Anti-herbivore silicon defences in a model grass are greatest under Miocene levels of atmospheric CO 2 .

Silicon (Si) has an important role in mitigating diverse biotic and abiotic stresses in plants, mainly via the silicification of plant tissues. Environmental changes such as atmospheric CO ₂ concentrations may affect grass Si concentrations which, in turn, can alter herbivore performance. We recently demonstrated that pre-industrial atmospheric CO ₂ increased Si accumulation in Brachypodium distachyon grass, yet the patterns of Si deposition in leaves and whether this affects insect herbivore performance remains unknown. Moreover, it is unclear whether CO ₂ -driven changes in Si accumulation are linked to changes in gas exchange (e.g. transpiration rates). We therefore investigated how pre-industrial (reduced; rCO ₂ , 200 ppm), ambient (aCO ₂ , 410 ppm) and elevated (eCO ₂ , 640 ppm) CO ₂ concentrations, in combination with Si-treatment (Si+ or Si-), affected Si accumulation in B. distachyon and its subsequent effect on the performance of the global insect pest, Helicoverpa armigera. rCO ₂ increased Si concentrations by 29% and 36% compared to aCO ₂ and eCO ₂ respectively. These changes were not related to observed changes in gas exchange under different CO ₂ regimes, however. The increased Si accumulation under rCO ₂ decreased herbivore relative growth rate (RGR) by 120% relative to eCO _2, whereas rCO ₂ caused herbivore RGR to decrease by 26% compared to eCO ₂ . Si supplementation also increased the density of macrohairs, silica and prickle cells, which was associated with reduced herbivore performance. There was a negative correlation among macrohair density, silica cell density, prickle cell density and herbivore RGR under rCO ₂ suggesting that these changes in leaf surface morphology were linked to reduced performance under this CO ₂ regime. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO ₂ reduces insect herbivore performance. Contrastingly, we found reduced Si accumulation under higher CO ₂ , which suggests that some grasses may become more susceptible to insect herbivores under projected climate change scenarios.
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