Adelheid Ehmke

Sites of synthesis, translocation and accumulation of pyrrolizidine alkaloid N-oxides in Senecio vulgaris L.

Abstract14C-Labelled alkaloid precursors (arginine, putrescine, spermidine) fed to Senecio vulgaris plants via the root system were rapidly taken up and efficiently incorporated into the pyrrolizidine alkaloid senecionine N-oxide (sen-Nox) with total incorporations of 3–6%. Considerable amounts of labelled sen-Nox were translocated into the shoot and were directed mainly into the inflorescences, the major sites of pyrrolizidine-alkaloid accumulation. Detached shoots of S. vulgaris were unable to synthesize pyrrolizidine alkaloids, indicating that the roots are the site of their biosynthesis. Further evidence was obtained from studies with in-vitro systems established from S. vulgaris: root cultures were found to synthesize pyrrolizidine alkaloids but not cell-suspension cultures, tumor cultures or shoot-like teratomas obtained by transformation with Agrobacterium tumefaciens. Studies on transport of [14C]sen-Nox, which was fed either to detached shoots or to the root system of intact plants, indicate that the alkaloid N-oxide does not simply follow the transpiration stream but is specifically channelled to the target tissues such as epidermal stem tissue and flower heads. Exogenously applied [14C]senecionine is rapidly N-oxidized. If the phloem path along the stem is blocked by a “steam girdle” translocation of labelled sen-Nox is blocked as well. Root-derived sen-Nox accumulated below the girdle and only trace amounts were found in the tissues above. It is most likely that the root-to-shoot transport of sen-Nox occurs mainly if not exclusively via the phloem. In accordance with previous studies the polar, salt-like N-oxides, which are often considered to be artifacts, were found to be the real products of pyrrolizidine-alkaloid biosynthesis as well as the physiological forms for long-distance transport, tissue-specific distribution and cellular accumulation.

Alkaloid N-oxides as transport and vacuolar storage compounds of pyrrolizidine alkaloids in Senecio vulgaris L

Cell-suspension cultures of pyrrolizidinealkaloid-producing species selectively take up and accumulate senecionine (sen) and its N-oxide (sen-Nox). Cultures established from non-alkaloid-producing species are unable to accumulate the alkaloids. The uptake and accumulation of 14C-labelled alkaloids was studied using a Senecio vulgaris cell-suspension culture as well as protoplasts and vacuoles derived from it. The alkaloid uptake exhibits all characteristics of a carrier-mediated transport. The uptake of sen-Nox follows a multiphasic saturation kinetics. The Km-values for sen Nox of 53 μM and 310 μM are evaluated. Senecionine competitively inhibits sen-Nox uptake, indicating that the tertiary alkaloid and its N-oxide share the same membrane carrier. The N-oxide of sen shows a pH optimum below 5.5, whereas sen is taken up over a range from pH 4 to 8. Activation energies of 90 and 53 kJ·mol-1 are calculated for sen-Nox and sen transport, respectively. At concentrations of 10 to 100 μM, sen-Nox is rapidly taken up by cells and protoplasts; within 2 h >90% of total N-oxide is within the cells. By contrast the uptake of sen is less efficient. Vacuoles isolated from protoplasts preloaded with sen-Nox totally retained the alkaloid N-oxide, whereas sen is rapidly lost during the procedure of vacuole preparation. N-oxidation converts the weak lipophilic tertiary base into a charged polar molecule which is excellently adapted to serve as the cellular transport and storage form of pyrrolizidine alkaloids.

