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A trade‐off between growth and defence against biotic stresses is common in plants. Fungal endophytes of the genus Epichloë may relieve this trade‐off in their host grasses since they can simultaneously induce plant growth and produce antiherbivore alkaloids that circumvent the need for host defence. The Epichloë ability to decouple the growth‐defence trade‐off was evaluated by subjecting ryegrass with and without Epichloë endophytes to an exogenous treatment with gibberellin (GA) followed by a challenge with Rhopalosiphum padi aphids. In agreement with the endophyte‐mediated trade‐off decoupling hypothesis, the GA‐derived promotion of plant growth increased the susceptibility to aphids in endophyte‐free plants but did not affect the insect resistance in endophyte‐symbiotic plants. In line with the unaltered insect resistance, the GA treatment did not reduce the concentration of Epichloë‐derived alkaloids. The Epichloë mycelial biomass was transiently increased by the GA treatment but at the expense of hyphal integrity. The response of the phyllosphere bacterial microbiota to both GA treatment and Epichloë was also evaluated. Only Epichloë, and not the GA treatment, altered the composition of the phyllosphere microbiota and the abundance of certain bacterial taxa. Our findings clearly demonstrate that Epichloë does indeed relieve the plant growth‐defence trade‐off. Summary statement: A debate has been raised if fungal endophytes can relieve the plant growth‐defence trade‐off. Epichloë endophytes decoupled the trade‐off: the growth‐mediated penalisation of plant defences was observed in E− but not in E+ plants. The Epichloë production of alkaloids was likely the decoupling mechanism. [ABSTRACT FROM AUTHOR]

Abstract: The plant hormone salicylic acid (SA) is recognized as an effective defence against biotrophic pathogens, but its role as regulator of beneficial plant symbionts has received little attention. We studied the relationship between the SA hormone and leaf fungal endophytes on herbivore defences in symbiotic grasses. We hypothesize that the SA exposure suppresses the endophyte reducing the fungal‐produced alkaloids. Because of the role that alkaloids play in anti‐herbivore defences, any reduction in their production should make host plants more susceptible to herbivores. Lolium multiflorum plants symbiotic and nonsymbiotic with the endophyte Epichloë occultans were exposed to SA followed by a challenge with the aphid Rhopalosiphum padi. We measured the level of plant resistance to aphids, and the defences conferred by endophytes and host plants. Symbiotic plants had lower concentrations of SA than did the nonsymbiotic counterparts. Consistent with our prediction, the hormonal treatment reduced the concentration of loline alkaloids (i.e., N‐formyllolines and N‐acetylnorlolines) and consequently decreased the endophyte‐conferred resistance against aphids. Our study highlights the importance of the interaction between the plant immune system and endophytes for the stability of the defensive mutualism. Our results indicate that the SA plays a critical role in regulating the endophyte‐conferred resistance against herbivores. [ABSTRACT FROM AUTHOR]