Elevated atmospheric CO₂ and silicon antagonistically regulate anti-herbivore phytohormone and defence gene expression levels in wheat

Silicon (Si) accumulation by grasses is a key mechanism for alleviating biotic and abiotic stresses, including insect herbivory. In addition to conferring physical resistance, tissue silicification may enhance anti-herbivore phytohormone production, such as the jasmonic and salicylic (JA and SA) acid pathways, and downstream regulation of defence genes, although this is poorly understood. Elevated atmospheric carbon dioxide (eCO₂) concentrations predicted by climate models are reported to reduce Si accumulation in several plant taxa and may therefore compromise Si-augmented resistance. We investigated how Si enrichment and eCO₂ regulate the JA and SA pathways and expression of defence genes in wheat (Triticum aestivum) challenged by a global insect pest (Helicoverpa armigera). Si treatments increased JA production and expression of β-1,3-ENDOGLUCANASE (GNS), and MITOGEN-ACTIVATED PROTEIN KINASE (MAPK; WCK-1) defence genes, while suppressing SA production, resulting in reduced feeding and growth of H. armigera. In contrast, under eCO₂ conditions, Si accumulation was reduced, GNS downregulated, but SA production was upregulated. Despite compromised plant defences, H. armigera growth rates were reduced under eCO₂. We conclude that eCO₂ and Si supplementation contrastingly regulate anti-herbivore defences in wheat; these important drivers operate independently and may influence future patterns of pest resistance in wheat under projected rises in atmospheric CO₂.