Sequestration of plant pyrrolizidine alkaloids by chrysomelid beetles and selective transfer into the defensive secretions

SummaryOreina cacaliae andO. speciosissima (Coleoptera, Chrysomelidae) sequester in their elytral and pronotal defensive secretions pyrrolizidine alkaloids (PAs) as Noxides (PA N-oxides). The PA N-oxide patterns found in the beetles and their host plants were evaluated qualitatively and quantitatively by capillary gas chromatography/mass spectrometry (GC-MS). Of the three host plantsAdenostyles alliariae (Asteraceae) is the exclusive source for PA N-oxide sequestration in the defensive secretions of the beetles. With the exception of O-acetylseneciphylline the N-oxides of all PAs ofA. alliariae, i.e. senecionine, seneciphylline, spartioidine, integerrimine, platyphylline and neoplatyphylline were identified in the secretion. PA N-oxides typical ofSenecio fuchsii (Asteraceae) were detected in the bodies of the beetles but not in their secretion. No PAs were found in the leaves of the third host plant,Petasites paradoxus (Asteraceae). The results suggest the existence of two distinctive storage compartments for PA N-oxides in the beetle: (1) the defensive secretion, containing specifically PA N-oxides acquired fromA. alliariae; (2) the body of the beetle, sequestering additionally but less selectively PA N-oxides from other sources,e.g. S. fuchsii or monocrotaline N-oxide fed in the laboratory. The concentration of PA N-oxides in the defensive secretion is in the range of 0.1 to 0.3 mol/1, which is more than 2.5 orders of magnitude higher than that found in the body of the beetle. No significant differences exist in the ability of the two species of beetles to sequester PA N-oxides fromA. alliariae, althoughO. speciosissima, but notO. cacaliae, produces autogenous cardenolides. A negative correlation seems to exist between the concentrations of plant-derived PA N-oxides andde novo synthesized cardenolides in the defensive secretion ofO. speciosissima.

Selective sequestration and metabolism of plant derived pyrrolizidine alkaloids by chrysomelid leaf beetles

Pyrrolizidine alkaloids (PAs) are assumed to function as plant defence compounds against herbivory. A number of adapted insects are known to sequester plant derived PAs for their own benefit. Here we summarize the chemical interactions between leaf beetles of the genus Oreina (Coleoptera, Chrysomelidae) and their host plants Adenostyles spp., Senecio nemorensis, and S. fuchsii (Asteraceae, tribe Senecioneae). Seneciphylline N-oxide and senecionine N-oxide, the main PAs of Adenostyles, are sequestered in the bodies and exocrine defensive glands of the leaf beetles. The comparison with the PA patterns of the Senecio host plant indicates a selective PA uptake. The uptake into the body (hemolymph) is less specific, whereas the translocation into the defensive glands is highly specific. Only the N-oxides of macrocyclic retronecine esters of the senecionine type are found in significant amounts in the defensive secretions. Many other PAs such as monoesters and open-chain diesters as well as PAs of other structural types (e.g. monocrotaline N-oxide and senkirkine) are not transferred into the defensive glands. Leaf beetles sequester PAs exclusively as N-oxides. A novel PA not found in the food plants was detected in the defensive secretions of Oreina elongata; it was identified as 13,19-expoxisenecionine N-oxide (oreine), the epoxidation product of seneciphylline N-oxide. Besides this transformation, leaf beetles are able to catalyse further transformations such as the O-dealkylation of heliotrine N-oxide to rinderine N-oxide and the O-deacetylation of acetylseneciphylline N-oxide to seneciphylline N-oxide. The plant-beetle interactions are discussed in the functional context of PAs as powerful plant defensive chemicals.

DISTRIBUTION OF AUTOGENOUS AND HOST-DERIVED CHEMICAL DEFENSES IN Oreina LEAF BEETLES (COLEOPTERA: CHRYSOMELIDAE)

