Plant-microbe symbioses occur commonly in nature and may influence the ability of plants to tolerate biotic and abiotic stress.  Many grasses host systemic fungal endophytes that grow in the intercellular spaces of aboveground tissues.  These endophytes are known to increase resistance to herbivory, enhance tolerance to drought, and increase nutrient availability.  However, the benefits and costs have not been fully explored.  Specifically, many grass species that occur in shaded, understory habitats support high frequencies of fungal endophytes.  Endophytes could benefit plant hosts by increasing shade tolerance. Alternatively, the endophytes could confer a greater cost under high shade as they acquire carbon from the plant.  We tested these alternative hypotheses in a greenhouse experiment that manipulated light availability for a suite of six grass species (Agrostis perennans, Elymus villosus, Festuca subverticillata, Lolium arundinaceum, Poa alsodes, and Poa autumnalis). The shade treatments consisted of 0, 30, 60, or 90% reduction of ambient light levels. Clones of naturally endophyte infected (E+) and experimentally disinfected (E-) plants were randomly assigned to each shade treatment and several plant and fungal responses were measured. We expected a significant interaction between the shade and endophyte treatments if symbiosis mediates plant response to shade. Results/Conclusions

The endophyte increased above- and below-ground biomass in Poa alsodes. In contrast, the endophyte decreased above-ground biomass in Agrostis perennans, suggesting a cost of symbiosis for this species under the conditions of our experiment.  Shade decreased above-ground biomass for Agrostis perennans, Lolium arundinaceum, P. alsodes, and P. autumnalis, and reduced belowground biomass for A. perennans, E. villosus. L. arundinaceum, and P. autumnalis.  The predicted endophyteXlight interaction was not detected for biomass, suggesting that endophytes do not strongly modulate plant responses to shade.  However, both specific leaf area and root:shoot ratio responded to the endophyteXlight interaction.  For Poa autumnalis, specific leaf area was higher for endophyte symbiotic plants, but only at the lowest shade. This result suggests that the endophyte alters this plastic plant response to shade.  For A. perennans, endophyte symbiosis increased the root:shoot ratio, but only under the highest shade. This result suggests that the endophyte can alter plant allocation under shade stress by increasing belowground storage. Our results suggest that endophytes neither increase shade tolerance nor increase the carbon costs to plant hosts under low light environments.  This work is among the first to investigate shade as a possible mechanism influencing endophyte costs and benefits to host plants.