| Summary: | <p>Abstract</p> <p>Background</p> <p>The adage from Shakespeare, "troubles, not as single spies, but in battalions come," holds true for <it>Nicotiana attenuata</it>, which is commonly attacked by both pathogens (<it>Pseudomonas spp</it>.) and herbivores (<it>Manduca sexta</it>) in its native habitats. Defense responses targeted against the pathogens can directly or indirectly influence the responses against the herbivores. Na<it>defensin </it>is an effective induced defense gene against the bacterial pathogen <it>Pseudomonas syringae </it>pv <it>tomato </it>(PST DC3000), which is also elicited by attack from <it>M. sexta </it>larvae, but whether this defense protein influences <it>M. sexta's </it>growth and whether <it>M. sexta</it>-induced Na<it>defensin </it>directly or indirectly influences PST DC3000 resistance are unknown.</p> <p>Results</p> <p><it>M. sexta </it>larvae consumed less on WT and on Na<it>defensin</it>-silenced <it>N. attenuata </it>plants that had previously been infected with PST DC3000 than on uninfected plants. WT plants infected with PST DC3000 showed enhanced resistance to PST DC3000 and decreased leaf consumption by <it>M. sexta </it>larvae, but larval mass gain was unaffected. PST DC3000-infected Na<it>defensin</it>-silenced plants were less resistant to subsequent PST DC3000 challenge, and on these plants, <it>M. sexta </it>larvae consumed less and gained less mass. WT and Na<it>defensin</it>-silenced plants previously damaged by <it>M. sexta </it>larvae were better able to resist subsequent PST DC3000 challenges than were undamaged plants.</p> <p>Conclusion</p> <p>These results demonstrate that Na-defensin directly mediates defense against PST DC3000 and indirectly against <it>M. sexta </it>in <it>N. attenuata</it>. In plants that were previously infected with PST DC3000, the altered leaf chemistry in PST DC3000-resistant WT plants and PST DC3000-susceptible Na<it>defensin</it>-silenced plants differentially reduced <it>M. sexta's </it>leaf consumption and mass gain. In plants that were previously damaged by <it>M. sexta</it>, the combined effect of the altered host plant chemistry and a broad spectrum of anti-herbivore induced metabolomic responses was more effective than Na<it>defensin </it>alone in resisting PST DC3000.</p>
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