of organic compounds: roots of perennial plants (except monocots) typically develop
a rigid protective structure called periderm (replaces the normal rhizoderm), which
comprises a large component of bark and the most outer layer called phellem,
consisting of suberized-dead cells [31]. These bark-like materials contain accumulations of lipophilic substances and may hence act as a sink for lipophilic compounds. In this context, the role of the protective root cap and its mucilage has not
been investigated as sink in depth.
Although chemical features of a molecule may be important predictors for the
uptake, the physiology of the plant root itself and its composition can also have
significant influence. Trapp and Pussemeir [32] critically reviewed the relationship
derived by Briggs and co-workers [25] as an overestimate of the uptake of some
herbicides by common bean (Phaseolus vulgaris) [33]. We are still lacking knowledge about the factors determining such differences.
Among all biological factors, root extractable lipid content seems to have the
strongest influence on the emerging compounds’ uptake [34]. Either way, lipophilic
compounds are expected to partition to root lipids (membrane and storage lipids) and
thus concentrate in roots, until an equilibrium between the chemical concentration in
the aqueous phase within the plant root and the external solution is reached. The
strong affinity of charged compounds or their metabolites in roots retards pharmaceutical transport to shoots and results in a significant accumulation in roots, making
tuberous vegetables critical sources of food and fodder [35]. However, protein
content was found to have a greater influence on the prediction of uptake than the
lipid content as described by González García and co-authors [36]. For weak acids
like ibuprofen, ketoprofen and naproxen, higher concentrations in roots than in
leaves were quantified, suggesting the adsorption to proteins and consequently
retention in roots, which supported their model.
Once a solute enters the root – through the growing tip of the root hair epidermis
passing by cortex, endodermis and pericycle, ending up with the entrance into the
vascular tissue – it can take two pathways to reach the xylem, along which it is
transported to the aerial plant parts:
In the apoplastic pathway, the solute travels along cell walls through the
intercellular space of the epidermis and cortex region of the root and across cell
membranes at the endodermis. Non-ionic pharmaceutics are able to cross cell
membranes easily and thus have higher potential to be taken up by the roots due
to their higher lipophilicity [37]. However, compounds taken up exclusively by the
apoplastic route cannot cross the Casparian strip; that is, they must cross at least one
lipid bilayer to enter the xylem or phloem; if not, they tend to accumulate in roots
[9]. Little research has been directed towards elucidating xenobiotic uptake mechanisms and pathways, knowledge that is needed to develop models to predict uptake
and accumulation. Chemical sorption to lipophilic root structures may be a significant factor influencing the available concentration.
In the symplastic pathway, the solute crosses cell membranes of root hairs,
epidermis and cortex and moves to the vascular cylinders by the plasmodesmata
and/or by membrane permeation [38], which means that only a small fraction of the
compounds is transported via the symplastic movement into cellular vacuoles
Uptake and Translocation of Pharmaceuticals in Plants:. . .
109
a rigid protective structure called periderm (replaces the normal rhizoderm), which
comprises a large component of bark and the most outer layer called phellem,
consisting of suberized-dead cells [31]. These bark-like materials contain accumulations of lipophilic substances and may hence act as a sink for lipophilic compounds. In this context, the role of the protective root cap and its mucilage has not
been investigated as sink in depth.
Although chemical features of a molecule may be important predictors for the
uptake, the physiology of the plant root itself and its composition can also have
significant influence. Trapp and Pussemeir [32] critically reviewed the relationship
derived by Briggs and co-workers [25] as an overestimate of the uptake of some
herbicides by common bean (Phaseolus vulgaris) [33]. We are still lacking knowledge about the factors determining such differences.
Among all biological factors, root extractable lipid content seems to have the
strongest influence on the emerging compounds’ uptake [34]. Either way, lipophilic
compounds are expected to partition to root lipids (membrane and storage lipids) and
thus concentrate in roots, until an equilibrium between the chemical concentration in
the aqueous phase within the plant root and the external solution is reached. The
strong affinity of charged compounds or their metabolites in roots retards pharmaceutical transport to shoots and results in a significant accumulation in roots, making
tuberous vegetables critical sources of food and fodder [35]. However, protein
content was found to have a greater influence on the prediction of uptake than the
lipid content as described by González García and co-authors [36]. For weak acids
like ibuprofen, ketoprofen and naproxen, higher concentrations in roots than in
leaves were quantified, suggesting the adsorption to proteins and consequently
retention in roots, which supported their model.
Once a solute enters the root – through the growing tip of the root hair epidermis
passing by cortex, endodermis and pericycle, ending up with the entrance into the
vascular tissue – it can take two pathways to reach the xylem, along which it is
transported to the aerial plant parts:
In the apoplastic pathway, the solute travels along cell walls through the
intercellular space of the epidermis and cortex region of the root and across cell
membranes at the endodermis. Non-ionic pharmaceutics are able to cross cell
membranes easily and thus have higher potential to be taken up by the roots due
to their higher lipophilicity [37]. However, compounds taken up exclusively by the
apoplastic route cannot cross the Casparian strip; that is, they must cross at least one
lipid bilayer to enter the xylem or phloem; if not, they tend to accumulate in roots
[9]. Little research has been directed towards elucidating xenobiotic uptake mechanisms and pathways, knowledge that is needed to develop models to predict uptake
and accumulation. Chemical sorption to lipophilic root structures may be a significant factor influencing the available concentration.
In the symplastic pathway, the solute crosses cell membranes of root hairs,
epidermis and cortex and moves to the vascular cylinders by the plasmodesmata
and/or by membrane permeation [38], which means that only a small fraction of the
compounds is transported via the symplastic movement into cellular vacuoles
Uptake and Translocation of Pharmaceuticals in Plants:. . .
109