1. Elevated atmospheric CO 2 (eCO 2 ) not only increases plant growth but can also interfere with defence against insect herbivory through the disruption of the jasmonic acid (JA) pathway. Silicon (Si) plays an important role in plant stress tolerance and resistance to herbivory, particularly in grasses, many of which accumulate high amounts of Si. Activation of the JA pathway has been reported to stimulate Si uptake, while Si supplementation can alter both constitutive and induced phytohormone levels. A reduction in JA concentration under eCO 2 has the potential to reduce Si uptake in plants. Using both Si supplemented (Si+) and control (Si−) plants (Brachypodium distachyon)grown under ambient (400 ppm) and elevated (640 ppm) CO 2 concentrations, we tested how plant growth, foliar Si concentration and endogenous JA responded to methyl jasmonate (MeJA) application and the subsequent effects on insect herbivore performance (Helicoverpa armigera). Elevated CO2 reduced Si concentration by 19% and endogenous JA by 70% on average. MeJA significantly increased Si concentration in Si+ plants. Si+ plants had higher baseline JA levels compared to Si− plants under control conditions (i.e. no stress), however, when plants were chemically induced with MeJA, the JA response was on average 84% lower in Si+ plants compared to Si− plants. Plants without MeJA treatment showed the opposite response, that is, Si+ plants had higher baseline JA levels compared to Si− plants. Si significantly reduced herbivore consumption and growth rate. Despite eCO 2 significantly reducing both Si and endogenous JA, no effect was seen on herbivores feeding on eCO 2 plants. 4. Collectively our results suggest that Si alters the JA response of plants. We show that JA induces Si uptake, however, Si then reduces the JA response of plants under induced stress conditions. However, predicted increases in CO 2 levels within this century may significantly reduce Si-based mechanical defences against herbivory via a reduction of endogenous JA. K E Y W O R D S elevated CO 2 , herbivory, jasmonic acid, plant defence, silica S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Hall CR, Mikhael M, Hartley SE, Johnson SN. Elevated atmospheric CO 2 suppresses jasmonate and silicon-based defences without affecting herbivores. Funct

1. Silicon (Si) is known to alleviate diverse biotic and abiotic stresses including insect herbivory. 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. 2. The model grass Brachypodium distachyon 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 hr to determine the temporal dynamics of Si accumulation and the interplay between Si, simulated herbivory and plant defence machinery. 3. MeJA-induced Si accumulation occurred as early as 6 hr 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 the phenolic precursor compound, phenylalanine. 4. We provide evidence for rapidly induced Si accumulation associated with herbivory, and that increased Si accumulation impacts on phytohormones and carbonbased defences over a 24-hr 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 shortterm temporal scales, Si accumulation responds to herbivore signals and impacts on plant defence machinery.

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