Insect-Induced Terpenoid Defenses in Spruce

Abstract

Conifers produce an array of hundreds of different terpenoids as part of their complex chemical, physical, and ecological defenses against insect pests and pathogens. Terpenoid chemicals exist both as constitutive and as massively induced defenses in conifers. It is thought that the diversity of terpenoid chemicals serves as a multilayered chemical shield in long-lived conifer trees that provides a lasting protection against the much faster evolving insect pests and potential pathogens. The formation of terpenoid defenses in conifers involves the activity of two pathways, the methylerythritol phosphate pathway and the mevalonate pathway, which lead to the five-carbon precursors of terpenoid biosynthesis. The many monoterpenoids, sesquiterpenoids, and diterpene resin acids, which are present in oleoresin mixtures, are then formed by families of enzymes belonging to the classes of prenyl transferases, terpenoid synthases, and cytochrome P450 dependent monooxygenases. The genes for almost all the enzymatic steps in terpenoid oleoresin biosynthesis have been identified in species of spruce, which have thus been established as a conifer reference system to study constitutive and induced terpenoid defenses using biochemical, molecular genetic, and genomic and proteomic approaches. At the histological and cellular levels, oleoresin terpenoids are produced and accumulate constitutively in large quantities in specialized anatomical structures that are found in most organs and tissues. In many conifer species, biosynthesis and accumulation of terpenoids is further enhanced as part of the induced defense in response to insect attack or fungal infection. In this chapter, I will discuss selected aspects of terpenoid defenses in conifers against insects and pathogens.