Contributions to the Max Planck Year Book

Monarch butterflies, milkweed bugs and tiger keelback snakes may be very different creatures, but they have some things in common. These master chemists all contain heart-stopping toxic steroids, and have distinctive bright orange markings that warn predators: “if you eat me, you’ll regret it.” It’s difficult to believe, then, that any predator would make a meal of such obviously dangerous prey. But our research is discovering that predators from Germany to Australia and from Mexico to Japan have evolved remarkable abilities to withstand these toxins, and to survive on a most dangerous diet.

Black poplar leaves are particularly susceptible to be attacked by gypsy moths if they are also infected by a fungus. Actually, young larvae of the gypsy moth that feed on leaves covered with fungal spores gain biomass much faster and pupate several days earlier than larvae that feed only on leaf tissue. The reason for this is probably that fungal spores improve the supply of nutrients in the caterpillars’ diet. Thus, some herbivores may be in fact also fungivores. Fungi and microorganisms probably play a more important role in the coevolution of plants and herbivores than previously thought.

Plants produce an enormous number of complex molecules. These compounds play various roles in the plants’ natural environment. Many of these compounds are also used in both traditional and modern medicine; some have even life-saving properties like the anti-cancer drug vinblastine from the Madagascar periwinkle. Our aim is to elucidate the fundamental chemical, biological and evolutionary processes that underlie the biosynthesis of these complex molecules in order to be able to optimize the production of these compounds using synthetic biology.

Plants produce an enormous number of complex molecules. These compounds play various roles in the plants’ natural environment. Many of these compounds are also used in both traditional and modern medicine; some have even life-saving properties. Our aim is to elucidate the fundamental chemical, biological and evolutionary processes that underlie the biosynthesis of these complex molecules in order to be able to optimize their production using synthetic biology.

The tobacco hawkmoth uses olfaction to localize suitable host plants for oviposition. Recent findings suggest that the moth not only decides based on odors emitted by host plants, but also can sense odors emitted from frass of conspecific larvae already present on this host plant. By avoiding oviposition in the presence of frass odors, the female moth avoids conspecific competition for its offspring. Using the novel genetic tool CRISPR/Cas9 we could identify the olfactory receptor detecting these frass odors and hence, governing the moth’s competition avoidance.

Marine microalgae are ubiquitous in the ocean plankton and biofilms. We aim to elucidate how chemical signals and gradients affect plankton communities and how large species assemblages as well as microscopically small clusters are controlled by chemical signaling. We showed that plankton microalgae as well as bacteria produce a previously unknown sulfur compound and thus might fundamentally impact microbial interactions and the global sulfur cycle.

ABC transporters have many useful functions in insects, such as determining the coloration of the caterpillar body and the adult eye, and likely the detoxification of plant defense metabolites. But ABC transporters are also vulnerable to attack by toxins made by the bacterium Bacillus thuringiensis. The use of these toxins in transgenic crop plants sets up a new evolutionary conflict between benefits and costs of ABC transporters, that can help in the control of insect pests of agriculture.

Many insect species emit aggregation pheromones to attract conspecifics to host plants. This can lead to rapid mass infestations and severe crop losses in agriculture. Recently, a novel family of terpene synthases was discovered in Phyllotreta flea beetles which are important pests of crucifer crops. One member of this enzyme family was shown to be responsible for the formation of the sesquiterpene aggregation pheromone of the pest species. This knowledge on insect pheromone biosynthesis may lead to new approaches in pest management.

Insect feeding often induces early defense signaling in plants that activates a cascade of anti-herbivore defenses, protecting the plants from further attack. However, defense responses can also reduce the plant`s ability to survive due to physiological trade-offs. Thus plants need to evolve a robust signaling network that regulates these herbivore-induced defenses. Phylogenomic analysis of the genes involved in herbivore induced transcriptomic responses in Nicotiana showed that genome multiplication likely played an important role in shaping the evolution of early defense signaling in plants.

Calcium ions (Ca²⁺) represent the most important intracellular second messengers in the signaling networks of plants. After herbivore damage the opening of specific ion channels achieve a rapid transient increase of the cytoplasmatic Ca²⁺-level. The enhanced concentrations can be monitored in planta after expression of the bioluminescent Aequorin, that emits light upon binding of Ca²⁺-ions. The signal spreads with ca. 1-2 cm/min in the directly connected vascular system and corresponds with the speed of electrical signals triggered by herbivore damage.

Flowers are reproductive plant organs, essential for the reproduction and dispersion of the respective species. The required visual and olfactory communication with the pollinators is mediated by floral pigments and scent. In both cases, chemicals serve as information transmitters. For their service, the pollinators are rewarded with nectar and pollen, which are rich in valuable nutrients such as sugars, proteins and lipids. The qualitative and quantitative chemical analysis of the different flower constituents is one of the missions of chemical ecology.