The pronotal and elytral defensive secretions of 10Oreina species were analyzed. Species feeding on Apiaceae, i.e.,O. frigida andO. viridis, or on Cardueae (Asteraceae), i.e.,O. bidentata, O. coerulea, andO. virgulata, produce species-specific complex mixtures of autogenous cardenolides.O. melanocephala, which feeds onDoronicum clusii (Senecioneae, Asteraceae), devoid of pyrrolizidine alkaloids (PAs) in its leaves, secretes, at best, traces of cardenolides. Sequestration of host-plant PAs was observed in all the other species when feeding on Senecioneae containing these alkaloids in their leaves.O. cacaliae is the only species that secretes host-derived PA N-oxides and no autogenous cardenolides. Differences were observed in the secretions of specimens collected in various localities, because of local differences in the vegetation. The other species, such asO. elongata, O. intricata, andO. speciosissima, have a mixed defensive strategy and are able both to synthesize de novo cardenolides and to sequester plant PA N-oxides. This allows a great flexibility in defense, especially inO. elongata andO. speciosissima, which feed on both PA and non-PA plants. Populations of these species were found exclusively producing cardenolides, or exclusively sequestering PA N-oxides, or still doing both, depending on the local availability of food-plants. Differences were observed between species in their ability to sequester different plant PA N-oxides and to transform them. Therefore sympatric species demonstrate differences in the composition of their host-derived secretions, also resulting from differences in host-plant preference. Finally, within-population individual differences were observed because of local plant heterogeneity in PAs. To some extent these intrapopulation variations in chemical defense are tempered by mixing diet and by the long-term storage of PA N-oxides in the insect body that are used to refill the defensive glands.

Sequestration, N-Oxidation and Transformation of Plant Pyrrolizidine Alkaloids by the Arctiid Moth Tyria jacobaeae L

Larvae of the arctiid moth Tyria jacobaeae reared on Senecio jacobaea or S. vulgaris take up and store pyrrolizidine alkaloids (PAs) from their host plants. Individual PAs are taken up without preference. The PA patterns found in the insect bodies correspond to the PA composition of their host plants. Like plants the insects store PAs as N-oxides, and larvae as well as pupae are specifically able to N -oxidize any tertiary PA. Callimorphine (O9-(2-methyl-2-acetoxybutanoyl)-retronecine), an insect PA well known from several arctiids, was found in pupae and imagines of Tyria which as larvae had been fed on S. jacobaea. It is accompanied by small amounts of its isomer O7-(2-methyl-2-acetoxybutanoyl)-retronecine named isocallimor-phine. The callimorphines may well account for 45% of total PAs found in the insect. Only small amounts of callimorphine were detected in pupae of Tyria which as larvae had been fed on S. vulgaris. [14C]Callimorphine N -oxide was isolated and identified from Tyria pupae which as larvae received [14C]retronecine. It is suggested that Tyria is able to esterify retronecine, derived from hydrolysis of ingested plant PAs with a necic acid produced by the insect. During metamorphosis the formation of callimorphine is restricted to the early stage of pupation.

Sequestration of ingested [14C]senecionine N-oxide in the exocrine defensive secretions of chrysomelid beetles

Oreina cacaliae (Chrysomelidae) sequesters in its elytral and pronotal defensive secretion theN-oxides of pyrrolizidine alkaloids (PAN-oxides) from its food plantAdenostyles alliariae (Asteraceae). [14C]SenecionineN-oxide was applied for detailed studies of PAN-oxide sequestration. An average of 11.4% of total radioactivity is taken up by individual beetles which had received [14C]senecionineN-oxide with their food leaves 8 days before. An average of 28.9% of the ingested radioactivity could be recovered from the defensive secretions collected twice, i.e., 5 and 8 days after tracer feeding. The tracer transfer into the secretion seems to be a slow but progressive process as indicated by the high percentage of tracer still recovered from the secretion sampled after 8 days. Chromatographic analysis revealed that [14C]senecionineN-oxide is the only labeled compound in the defensive secretion. Beetles that fed on tertiary [14C]senecionine sequestered only trace amounts of radioactivity (exclusively present as labeled IV-oxide) in their secretions.O. speciosissima, a species also adapted to PA containing food plants, was shown to sequester [14C]senecionineN-oxide with the same efficiency asO. cacaliae. O. bifrons, a specialist feeding onChaerophyllum hirsutum (Apiaceae), rejected PA treated leaf samples already at very low PA concentrations (10 nmol/leaf piece). In bothO. cacaliae andO. speciosissima, [14C]senecionineN-oxide applied by injection into the hemolymph is rapidly transferred into the glands.O. bifrons, not adapted to pyrrolizidine alkaloid containing plants was unable to sequester [14C]-senecionineN- oxide in the secretion but rapidly eliminated the tracer with the frass. Again, only traces of labeled [14C]senecionineN-oxide were found in the defensive secretions of the two PA adapted species if labeled senecionine was injected. It is suggested that the beetles are adapted to theN-oxide form of PAs, similarly as their food plants, and that they lack the ability to efficientlyN-oxidize tertiary PAs. No indication forde novo PA synthesis by the beetles was found in tracer feeding experiments with the biogenetic PA precursor putrescine.

