Resin-based defenses in conifers.
Bark beetle infestation and associated fungal infection are a serious disease problem in conifer species. Conifers have evolved elaborate, constitutive and inducible, terpene-based defense mechanisms to deter insect pests and their symbiotic fungal pathogens. This process involves the secretion of oleoresin, a complex mixture of monoterpenes, sesquiterpenes and diterpenoid acids. Induced oleoresinosis in grand fir (Abies grandis) provides a model system for studying the regulation of defensive terpene biosynthesis and for identifying relevant genes. The ecological relationships between conifers, beetle pests, beetle predators and fungal pathogens present several possible avenues for manipulating oleoresin composition to improve tree resistance. Possible examples include chemically disguising the host, adding toxins and altering the levels of pheromone precursors, attractants for predators or hormone mimics to disrupt insect development. Strategies and prospects for generating transgenic conifers with increased defense capability are discussed.