and plant resistance to biotic and abiotic stresses [105]. Researchers are addressing
the impact of pharmaceuticals on plant microbiome in wetlands [106, 107] and crops
[108], contributing to elucidate the role of the plant microbiome in xenobiotic
metabolism as well as its contribution to plant fitness in polluted environments.
5.1 The Rhizosphere Is a Hot Spot for Pharmaceutical
Metabolism and Metabolite Exchange Between Plant
and Microorganisms
The contaminant concentration in soil is a major factor determining rhizosphere and
root endosphere microbiome structure and function [109]. Bacteria abundance and
diversity are high in the rhizosphere and decrease from the outer region to the inner
compartments. Bacterial abundance has been estimated to be 10
7
–10
9 cfu per g of
fresh soil in the rhizosphere, 10
5
–10
7 in the rhizoplane, and 10
3
–10
4 cfu/gfw in the
aerial endosphere [110]. The rhizosphere is a zone of exchange between the plant
and the soil, where soil nutrients and root exudates allow the growth of a complex
microbial community. Rhizospheric bacteria are key players in the metabolism of
xenobiotics present in the soil, being the first to enter in contact with foreign
molecules. It has been observed that degradative genes are enriched [111] and
expressed [112] in the rhizosphere microbiome of plants growing in contaminated
sites, revealing a selective control of the plant over rhizospheric microbial communities, favoring microbes with effective degradative traits. Plant selection of
microbes can be exploited for cultivation and isolation of plant-associated bacteria
with degradation and beneficial properties. Several studies have addressed
microbiome composition and role in plants exposed to pharmaceuticals, some of
them using culture-dependent methods (Table 3). Isolated microorganisms with
in vitro degradation capabilities can be used for bioaugmentation in nature-based
solutions for wastewater treatment.
Several studies have revealed how plant-associated microbial communities
respond to the presence of pharmaceuticals in constructed wetlands (CW) for wastewater treatment highlighting the importance of plant-bacteria interplay for the
remediation of pharmaceuticals [119]. In general, the concentration of pharmaceuticals affects bacterial community richness and diversity. It was observed that TCS
affects the development of certain bacteria and, eventually, the bacterial community
structures in CWs [117]. Moreover, the plant species selected for the CWs affected
the selection of microbial strains involved in TCS degradation. Whereas in Typha
angustifolia and Hydrilla verticillata CWs, beta-Proteobacteria were enriched after
exposure to TCS, in Salvinia natans CWs, delta- and gamma-Proteobacteria and
Sphingobacteria populations were significantly increased, and could relate to TCS
biodegradation [117]. Similar results were obtained by Liu and coworkers in CWs
planted with emergent cattail, submerged hornwort, and floating duckweed
[118]. Accumulation of TCS in sediment and plants was high in hornwort and
250
A. Sauvêtre et al.
the impact of pharmaceuticals on plant microbiome in wetlands [106, 107] and crops
[108], contributing to elucidate the role of the plant microbiome in xenobiotic
metabolism as well as its contribution to plant fitness in polluted environments.
5.1 The Rhizosphere Is a Hot Spot for Pharmaceutical
Metabolism and Metabolite Exchange Between Plant
and Microorganisms
The contaminant concentration in soil is a major factor determining rhizosphere and
root endosphere microbiome structure and function [109]. Bacteria abundance and
diversity are high in the rhizosphere and decrease from the outer region to the inner
compartments. Bacterial abundance has been estimated to be 10
7
–10
9 cfu per g of
fresh soil in the rhizosphere, 10
5
–10
7 in the rhizoplane, and 10
3
–10
4 cfu/gfw in the
aerial endosphere [110]. The rhizosphere is a zone of exchange between the plant
and the soil, where soil nutrients and root exudates allow the growth of a complex
microbial community. Rhizospheric bacteria are key players in the metabolism of
xenobiotics present in the soil, being the first to enter in contact with foreign
molecules. It has been observed that degradative genes are enriched [111] and
expressed [112] in the rhizosphere microbiome of plants growing in contaminated
sites, revealing a selective control of the plant over rhizospheric microbial communities, favoring microbes with effective degradative traits. Plant selection of
microbes can be exploited for cultivation and isolation of plant-associated bacteria
with degradation and beneficial properties. Several studies have addressed
microbiome composition and role in plants exposed to pharmaceuticals, some of
them using culture-dependent methods (Table 3). Isolated microorganisms with
in vitro degradation capabilities can be used for bioaugmentation in nature-based
solutions for wastewater treatment.
Several studies have revealed how plant-associated microbial communities
respond to the presence of pharmaceuticals in constructed wetlands (CW) for wastewater treatment highlighting the importance of plant-bacteria interplay for the
remediation of pharmaceuticals [119]. In general, the concentration of pharmaceuticals affects bacterial community richness and diversity. It was observed that TCS
affects the development of certain bacteria and, eventually, the bacterial community
structures in CWs [117]. Moreover, the plant species selected for the CWs affected
the selection of microbial strains involved in TCS degradation. Whereas in Typha
angustifolia and Hydrilla verticillata CWs, beta-Proteobacteria were enriched after
exposure to TCS, in Salvinia natans CWs, delta- and gamma-Proteobacteria and
Sphingobacteria populations were significantly increased, and could relate to TCS
biodegradation [117]. Similar results were obtained by Liu and coworkers in CWs
planted with emergent cattail, submerged hornwort, and floating duckweed
[118]. Accumulation of TCS in sediment and plants was high in hornwort and
250
A. Sauvêtre et al.