Uptake and modification of coumarins by plants – a coherent phenomenon extending the concept of Horizontal Natural Product Transfer

It was recently stated that various alkaloids which are leached from rotting plants, or exuded from living plants into the soil are taken up by acceptor plants growing in the vicinity. This phenomenon is denoted as “Horizontal Natural Product Transfer”. In analogy to alkaloids, also other natural products should be taken up from the soil. Consequently, in this thesis, the uptake of phenolic compounds, and especially of coumarins, was examined. Umbelliferone was chosen as a model compound since its uptake and derivatization can be easily detected and determined, due to its high fluorescence. 
Seedlings of various plant species, i.e., barley (Hordeum vulgare L.), radish (Raphanus sativus L.), pea (Pisum sativum L.), flax (Linum usitatissimum L.), and garden cress (Lepidium sativum L.) were grown in hydroponic media containing the coumarins. All plants took up umbelliferone and translocated it via xylem into the leaves to a high extent. In barley and garden cress, the imported umbelliferone was modified to scopoletin and esculin, respectively. Based on the occurrence of umbelliferone in guttation droplets, it was assumed that these modifications most probably take place in the leaves of the acceptor plants. To verify this, isolated leaves and roots of barley were incubated with umbelliferone. Indeed, in the roots, no derivatives were detected, and thus any derivatization in the roots could be ruled out. Surprisingly, in the cut leaves, in addition to scopoletin, large amounts of glucosidic derivatives, i.e., esculin and scopolin were detected. Obviously, the huge amounts of umbelliferone imported into the cut leaves induced corresponding glucosyltransferases.
Since related modifications of xenobiotics are known to be catalyzed by cytochrome P450 enzymes, umbelliferone was applied to the seedlings together with naproxen, a well-known inhibitor of P450 enzymes. As expected, the conversion of umbelliferone to scopoletin in barley as well as the modification to esculin in garden cress was strongly reduced by naproxen. 
The results on the uptake of coumarins display that the horizontal natural product transfer represents a more general phenomenon in plant ecology. In analogy to xenobiotics, the imported substances are modified. However, these modifications are restricted to some species and are different in the plant species. In consequence, these findings contradict the so-called “Green liver concept” that postulated a general detoxification process in plants.