Several metabolites have been identified in HRs exposed to pharmaceuticals
(Table 2). With the development of new analytical methods, HRs are helping
researchers to identify novel metabolites and describe metabolic pathways in plants.
The first study of uptake and metabolism of pharmaceuticals was done using
sunflower HRs to biotransform tetracycline and oxytetracycline [74]. Even if the
transformation products were not fully identified, authors could evidence oxidation
reactions affecting the UV absorption spectra of the parent compounds. Interestingly, it was observed that these modifications originated also in the liquid media,
suggesting an active role of root exudates. It is known that roots can exudate organic
acids, sugars, or amino acids but also enzymes such laccases or peroxidases.
Mechanisms of DCF oxidation by peroxidases were recently studied using crude
enzyme extracts from HRs [72]. Using stopped flow spectroscopy in combination
with liquid chromatography-mass spectrometric analysis, authors could identify the
formation of the highly reactive diclofenac-2,5-iminoquinone, which may be the
precursor of several biological conjugates and breakdown products in plants.
Dismissed of the aerial part, HRs are deprived of transpiration, the main driving
force of water uptake in whole plants. As a consequence, pharmaceutical uptake is
the result of passive diffusion through the membrane, direct entrance into wounds, or
active transport in case of molecules requiring transporters (e.g., MFM). For the
same reason, phase I metabolites that are normally free in the cytosol can cross
membranes by diffusion, being therefore detectable in the culture media [29]. Phase
II metabolites are normally immobilized in the vacuole or in the cell wall and require
a tissue extraction for their identification. Following this procedure, glucoside and
GSH conjugates were identified in Armoracia rusticana HRs after exposure to
acetaminophen [26], DCF [37], and CBZ [29]. In some cases, glucoside conjugates
are exudated into the medium where the parent compound can be released after
cleavage of the glucose. Malonylation has been hypothesized to protect the saccharide conjugates against enzyme cleavage and to render the products ready for storage
in vacuole or cell walls [86]. A mass balance can be calculated by combining the
analysis of both intra- and extracellular matrices. Using this approach, it was
observed that A. rusticana could metabolize up to 82% of the initial acetaminophen
amount after 6 hours of incubation with a distribution in the cells of 18% acetaminophen, 64% acetaminophen-glucoside, 17% acetaminophen GSH conjugate, and 1%
of the corresponding cysteine conjugate [26].
5 Role of Microbiome in Pharmaceutical Metabolism
and Plant-Microbe Interactions
With the recent developments on the omics technologies, the study of the
microbiome has gained attention. Plants are no longer considered as standalone
organisms but as holobionts in which many different microbiomes interact in a
specific ecological context, contributing to major functions such as plant nutrition
Metabolism of Pharmaceuticals in Plants and Their Associated Microbiota
249
(Table 2). With the development of new analytical methods, HRs are helping
researchers to identify novel metabolites and describe metabolic pathways in plants.
The first study of uptake and metabolism of pharmaceuticals was done using
sunflower HRs to biotransform tetracycline and oxytetracycline [74]. Even if the
transformation products were not fully identified, authors could evidence oxidation
reactions affecting the UV absorption spectra of the parent compounds. Interestingly, it was observed that these modifications originated also in the liquid media,
suggesting an active role of root exudates. It is known that roots can exudate organic
acids, sugars, or amino acids but also enzymes such laccases or peroxidases.
Mechanisms of DCF oxidation by peroxidases were recently studied using crude
enzyme extracts from HRs [72]. Using stopped flow spectroscopy in combination
with liquid chromatography-mass spectrometric analysis, authors could identify the
formation of the highly reactive diclofenac-2,5-iminoquinone, which may be the
precursor of several biological conjugates and breakdown products in plants.
Dismissed of the aerial part, HRs are deprived of transpiration, the main driving
force of water uptake in whole plants. As a consequence, pharmaceutical uptake is
the result of passive diffusion through the membrane, direct entrance into wounds, or
active transport in case of molecules requiring transporters (e.g., MFM). For the
same reason, phase I metabolites that are normally free in the cytosol can cross
membranes by diffusion, being therefore detectable in the culture media [29]. Phase
II metabolites are normally immobilized in the vacuole or in the cell wall and require
a tissue extraction for their identification. Following this procedure, glucoside and
GSH conjugates were identified in Armoracia rusticana HRs after exposure to
acetaminophen [26], DCF [37], and CBZ [29]. In some cases, glucoside conjugates
are exudated into the medium where the parent compound can be released after
cleavage of the glucose. Malonylation has been hypothesized to protect the saccharide conjugates against enzyme cleavage and to render the products ready for storage
in vacuole or cell walls [86]. A mass balance can be calculated by combining the
analysis of both intra- and extracellular matrices. Using this approach, it was
observed that A. rusticana could metabolize up to 82% of the initial acetaminophen
amount after 6 hours of incubation with a distribution in the cells of 18% acetaminophen, 64% acetaminophen-glucoside, 17% acetaminophen GSH conjugate, and 1%
of the corresponding cysteine conjugate [26].
5 Role of Microbiome in Pharmaceutical Metabolism
and Plant-Microbe Interactions
With the recent developments on the omics technologies, the study of the
microbiome has gained attention. Plants are no longer considered as standalone
organisms but as holobionts in which many different microbiomes interact in a
specific ecological context, contributing to major functions such as plant nutrition
Metabolism of Pharmaceuticals in Plants and Their Associated Microbiota
249