The plant hormone salicylic acid interacts with the mechanism of anti-herbivory conferred by fungal endophytes in grasses

• The plant salicylic acid (SA) pathway is recognized as an effective defense against biotrophic pathogens, but its role as a regulator of beneficial plant symbionts has received little attention. We studied the relationship between the SA hormone and leaf fungal endophytes on herbivore defenses in symbiotic grasses. We hypothesized that the exogenous application of SA suppresses the endophyte, reducing the fungal-produced alkaloids. Because of the role that alkaloids play in anti-herbivore defenses, any reduction in their production should make host plants more susceptible to herbivores. • Lolium multiflorum plants, symbiotic and non-symbiotic with the endophyte Epichloë occultans , were exposed to the SA hormone followed by a challenge with the aphid Rhopalosiphum padi. We measured the level of plant resistance to aphids, in addition to the defences conferred by fungal endophytes and host plants. • Symbiotic plants had lower concentrations of the SA hormone than did the endophyte-free plants. Following the hormone exposure, all plants achieved very high physiological concentrations of SA. In line with our prediction, the hormonal treatment reduced the concentration of fungal alkaloids, and consequently decreased the endophyte-conferred resistance against aphids. • Our study highlights the importance of the interaction between the plant immune system and the presence of fungal endophytes for the stability and persistence of the defensive mutualism. Specifically, our results indicate that the hormone SA plays a critical role in regulating the endophyte-conferred resistance against sap-sucking insects.