Chemical Ecology of Anti-Herbivore Defenses in Poplar
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
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
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.
The response of poplar to a gall-inducing sucking herbivore, the aphid Pemphigus spirothecae
During their lifetime, poplar trees encounter an enormous variety of herbivore attacks. In a long lived organism such as a tree, these interactions can last from just seconds to months. My research focuses on the reaction of the black poplar to one of these long lasting herbivore attacks, that of Pemphigus spirothecae, a gall forming, colonial aphid. P. spirothecae begins its interaction with the tree right after budbreak, when it chooses a leaf petiole and starts sucking on it, thereby inducing the twisting of the petiole that leads to gall formation (see pictures). However, it is unknown how this “extended phenotype” is created or how the tree defends itself against this attack. To better understand this system we are analyzing poplar metabolites with mass spectrometry as well as carrying out transcriptional profiling with customized microarrays. We will attempt to correlate phenotypic changes in poplar defense compounds and phytohormones with transcriptional changes.
Other research projects in the group of Dr. Sybille Unsicker see Annegret Miehe-Steier and Rose Kigathi
The impact of abiotic factors on the iridoid glycosides in ribwort plantain (Plantago lanceolata L.)
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.
