26 T. Macek et al. Biotechnology Advances 18 2000 23–34
wastes from soil or water through their roots. Although it is not discussed in this review, plants have a great capacity to bind organic compounds from the air [7]. Classes of organic
compounds that are more rapidly degraded in the rhizosphere than in bulk soil include poly- cyclic aromatic hydrocarbons [12,13], total petroleum hydrocarbons [14], chlorinated pesti-
cides PCP, 2,4-D, other chlorinated compounds like polychlorinated biphenyls PCBs [15,16], TCE [17], explosives [2,4,6,-trinitrotoluene TNT, hexahydro-1,3,5-trinitro-1,3,5-
triazine RDX, dinitrotoluene DNT], organophosphate insecticides diazinon, parathion, and surfactants detergents. Burken and Schnoor [18] discussed beneficial impacts of root
exudates and biomass on mineralization of atrazine by poplar trees.
Betts [19] reported a field trial in which a substantial decrease in total petroleum hydrocar- bon TPH content using bermuda grass, rye grass, white clover, and tall fescue was ob-
served in plots at a large fuel facility in the United States. Recently, it was shown that some compounds present in root exudates can serve not only as nutrients for microorganisms
[11,20], but they can also induce the bacterial degradation of PCBs [21–23]. These com- pounds phenolics, flavonoids, terpenes served as growth substrates and they stimulated
PCB degradation in the same manner as biphenyl.
6. Metabolism
Vegetative caps at landfill sites and other contaminated areas are used as alternative tech- nologies to help contain toxic chemicals and reduce their movement to other sites. The dif-
ferent mechanisms mentioned earlier can be involved. Organic compounds can be translo- cated to other plant tissues [7] and subsequently volatilized, they may undergo partial or
complete degradation, or they may be transformed to less toxic, especially less phytotoxic, compounds and bound in plant tissues. In general, most organics appear to undergo some de-
gree of transformation in plant cells before being sequestered in vacuoles or bound to insolu- ble cellular structures such as lignin. Metabolism of herbicides and pesticides was exten-
sively studied many years ago [24–27]. During recent years metabolism of nonagricultural xenobiotics such as trichloroethylene TCE, TNT, glyceroltrinitrate GTN, polyaromatic
hydrocarbons PAHs, PCBs [12–14,16,17] and other chlorinated compounds has been stud- ied [7,28]. It was shown that most of these compounds are metabolized but only a few chem-
icals appear to be fully mineralized. Some plant metabolites of pollutants may be more toxic than the original compounds, making plants less attractive compared with bacteria, which to-
tally degrade organic pollutants.
Among the most persistent compounds present in the environment are polychlorinated bi- phenyls. Metabolism of these compounds by mammalian cells was extensively studied dur-
ing the 1970s [29,30]. Recently it was shown that plants are able to metabolize PCBs, but the data for such models are scarce in comparison with the vast literature on bacterial or fungal
pathways of PCB degradation. Fletcher et al. [31] reported transformation of 2-chlorobiphe- nyl but without any description of the products obtained. Beginning in the 1970s, there were
some studies discussing metabolism of PCBs in plants with the identification of oxygenated metabolites hydroxychlorobiphenyls confirmed by different analytical methods [32–34].
Two monohydroxychlorobiphenyls and one dihydroxychloro derivative, which were mostly
T. Macek et al. Biotechnology Advances 18 2000 23–34 27
conjugated, have been identified, together with some dechlorinated products. Wilken et al. [35] studied the metabolism of 10 different congeners of PCBs in 12 cell cultures of different
plant species one culture of each species. The authors observed that metabolism of defined PCBs congeners was dependent on plant species, detecting various monohydroxylated and
dihydroxylated compounds after acid hydrolysis of polar metabolites.
Macková et al. [16,36,37] studied the ability of cultures of various species cultivated in vitro to degrade Delor 103 a mixture of PCBs commercially produced in the former Czech-
oslovakia until the mid-1980s [38,39]. The PCB mixture contained about 59 congeners with differing degrees of chlorination, with an average number of three chlorines per biphenyl
molecule. The analytical procedure was optimized by Burkhard et al. [40]. About 40 axenic cell cultures of different plant species were screened for the ability to transform PCBs. When
the PCB degradative ability in relation to the origin and morphology of the cultures was eval- uated, the results showed a great variability in the capability to convert PCBs within different
cultures of the same plant species [41,42]. The best results were obtained with
Solanum ni- grum
black nightshade hairy root clone SNC-9O. Such cultures, obtained after transforma- tion of plant cells by
Agrobacterium rhizogenes , proved to be a very useful tool in basic re-
search for phytoremediation purposes [37,43,44]. Metabolism of individual congeners of all three monochlorobiphenyls were studied and various monohydroxylated and dihydroxylated
chlorobiphenyls were detected. 4-Hydroxy derivatives were identified as the major products in all three cases of monochlorobiphenyls transformation [16].
7. Enzymes involved
