Leaf endophytes mediate fertilizer effects on plant yield and traits in northern oat grass (Trisetum spicatum)

AimsSymbiotic fungi commonly increase plant acquisition of soil nutrients. Because prior work has focused on root fungi, we examined how leaf endophytes (Epichloe) influenced plant responses to fertilization and altered plant traits that may cascade to food webs and ecosystem processes.MethodsWe manipulated endophyte presence/absence in two populations of Trisetum spicatum, a wild relative of oat, under a 2x2 addition of soil nitrogen (N) and phosphorus (P) in the greenhouse.ResultsEndophyte symbiosis altered how plant biomass responded to soil N and how plant traits responded to soil P. Endophytes boosted the biomass gains from N-fertilization in one population. Plants from a second population had weak benefits of symbiosis, but the endophyte altered plant traits, by increasing specific leaf area under P-fertilization, root diameter under low P, and concentration of the fungal alkaloid AcAP under N fertilization. Endophyte presence suppressed the typically observed increase in root hair density in response to soil P limitation. Under low P, symbiotic plants from both populations had improved forage quality relative to symbiont-free plants, although N-fertilization had a larger effect size on forage quality than did symbiosis. Finally, the two populations differed in production of fungal alkaloids, which generally increased in response to fertilization.ConclusionsPredicting how microbial symbionts mediate plant acquisition of nutrients requires understanding how much their effects vary among plant and endophyte genotypes. Here, the magnitude and direction of leaf symbionts' effects on plant yield and traits varied between populations and with soil nutrient availability.