Update on Recognition of Herbivory Recognition of Herbivory-Associated Molecular Patterns

During their long, approximately 350 million-year period of coexistence, plants, insects, and other arthropods evolved a variety of different interactions (Gatehouse, 2002). Some interactions can be beneficial for the plant, as in the case of insect-mediated pollination or seed dispersion, and others are deleterious, as in the case of attack by herbivorous insects (Fig. 1). To successfully combat aggressors, plants must be equipped with a sophisticated sensory system to perceive signals fast and efficiently from their environment and thereby detect potential enemies and subsequently translate and integrate such signals into appropriate biochemical and physiological responses. Thus, upon attack, a number of reactions are detectable in plant cells, including changes in ion flux and protein phosphorylation, formation of reactive oxygen species and oxylipins, as well as initiation of various defense reactions in the host plant (Kessler and Baldwin, 2002; Maffei et al., 2007b). Intriguing questions arising from these observations are how plants recognize the particular herbivores, what kinds of signals are involved, how such signals are perceived, and how they are converted into downstream signaling pathways involved in plant defense activation. Signal perception in the plant cell may rely on the presence of specific receptors for chemical signals or on general recognition processes based on localized tissue injuries. In principle, the feeding process combines two sites of the same coin: mechanical wounding of the infested tissue and introduction of oral secretions that are delivered from the feeding organism into the wounded tissue (i.e. the attacked plant is challenged by both a mechanical as well as a chemical stimulus). This Update introduces herbivore-derived metabolites, which represent serious candidates for signaling compounds; we will also discuss advances in herbivore recognition, namely, the perception of insect-derived signals by specific binding proteins. The properties of these binding proteins suggest their involvement in signal perception.