Anthropogenic increase in carbon dioxide compromises plant defense against invasive insects

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Abstract
Elevated levels of atmospheric carbon dioxide (CO 2 ), 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 CO 2 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 CO 2 . Gut cysteine proteinase activity was higher in beetles consuming foliage of soybeans grown under elevated CO 2 than in beetles consuming soybeans grown in ambient CO 2 , 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 CO 2 levels may be reduced by increased susceptibility to invasive crop pests.