AG Hartmann (Emeritus seit 2005)

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A.  Plant Secondary Metabolism

  • Plants produce a rich diversity of chemical constituents that we classify as products of secondary metabolism. Till now some 200 000 secondary compound are known. They account for those characters that make plants unique and distinctive to humans: the scent of flowers which we appreciate in perfumes, the various pigments that make our world so colorful, or the hot and aromatic ingredients of spices like chili, pepper or mustard, that we do not like to miss in our meals. more...

B.  The Pyrrolizidine Alkaloids (PAs)

  •   The pyrrolizidine alkaloids (PAs) represent a class of typical plant secondary compounds (Fig. 1). Their sporadic occurrence is restricted to some families of the angiosperms (Fig. 2). The PAs comprise an important component within constitutive plant defense against herbivores, particularly, insects. Convincing evidence favoring a defensive role of PAs comes from adapted insects of various unrelated taxa that sequester PAs from their host-plants and utilize them for their own defense. We are concerned with PAs for almost 20 years. They represent an example of a system of secondary compounds that most likely originated under the selective pressure of herbivorous insects. more...

     1.  Physiology and Biochemistry of PAs in the plant

  • For different Senecio species (Asteraceae) we established that  PAs are synthesized exclusively in the roots (Hartmann and Toppel, 1987). From the roots they are translocated to the shoots via the phloem-path (Hartmann et al., 1989). Within flowering shoots PAs accumulate mainly in the inflorescences (Hartmann and Zimmer, 1986) (Fig. 3). Intracellularly the PAs are specifically transferred into and stored within the vacuole (Ehmke et al., 1988). PAs do not show turnover but are spatially mobile within  the plant. In the roots of all Senecio species studied so far senecionine N-oxide is the common product of biosynthesis. more...

     2.  Biosynthesis and Molecular Evolution

  • PAs originate biosynthetically from amino acids. The necine base (see Fig. 1) originates from arginine via putrescine and spermidine, the necic acids are formed from the C-skeletons of aliphatic amino acids (isoleucine, valine, leucine) (Fig. 6). The discovery, purification (Böttcher et al., 1993 and 1994) and the molecular characterization (Ober and Hartmann, 1999) of a key-enzyme of PA biosynthesis, i.e. homospermidine synthase (HSS), led to an intriguing discovery: HSS was shown to be a gene duplicate of ubiquitous deoxyhypusine synthase (DHS). Apparently this gene duplicate lost its intrinsic activity (i.e. more...

     3.  PA Analytics and Preparation of Isotope labeled PAs

  • A basic requirement for studies of PA containing plants is a sensitive and high-resolution method for the separation of the mostly complex alkaloid mixtures and a rapid and explicit assignment of the discrete components. From the very first gas chromatography and its combination with mass spectrometry (GC-MS) were found to be excellent methods (Witte et al., 1993). Over the years Ludger Witte, who died two years ago in a tragic exident, had established a MS library with mass-spectra of more than 300 structures. This library is carried on and continuously expanded by Till Beuerle (t.beuerle@tu-bs.de). more...

     4.  The two Faces of PAs: the Non-Toxic N-Oxide and the Toxic Free Base

  • One of the most important features of PAs is that they may exist in two molecular states, as lipophilic free base (tertiary amine) and as polar, salt-like N-oxide. (Fig. 9). PA containing species of the families Asteraceae, Boraginaceae and Fabaceae synthesize, transport, and store PAs exclusively as N-oxides. The only known exception is seeds of the genus Crotalaria (Fabaceae) (Toppel et al., 1988), which store PAs as lipophilic free bases. In other families with PA containing species, e.g. Apocynaceae, Convolvulaceae and Orchidaceae PAs may occur as free base. PAs are pro-toxins. more...

     5.  About Insects that Acquire Plant PAs for their Own Benefit

  • PAs are strong deterrents, feeding inhibitors and toxins for vertebrates and insects. During co-evolutionary adaptation some herbivorous insects succeeded not only to overcome this efficient chemical barrier, but also to adopt the plant chemical defense for their own benefit (Fig. 11). The ability to sequester PAs appears to be phylogenetically widespread. Single PA sequestering species are known from grass-hoppers (Orthoptera) and aphids (Homoptera). PA sequestration is especially popular among lepidopterans (butterflies and moths; Lepidoptera) und various leaf-beetles (Chrysomelidae; Coleoptera). more...

          a)  Acquisition of Plant PAs by Arctiid Moths (Arctiidae, Lepidoptera)

  • A number of arctiid larvae are able to utilize PA containing plants as both food-source and PA source. Well known examples are the monophagous European cinnabar moth (Tyria jacobaeae) whose larvae develop almost exclusively on the tansy ragwort (Senecio jacobaea)  (Fig. 12). Another example is the polyphagous East-Asian arctiid Creatonotos transiens (Fig. 13) whose larvae can feed on a great number of plants, among them PA containing species. PAs are strong feeding stimulants for Creatonotos larvae which even feed with great appetite on glass-fiber filters treated with PAs (Fig. 13). more...

