Plant defense: signals in insect eggs

Abstract

Like many adaptations, plant defenses operate with a high level of economy. Given the variable nature of herbivory, and the potential ecological and energetic costs of defense, many plants employ defenses that are induced following initial attack. In addition, some defenses are more effective when inducible. Three recent papers1xBruchins (insect-derived plant regulators that stimulate neoplasm formation) . Doss, R.P. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6218–6223Crossref | PubMed | Scopus (127)See all References, 2xInduction of plant synomones by oviposition of a phytophagous insect. Meiners, T. and Hilker, M. J. Chem. Ecol. 2000; 26: 221–232Crossref | Scopus (128)See all References, 3xSpecificity of chemical cues used by a specialist egg parasitoid during host location. Meiners, T. et al. Entomol. Exp. Appl. 2000; 95: 2000CrossrefSee all References describe an extreme level of precision in a plant’s response to the least damaging (yet with the most potential to damage) stage of an herbivore: its eggs. A useful distinction in the type of plant defense, be it induced or constitutively expressed, is that some act directly on herbivores and others act indirectly by involving a third party – usually predators or parasitoids. Although both can be effective, direct defenses are not generally lethal and indirect defenses might act late or reduce plant damage only minimally. These observations have led to the questions: (1) how strong a selective impact do herbivores really have on plants?; and (2) has coevolution resulted in moderately strong defenses and counterdefenses, with nowhere left to go? These articles describe wonderful and detailed natural history that challenges these questions, by describing systems in which plants actively respond to oviposition by an herbivore by killing the eggs directly or by attracting natural enemies that eat them before they hatch. In both of these cases, a constitutive strategy would probably not be an effective defense.Doss et al.1xBruchins (insect-derived plant regulators that stimulate neoplasm formation) . Doss, R.P. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 6218–6223Crossref | PubMed | Scopus (127)See all References1 show, using field trials, that it is probable that a single gene in peas is responsible for the growth of undifferentiated cells (neoplasm) upon oviposition by the pea weevil. The small, tumor-like growth impedes the entrance of neonate larvae (direct defense). The authors characterize a new class of compounds from weevils (called Bruchins) that elicit the neoplasm formation on the pods, but not on stems or leaves of peas and several other legumes. Meiners et al.3xSpecificity of chemical cues used by a specialist egg parasitoid during host location. Meiners, T. et al. Entomol. Exp. Appl. 2000; 95: 2000CrossrefSee all References3 show that female elm leaf beetles cause elms to emit volatiles upon oviposition. These induced volatiles attract parasitoids of the beetle eggs in the laboratory (indirect defense). Amazingly, the volatiles are mediated by the highly conserved jasmonic acid pathway, they are systemically released by the plant, and they are specific to the species of elm and ovipositing beetle. These studies are reminiscent of previous reports of plant hypersensitivity in response to herbivore oviposition that resulted in egg desiccation or drop from the plant. However, the mechanistic detail in both studies reduces the possibility that the induction was a result of bacterial contamination.Induced defense against the eggs of herbivores provides a novel level of complexity in plant–herbivore interactions, hinting towards how strong herbivore impacts can be, and opens the door for ongoing coevolution. There must be strong selection for herbivores to avoid producing signals in eggs that the plants perceive – the authors suggest that perhaps these signals are intimately involved in primary insect functions. Such interactions between not-so-passive plants and the apparently passive stages of herbivores are probably common in nature, but herbivore eggs are small and thus often out of sight.