Horizontal natural product transfer: A potential source of alkaloidal contaminants in phytopharmaceuticals

Background: It was recently shown that nicotine and pyrrolizidine alkaloids that leach out from decomposing plant material (donor plants) are subsequently taken up by the roots of acceptor plants and translocated into their leaves. Furthermore, it is well established that plant roots take up xenobiotics, generally by simple diffusion, and that this passive import depends on the physico-chemical properties of the substances. Hypothesis: Based on the well-known uptake of xenobiotics, we assumed that in analogy, the uptake of alkaloids, which are leached out from plant material (donor plants) represents a quite general feature of plant biology. Methods: Using barley as a model plant, we analyzed the uptake of alkaloids by applying them to Hordeum vulgare seedlings. Based on HPLC analyses, the presence of the particular alkaloids in the acceptor plants was determined. Results: We demonstrated that numerous alkaloids of different structural types are able to diffuse through biomembranes and are taken up by acceptor plants. In contrast, an uptake of quaternary alkaloids, with a permanent positive charge, could not be detected. Conclusion: As most alkaloidal plants generally die back afield, and the corresponding natural products are leached out into the soil. Our findings have substantial relevance for all plant-derived commodities, especially for the production of phytopharmaceuticals and the related safety issues. Moreover, the evidence that plants are inherently able to take up alkaloids from the soil, which are derived from other plants, will alter our appraisal of plant-plant interactions. In this context, the classical definition of xenobiotics, which are considered as nonnatural substances, might be also extended by including natural products leached out into the soil.