Herbivory and caterpillar regurgitants amplify the wound-induced increases in jasmonic acid but not nicotine in Nicotiana sylvestris

Both herbivory and mechanical damage result in increases in the concentration of the wound-signal molecule, jasmonic acid (JA), and the defense metabolite, nicotine, in native tobacco plants, Nicotiana sylvestris Speg. et Comes (Solanaceae). We found that higher concentrations of JA resulted from herbivory by Manduca sexta (L.) larvae than from the mechanical damage designed to mimic the herbivory. While both herbivory and mechanical damage increased JA concentrations in roots of wounded plants, herbivory did not induce either higher root JA or nicotine responses than mechanical damage. In a separate experiment in which mechanical damage was not designed to mimic herbivory, JA responses to herbivory were higher than those to mechanical damage, but the whole-plant (WP) nicotine responses were smaller. Furthermore, when regurgitants from M. sexta larvae were applied to standardized mechanical leaf wounds, leaf JA responses were dramatically amplified. However, neither the root JA response nor the WP nicotine response was comparably amplified by application of regurgitants. Our findings demonstrate that the response of N. sylvestris to herbivory is different from its response to mechanical damage; moreover, oral secretions from larvae may be partly responsible for the difference. During feeding, M. sexta larvae appear to modify the plant's normal defensive response to leaf wounding by reducing the systemic increase in root JA after leaf damage and the subsequent WP nicotine response.