Phytoremediation of polychlorinated biphenyls: new trends and promises

Abstract

Transgenic plants and associated bacteria constitute a new generation of genetically modified organisms for efficient and environment-friendly treatment of soil and water contaminated with polychlorinated biphenyls (PCBs). This review focuses on recent advances in phytoremediation for the treatment of PCBs, including the development of transgenic plants and associated bacteria. Phytoremediation, or the use of higher plants for rehabilitation of soil and groundwater, is a promising strategy for cost-effective treatment of sites contaminated by toxic compounds, including PCBs. Plants can help mitigate environmental pollution by PCBs through a range of mechanisms: besides uptake from soil (phytoextraction), plants are capable of enzymatic transformation of PCBs (phytotransformation); by releasing a variety of secondary metabolites, plants also enhance the microbial activity in the root zone, improving biodegradation of PCBs (rhizoremediation). However, because of their hydrophobicity and chemical stability, PCBs are only slowly taken up and degraded by plants and associated bacteria, resulting in incomplete treatment and potential release of toxic metabolites into the environment. Moreover, naturally occurring plant-associated bacteria may not possess the enzymatic machinery necessary for PCB degradation. To overcome these limitations, bacterial genes involved in the metabolism of PCBs, such as biphenyl dioxygenases, have been introduced into higher plants, following a strategy similar to the development of transgenic crops. Similarly, bacteria have been genetically modified that exhibit improved biodegradation capabilities and are able to maintain stable relationships with plants. Transgenic plants and associated bacteria bring hope for a broader and more efficient application of phytoremediation for the treatment of PCBs.

Figure 1

Phytoremediation of organic pollutants, such as PCBs, may involve several processes: pollutants in soil and groundwater can be taken up inside plant tissues (phytoextraction) or adsorbed to the roots (rhizofiltration); pollutants inside plant tissues can be transformed by plant enzymes (phytotransformation) or can volatilize into the atmosphere (phytovolatilization); pollutants in soil can be degraded by microbes in the root zone (rhizoremediation) (1,6,9). Adapted from Van Aken (104).

Figure 2

The three phases of the green liver model. Hypothetical pathway representing the metabolism of 2,3'-dichlorobiphenyl in plant tissues: Phase I, activation of the PCB by hydroxylation; Phase II, conjugation with a plant molecule (sugar); Phase III, sequestration of the conjugate into the vacuole or cell wall. Adapted from Van Aken (104).

Figure 3

Bacterial aerobic degradation of lower-chlorinated PCBs is catalyzed by biphenyl dioxygenase (bph) gene cluster (upper pathway). Adapted from Furukawa et al. (61).