               I.  Detoxification of PAs by Specific N-Oxidation

  • How can PA sequestering arctiids handle pro-toxic PAs in a safe manner that exclude detrimental effects following P450-dependent bioactivation? We showed that ingested PAs, independently of whether they are offered as free base or N-oxide, are absorbed exclusively as pro-toxic free base. PA N-oxides are reduced in the gut before absorption; in arctiids they are never taken up as N-oxides, although claimed previously. Larval hemolymph was found to possess a soluble N-oxygenase that efficiently converts the absorbed free base into its N-oxide (Lindigkeit et al., 1997). more...

               II.  Specific Sensory, Physiological, Biochemical and Ecological Mechanisms of PA Sequestration

  • A PA adapted insect that utilizes plant PAs for its own benefit requires specific mechanisms to recognize a suitable PA source. Little is known about such mechanisms, as well as the behavioral mechanisms involved. In a close co-operation with Liz Bernays, Reg Chapman (deceased) und Mike Singer (University of Arizona, Tucson) we address these questions in studies with two highly polyphagous arctiids, i.e. Estigmene acrea (Fig. 18) und Grammia geneura (Fig. 19). In electrophysiological studies L. Bernays and R. Chapman were able to localize two taste receptors. One receptor is highly specific for PAs (Bernays et al. more...

          b)  Recruitment of Plant PAs by Leaf-Beetles (Chrysomelidae, Coleoptera)

  • Among the many known leaf-beetle species (Chrysomelidae) to date three genera are known that live and feed on PA containing plants. All of them sequester PAs ingested with their food and incorporate them into their chemical defense. They are the palaearctic genus Oreina (Fig. 20), the neotropical genus Platyphora (Fig. 21) and the European flea-beetle genus Longitarsus (Fig. 22). The mechanisms of PA sequestration by these beetles are being studied in fruitful and efficient co-operations: Oreina in co-operation with M. Rahier (Neuchâtel) and J. M. Pasteels (Brussels), Platyphora with J. M. Pasteels (Brussels) und Longitarsus with S. more...

               I. Uptake, Transport and Accumulation of PAs in Leaf-Beetles of the Genus Oreina

  • Most Oreina species contain in their defensive secretions (Fig. 23) cardenolides whixh they de novo synthesis from sterol precursors of plant origin. In the late eighties J. M. Pasteels coworkers observed that O. cacaliae sequester in its defensive secretions the PA seneciphylline N-oxide from its food-plant Adenostyles aliariae (Asteraceae). The mechanism of this sequestration was characterized in mutual studies with J. M. Pasteels and M. Rahier (Pasteels et al., 1995; Hartmann et al., 1997). In comparison to arctiid larvae, Oreina beetles and karvae developed a completely different sequestration strategy. more...

               II.  Uptake, Storage and Metabolism of PAs in Leaf-Beetles of the Genus Platyphora

  • J. M. Pasteels discovered in Panama Platyphora species living on PA-containing host-plants. Mutual studies revealed that these species beside autogenously produced triterpene saponins accumulate plant acquired PAs in their defensive secretions (Pasteels et al., 2001). In contrast to Oreina Platyphora absorb PAs as pro-toxic free base. As in arctiid larvae ingested N-oxides are reduced in the gut before absorption. However, the absorbed PA free bases do not accumulate in the hemolymph, they are efficiently "pumped" into the exocrine glands and accumulate as free bases in the defensive secretions. The tissues outside the glands are almost devoid of PAs. more...

               III.  PA Sequestration by Flea-Beetles of the Genus Longitarsus

  • The flea-beetles are the research topic of S. Dobler (Hamburg) and her co-workers. We are studying in co-operation with S. Dobler's group the occurrence of PAs in the various Longitarsus species and their host-plants (Dobler et al., 2000). Tracer feeding experiments revealed that Longitarsus specifically sequester PAs. Recent results indicate that flea-beetles most likely absorb PAs as both free base and N-oxide and that they are able to N-oxidize the free base (Narberhaus et al., 2003a und 2003b). more...