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Abstract

Plants collectively produce hundreds of thousands of specialized metabolites
that are not required for growth or development. Each species has a qualitatively
unique profile, with variation among individuals, growth stages,
and tissues. By the 1950s, entomologists began to recognize the supreme
importance of these metabolites in shaping insect herbivore communities.
Plant defense theories arose to address observed patterns of variation, but
provided few testable hypotheses because they did not distinguish clearly
among proximate and ultimate causes. Molecular plant-insect interaction
research has since revealed the sophistication of plant metabolic, developmental,
and signaling networks. This understanding at the molecular level,
rather than theoretical predictions, has driven the development of new hypotheses
and tools and pushed the field forward. We reflect on the utility
of the functional perspective provided by the optimal defense theory, and
propose a conceptual model of plant defense as a series of layers each at a
different level of analysis, illustrated by advances in the molecular ecology
of plant-insect interactions.