The growth promotion in endophyte symbiotic plants does not penalise the resistance to herbivores and bacterial microbiota

<p dir="ltr">A trade-off between growth and defence against biotic stresses is common in plants. Fungal endophytes of the genus <i>Epichloë </i>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 <i>Epichloë </i>ability to decouple the growth-defence trade-off was evaluated by subjecting ryegrass with and without <i>Epichloë </i>endophytes to an exogenous treatment with gibberellin (GA) followed by a challenge with <i>Rhopalosiphum padi</i> 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 <i>Epichloë</i>-derived alkaloids. The <i>Epichloë </i>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 <i>Epichloë </i>was also evaluated. Only <i>Epichloë</i>, and not the GA treatment, altered the composition of the phyllosphere microbiota and the abundance of certain bacterial taxa. Our findings clearly demonstrate that <i>Epichloë</i> does indeed relieve the plant growth-defence trade-off.</p>