The eco-physiological complexity of plant responses to insect herbivores

Plants must fend off a myriad of aggressive biotic agents and this biotic onslaught occurs across the complete spectrum of spatial scales from pathogens that attack single cells (Somssich and Hahlbrock 1998) to herbivores that select their hosts by characteristics of the community in which a plant grows (Hay 1986). The recent advances in understanding plant defense responses that function on the cellular level against pathogens have been awe-inspiring, particularly with regard to the mechanisms of pathogen recognition (Baker et al. 1997). In contrast, our understanding of how plants recognize attack from herbivores is in its infancy; the first herbivore-specific elicitor (volicitin) was only just recently characterized (Alborn et al. 1997). While the mechanisms of many plant-herbivore interactions are not well understood, the ecological complexity of plant responses against herbivores is well recognized, as the defense response elicited by volicitin underscores. Volicitin [N-(17-hydroxylinolenoyl)-L-glutamine], isolated from the oral secretions of beet armyworm (Spodoptera Exigua) larvae, increases the emission of volatile organic compounds when applied to corn (Alborn et al. 1997). How these volatile emissions function as a defense highlights a key difference between plant defenses against pathogens and herbivores: the physiological autonomy of herbivores, which allow plants to utilize components of their ecological community for defense.