Thomas HARTMANN
lnstitut für Pharmazeutische Biologie der Technischen Universität Braunschweig, Mendelssohnskasse 1, D-38106 Braunschweig, Germany
Structural diversity and intraspecific variability are striking characteristics of plant secondary metabolism. Following the hypothesis that secondary metabolism evolved under selection pressure of a competitive and continuously changing environment, chemical diversification is one of the essential needs in secondary metsbolism to cope with and adapt to an ever changing environment. At least three features become visible that characterise secondary metabolism and form the mechanistic basis of diversification: (i) a high chemical degree of freedom, i.e. freedom of structural modification with almost no restrictions: (ii) the existence ot central pathways leading to complex key-metabolites well suited for chemical diversification; (iii) recruitment and optimisation of enzymes (and the respective genes) responsible for the specific and controllled structural diviersification.
The pyrrolizidine alkaloid N-oxides (PAs) of Senecio species (Astersceae) are selected as a typical class of secondary compounds to exemplify the general aspects summarised above. In plants they are synthesised site-specifically in the roots, translocated into the shoot vis the phloem-path and channelled to the inflorescence as the preferred sites of storage. Both long-distance translocation and vacuoIar storage require specific membrane carriers. In a number species senecionine N-oxide is synthesised as key-metabolite in the roots, translocated into the shoot where it is converted into the species-specific aIkaIoid patterns by simple enzymatic transformations (e.g., hydroxylation, dehydrogenation, epoxidation O-acetylation, retronecine-otonecine transformation).
PAs are well known defence compounds. This is evidenced by the fact that a number of insects from diverse taxa have evolved adaptations not only to overcome PA mediated defence but aIso sequester and utiIise PAs against predators. PA sequestering lepidopterans contain a specific enzyme (pyrrolizidine aIkaloid N-oxigenase) which selectively N-oxidise toxic PAs and thus maintain them in the state of a nontoxic pro-toxin. This enzyme must have been evolved in the course of the coevolutionary adaptation process of PA storing insects to PA produclng plants.