with an uptake of xenobiotics, they are also relevant and suitable for all natural
products, e.g., alkaloids. However, when considering the membrane permeability of
alkaloids, an additional factor, i.e., the pH-dependent protonation of these natural
compounds has to be considered [29]. In contrast to the free bases of most alkaloids,
which are simply able to pass through membranes, their protonated forms – due to
the positive charge – in general cannot penetrate biomembranes. This peculiarity is
in accordance with the quite negative logP values of charged alkaloid salts. Thus, in
addition to the logP of the various alkaloids, the pH of the medium also determines
their membrane permeability. Consequently, the pH of the soil will massively
influence the extent of the uptake of alkaloids from the soil. The higher the pH of
the soil, the greater is the proportion of unprotonated alkaloids, and thus, their
membrane permeability [29]. In contrast, in acidic soils, due to a high degree of
protonated alkaloids, the uptake of alkaloids is drastically decreased [5].
Just recently, Yahyazadeh et al. [30] evinced that many different classes of
alkaloids are also imported into putative acceptor plants. As prognosticated,
all tested alkaloids revealing logP values between À1 and 3 are taken up. Consequently, in addition to nicotine and PAs, also tropane and purine alkaloids as well as
benzylisoquinoline and indole alkaloids had been detected in the leaves of the
acceptor plants. In contrast, when quaternary alkaloids such as coptisine, palmatine,
or berberine had been applied to the soil, they were not taken up. The explanation
for this difference is simple: these alkaloids – independently to the pH of the medium
– always reveal a positive charge. Consequently, they are not able to pass through
biomembranes. This notably is underlined by the quite negative logP values of these
alkaloids [30].
Based on the nexus between the membrane permeability and logP values,
it can be deduced that also substances representing other classes of natural
products than alkaloids should be taken up in the same manner. In this sense,
phenolic compounds, terpenoids, etc. should also be able to diffuse through biomembranes as long the hydrophilic as well as the lipophilic properties of the
substance are within an appropriate range, i.e., when the logP values are between
À1 and 3 [5, 25]. Nevertheless, up to recently, no experimental data on a putative
transfer of natural products between donor and acceptor plants had been available.
Notwithstanding, in the literature, various corresponding hints could be
detected. In this sense, investigations of the uptake of so-called “emerging organic
contaminants” (EOCs) revealed that indeed many different classes of organic
compounds are able to pass biomembranes and are taken up by acceptor plants
[31]. Moreover, in earlier studies dealing with the glucosylation of coumarins, it is
stated that esculetin and scopoletin are able to pass the plasmalemma and are
imported substantially by protoplasts from barley leaves [32]. Despite these
indications, no investigations on the uptake of natural compounds representing
other classes of natural products were available.
In order to visualize the phenomenon of horizontal natural product transfer, we
applied colorants like betanidines to acceptor plants. Indeed, the green color of vital
leaves overlays all other hues and the presence of these red dyes cannot be
simply detected visually. However, when using etiolated seedlings, e.g., of peas or
432
D. Selmar et al.
products, e.g., alkaloids. However, when considering the membrane permeability of
alkaloids, an additional factor, i.e., the pH-dependent protonation of these natural
compounds has to be considered [29]. In contrast to the free bases of most alkaloids,
which are simply able to pass through membranes, their protonated forms – due to
the positive charge – in general cannot penetrate biomembranes. This peculiarity is
in accordance with the quite negative logP values of charged alkaloid salts. Thus, in
addition to the logP of the various alkaloids, the pH of the medium also determines
their membrane permeability. Consequently, the pH of the soil will massively
influence the extent of the uptake of alkaloids from the soil. The higher the pH of
the soil, the greater is the proportion of unprotonated alkaloids, and thus, their
membrane permeability [29]. In contrast, in acidic soils, due to a high degree of
protonated alkaloids, the uptake of alkaloids is drastically decreased [5].
Just recently, Yahyazadeh et al. [30] evinced that many different classes of
alkaloids are also imported into putative acceptor plants. As prognosticated,
all tested alkaloids revealing logP values between À1 and 3 are taken up. Consequently, in addition to nicotine and PAs, also tropane and purine alkaloids as well as
benzylisoquinoline and indole alkaloids had been detected in the leaves of the
acceptor plants. In contrast, when quaternary alkaloids such as coptisine, palmatine,
or berberine had been applied to the soil, they were not taken up. The explanation
for this difference is simple: these alkaloids – independently to the pH of the medium
– always reveal a positive charge. Consequently, they are not able to pass through
biomembranes. This notably is underlined by the quite negative logP values of these
alkaloids [30].
Based on the nexus between the membrane permeability and logP values,
it can be deduced that also substances representing other classes of natural
products than alkaloids should be taken up in the same manner. In this sense,
phenolic compounds, terpenoids, etc. should also be able to diffuse through biomembranes as long the hydrophilic as well as the lipophilic properties of the
substance are within an appropriate range, i.e., when the logP values are between
À1 and 3 [5, 25]. Nevertheless, up to recently, no experimental data on a putative
transfer of natural products between donor and acceptor plants had been available.
Notwithstanding, in the literature, various corresponding hints could be
detected. In this sense, investigations of the uptake of so-called “emerging organic
contaminants” (EOCs) revealed that indeed many different classes of organic
compounds are able to pass biomembranes and are taken up by acceptor plants
[31]. Moreover, in earlier studies dealing with the glucosylation of coumarins, it is
stated that esculetin and scopoletin are able to pass the plasmalemma and are
imported substantially by protoplasts from barley leaves [32]. Despite these
indications, no investigations on the uptake of natural compounds representing
other classes of natural products were available.
In order to visualize the phenomenon of horizontal natural product transfer, we
applied colorants like betanidines to acceptor plants. Indeed, the green color of vital
leaves overlays all other hues and the presence of these red dyes cannot be
simply detected visually. However, when using etiolated seedlings, e.g., of peas or
432
D. Selmar et al.