Plant chemical defense versus insect counter-defense

Plants synthesize a broad range of secondary metabolites (allelochemicals) to defend themselves against herbivores, pathogens and their competing neighbors (allelopathy). Enemy attack on plants activate a series of signal transduction processes, which in turn induce an array of defense genes to produce toxic compounds that can mount plant resistance against subsequent attack. RNA interference (RNAi) was used to characterize functions of specific allelochemicals in rice defense against insects and pathogens. Roles of jasmonate and salicylate signaling pathways in regulating rice chemical defense were determined. We also investigated insect responses to plant defensive chemicals. Polyphagous herbivores encounter numerous allelochemicals in their many host plants. Certain plant allelochemicals (e.g. coumarin and flavone) reduced toxicity of co-occurring compounds and insecticides to insects by inducing detoxification systems, including cytochrome P450 monooxygenases (P450s), which can metabolize a broad range of substances. Ecological significances of xenobiotic resistance induced by plant allelochemicals will be discussed. These results suggest that plant allelochemicals play a vital role in plant defense, but they also serve as antidotes of herbivorous insects. Appropriate manipulation of plant chemical defense and insect counter-defense is a promising approach to reduce human dependency on syntactic chemicals.