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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Pyrrolizidine Alkaloid Biosynthesis in Phalaenopsis Orchids: Developmental Expression of Alkaloid-Specific Homospermidine Synthase in Root Tips and Young Flower Buds^1,[OA]

Institut für Pharmazeutische Biologie (S.A., C.T.) and Institut für Pflanzenbiologie (R.H.), Technische Universität Braunschweig, D–38106 Braunschweig, Germany; and Botanisches Institut und Botanischer Garten, Universität Kiel, Olshausenstrasse 40, D–24098 Kiel, Germany (D.G., E.K., D.O.)

Pyrrolizidine alkaloids (PAs) are typical compounds of plant secondary metabolism and are believed to be part of the plant's chemical defense. Within the monocotyledonous plants, PAs have been described in only a few genera, mainly orchids, including Phalaenopsis. Because phylogenetic analyses suggest an independent origin of PA biosynthesis within the monocot lineage, we have analyzed the developmentally regulated expression of homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis, at the cell level. HSS is expressed in the tips of aerial roots exclusively in mitotically active cells. Raphide crystal idioblasts present within the root apical meristem do not show HSS expression. In addition, young flower buds, but not mature flowers, express HSS and have been shown by tracer feeding experiments to be able to catalyze PAs. This second site of PA biosynthesis ensures high concentrations of PAs in the reproductive structures of the Phalaenopsis flower, even after the flower opens. Thus, in spite of its identical function in PA biosynthesis, HSS shows in Phalaenopsis a completely different spatial and developmental expression pattern in comparison to other PA-producing species. These results show that the proverbial diversity of plant secondary metabolism is not just a matter of structural diversity, but is also multifaceted in terms of pathway regulation and expression.

² Present address: Centre de Recherche en Cancérologie de l'Université LAVAL, Pavillon Hôtel-Dieu de Québec, 9 rue Mc Mahon, Québec, Canada G1R 2J6.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Dietrich Ober (dober{at}bot.uni-kiel.de).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.124859

^* Corresponding author; e-mail dober{at}bot.uni-kiel.de.

Received June 16, 2008; accepted August 10, 2008; published August 13, 2008.