Background/Question/Methods   By altering leaf physiology and chemistry, exposure to elevated levels of atmospheric carbon dioxide may fundamentally change the trophic interactions between plants and the insects and pathogens that attack them. Using free air concentration enrichment (FACE) technology, large plots of soybean were exposed to ambient and elevated carbon dioxide expected by 2050 (550 ppm), under otherwise realistic field conditions where pests and pathogens had unfettered access to the crop.

Results/Conclusions   In years with large infestations of insects, damage by chewing (Japanese beetle) and sucking insects (aphid) were greater in elevated than ambient carbon dioxide plots. The incidence of pathogen attack was greatest in warm, wet years, but, in contrast to insects, damage by viral, bacterial and fungal pathogens was reduced in elevated carbon dioxide plots. Transcriptional and biochemical analyses revealed that exposure to elevated carbon dioxide reduced the expression genes leading to the production of jasmonic acid (JA), a primary defense hormone, leaving soybean with reduced defenses against insects. A corresponding increase in salicylic acid (SA), a phytohormone that accumulates after pathogen attack, may have increased defenses against pathogens. These phytohormones often act antagonistically and are critical in mounting a proper defensive response to the array of challenges encountered in the field. Alterations in the relative production of JA and SA under elevated carbon dioxide may directly affect the trophic dynamics of plants in the field. By altering key aspects of primary and secondary metabolism, projected increases in atmospheric carbon dioxide will reset the co-evolutionary relationship between plants and organisms that attack them.