Herbivore and Pathogen Induced Defense Strategies in Conifers
Although our understanding of plant defense mechanisms has grown rapidly in recent years, most of the new knowledge has been obtained from studies on herbaceous species, especially the model plants Arabidopsis thaliana, Medicago truncatula, tomato, potato, maize and rice. 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 herbaceous plants. Woody plants are usually larger and live longer than herbaceous plants, have a different life history as well, 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.
In order 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. They are a 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 have chosen Picea abies (Norway spruce), the most abundant and economically-important conifer species in northern and central Europe. Much is already known about the herbivore and pest problems of P. abies, and this information will be valuable in studying its defense mechanisms. In our group, we are examining 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. We study the induction of several different classes of induced defenses in P. abies, including terpene-containing resins and phenolic compounds. 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 examining their ecological role in the plant.
Characterization of isoprenyl diphosphate synthases as branch point of terpenoid biosynthesis
The conifer Picea abies (Norway spruce) employs terpenoid-based oleoresins as part of its constitutive and induced defense responses against herbivores and pathogens. Attack by herbivores, such as bark beetles and their associated fungi Ceratocystis polonica, often induces the production of additional newly composed resin in newly formed traumatic resin ducts (TRD). Conifer resins are complex mixtures of monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20) whose composition varies among and within species and with age and development of the tree. In order to understand how metabolic flux is channeled among the different terpene types, we are investigating the short-chain isoprenyl diphosphate synthases (IDS) that catalyze the condensation of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) to form geranyl diphosphate (GPP), farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP). These branch point reactions control the flow of metabolites which act as precursors to each of the major terpene classes in the trunk resin of Picea abies (Norway spruce).
Molecular characterization of pine response to insect egg deposition
Conifers are known to produce high amounts of terpenoids, which are both stored within the plant (e.g. in resins) and also released as volatiles. These volatile compounds often mediate direct and indirect defense mechanisms in plants. Scots pine (Pinus sylvestris, Pinaceae) is known to change its terpenoid volatile blend after egg deposition by the herbivorous sawfly (Diprion pini, Diprionidae), which results in the attraction of a specialist egg parasitoid (Closterocerus ruforum, Eulophidae). In cooperation with Prof. Monika Hilker (FU Berlin, Germany) we are investigating if herbivore-induced changes in pine volatile emission that result in egg parasitoid attraction are also reflected at the molecular level. We are also investigating whether D. pini egg deposition activates genes encoding pine terpene synthases and whether their activation (transcriptional level) is species-specific.
Role of endosymbiotic gut bacteria of Hylobius abietis in detoxification of terpenes
This project is a collaboration between the Herbivore and Pathogen Induced Defense Strategies in Conifers Research Group and the Insect Symbiosis Research Group.
The pine weevil (Hylobius abietis) is a major pest in European conifer forests where adults feed on the bark and cambium of Norway spruce and pine seedlings. Conifers are protected against bark-feeding herbivores by a complex mixture of secondary metabolites, mainly terpenoid-based oleoresins, and phenolic compounds. Therefore, adult pine weevils must cope with a complex mixture of noxious secondary metabolites in their diet, in addition to the problem of utilizing a food source that mainly consists of lignocellulose.
Many insects are known to harbor symbiotic microorganisms in their digestive system that allow the host to subsist on suboptimal diets by enhancing digestion efficiency, supplementing the diet with limiting vitamins or amino acids, or detoxifying plant secondary metabolites. However, little is known about degradation of lignin or mono-, and sesquiterpenes, as well as diterpene resin acids, the components of coniferous resin by symbiotic microorganisms.
We are exploring how the pine weevil copes with high concentrations of terpenes present in the host bark and cambium, by elucidating the metabolic fate of these compounds and their effect on the beetle. In order to understand the possible role of gut microorganisms in terpene detoxification as well as in wood digestion, we are using culture-dependent and -independent methods as well as metabolic and genomic analyses to functionally characterize the pine weevil’s gut microbiota. Elucidating the fate of terpenes in this system will shed some light on how insects cope with plant induced defenses and how some symbionts allow their hosts to exploit otherwise inaccessible food sources.
Jörg Bohlmann (University of British Columbia, Michael Smith Laboratories, Vancouver, Canada)
Wilhelm Boland (Max Planck Institute for Chemical Ecology, Bioorganic Chemistry, Jena, Germany)
Antje Burse (Max Planck Institute for Chemical Ecology, Bioorganic Chemistry, Jena, Germany)
Thilo Fischer (Ludwig Maximilian University, Munich, Germany)
Monika Hilker (FU Berlin, Institute of Biology, Dept. of Applied Zoology/Animal Ecology, Berlin, Germany)
Trygve Krekling (Norwegian University of Life Sciences, Dept. of Plant and Environmental Sciences, Ås, Norway)
Paal Krokene (The Norwegian Forest and Landscape Institute, Forest Health, Ås, Norway)
Nina Elisabeth Nagy (The Norwegian Forest and Landscape Institute, Forest Health, Ås, Norway)
Marco Michelozzi (Institute of Plant Genetics, Divisions Florence, Florence, Italy)
Christian Paetz (Max Planck Institute for Chemical Ecology, NMR group, Jena, Germany)
Frederik Schlyter (Swedish University of Agricultural Sciences, Dept. of Plant Protection Biology, Alnarp, Sweden)
Bernd Schneider (Max Planck Institute for Chemical Ecology, NMR group, Jena, Germany
Jörg-Peter Schnitzler (German Research Center for Environmental Health (Munich, Germany)
Armand Séguin (Canadian Forest Service, Laurentian Forestry Centre, Quebec, Canada)