Biosynthesis and Function of Volatile Formation in Woody Plants and Grasses
Many plants respond to herbivore feeding by producing complex volatile blends. These blends consist of compounds from several chemical classes such as terpenoids, aromatic esters, and green-leaf volatiles. We are investigating the biochemical and genetic basis of herbivore-induced volatile production in woody plants and grasses. Gaining basic knowledge about these processes will help us to elucidate the biological role of single volatile compounds in plant-insect interactions.
The evolution of insect-induced terpene biosynthesis in the grasses
Dr. Tobias Köllner
(in collaboration with Dr. Feng Chen, University of Tennessee)
The aim of this project is to investigate the molecular basis of insect-induced terpene biosynthesis in sorghum and the evolution of responsible terpene synthases in different grasses. Currently we identified three terpene synthase genes from sorghum which showed increased expression after herbivore-feeding. Biochemical characterization revealed that the three TPS produce the same sesquiterpenes but with completely different product distributions. Comparative studies with terpene synthase orthologs from sorghum, rice and maize will help us to identify key amino acids which changed during evolution and resulted in altered product specificities.
Exploring the underground: How poplar roots defend against herbivores
In response to herbivory, many plants produce defense compounds which can either be toxic or repellant for the herbivore or attractive for natural enemies of the herbivore. These direct and indirect defense mechanisms often present strong barriers against various attackers. Such induced plant defense mechanisms have been intensively studied above-ground, but little is known about the induced defenses of roots, especially those of trees. In this regard, we investigate the herbivory-induced accumulation of chemical metabolites in roots of the Western balsam poplar (Populus trichocarpa). Feeding of cockchafer (Melolontha melolontha) larvae on poplar roots leads to a highly increased accumulation of salicylaldehyde. To investigate the biochemical mechanisms of the observed herbivory-induced root response, a transcriptome dataset was generated to identify all differentially expressed genes in poplar roots. The further aim of the study is the identification and characterization of enzymes which are responsible for the formation of salicylaldehyde. Putative candidate genes of a β-oxidative biosynthetic pathway were identified and we are now characterizing the cinnamic acid-CoA ligases (CNL), cinnamoyl-CoA hydratases/dehydrogenases (CHD) and 3-ketoacyl-CoA-thiolases (KAT) in poplar. Further on, transgenic trees will be generated and then used in behavioral assays with cockchafer larvae in order to elucidate the biological role of salicylaldehyde as defense compund.
The odor of roots: Biochemical basis of terpene biosynthesis in poplar roots
Plants produce and emit a large variety of volatile organic compounds that belong to several chemical classes as for instance terpenoids or green leaf volatiles. Preliminary experiments revealed that feeding of cockchafer (Melolontha melolontha) larvae on poplar roots induces the emission of volatile monoterpenes. As other plants, poplar contains a large family of terpene synthase genes (tps) which encode key enzymes of terpene biosynthesis. The aim of the study is the identification of terpene synthases responsible for the formation of volatile monoterpenes in poplar roots. Identified candidate genes will be further characterized and their enzymatic activity will be examined in vitro. Further on, amino acids in the active site of the terpene synthases will be specifically changed to alter the product formation of the terpene synthases.