The sense of olfaction is crucial for many insect species. Up until now, a major hypothesis stated that the most important of the involved receptor families, the olfactory receptors, appeared in evolution when insects emerged unto land. However, the analysis of basal flightless insects demonstrated that this is not true. The more likely scenario is that these receptors appeared when insects started to fly: Due to the higher speeds, insects have to resolve odor vanes more quickly, a task for which older receptors were probably insufficient.

The novel method of matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) allows to uncover functions of surface-occurring glucosinolates e.g. as oviposition clues for female moths during egg-laying. Similarly, spatial distribution fatty-acid-derived semiochemicals can be determined on fruit flies cuticles which actually matched with their expected biological function. The developed protocol can be further used to study other classes of chemicals presented on a biological surface helping to understand the chemical communication between organisms.

Female moths attract mates using their pheromones. Any deviation from producing the optimal blend by females, or any tendency to follow a different blend by males, results in reduced mating success and should be selected against. This poses an enigma: in the face of such strong stabilizing selection, how could the different pheromone blends now used by different moth species have evolved? Genetic studies provide a clue: a surprising amount of genetic variation can exist within species, and some of this variation can account for the evolution of differences between species.

To sustain 10 billion humans, we must imbue intensive agricultural monocultures with the ecosystem services once afforded by our planet’s biodiverse terrestrial habitats. Biodiversity is linked to ecosystem productivity and stability, and functional diversity may be the cause. Yet past research on biodiversity has confounded this analysis with profuse traits distinguishing species, regardless of their function. The new project group will investigate functional diversity’s role by amortizing over 300 isogenic, but functionally distinct, Nicotiana attenuata lines created at the institute.

Symbioses are ubiquitous in the natural environment and enormously important for the survival of animals and plants. A group of digger wasps, the so-called beewolves, engage in a remarkable defensive alliance with bacteria: By producing a cocktail of different antibiotics, the symbionts protect the wasps’ offspring in the subterranean cocoon against mold fungi and bacteria. In turn, the beewolf provides nutrition and shelter for the bacteria in its antennae. This symbiosis already evolved in the cretaceous and may have represented a key adaptation for the evolutionary success of beewolves.

Norway spruce developed constitutive and induced defense responses against herbivores and pathogens. Attack by herbivores like the bark beetle and his associated fungus induces the production of additional new resin in newly formed traumatic resin ducts and the accumulation of a mixture of polyphenolic compounds in specialized cambial cells. We investigate the regulation of terpenoids and polyphenolics by characterizing branch point enzymes that control the flow of metabolites during biosynthesis for a better understanding how both defense strategies function in an ecological context.

Larvae of the leaf beetle Chrysomela lapponica feed on birches or willows. Both beetle populations utilize precursor molecules of the plants to produce chemical defenses. As a toxin, the willow population produces salicylaldehyde. The birch population does not produce salicylaldehyde, since birches do not contain the precursor salicin. During evolution this resulted in a defect aldehyde producing enzyme, a salicylalcohol oxidase. The strong association of the leaf beetles with their host plants can be considered as the beginning of a newly emerging species.

The occurrence of specialized natural products in single cells or cell types of plants and other organisms, the composition of the mixture of metabolites and their temporal concentration changes are challenging for chemical analytics because of tiny amounts of material. Despite it is of moderate sensitivity, nuclear magnetic resonance spectroscopic methods are useful to precisely determine the spatio-temporal distribution of metabolites. This is feasible especially if NMR is applied in combination with laser microdissection.

Visualizing biological material helps to map the distribution of cells and organelles to better understand the principles of life. Most methods to date rely on microscopy and different labeling techniques. However, imaged elements are observed indirectly and lack molecular specificity. Mass spectrometry imaging (MSI) directly provides information related to the mass of the compound and possibly structural data taken from collision-induced-dissociation (CID) mass spectra. Distribution maps of secondary metabolites, obtained by MSI, already answered pending questions in chemical ecology.

Plants of the Arum family and many orchids have something in common: they deceive flying insects using chemical deception to be pollinated and to transmit pollen to neighboring flowers. Flowers emit volatile compounds that e.g. imitate yeast-dependent fermentation products to attract vinegar flies, while others mimic female sex pheromones to seduce male insects. The duped insects are not rewarded with nectar for their indeliberate pollination service. Analysis of the volatiles involved in these deceptive mechanisms allows new insights into the ecology and co-evolution of plants and insects.

Herbivorous insects encounter many different types of stresses in their environment. The digestive system must cope with toxins made by their host plants to defend against herbivory, and the immune system must defend against attack by pathogens and parasites. Scientists from the Max Planck Institute for Chemical Ecology have discovered that these two physiological systems – digestion and immunity – interact in some unexpected ways.

The ability of plants to recognize herbivores constitutes a form of plant immunity that is essential for plant survival. This process relies on the ability to perceive signals from the insect, to transmit this information to unattacked tissues to anticipate future attacks and to mount defenses that reduce insect performance and/or activate mechanisms that allow plants to tolerate the damage. Little is known about recognition events that trigger plant responses. One of these recognition systems involves the perception of insect-derived molecules delivered to plant cells during larval feeding.

