Guild-specific herbivory alters physiology and the induction of plant defenses in Nicotiana attenuata

Insect herbivory elicits guild-specific responses in gene expression in tobacco depending upon the insect and its constituent feeding guild (e.g., defoliators vs. phloem or xylem feeders), yet the known physiological responses of these plants to herbivory are not well investigated. In this study we tested guild-specific affects on photosynthesis and the induction of plant defense by examining the responses of N. attenuata to phloem and leaf mass consumption. If plants respond to insect herbivory depending upon the feeding mechanism, then the induction of plant defenses should be correlated to the type of physiological response and the mechanism of injury. We characterized photosynthesis using an open-system infrared gas analyzer and a novel chlorophyll fluorescence imaging system. Defoliation by a native specialist lepidopteran, Manduca sexta, and a generalist lepidopteran, Helicoverpa zea, did not alter photosynthesis; there were no short or long term decreases in carbon assimilation or photosystem efficiency. However, phloem feeding by the tobacco-adapted aphid, Myzus persicae, decreased assimilation and reduced quantum yield revealing an insect-induced spatial pattern in chlorophyll fluorescence. Physical damage has been shown to induce the jasmonic acid (JA) signaling cascade but did not alter assimilation in this study. Artificial stimulation of the defensive pathway using methyl jasmonate, a JA precursor, reduced assimilation over time. Phloem consumption did not induce the signaling cascade indicating context dependent physiological induction of the JA signaling cascade. This connection between elicited physiological responses and induced plant defenses may help explain the differences seen in guild-specific gene expression.