the three type ofsymbionts in another mesocosms experiment, we detected interactive effectsamong symbioses, with consequences on plant productivity. Endophyte symbiosishad no effect on legume growth, but erased the positive effect of AMF onbiomass production of both the host grass and the whole community. However, theability of AMF to form common mycorrhizal networks increased threefold thetransfer of N fixed from the legume to the grass, regardless of its endophytesymbiotic status. Overall, our findings suggest a significant, but hithertounknown, endophyte impact on the ability of rhizobia and arbuscular mycorrhizalfungi to provide nutrients to neighbouring plants. Furthermore, we propose sustainableagriculture may be achieved by considering that all plants establish symbioticrelationships, with potential benefits beyond those to the host, and that synergisticcollaboration of plant symbionts with different specificity and functionalroles may be maximized.