Alkaloid recruitment

Insects that recruit plant defense for their own protection are promising candidates for understanding the specific mechanisms that insects have evolved to cope with alkaloid-mediated plant defense. Several fascinating examples of alkaloid-recruitment have been described for the pyrrolizidine alkaloid system, some of which are given below. Most likely this is due to the unique feature of these alkaloids to exist in two interchangeable forms, as Y-oxide or as free base (tertiary alkaloid form). 

NAUMANN, C., HARTMANN, T., OBER, D., Evolutionary recruitment of a flavin-dependent monooxygenase for the detoxification of host plant-acquired pyrrolizidine alkaloids in the alkaloid-defended arctiid moth Tyria jacobaeae. Proc. Natl. Acad. Sci. U. S. A., 2002,99, 6085-6090. 

The synthesis of several other plant metabolites, such as auxin, indole glucosinolates, anthranilate-derived alkaloids, and tryptamine derivatives, could depend on indole as an intermediate. Indole is also found in the scent of flowers such as lilac and robinia. Therefore, it is possible that the recruitment of an indole-3-glycerol phosphate lyase function from TSA genes might have occurred independently several times during plant evolution. 

The precursor substrates and enzymes necessary for the first committed steps often appear to have been recruited from primary metabolic pathways, such as glycolysis, the Krebs cycle, the pentose phosphate pathway and the shikimate pathway [32], For example, the aromatic amino acid s L-phenylalanine and L-tyrosine, produced by the shikimate pathway, are precursors for a wide spectrum of natural products including phenylpropanoids, flavonoids, lignins, coumarins, cyanogenic glycosides, glucosinolates, and alkaloids [33],... 

Exogenous Isbelled phenylalanine rapidly equilibrates with cytoplasmic pools in hyphae of emerged cultures of P, cycloplum. If not incorporated into protein it is accumulated in the vacuole (expandable pool). At high exogenous concentrations practically all phenylalanine needed for protein synthesis comes from the extracellular source, contrary to alkaloid synthesis which under all conditions tested recruits more than 90 % of the required phenylalanine from intracellular sources. Two pathways of alkaloid labelling can be distinguished ... 

Hartmann T, Ober D. Defense by pyrrolizidine alkaloids developed by plants and recruited by insects. In Schaller, A., editor. Induced Plant Resistance to Herbivory. Heidelberg, Germany Springer Science + Business Media 2008, p. 213-231. 

L-Phenylalanine in the cytoplasm serves protein and alkaloid biosynthesis. Excess L-phenyl-alanine is accumulated in the vacuoles and can be reused later in alkaloid formation but not in protein biosynthesis. Hence, with respect to alkaloid formation there are two channels for L-phenylalanine a direct, low capacity pathway via the peripheral pool (primary labeling of alkaloids) and an indirect, high capacity pathway from the expandable pool (secondary labeling of the alkaloids). The relative contributions of these two channels vary with the concentration of L-phenylalanine, the time of incubation, etc. Under all experimental conditions, however, in contrast to protein biosynthesis, about 90 % of the L-phenylalanine incorporated into the alkaloids is recruited from endogenous sources, i.e., de novo synthesis and protein degradation. 

Langel D, Ober D (2011) Evolutionary recruitment of a flavin-dependent monooxygenase for stabilization of sequestmed pyrrolizidine alkaloids in arctiids. Phytochemistry 72(13) ... 

Hartmann T, Ober D (2008) Defense by pyrrolizidine alkaloids developed by plants and recruited by insects. In SchaUer A (ed) Induced plant resistance to herbivory. Springer, New York Hartmaim T, Witte L (1995) PytroUzidine alkaloids chemical, biological and chemoecological aspects. In Pelletier SW (ed) Alkaloids chemical and biological perspectives. Pergamon, Oxford Hashimoto T, Yamada Y (1994) Alkaloid biogenesis molecular aspects. Annu Rev Plant Physiol Plant Mol Biol 45 257-285... 

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