Abstract
Plants employ hormone-mediated signaling pathways to defend against pathogens and insects. We tested predictions about the relative effect of jasmonate and
salicylate pathways and how they mediate interactions between pathogens and herbivores. We
employed two pathogens of tomato, Pseudomonas syringae (Pst) and tobacco mosaic virus
(TMV), that are known to elicit distinct components of the two pathways, and we address the
consequences of their induction for resistance in wild-type and salicylate-deficient transgenic
plants in field experiments. We report that Pst infection induced jasmonic acid and proteinase
inhibitors (PIs), and reduced the growth of Spodoptera exigua caterpillars on wild-type and
salicylate-deficient plants. Pst and TMV both induced salicylic acid in wild-type but not
salicylate-deficient plants. Although TMV did not affect jasmonic acid or PIs, infection
increased caterpillar growth on wild-type plants, but not on salicylate-deficient plants. Aphid
population growth was higher on salicylate-deficient compared to wild-type plants, and lower
on salicylate-induced plants compared to controls. Natural aphid colonization was reduced on
TMV-infected wild types, but not on salicylate-deficient plants. In sum, jasmonate-mediated
resistance is induced by some pathogens, independent of salicylate, and salicylate-mediated
induction by other pathogens results in induced susceptibility to a chewer and resistance to an
aphid. We conclude with a predictive model for the expression of defense pathways and their consequences.
