Uptake and modification of umbelliferone by various seedlings

Inspired by the recently discovered phenomenon of horizontal natural product transfer we investigated the putative uptake of phenolic specialized metabolites. Umbelliferone was chosen for this case study, since this coumarin as well as its derivatives can easily be determined by HPLC analyses. Barley (Hordeum vulgare L.), radish (Raphanus sativus L.), pea (Pisum sativum L.), flax (Linum usitatissimum L.), and garden cress (Lepidium sativum L.) were cultivated in hydroponic media, to which the coumarin was applied. Uptake of umbelliferone was verified by corresponding HPLC analyses of extracts obtained from the aerial parts of the seedlings. In all cases, a tremendous uptake of umbelliferone was observed. In plants that genuinely contain coumarins, the umbelliferone taken up was modified: in garden cress, it was hydroxylated and glucosylated to yield esculin, while in barley seedlings, the imported umbelliferone was modified by methoxylation to yield scopoletin. Corresponding reactions are known from modifications of xenobiotics to be catalyzed by cytochrome P450 enzymes. Accordingly, in an additional approach, umbelliferone was applied together with naproxen, which is reported to reduce enzyme activity of P450 enzymes. As predicted, the conversion of umbelliferone to scopoletin in barley as well as the modification to esculin in garden cress was strongly reduced by the addition of naproxen. These data for the first time demonstrate that - in addition to alkaloids - also phenolic compounds are taken up by various acceptor plants. Apart from the leaching of rotting plants, coumarins are known to be exuded by many plants. Accordingly, these compounds are frequently present in soils and will be taken up. These coherences imply that the horizontal natural product transfer might represent a more general phenomenon in plant ecology. Moreover, this study outlines that - in analogy to the modification of xenobiotics - also natural products taken up are modified in the acceptor plants.