ARE IRIDOIDS IN LEAF BEETLE LARVAE SYNTHESIZED DE NOVO OR DERIVED FROM PLANT PRECURSORS? A METHODOLOGICAL APPROACH*

Abstract

Iridoids, belonging to a group of cyclopentanoid monoterpenoids, are secreted by many species of leaf beetles as a defense against predators. Using chemically modified precursors of iridoid biosynthesis, it has been shown that some leaf beetle larvae can synthesize these iridoids de novo as well as sequester plant-produced molecules. Stable isotope techniques can provide useful methods for studying terpenoid biosynthesis without disturbing the natural conditions much. Two terpenoid biosynthesis pathways (mevalonic acid (MVA) pathway and methylerythritol-4-phosphate (MEP) pathway) may lead to different δ¹³C signatures of the products. Our results from natural abundance ¹³C and ¹³C-labelled iridoid precursors in Gastrophysa viridula and Phaedon cochleariae suggested that the two leaf beetle species use only de novo synthesis of their defensive iridoids. We observed that the isotope signature of the leaf-beetle-produced iridoids (via the MVA pathway) resembled that of the MEP-derived monoterpenoids from plants. Owing to this close similarity in the natural ¹³C abundances in the plant and insect compounds, a determination of iridoid-origin in leaf beetle secretion may only be possible by use of isotopically labelled compounds.

Extended version of the poster prize winner's contribution to the 26^th Annual Meeting of the German Association for Stable Isotope Research (GASIR) October, 6 to 8, 2003, Cologne, Germany

Keywords:

• Carbon-13
• IRMS
• Leaf beetle
• Natural isotope variations
• Sequestration
• Terpenoid biosynthesis
• Tracer technique

Acknowledgement

We would like to thank Anja Biedermann for assistance in the laboratory, Heike Geilmann (MPI-BGC, Jena) for stable isotope measurements of bulk plant material, Gerd Gleixner and Susanna Andersson for fruitful discussions, Emily Wheeler for manuscript proof reading, and Fonds der Chemischen Industrie, Frankfurt/M. and the Max-Planck-Gesellschaft for financial support.

Notes

Extended version of the poster prize winner's contribution to the 26^th Annual Meeting of the German Association for Stable Isotope Research (GASIR) October, 6 to 8, 2003, Cologne, Germany