Induced plant defense responses against chewing insects. Ethylene signaling reduces resistance of Arabidopsis against Egyptian cotton worm but not diamondback moth

The induction of plant defenses by insect feeding is regulated via multiple signaling cascades. One of them, ethylene signaling, increases susceptibil ity of Arabidopsis to the generalist herbivore Egyptian cotton worm (Spodop tera littoralis; Lepidoptera: Noctuidae). The hookless1 mutation, which aff ects a downstream component of ethylene signaling, conferred resistance to Egyptian cotton worm as compared with wild-type plants. Likewise, ein2, a m utant in a central component of the ethylene signaling pathway, caused enha nced resistance to Egyptian cotton worm that was similar in magnitude to ho okless1. Moreover, pretreatment of plants with ethephon (2-chloroethanephos phonic acid), a chemical that releases ethylene, elevated plant susceptibil ity to Egyptian cotton worm. By contrast, these mutations in the ethylene-s ignaling pathway had no detectable effects on diamondback moth (Plutella xy lostella) feeding. It is surprising that this is not due to nonactivation o f defense signaling, because diamondback moth does induce genes that relate to wound-response pathways. Of these wound-related genes, jasmonic acid re gulates a novel beta -glucosidase 1 (BGL1), whereas ethylene controls a put ative calcium-binding elongation factor hand protein. These results suggest that a specialist insect herbivore triggers general wound-response pathway s in Arabidopsis but, unlike a generalist herbivore, does not react to ethy lene-mediated physiological changes.