Plants make a diverse array of chemicals in order to mediate the many interactions they have with their environment. To increase the diversity of these compounds, core structures are modified by certain families of enzymes. One of the most important modifications is the formation of esters, catalyzed by acyltransferases. We have used modern techniques to characterize the genes, enzymes, and products of a family of plant acyltransferases known as the BAHD family. The sequenced genome of the model plant Arabidopsis thaliana presents the opportunity to study a number of BAHD members.

In response to herbivorous insects plants produce a variety of natural compounds. Many beetle species developed sophisticated strategies to deal with these substances allowing colonization of habitats non attractive for other organisms. Frequently the plant derived compounds are used by the herbivores for their own interaction with the environment. Studying such detoxification strategies is one of the important topics in chemical ecology. They manipulate not only the evolution of beetles and plants but also of other species living in an ecosystem.

A gene’s influence on an organism’s Darwinian fitness ultimately determines whether it will be lost, maintained or modified by natural selection, yet biologists have few gene expression systems in which to measure whole-organism gene function. In the Department of Molecular Ecology scientists are training “molecularly-enabled field biologists” to use transformed plants silenced in the expression of environmentally-regulated genes and the plant’s native habitats as “laboratories". Research done in these labs will, so they hope, increase our understanding of the influence of a gene on plants’ Darwinian fitness.

Most animals are strongly dependent on odour information to survive and to reproduce. This dependency has in many species created very sensitive and specific odour-detecting systems – olfaction. One well-known interaction is the strong attraction of male dogs to a bitch in heat. In science a considerable amount of information is today available regarding olfactory structure and function from several model systems, including mice and fruitflies. Insects have proven to be interesting objects for olfactory studies, mainly because most of them are extremely odour-dependent, but also because their olfactory system can be used as a model both for olfactory functions and for sensory structure and evolution in general.

In response to damage, plants produce a large amount of natural compounds. Establishing the functional role of these natural compounds in plant defense is an important topic of chemical ecology. Recently, special attention has been given to volatile plant defense compounds that are emitted by the plant attracting natural enemies of the attacker. To attract the correct enemies, plants emit specific volatile blends which respond to the different types of damage inflicted by their enemies.

In the coevolutionary interplay between herbivorous insects and their foodplants, specific chemicals play a central role: Plants and insects use novel genes to control chemicals for offensive or defensive purposes. Many of these genes arise by minor modifications of pre-existing ones, but surprisingly, some originate from unexpected sources, including the genomes of completely different species. Scientists from the MPI for Chemical Ecology review several cases that illustrate this opportunistic nature of evolution.

Plants respond to herbivore attack by releasing volatile organic compounds (VOCs) that function as a defense by attracting natural enemies of the herbivore, thereby establishing defensive mutualisms with insects. The fact that plants use VOCs to communicate with insects generates the expectation that they also use VOCs to communicate with each other. Numerous studies have examined the question of whether un-attacked neighboring plants growing adjacently to attacked plants use these VOCs to anticipate future attack and preemptively activate defense responses. Most of these experiments have been carried out under experimental conditions that unnaturally amplify or distort the effects of the VOCs on neighboring plants and none have conclusively identified the active constituents of the VOC bouquet that function as signals. Scientists from the Max Planck Institute for Chemical Ecology in Jena present a new experimental approach to these challenges that allows for the study of plant-plant signaling under natural conditions.

Insect feeding elicits the synthesis and emission of volatile compounds in the infested plants as part of the indirect defense against herbivory. By using an artificial caterpillar, MecWorm, it is possible to analyze the impact of mechanical wounding and chemical signals separately. Studies with lima bean revealed that long lasting continuous wounding of plant tissues is sufficient to induce volatile blends which are similar to those emitted after insect-feeding. Microarray techniques were used to investigate gene regulation processes on transcript levels in Arabidopsis thaliana upon insect feeding and MecWorm treatment, respectively. On the whole genome background, significant changes in transcript levels have been found locally as well as systemically in both cases for about 5700 genes. Among these genes, 4100 were identically regulated, independently from the presence or absence of insect chemical components. In contrast, the observation of about 3200 regulated genes in systemically induced leaves indicates that insect signal compounds are involved in long distance responses.

We seek to understand the evolutionary forces which influence genetic variation of ecologically important traits within and among species. Following our earlier functional genomics studies in the model organism Arabidopsis thaliana, evolutionary inquiries are now focused on wild, perennial species of Arabidopsis and Boechera growing in natural populations in Europe and North America.

Plants produce a large variety of chemical compounds that are believed to protect them from herbivore or pathogen attack. However, it has been difficult to prove these defensive roles, especially since certain herbivores feed without any apparent negative effects on plants with high levels of defensive molecules. One of the most interesting groups of plant defense compounds are the glucosinolates, representing sulfur-containing metabolites that are precursors of the mustard oils. Modern molecular and biochemical methods now provide researchers with new tools to test the function of plant chemical defenses in a rigorous manner, as well as to explain how defenses may be circumvented. Here we describe how herbivorous insect species biochemically manage to disarm the plants' mustard oil bombs.