Interactions between silicon and alkaloid defences in endophyte‐infected grasses and the consequences for a folivore

Grasses have developed a wide range of morphological and physiological mechanisms to resist herbivory. For instance, they accumulate silicon (Si) in tissue, as physical defence, associate symbiotically with foliar Epichloë-endophytes that provide chemical defence via antiherbivore alkaloids. Recent evidence showed some increase Si forage grasses; however, whether this impacts insect herbivores is unknown. Furthermore, while primarily it also affects production plant defensive secondary metabolites; supply might therefore affect Epichloë-alkaloids, although remains untested. We grew endophyte-free (Nil) Epichloë-infected tall fescue perennial ryegrass factorial combination or without supplementation, the absence presence Helicoverpa armigera. Epichloë-endophyte strains were AR584 for fescue, AR37, AR1 Wild-type (WT) ryegrass. assessed how interaction herbivory affected mutualist-derived alkaloid concentrations. Subsequently, their effects on H. armigera relative growth rates (RGRs) evaluated. Endophytes generally increased concentrations Si-supplied plants. In endophytes constitutive (herbivore-free) induced (herbivore-inoculated) by at least 25%; contrast, ryegrass, endophyte only levels. did not alkaloids produced AR1/WT endophytes; herbivory, decreased induction AR37 33%. reduced RGR 76%, regardless endophytic status, whereas, endophyte-alkaloids played role reducing herbivore supply. Conversely, both (regardless supply) synergised. Novel interactions between Si- alkaloid-based defences grasses observed. Overall, had greater effect folivore than grasses. Endophyte contributed more resistance fescue. demonstrate antagonistic highlight protective nature varies grass-endophyte species tested. A free Plain Language Summary can be found within Supporting Information article.