A fungal endophyte mediates reversal of wound-induced resistance and constrains tolerance in a grass

Most plants respond to herbivore damage with elevated chemical and/or structural mechanisms of resistance. However, some plants, particularly those with few chemical defenses, may actually become more susceptible to herbivores following damage; that is, damage may induce susceptibility. We tested the hypothesis that a fungal symbiont of tall fescue grass provides induced resistance to aphids by manipulating fungal infection, plant damage (clipping), and level of nutrient fertilizer in greenhouse/laboratory experiments. The fungus had a negative effect on aphid reproduction that was most pronounced following damage. This result appears to be caused by elevated levels of alkaloids produced by the fungus after damage. Furthermore, plants lacking fungal infection were more susceptible to aphids following damage. Hence, while plant damage induces susceptibility in uninfected plants, it does the opposite (induced resistance) in infected plants. Additional experiments showed that the reversal of induced susceptibility is influenced by intrinsic (fungal strain) as well as extrinsic (nutrient application to plants) factors. We also found that the induced resistance conferred by the fungus comes at the cost of tolerance (reduced regrowth) in infected plants. As a modulator of both resistance and tolerance, the fungus plays a central role in influencing defense of host plants against herbivores and should have important implications for plant-herbivore interactions in natural and agricultural systems.