Chemical Ecology & Phytochemistry-2006

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2 INTRODUCTION SCHMIDT, et al. Although our understanding of plant defense mechanisms has grown rapidly in recent years, most of the new knowledge has been obtained through studies on herbaceous species, especially the model plants Arabidopsis thaliana, Medicago truncatula, tomato, potato, maize, and rice. 1 Much less is known about the types of defenses employed by woody plants. Consequently, it is not clear if the deployment of chemical defense in woody taxa is fundamentally the same as that in herbs. Woody plants usually have a much greater size and longer lifetime than herbaceous plants, as well as a different life history, 2 and thus may be subject to different patterns of herbivore and pathogen pressure. In addition, woody plants have unique tissues, such as those resulting from secondary growth of the stem, and so may require different modes of protection. To gain a complete picture of defense in the plant kingdom, it is essential to know more about the defenses of a variety of woody plants, both angiosperms and gymnosperms. Conifers are a distinctive and widespread group of woody gymnosperms whose 500-600 species include some of the largest and longest lived representatives of the plant kingdom. 3,4 They are significant climax species, dominating most of the major forest ecosystems of Europe, Asia, and North America. Of the 8-9 recognized families, the largest and geographically most widespread is the Pinaceae which includes Pinus, Abies, Larix, Pseudotsuga, and Picea. As a model species for studying conifer defense, we chose Picea abies (Norway spruce), the most abundant and economically-important conifer species in northern and central Europe. 5 In addition, much is already known about the herbivore and pest problems of P. abies, 6-11 which will be valuable in studying its defense mechanisms. In this review, we examine the induced chemical defenses of P. abies, defenses whose levels increase following herbivore or pathogen attack. Induced defenses have attracted much attention in recent years because of their widespread occurrence in plants and their usefulness as subjects for study. 12 Here, we cover the induction of several different classes of induced defenses in P. abies, including terpene-containing resins, phenolic compounds, and chitinases. Our focus is not only on their defensive roles, but also on how the levels of these compounds may be manipulated by biochemical and molecular methods while minimizing other phenotypic changes. Manipulation of defense compounds in intact plants is a valuable approach to assessing their value to the plant. TERPENES The best studied chemical defense of P. abies and other conifers is the oleoresin found in foliage, stems, and other organs, a defense system that has existed
INDUCED CHEMICAL DEFENSES IN CONIFERS 3 for at least 50 million years. 13 Oleoresin is composed largely of terpenes, the largest class of plant secondary compounds. 14 Terpenes are formed by the fusion of C 5 isopentenoid units and classified by the number of such units present in their basic skeletons. Conifer resin is composed chiefly of monoterpenes (C 10 ) and diterpenes (C 20 ), with small amounts of sesquiterpenes (C 15 ) and other types of compounds. Oleoresin has long been believed to play a crucial role in conifer defense because of its physical properties (viscosity) and repellency to many herbivores and pathogens. In addition, oleoresin exudes under pressure from the tree following rupture of the ducts or blisters in which it is stored, often expelling or trapping invaders. 15-18 After rupture, the monoterpenes volatilize upon exposure to the air, while the diterpenes polymerize sealing the wound. However, it is still not clear to what extent the defensive properties of terpene resins are based on the repellency and toxicity of individual components or on the physical properties of the total resin (see chapter by Raffa this book). In P. abies, oleoresin is found constitutively, but may also be induced by herbivore or pathogen attack. 19,20 We are focusing on the induced resin because of the potential of altering its production to test its protective role. In preliminary studies, we tried wounding and fungal inoculation in an attempt to induce terpene formation in large trees, but this gave variable and inconsistent results (Martin, D., Krokene, P., Gershenzon, J., and Christiansen, E., unpublished data). Since wounding itself can cause the loss of resin, especially the volatile components, we explored the utility of a non-invasive procedure for resin induction involving the application of methyl jasmonate, an elicitor of plant defense responses in many species. 21,22 Methyl Jasmonate Application to Saplings in the Laboratory When methyl jasmonate was sprayed on the foliage of 1-2 year-old P. abies saplings from a uniform genetic background, this treatment triggered a dramatic increase in terpene levels. 23 There was a more than 10-fold increase in monoterpenes and a nearly 40-fold increase in diterpenes in wood tissue. In contrast, in the bark there was a much smaller increase in monoterpenes and no significant change in diterpene levels. Curiously, the response to methyl jasmonate took much longer than previously-observed inductions of plant defenses with this elicitor. Significant increases were not seen until 15 days after application. 23 Examination of the anatomy of the treated saplings revealed that methyl jasmonate had stimulated the formation of a ring of new resin ducts (traumatic resin ducts) in the newlyformed xylem (Fig. 1.1). Franceschi and co-workers had previously shown that wounding of P. abies or infection with Ceratocystis polonica, a blue-stain fungus vectored by the bark beetle Ips typographus, could induce the appearance of traumatic resin ducts over a 36 day period. 19 Apparently, this response also occurred with methyl jasmonate. 24 A change is triggered in the developmental program of the
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