Chemical Ecology of Anti-Herbivore Defenses in Poplar

Dr. Sybille Unsicker

We are investigating the direct and indirect defenses of black poplar against insect herbivore attack using both old growth-trees growing under natural conditions and transplants of these trees, raised in a greenhouse. Our approach is interdisciplinary, combining classical ecological methods with analytical chemistry and molecular biology.

Priming of herbivore-induced chemical defenses in Populus nigra

Plants that suffer herbivory may not induce anti-herbivore defenses immediately, but instead be primed to increase their induction of defenses after a second bout of feeding. Such priming of defense induction has been described for a number of herbaceous species (e.g. maize, tobacco, lima bean and poplar), but most of these experiments were performed under laboratory conditions. The goal of our project is to determine if priming of anti-herbivore defense induction occurs in nature using a natural population of old growth Populus nigra L. (black poplar). This species is a close relative of Populus trichocarpa, allowing the genomic resources of this model woody plant taxon to be applied. We want to look for priming of defense induction in natural populations of P. nigra, discover what anti-herbivore defenses are induced and examine some of the internal and external signals mediating this phenomenon. This is a joint research together with the project of Dr. Tobias Köllner.

 

Volatiles as indirect defenses of black poplar

Andrea Clavijo Mc Cormick

Plants emit volatile organic compounds (VOCs) in response to herbivore attack. These volatiles are believed to function in attracting natural enemies of herbivores and thus reduce herbivore damage. Many studies of such "indirect defenses" have been carried out in crop species under controlled settings. However, this phenomenon has not been well demonstrated under natural conditions, especially in woody plants. Nor has it been determined if the herbivore's enemies can obtain specific information on prey species or their stage of development from herbivore-induced volatiles. The aim of this study is to determine the major components of herbivore-induced volatiles in black poplar (P. nigra L.) that are responsible for the attraction of natural predators and parasitoids. Currently caterpillars of the generalist-feeding gypsy moth (Lymantria dispar L.) and its hymenopteran parasitoid (Glyptapanteles liparidis) are being used as model organisms.

 

Direct defenses of black poplar

Andreas Boeckler

Plants of the family of the Salicaceae synthesize a variety of phenylpropanoid derived secondary metabolites, primarily flavan-3-ol based oligomers (condensed tannins) and derivatives of salicin ("phenolic glycosides" or "PG"). Salicin, the 1-β-D-glucopyranoside of salicyl alcohol, and especially more complex PGs function as important mediators in the plant-herbivore interactions of the Salicaceae. The performance of generalist herbivores is negatively affected by PG uptake, but specialized species can tolerate or even sequester ingested PGs for their own defense (see department of bio-organic chemistry). Although these patterns have been described in numerous studies in the past two decades, there is no definitive information on the processing of PG in herbivores on a molecular level. I am investigating the metabolic conversion of PG to bioactive or innocuous molecules using chemical analytical techniques.

 

Pathogen-mediated changes in black poplar defense against insect herbivores

Franziska Eberl

In their natural environment plants are constantly exposed to antagonists such as herbivorous insects or bacterial and fungal pathogens. When a plant is attacked by either of these organisms, a complex signaling network is triggered, including ion fluxes, phytohormone and gene expression changes. This regulatory network finally results in metabolic changes to defend the plant against the attacker. Most of the studies investigating these plant defense reactions have been conducted with only one pest organism and its host. However, these single-species interactions are very simplified and often unrealistic since under natural conditions plants face a plethora of simultaneously occurring antagonists. Furthermore, almost all of these investigations have been carried out with herbaceous species such as tobacco, lima bean or maize. But how do woody plants react to an infection by a fungus or the feeding of an insect? Is the signaling, e.g. phytohormone levels, analogous to those in herbs? What happens if a plant is attacked by two pests simultaneously? Has such a multiple attack an effect on the attacking species themselves or their natural enemies? To answer these and related questions is the aim of my study. In order to do this, I use black poplar trees (Populus nigra) and different herbivorous insect species. Additionally to those leaf-feeding antagonists, I infect the trees with the biotrophic fungus Melampsora larici-populina, a wide-spread pest in natural poplar populations and plantations. By applying both pests subsequently as well as simultaneously I try to unveil possible effects of the pathogen on the development and host-interaction of the insects, and vice versa. By measuring phytohormones, photosynthesis rate, gene expression and volatile emission of the plants I further want to examine the signaling and defense reactions in the poplar tree. Additionally, I want to investigate the ecological consequences by monitoring the fitness and behavior of the feeding insects and their natural enemies as well as the development of the fungal pathogen.

 

The impact of abiotic factors on the iridoid glycosides in ribwort plantain (Plantago lanceolata L.)

Annegret Miehe-Steier

The iridoid glycosides aucubin and catalpol function as direct defense compounds in ribwort plantain (Plantago lanceolata L.). These compounds are monoterpene derived and constitutively present in plant tissue but can also be induced by e.g. insect herbivore feeding. Furthermore, abiotic parameters such as light and nutrients can also affect the concentration of these compounds in plantain.
The production of aucubin and catalpol is costly but necessary as it can reduce levels of herbivory and thus protect precious photosynthetically active plant tissue. On the other hand the biosynthesis of these compounds requires essential nutrients that are then no longer available for other processes such as growth.

I study the effects of varying abiotic parameters – light and nutrients - on the concentration and composition of iridoid glycosides in ribwort plantain. To understand the trade-off between primary and secondary metabolism in Plantago lanceolata I also investigate nitrogen and carbon allocation patterns along with crude measurements of photosynthetic efficiency under different abiotic conditions.