Biochemical and functional aspects of the metabolism of DIMBOA-Glc in maize and wheat
Benzoxazinoids (BXDs) are plant secondary metabolites that are well known for their defensive and allelopathic properties. They are mainly produced in plants of the grass family (Poaceae), including agricultural crops such as maize, wheat, and rye. The biosynthesis of BXDs has been extensively studied in maize and especially the biosynthetic pathway for the benzoxazinoid hydroxamic acids is fully elucidated in this plant. The core pathway starts from indole-3-glycerol phosphate, which undergoes several oxidations, a glucosylation, and a methylation leading to 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside (DIMBOA-Glc). DIMBOA-Glc is the most abundant BXD in undamaged maize, however, after herbivore feeding it is converted to a variety of other compounds such as HDMBOA-Glc, DIM2BOA-Glc , and HDM2BOA-Glc. In collaboration with the groups of Dr. Georg Jander (Boyce Thompson Institute, Ithaca, USA) and Prof. Dr. Matthias Erb (University of Bern, Switzerland), we have recently identified and characterized the enzymes involved in the formation of HDMBOA-Glc, DIM2BOA-Glc, and HDM2BOA-Glc in maize (see figure). The aim of the present study is to identify respective enzyme homologues in wheat and to investigate how they might have evolved. Preliminary BLAST analysis with the wheat genome revealed no direct orthologues, suggesting a convergent evolution of DIMBOA-Glc metabolism in the grasses. Since fungus infestation can induce the conversion of DIMBOA-Glc into other BXDs in wheat, a transcriptomic dataset derived from non-infested and fungus-infested wheat plants will help us to identify upregulated candidate genes. Further on, we are interested in elucidating the biosynthesis of benzoxazinoid lactams such as HMBOA-Glc in both maize and wheat. These compounds are frequently found in the leaves besides benzoxazinoid hydroxamid acids, but the enzymes responsible for their formation are still unknown and the biological roles of these compounds are unclear.
Consequences of interspecific hybridization on the plant metabolome
Hybridization is an important evolutionary force that is estimated to have affected 30-70% of our modern plant species. The genus Baccharis (Asteraceae), native to South America, provides a good model system for investigating hybridization and its consequences, as more than 30 putative interspecific hybrid species have been described here. One of these examples is Baccharis intermedia, a hybrid of the Chilean species B. linearis and B. macraei. While B. linearis is a widespread shrub in inland areas, B. macraei grows exclusively on sandy and coastal soils. The hybrid B. intermedia, which occur at the points of contact between the two parental species, appear to be able to bridge the spatially separated ecological niches occupied by B. linearis and B. macraei. Although B. linearis and B. macraei are closely related species, their secondary metabolic profiles differ significantly. This provides an opportunity to study the consequences of mixing very different plant metabolomes. In this project, we aim to a) characterize the chemical diversity of natural populations of Baccharis spp., b) describe the effects of hybridization on the chemical profile of the hybrids, and c) determine whether combining the enzymatic repertoire of parental species through allelic complementarity can lead to novel chemistry in the hybrids.
Elucidation of the biosynthesis of neo-clerodane diterpenes and their role in insect deterrence
The genus Baccharis is known to produce a wide array of secondary metabolites with different pharmacological, antimicrobial, and antiparasitic properties. Baccharis diterpenoids have displayed three main types of carbon skeletons: kaurane, labdane, and neo-clerodane skeletons. While neo-clerodane-type diterpenes have been widely associated with insect deterrence, little is yet known about their biosynthetic pathway and the various enzymatic steps leading to their characteristic furan ring and the α,β-unsaturated lactone, both of which are structurally associated with enhanced antifeedant properties.
One of the most abundant neo-clerodane diterpenes found in B. macraei is bacchasmacranone. Our proposed metabolic pathway suggests that bacchasmacranone is likely produced from hardwickiic acid, which is known for its anti-inflammatory and antiparasitic properties. While bacchasmacranone accumulates strongly in the aerial parts of B. macraei, neither it itself nor its putative intermediates are found in root tissues. Furthermore, the closely related species B. linearis appears to lack the final enzymatic steps responsible for the formation of bacchasmacranone, while it is capable of producing hardwickiic acid and a distinct diterpenoid profile. Therefore, in this project we aim to a) elucidate the metabolic pathway responsible for the formation of bacchasmacranone by exploiting the interspecific and intraspecific diversity of the diterpene profiles of B. macraei and B. linearis, b) determine the insect deterrent potential of bacchasmacranone and its various metabolic intermediates, and c) identify novel diterpenes by examining the chemical space of B. macraei and B. linearis.