Interspecific transfer of pyrrolizidine alkaloids: An unconsidered source of contaminations of phytopharmaceuticals and plant derived commodities
Introduction
Within the last decade, it became quite evident that a tremendous number of plant derived commodities, including herbal teas, spices, or phytopharmaceuticals contain toxic pyrrolizidine alkaloids (PAs; EFSA Panel on Contaminants in the Food Chain, 2011). An actual evaluation of Mulder, Sánchez, These, Preiss-Weigert, and Castellari (2015), supported by the European Food Safety Authority (EFSA), revealed that PAs are present in more than 90% of all herbal tea samples. However, the origin of these PAs is still unknown. In Central Europe, up to now, only a limited number of species, including some widespread crop weeds, such as various Senecio species, are known to be able to synthesize pyrrolizidine alkaloids (Pichersky & Lewinsohn, 2001). Pyrrolizidine alkaloids, representing typical secondary plant products, are thought to protect the plants against herbivores and pathogens. In PA containing plants the alkaloid content varies between different plant organs (e.g., Frölich, Ober, & Hartmann, 2007) as well as plants species (e.g., Johnson, Molyneux, & Merrill, 1985). Moreover, the PA content is reported to be modulated by several abiotic factors such as soil quality, water availability, and temperature (Hol et al., 2003, Kirk et al., 2010, Vrieling et al., 1993). In typical PA-plants like Senecio jacobaea, the concentration of PAs could be extremely high, and values between 1000 and 10,000 mg alkaloids/kg d.w. are frequently reported (e.g., Johnson et al., 1985, Vrieling et al., 1993). Accordingly, in any case where just one Senecio plant is co-harvested with several thousands of PA-free crop or medicinal plants, the entire batch is contaminated, exhibiting PA concentrations of 1 mg/kg d.w. or more. Consequently, the accidental co-harvest of PA-containing weeds is considered to be the main source of the PA-contaminations of herbal teas, spices, or phytopharmaceuticals (e.g., Mulder et al., 2015). However, in various cases, where an accidental co-harvest of PA containing weeds could be excluded, e.g., when the plant material is obtained by thorough hand-picking like green or black teas (two leaves and a bud), the observed PA-contaminations must have another origin.
In this context, the recently discovered horizontal transfer of natural products comes into focus. This phenomenon, which describes the uptake of natural products leached out from rotting plants (Selmar, Radwan, & Nowak, 2015), is reported to be responsible, at least in part, for the nicotine contaminations of several plant derived commodities (Selmar, Engelhardt et al., 2015). Consequently, horizontal transfer of natural products could conceivably also be the cause for contaminations of plant derived commodities with PAs. In this paper, various plant species were employed that are commonly used as herbal teas or spices, i.e., melissa (Melissa officinalis), peppermint (Mentha × piperita), chamomile (Matricaria chamomilla), and parsley (Petroselinum crispum), to investigate the putative uptake of PAs from soil, mulched with dried plant material from Senecio jacobaea.
Section snippets
Plant material
All experimental plants (Melissa officinalis, Mentha × piperita, Matricaria chamomilla, Petroselinum crispum) were grown during spring to early summer, outdoors, in pots. In the case of the peppermint, chamomile, and parsley plants, cultivation was performed in square pots (9.0 × 9.0 cm × 9.5 cm) containing about 0.75 L of a soil-sand mixture (3:1). For the melissa plants, due to their vigorous growth, round pots (Ø = 15 cm) containing about 1.5 L of the soil-sand mixture were used.
For the mulching,
PA-uptake and translocation into the shoots
The plants had been mulched with dried plant material of Senecio jacobaea, which revealed about 3200 mg/kg of total PAs. When the plants were mulched with 1 g of this material, 3.2 mg PAs were added to each pot. One week after application, every mulched plant exhibited marked concentrations of PAs (Fig. 1a–d), whereas the untreated controls were totally free of these alkaloids. These results demonstrated that PAs indeed are leached out from the dried Senecio material into the soil and confirm that
Acknowledgements
Financial support by the German Federal Ministry of Food and Agriculture via the “Fachagentur für Nachwachsende Rohstoffe” (Project No.: 95667-002) is greatly acknowledged. We wish to thank Dr. Andreas Plescher (PHARMAPLANT, Artern, Germany) for providing the experimental plants.
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