The role of cytochrome P450 monooxygenases in plant-insect interactions.

Plants and herbivorous insects interact at the most fundamental of levels. With few exceptions, in the battle waged daily between these two groups of organisms, one serves as food source, the other as consumer. From a plant’s perspective, success in this interaction is measured by its ability to defend itself from devastation by insect feeding. From an insect’s perspective, success is measured by its ability to protect itself from a variety of toxic plant defense compounds, thereby allowing it to utilize plants as its sole food source. These interactions are multifaceted and dynamic. Many classes of insect repellents and toxic substances, including isoflavonoids, furanocoumarins, terpenoids, alkaloids, and cyanogenic glycosides, are synthesized in plants. The biosynthetic pathways leading to these allelochemicals are continually evolving to generate new secondary metabolites. A variety of defense mechanisms, including enzymatic detoxification systems, physiological tolerance, and behavioral avoidance, protect insects from the hazards of these compounds. These mechanisms continue to evolve as insects attempt to colonize new plant species. P450s have a fundamental role on both sides of the plant-versus-insect equation. They define the spectrum of the defense compounds synthesized by plants and the plant toxins metabolized by insects. Although many other insect enzyme systems participate in the synthesis and degradation of plant allelochemicals, P450s occupy a position of unique importance in the evolution of interspecies adaptive strategies due to their extraordinary versatility. The multiplicity and diversity of their substrate recognition sites, as well as their transcriptional regulatory cascades, have allowed for tremendous biochemical flexibility in the metabolic profiles of individual organisms. This flexibility has in turn provided the necessary genetic variability required for reciproca1 evolutionary responses between these trophically linked taxa. Although their common name implies similarity in function, P450s actually include a highly diverse array of pro-