therefore blocked by the Casparian strip [67]. Hence, polar and ionizable compounds
absorbed exclusively by the apoplastic pathway cannot cross the Casparian strip and
will not reach the vascular tissue. Consequently, they remain confined at the root
level and accumulate there [68]. Furthermore, given the negative charge of the plant
cell wall, positively charged compounds will also be hindered from entering through
the cell wall matrix. Transport through the cell membrane could only be allowed
through the passage through non-selective channels which would allow to bypass the
Casparian strip. It has also been suggested that the absorption of basic compounds
would occur through processes mediated actively by carrier proteins due to similarities of natural compounds absorbed by these pathways [68]. Polar compounds that
are able to pass the Casparian strip or enter the root through passive diffusion into the
symplastic path or by active absorption can move through the roots and reach and
accumulate in the aerial parts of the plant. Once in the xylem, transpiration guides
these compounds from the roots to the shoots and leaves with the water flow.
However, it seems that many PhACs tend to accumulate mainly in the roots and in
the green parts, shoots and leaves, rather than in the fruits, and that translocation
occurs mainly via xylem [69–71].
The biological characteristics of the plants, the physical-chemical properties of
the PhACs (molecular weight, Kow and pKa), the ionic nature of the PhACs, and the
characteristics of the soil are all factors that influence absorption and translocation of
PhACs in roots and aerial parts of the plant. PhACs with log Kow between 1 and
4 can easily be translocated in the different compartments of the plant [72]. In a
recent review based mainly on hydroponic studies, it is assumed that anionic PhACs
preferably accumulate in the roots, while neutral and cationic PhACs preferentially
move in the green parts of the plant or even into the fruits [73].
Soil also plays a key role in the absorption and distribution of drugs in cropping
systems. In fact, the soil is the first bulk receptor of organic contaminants when
agricultural fields are irrigated with wastewater [74]. The concentration of PhACs in
the water of the soil pores, and then in the availability of PhACs for the absorption of
plants, depends on the physico-chemical characteristics of the soil, in particular from
soil texture. The negative charge of organo-mineral colloids is the resultant of the
sum of the permanent negative charge of the clay minerals and the pH-dependent
charge of the humic matrices. High percentages of silt and clay make soil less fertile
and reduce the availability of ionizable compounds due to the presence of a greater
number of negative charges due mainly to clays. A strong electrostatic bond of
PhACs to soil particles generally reduces availability for plants, especially for those
chemicals with strong hydrophobicity or positive charge.
Once absorbed by plant cells, contaminants can accumulate as they are or
undergo metabolic processes of plants, which have similarities to those of the
mammalian detox system since many enzymes responsible for cell detoxification,
including cytochromes P450 (CYP450s), present high similarity. The processes that
the plant cell puts in place to reduce the toxicity of these exogenous compounds
begin with Phase I metabolism processes (oxidation, reduction, and hydrolysis) and
then move on to Phase II processes where they are conjugated with a polar molecule
such as sugars or amino acids, or glutathione [68]. Compared to mammalian cells, a
The Journey of Human Drugs from Their Design at the Bench to Their Fate in Crops
21
absorbed exclusively by the apoplastic pathway cannot cross the Casparian strip and
will not reach the vascular tissue. Consequently, they remain confined at the root
level and accumulate there [68]. Furthermore, given the negative charge of the plant
cell wall, positively charged compounds will also be hindered from entering through
the cell wall matrix. Transport through the cell membrane could only be allowed
through the passage through non-selective channels which would allow to bypass the
Casparian strip. It has also been suggested that the absorption of basic compounds
would occur through processes mediated actively by carrier proteins due to similarities of natural compounds absorbed by these pathways [68]. Polar compounds that
are able to pass the Casparian strip or enter the root through passive diffusion into the
symplastic path or by active absorption can move through the roots and reach and
accumulate in the aerial parts of the plant. Once in the xylem, transpiration guides
these compounds from the roots to the shoots and leaves with the water flow.
However, it seems that many PhACs tend to accumulate mainly in the roots and in
the green parts, shoots and leaves, rather than in the fruits, and that translocation
occurs mainly via xylem [69–71].
The biological characteristics of the plants, the physical-chemical properties of
the PhACs (molecular weight, Kow and pKa), the ionic nature of the PhACs, and the
characteristics of the soil are all factors that influence absorption and translocation of
PhACs in roots and aerial parts of the plant. PhACs with log Kow between 1 and
4 can easily be translocated in the different compartments of the plant [72]. In a
recent review based mainly on hydroponic studies, it is assumed that anionic PhACs
preferably accumulate in the roots, while neutral and cationic PhACs preferentially
move in the green parts of the plant or even into the fruits [73].
Soil also plays a key role in the absorption and distribution of drugs in cropping
systems. In fact, the soil is the first bulk receptor of organic contaminants when
agricultural fields are irrigated with wastewater [74]. The concentration of PhACs in
the water of the soil pores, and then in the availability of PhACs for the absorption of
plants, depends on the physico-chemical characteristics of the soil, in particular from
soil texture. The negative charge of organo-mineral colloids is the resultant of the
sum of the permanent negative charge of the clay minerals and the pH-dependent
charge of the humic matrices. High percentages of silt and clay make soil less fertile
and reduce the availability of ionizable compounds due to the presence of a greater
number of negative charges due mainly to clays. A strong electrostatic bond of
PhACs to soil particles generally reduces availability for plants, especially for those
chemicals with strong hydrophobicity or positive charge.
Once absorbed by plant cells, contaminants can accumulate as they are or
undergo metabolic processes of plants, which have similarities to those of the
mammalian detox system since many enzymes responsible for cell detoxification,
including cytochromes P450 (CYP450s), present high similarity. The processes that
the plant cell puts in place to reduce the toxicity of these exogenous compounds
begin with Phase I metabolism processes (oxidation, reduction, and hydrolysis) and
then move on to Phase II processes where they are conjugated with a polar molecule
such as sugars or amino acids, or glutathione [68]. Compared to mammalian cells, a
The Journey of Human Drugs from Their Design at the Bench to Their Fate in Crops
21