products, we always have to be aware that the actual manifestation and extent of
this phenomenon are determined by the outcome of several simultaneously occurring processes, i.e., the degradation of the substances by soil microorganisms, or
the uptake by the acceptor plants. Unfortunately, up to now, no solid information
on this topic is available.
3
Expanding the Concept of Horizontal Natural Product
Transfer: Co-cultures
When deliberating the coherences and central issues of horizontal natural product
transfer, the question will arise, whether – in analogy – to the leaching from dead,
rotting plant material a corresponding transfer also might occur between living
plants.
Allelopathy taught us that allelochemicals are released into the environment by
various processes [37]. In addition to a leaching out of decomposing plant residues,
allelochemicals also are actively exuded from living plants by their roots [2, 3] or by
their leaves [37, 38]. Consequently, it seems to be quite reasonable to assume that
analogously also common natural products might be released into the soil from
living donor plants. Recently, so-called co-culture experiments have been conducted
to verify such assumption: potential vital donor plants, e.g., pyrrolizidine alkaloids
(PAs) containing Senecio jacobaea plants, had been cultivated in single pots together
with potential acceptor plants, e.g., parsley. After 2 months of co-cultivation, the
plants had been harvested and the PAs were quantified. Astonishingly, in all parsley
plants co-cultivated with Senecio jacobaea, significant concentrations of PAs
were present; the average content was more than 200 μg/kg d.w. [39]. Based on
these results, further experiments and field trials had been conducted employing
a wide array of acceptor plants, which had been co-cultivated with S. jacobaea. In all
cases, considerable amounts of PAs, which previously had been synthesized in the
donor plants, were present in the acceptor plants. Thus, there is no doubt that – at
least in the case of PAs – the alkaloids are transferred from vital and living donor
plants via the soil into acceptor plants. However, up to now, there is no indication
about the mode of this transfer.
Considering the well-established active exudation of allelochemicals [2, 3, 37],
it seems obvious to assume that the PAs – and maybe other alkaloids – might be
exuded, either by the roots or by the leaves. However, we have to consider that all
plants frequently are faced with attacks by herbivores and infections by pathogens.
Accordingly, even vital and healthy plants exhibit numerous minor or larger injuries
due to previous attacks. Moreover, due to regular mechanical interactions with the
soil, frequently a certain number of root cells is destroyed, e.g., those of the calyptra.
Therefore, it could not be excluded that the observed transfer of PA is related to
natural injuries of the donor plant. To elucidate the actual mode of transfer, further
research is required. Nonetheless, the fact that alkaloids are transferred from living
and vital donor plants into acceptor plants necessitates a broadening of the concept of
horizontal natural product transfer, which is displayed in Fig. 3.
434
D. Selmar et al.
this phenomenon are determined by the outcome of several simultaneously occurring processes, i.e., the degradation of the substances by soil microorganisms, or
the uptake by the acceptor plants. Unfortunately, up to now, no solid information
on this topic is available.
3
Expanding the Concept of Horizontal Natural Product
Transfer: Co-cultures
When deliberating the coherences and central issues of horizontal natural product
transfer, the question will arise, whether – in analogy – to the leaching from dead,
rotting plant material a corresponding transfer also might occur between living
plants.
Allelopathy taught us that allelochemicals are released into the environment by
various processes [37]. In addition to a leaching out of decomposing plant residues,
allelochemicals also are actively exuded from living plants by their roots [2, 3] or by
their leaves [37, 38]. Consequently, it seems to be quite reasonable to assume that
analogously also common natural products might be released into the soil from
living donor plants. Recently, so-called co-culture experiments have been conducted
to verify such assumption: potential vital donor plants, e.g., pyrrolizidine alkaloids
(PAs) containing Senecio jacobaea plants, had been cultivated in single pots together
with potential acceptor plants, e.g., parsley. After 2 months of co-cultivation, the
plants had been harvested and the PAs were quantified. Astonishingly, in all parsley
plants co-cultivated with Senecio jacobaea, significant concentrations of PAs
were present; the average content was more than 200 μg/kg d.w. [39]. Based on
these results, further experiments and field trials had been conducted employing
a wide array of acceptor plants, which had been co-cultivated with S. jacobaea. In all
cases, considerable amounts of PAs, which previously had been synthesized in the
donor plants, were present in the acceptor plants. Thus, there is no doubt that – at
least in the case of PAs – the alkaloids are transferred from vital and living donor
plants via the soil into acceptor plants. However, up to now, there is no indication
about the mode of this transfer.
Considering the well-established active exudation of allelochemicals [2, 3, 37],
it seems obvious to assume that the PAs – and maybe other alkaloids – might be
exuded, either by the roots or by the leaves. However, we have to consider that all
plants frequently are faced with attacks by herbivores and infections by pathogens.
Accordingly, even vital and healthy plants exhibit numerous minor or larger injuries
due to previous attacks. Moreover, due to regular mechanical interactions with the
soil, frequently a certain number of root cells is destroyed, e.g., those of the calyptra.
Therefore, it could not be excluded that the observed transfer of PA is related to
natural injuries of the donor plant. To elucidate the actual mode of transfer, further
research is required. Nonetheless, the fact that alkaloids are transferred from living
and vital donor plants into acceptor plants necessitates a broadening of the concept of
horizontal natural product transfer, which is displayed in Fig. 3.
434
D. Selmar et al.