Chemical defense in chrysomelid leaf beetles

Exocrine chemical defense is widespread in the Chrysomelinae [1]. We have recently demonstrated the sequestration of N-oxides of pyrrolizidine alkaloids (PA N-oxides) in specialized exocrine glands as well as in the body of the leaf beetle Oreina cacaliae (Chrysomelinae) [2, 3]. There thus seems to be two compartments for the storage of PA N-oxides in this species: first, large quantities are stored at low concentration in the body; and secondly, small quantities are stored at high concentration in the defensive glands. These results suggested that the PA stored in the body can be translocated into the secretion. Here, to test this hypothesis, we examine the fate of ingested 14C-labeled PA N-oxide over a period of 25 days, paying particular attention to the replacement of the secretion after it has been experimentally removed. The storage of PA N-oxides for defensive purposes in O. cacaliae contrasts with the defensive strategy of other Oreina species which produce complex mixtures of cardenolides not present in their food plant. Here, we select O. gloriosa as a typical cardenolide producer. That cardenolides are biosynthesized de novo from ubiquitous phytosterols was demonstrated clearly in the related genus Chrysolina [4]. In Chrysolina, the cardenolides (site of biosynthesis still unknown) are found mostly, if not exclusively, in the defensive glands and not in the body of the beetle (D. Daloze, personal communication). If this was also the case in the Oreina producing cardenolides, the chemical defense of the different

Properties of glutamate dehydrogenase from Lemna minor

Abstract Sterile cultures of Lemna minor grown in the presence of either nitrate, ammonium or amino acids failed to show significant changes in glutamate dehydrogenase (GDH) levels in response to nitrogen source. Crude and partially purified GDH preparations exhibit NADH and NADPH dependent activities. The ratio of these activities remain ca 12:1 during various treatments. Mixed substrate and product inhibition studies as well as electrophoretic behaviour suggest the existence of a single enzyme which is active in the presence of both coenzymes. GDH activity was found to be localized mainly in mitochondria. Kinetic studies revealed normal Michaelis kinetics with most substrates but showed deviations with NADPH and glutamate. A Hill-coefficient of 1.9 determined with NADPH indicates positive cooperative interactions, whereas a Hill-coefficient of 0.75 found with glutamate may be interpreted in terms of negative cooperative interactions. NADH dependent activity decreases rapidly during gel filtration whereas the NAD + and NADPH activities remain unchanged. GDH preparations which have been pretreated with EDTA show almost complete loss of NADH and NAD + activities. NADPH activity again remains unaffected. NAD + activity is fully restored by adding Ca 2+ or Mg 2+ , whereas the NADH activity can only be recovered by Ca 2+ but not at all by Mg 2+ . Moderate inhibition of GDH reactions observed with various adenylates are fully reversed by adding Ca 2+ , indicating that the adenylate inhibition is due solely to the chelating properties of these compounds.

Role of mitochondrial glutamate dehydrogenase in the reassimilation of ammonia produced by glycine serine transformation.

The ability of isolated pea-shoot mitochondria conditioned to incorporate ammonia into glutamate to reassimilate endogenously produced ammonia from glycine transformation was investigated. In the presence of 1 mM to 20 mM glycine less than 15% of the ammonia liberated was found to be incorporated into glutamate. Thus, a prominent role of mitochondrial glutamate dehydrogenase in the reassimilation of intramitochondrially produced ammonia can be excluded.