mechanisms that A. rhizogenes uses to induce HR formation are similar to those used
by A. tumefaciens to induce gall formation and consist in the transfer of a DNA
segment (T-DNA) to plant cells. The T-DNA of the Ri (root inducing) plasmid
contains several virulent genes causing rhizogenic growth to the transformed cells
[90, 91]. Both bacteria transform plant tissues introducing genes through their
T-DNA to produce opines, which serve as specific nutrients for the bacteria
[92]. However, the physiologic basis of the tumorigenesis disease is different.
Alteration of auxin metabolism has been proposed to contribute significantly to the
expression of the HR phenotype [93, 94]. These findings have established the
foundations for the development of HR cultures with different valuable biotechnological applications such as secondary metabolites production, biotransformation
processes, or phytoremediation studies [73, 95].
In the last two decades, the use of HRs has been consolidated as study model for
the degradation and metabolism of organic pollutants, heavy metals, radionuclides,
and more recently of pharmaceuticals [96]. Additionally, HRs can be used to study
plant detoxification mechanisms and activity of detoxifying enzymes such as peroxidases and laccases [97, 98]. Considering (as mentioned previously) that metabolic pathways are conserved in plant cell cultures, HRs have emerged as a model for
the description of pharmaceutical metabolism in plants. HRs have some advantages
over whole plants or other models like cell suspensions. They have a stable genotype
and phenotype, a fast in vitro growth with no requirement of additional phytohormones, and a high production of secondary metabolites, a reason for which they are
often called “phytochemical factories” [99]. Additionally, they are easy to maintain,
by subculturing in sterile media, avoiding interactions with rhizosphere microbial
populations present in whole plants. For this reason, they can be used also to study
interactions with single microorganisms, especially rhizospheric and endophytic
bacteria or fungi [29, 73, 100, 101]. To date, HR cultures have been obtained from
a large number of plant species, predominantly dicotyledonous. Monocotyledonous
species (like many macrophytes used for phytoremediation) have remained recalcitrant to the transformation by A. rhizogenes. Nonetheless, in the last years, advances
in transformation techniques have allowed to obtain HR cultures from several
monocotyledonous species [102].
HRs have been used for their enzyme activity in pharmaceutical research.
Brugmansia candida HRs were shown to possess a glycosyl transferase very
effective in biotransforming the toxic depigmenting agent hydroquinone into a less
toxic alternative as arbutin [103]. For a long time, this model has been used for the
biotransformation of natural compounds (e.g., thymol, geraniol, coumarin, or flavone derivatives) into molecules of improved pharmaceutical properties
[104]. These biotransformations rely on HRs inherent enzymes and are governed
by hydroxylation, glycosylation, oxidoreduction, and hydrolysis reactions. As these
enzymes are normally present in roots of the selected plant species, this system is
now used for the identification of xenobiotic metabolites, using plant species known
for their production of enzymes involved in phase I metabolism such as peroxidases
and laccases [97, 98].
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A. Sauvêtre et al.
by A. tumefaciens to induce gall formation and consist in the transfer of a DNA
segment (T-DNA) to plant cells. The T-DNA of the Ri (root inducing) plasmid
contains several virulent genes causing rhizogenic growth to the transformed cells
[90, 91]. Both bacteria transform plant tissues introducing genes through their
T-DNA to produce opines, which serve as specific nutrients for the bacteria
[92]. However, the physiologic basis of the tumorigenesis disease is different.
Alteration of auxin metabolism has been proposed to contribute significantly to the
expression of the HR phenotype [93, 94]. These findings have established the
foundations for the development of HR cultures with different valuable biotechnological applications such as secondary metabolites production, biotransformation
processes, or phytoremediation studies [73, 95].
In the last two decades, the use of HRs has been consolidated as study model for
the degradation and metabolism of organic pollutants, heavy metals, radionuclides,
and more recently of pharmaceuticals [96]. Additionally, HRs can be used to study
plant detoxification mechanisms and activity of detoxifying enzymes such as peroxidases and laccases [97, 98]. Considering (as mentioned previously) that metabolic pathways are conserved in plant cell cultures, HRs have emerged as a model for
the description of pharmaceutical metabolism in plants. HRs have some advantages
over whole plants or other models like cell suspensions. They have a stable genotype
and phenotype, a fast in vitro growth with no requirement of additional phytohormones, and a high production of secondary metabolites, a reason for which they are
often called “phytochemical factories” [99]. Additionally, they are easy to maintain,
by subculturing in sterile media, avoiding interactions with rhizosphere microbial
populations present in whole plants. For this reason, they can be used also to study
interactions with single microorganisms, especially rhizospheric and endophytic
bacteria or fungi [29, 73, 100, 101]. To date, HR cultures have been obtained from
a large number of plant species, predominantly dicotyledonous. Monocotyledonous
species (like many macrophytes used for phytoremediation) have remained recalcitrant to the transformation by A. rhizogenes. Nonetheless, in the last years, advances
in transformation techniques have allowed to obtain HR cultures from several
monocotyledonous species [102].
HRs have been used for their enzyme activity in pharmaceutical research.
Brugmansia candida HRs were shown to possess a glycosyl transferase very
effective in biotransforming the toxic depigmenting agent hydroquinone into a less
toxic alternative as arbutin [103]. For a long time, this model has been used for the
biotransformation of natural compounds (e.g., thymol, geraniol, coumarin, or flavone derivatives) into molecules of improved pharmaceutical properties
[104]. These biotransformations rely on HRs inherent enzymes and are governed
by hydroxylation, glycosylation, oxidoreduction, and hydrolysis reactions. As these
enzymes are normally present in roots of the selected plant species, this system is
now used for the identification of xenobiotic metabolites, using plant species known
for their production of enzymes involved in phase I metabolism such as peroxidases
and laccases [97, 98].
248
A. Sauvêtre et al.