phytoextraction potential of a wetland plant like Juncus acutus [115]. Another study
focused on the isolation and identification of endophytic bacteria from Phragmites
australis exposed to the antiepileptic CBZ [116]. The authors could identify several
strains with abilities to degrade CBZ and with potential plant-growth properties.
CBZ metabolism has been studied in rats, humans, fungi, bacteria, and plants, being
probably one of the most studied pharmaceuticals in plants. Following uptake,
transformation products have been identified in a wide array of plant species
including macrophytes like Typha spp. [30], vegetables like tomato [28], carrot
[80, 88], lettuce [88, 120], potato, and zucchini [88]. Most of these metabolites are
found also in human urine, rat liver, fungi, or bacterial [121–123]. Only a few
metabolites seem to be exclusive to one organism. This specialization depends on
the detoxifying enzymes that have evolved in different organisms. While plants do
not rely on organic xenobiotics to obtain energy for growth and development,
endophytic bacteria may find in these compounds an available source of carbon.
Equipped with enzymes for the complete degradation of organic compounds, they
contribute to the metabolism of pharmaceuticals in the plant endosphere.
Attempts to identify the contribution of endophytic bacteria to the metabolism of
pharmaceuticals in the plant holobiont are scarce. Root cultures have been used to
study the interaction between plants and endophytic bacteria during CBZ metabolism [29]. Two strains isolated from Phragmites australis plants exposed to CBZ
were shown to use specific metabolic pathways in synergy with the plant (Fig. 14).
Metabolites accumulated in Armoracia rusticana roots belonged to phase I transformations and to a GSH conjugation (phase II) both occurring after 10,11epoxidation (Fig. 14). Enzymes involved in these steps were likely CYP or peroxidases (epoxidation), epoxide hydrolase (cleavage and hydroxylation), and GST
(GSH conjugation). When HRs were inoculated with endophytic bacteria, two
other pathways were favored. Application of Diaphorobacter nitroreducens resulted
in the formation of a group of metabolites with an acridine-related structure. This
pathway involves the cleavage of the carbamoyl group and rearrangement of the
central ring of CBZ and had been previously described in fungal cultures growing in
anoxic conditions [122] or as result of photo-oxidation [124]. The enzymatic
machinery behind such transformations is still unclear. Rhizobium radiobacter
activated a pathway involving successive oxidation reactions at the carbons of the
side aromatic benzene and leading to the formation of 2,3 dihydrodiols and subsequent 2,3 diol compounds. This is a conserved mechanism in bacterial degradation
of polycyclic aromatic hydrocarbons (PAHs) such as phenanthrene, naphthalene,
fluoranthene, pyrene, and benzopyrene and is generally catalyzed by
dehydrogenases.
6 Conclusion and Perspectives
Wastewater reuse for agricultural irrigation has been increasing over the years. In the
last 20 years, there has been an explosion of studies concerning the occurrence and
fate of pharmaceuticals and other emerging contaminants in crops and their possible
254
A. Sauvêtre et al.
focused on the isolation and identification of endophytic bacteria from Phragmites
australis exposed to the antiepileptic CBZ [116]. The authors could identify several
strains with abilities to degrade CBZ and with potential plant-growth properties.
CBZ metabolism has been studied in rats, humans, fungi, bacteria, and plants, being
probably one of the most studied pharmaceuticals in plants. Following uptake,
transformation products have been identified in a wide array of plant species
including macrophytes like Typha spp. [30], vegetables like tomato [28], carrot
[80, 88], lettuce [88, 120], potato, and zucchini [88]. Most of these metabolites are
found also in human urine, rat liver, fungi, or bacterial [121–123]. Only a few
metabolites seem to be exclusive to one organism. This specialization depends on
the detoxifying enzymes that have evolved in different organisms. While plants do
not rely on organic xenobiotics to obtain energy for growth and development,
endophytic bacteria may find in these compounds an available source of carbon.
Equipped with enzymes for the complete degradation of organic compounds, they
contribute to the metabolism of pharmaceuticals in the plant endosphere.
Attempts to identify the contribution of endophytic bacteria to the metabolism of
pharmaceuticals in the plant holobiont are scarce. Root cultures have been used to
study the interaction between plants and endophytic bacteria during CBZ metabolism [29]. Two strains isolated from Phragmites australis plants exposed to CBZ
were shown to use specific metabolic pathways in synergy with the plant (Fig. 14).
Metabolites accumulated in Armoracia rusticana roots belonged to phase I transformations and to a GSH conjugation (phase II) both occurring after 10,11epoxidation (Fig. 14). Enzymes involved in these steps were likely CYP or peroxidases (epoxidation), epoxide hydrolase (cleavage and hydroxylation), and GST
(GSH conjugation). When HRs were inoculated with endophytic bacteria, two
other pathways were favored. Application of Diaphorobacter nitroreducens resulted
in the formation of a group of metabolites with an acridine-related structure. This
pathway involves the cleavage of the carbamoyl group and rearrangement of the
central ring of CBZ and had been previously described in fungal cultures growing in
anoxic conditions [122] or as result of photo-oxidation [124]. The enzymatic
machinery behind such transformations is still unclear. Rhizobium radiobacter
activated a pathway involving successive oxidation reactions at the carbons of the
side aromatic benzene and leading to the formation of 2,3 dihydrodiols and subsequent 2,3 diol compounds. This is a conserved mechanism in bacterial degradation
of polycyclic aromatic hydrocarbons (PAHs) such as phenanthrene, naphthalene,
fluoranthene, pyrene, and benzopyrene and is generally catalyzed by
dehydrogenases.
6 Conclusion and Perspectives
Wastewater reuse for agricultural irrigation has been increasing over the years. In the
last 20 years, there has been an explosion of studies concerning the occurrence and
fate of pharmaceuticals and other emerging contaminants in crops and their possible
254
A. Sauvêtre et al.