Anthropogenic increase in carbon dioxide compromises plant defense against invasive insects

Elevated levels of atmospheric carbon dioxide (CO₂), a consequence of anthropogenic global change, can profoundly affect the interactions between crop plants and insect pests and may promote yet another form of global change: the rapid establishment of invasive species. Elevated CO₂ increased the susceptibility of soybean plants grown under field conditions to the invasive Japanese beetle (Popillia japonica) and to a variant of western corn rootworm (Diabrotica virgifera virgifera) resistant to crop rotation by down-regulating gene expression related to defense signaling [lipoxygenase 7 (lox7), lipoxygenase 8 (lox8), and 1-aminocyclopropane-1- carboxylate synthase (acc-s)]. The down-regulation of these genes, in turn, reduced the production of cysteine proteinase inhibitors (CystPIs), which are specific deterrents to coleopteran herbivores. Beetle herbivory increased CystPI activity to a greater degree in plants grown under ambient than under elevated CO₂. Gut cysteine proteinase activity was higher in beetles consuming foliage of soybeans grown under elevated CO₂ than in beetles consuming soybeans grown in ambient CO₂, consistent with enhanced growth and development of these beetles on plants grown in elevated CO ₂. These findings suggest that predicted increases in soybean productivity under projected elevated CO₂ levels may be reduced by increased susceptibility to invasive crop pests.