Short-term resistance that persists: Rapidly induced silicon anti-herbivore defence affects carbon-based plant defences

<p><b>1. </b>Silicon (Si) is known to alleviate diverse biotic and abiotic stresses including insect herbivory.<b> </b>Si accumulation in plants, notably the Poaceae, can be induced through stimulation of the jasmonic acid (JA) pathway (associated with chewing herbivores). Nevertheless, the temporal dynamics of Si accumulation as a defence response and its consequential effects on carbon-based defences (e.g. phenolics), particularly in the short-term, remain unclear.</p> <p><b>2. </b>The model grass <i>Brachypodium distachyon</i> was grown in a hydroponic solution where half the plants were supplemented with 2 mM potassium silicate and half had no Si supplied. Plants were treated with methyl jasmonate (MeJA) as a form of standardised simulated herbivory. We measured Si accumulation, the phytohormones JA and salicylic acid (SA), and carbon-based defences over 24 hours to determine the temporal dynamics of Si accumulation and the interplay between Si, simulated herbivory and plant defence machinery.</p> <p><b>3. </b>MeJA-induced Si accumulation occurred as early as 6 hours after treatment via increased JA concentrations. Si supplementation decreased SA concentrations, which could have implications on additional downstream defences. We show a trade-off between Si and phenolics in untreated plants, but this relationship was weakened upon MeJA treatment. Further, this trade-off did not apply to phenolic precursor compounds such as phenylalanine.</p> <p><span><b>4. </b>We provide evidence for rapidly induced Si accumulation associated with herbivory, and that increased Si accumulation impacts on phytohormones and carbon-based defences over a 24-hour period. Additionally, herbivory modifies the relationship between Si- and carbon-based defences. Thus, in addition to its well-documented role as a long-term defence against herbivores, we demonstrate that, over short-term temporal scales, Si accumulation responds to herbivore signals and impacts on plant defence machinery. </span></p>