Anthropogenic increase in carbon dioxide modifies plant-insect interactions

Industrialisation has elevated atmospheric levels of CO2 from original 280 ppm to current levels at 400 ppm, which is estimated to double by 2050. Although high atmospheric CO2 levels affect insect interactions with host plants, the impact of global change on plant defences in response to insect attack is not completely understood. Recent studies have made advances in elucidating the mechanisms of the effects of high CO2 levels in plant-insect interactions. New studies have proposed that gene regulation and phytohormones regulate resource allocation from photosynthesis to plant defences against insects. Biochemical and molecular studies demonstrated that both defensive hormones jasmonic acid (JA) and ethylene (ET) participate in modulating chemical defences against herbivores in plants grown under elevated CO2 atmosphere rather than changes in C:N ratio. High atmospheric CO2 levels increase vulnerability to insect damage by down-regulating both inducive and constitutive chemical defences regulated by JA and ET. However, elevated CO2 levels increase the JA antagonistic hormone salicylic acid that increases other chemical defences. How plants grown under elevated CO2 environment allocate primary metabolites from photosynthesis to secondary metabolism would help to understand innate defences and prevent future herbivory in field crops. We present evidence demonstrating that changes in chemical defences in plants grown under elevated CO2 environment are hormonal regulated and reject the C:N hypothesis. In addition, we discuss current knowledge of the mechanisms that regulate plants defences against insects in elevated CO2 atmospheres.