Terpenoid biosynthesis and specialized vascular cells of conifer defense

Defense-related terpenoid biosynthesis in conifers is a dynamic process closely associated with specialized anatomical structures that allows conifers to cope with attack from many potential pests and pathogens. The constitutive and inducible terpenoid defense of conifers involves several hundred different monoterpenes, sesquiterpenes and diterpenes. Changing arrays of these many compounds are formed from the general isoprenoid pathway by activities of large gene families for two classes of enzymes, the terpene synthases and the cytochrome P450-dependent monooxygenases of the CYP720B group. Extensive studies have been conducted on the genomics, proteomics and molecular biochemical characterization of these enzymes. Many of the conifer terpene synthases are multi-product enzymes, and the P450 enzymes of the CYP720B group are promiscuous in catalyzing multiple oxidations, along homologous series of diterpenoids, from a broad spectrum of substrates. The terpene synthases and CYP720B genes respond to authentic or simulated insect attack with increased transcript levels, protein abundance and enzyme activity. The constitutive and induced oleoresin terpenoids for conifer defense accumulate in preformed cortical resin ducts and in xylem trauma-associated resin ducts. Formation of these resin ducts de novo in the cambium zone and developing xylem, following insect attack or treatment of trees with methyl jasmonate, is a unique feature of the induced defense of long-lived conifer trees.