First record of pyrrolizidine alkaloids in the southern African biotype of Chromolaena odorata (Asteraceae)

Abstract

Pyrrolizidine alkaloids (PAs) are amino alcohols, often esterified with mono- or dicarboxylic acids, found in several plant families, including the Asteraceae. Pyrrolizidine alkaloids are used for plant defence against generalist invertebrate herbivores. However, specialist herbivores that are able to sequester PAs from the host plant species derive a number of benefits from these defence compounds. This study investigated alkaloids and PAs from the leaves and roots of the southern African biotype (SAB) of Chromolaena odorata in different regions and habitats in South Africa, with and without the specialist herbivore Pareuchaetes insulata. Alkaloid N-oxides were initially detected in leaf samples by Mayor's and Dragendorff's reagents and later by thin-layer chromatography on root samples with detection by successive treatment with acetic anhydride and Ehrlich's reagent. Two pyrrolizidine alkaloids, rinderine and the stereoisomer intermidine, were isolated from the reduced root extract of the SAB C. odorata and identified by Gas Chromatography-Mass Spectrometry. The structures and configuration of the compounds were confirmed by chemical and spectroscopic methods, especially one-dimensional 1H Nuclear Magnetic Resonance analysis. Pyrrolizidine alkaloids are known to be used by arctiine moths to find their host plant and are sequestered for mating purposes, and furthermore, these compounds render these lepidopterans unpalatable to predators. This study is the first to detect the presence of alkaloids and pyrrolizidine alkaloids in the SAB C. odorata.