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About the Authors:
Linzhu Wang
Current address: Botanik I, University of Würzburg, Würzburg, Germany
Affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Garden, Christian-Albrechts-Universität, Kiel, Germany
Till Beuerle
Affiliation: Institute for Pharmaceutical Biology, Technical University Braunschweig, Braunschweig, Germany
James Timbilla
Affiliation: Queensborough Community College, City University of New York, New York, New York, United States of America
Dietrich Ober
* E-mail: [email protected]
Affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Garden, Christian-Albrechts-Universität, Kiel, Germany
Introduction
Chemical defense against herbivory is essential for plants to be able to survive in their natural habitat. During evolution, many insect herbivores have developed counterstrategies to cope with these toxic compounds. In some cases, they have even acquired these chemicals for their own benefit. One of the best studied examples of plant toxins sequestered by adapted insects are the pyrrolizidine alkaloids (PAs) that are found in certain lineages scattered within the angiosperms [1]. PAs occur in plants usually in their polar non-toxic N-oxide form (Figure 1). After ingestion by a vertebrate or insect herbivore, the N-oxides are easily reduced to the protoxic free base, the substrate for cytochrome P450-mediated bioactivation [2], [3].
[Figure omitted. See PDF.]
Figure 1. Structures of characteristic pyrrolizidine alkaloids.
Structures are given in the N-oxide form with the exception of the otonecine derivative, senkirkine.
https://doi.org/10.1371/journal.pone.0031796.g001
Strategies for PA sequestration in adapted insects have evolved in various insect lineages under the identical selection pressure to avoid higher concentrations of PAs in the form of their free base in the hemolymph (for recent reviews see [1], [3], [4], [5]). Leaf beetles of the genus Platyphora (Chrysomelidae, Coleoptera) have developed a strategy to transfer the free base from the hemolymph into defense secretions with such an efficiency that all other tissues outside the secretory glands are almost devoid of PAs [6]. In the related leaf beetle genus Oreina (Chrysomelidae, Coleoptera), the reduction of ingested PAs is suppressed. Instead, the N-oxides are directly absorbed and accumulated in their hemolymph and defense glands [7], [8]. A third strategy to handle sequestered PAs is the stabilization of the PAs by enzyme-catalyzed N-oxidation of the alkaloids within the insect. This mechanism is realized in larvae of the tiger moth family (Arctiidae, Lepidoptera) [9]–[13], in...