Catabolic Enzymes and Metabolic Pathways in Phytoremediation

The Beginning
Phytoremediation of organic xenobiotic chemicals is a technology developed at The University of Iowa for treating soils, sediments, and groundwater. It recognizes the power of plants to recycle and treat waste chemicals through nature’s cycles. It is already in use at hundreds of waste sites throughout the United States, and the Environmental Engineering and Science (EES) group has won several awards for its introduction, including the Rudolf Hering Medal from the American Society of Civil Engineers and the Montgomery-Watson Best Thesis Award from the Water Environment Federation. Phytoremediation promises cost-effective treatment of wastes by utilizing the largest source of biocatalysts on earth – enzymes from the plant biosphere.

The Promise
First mentioned in the literature in 1993, phytoremediation has been applied to contaminated sites with chlorinated solvents, petrochemicals, explosives compounds, metals and other industrial chemicals. But a detailed scientific basis for the technology is still lacking. There is little documentation of the basic enzymes and metabolic pathways that occur when plant tissues are exposed to xenobiotic chemicals. In this proposal, we seek to investigate the metabolic pathways and catabolic enzymes involved in the transformation of toxic organic pollutants by plants (objective 1). As a consequence of this lack of fundamental insights about plant metabolism, an objective screening methodology does not exist to answer the simple question, “which plants should be utilized for which chemicals?” We intend to answer that question by developing a screening method for systematic plant selection based on the tracking of DNA-sequences coding for specific catabolic enzymes involved in the transformation of toxic pollutants (objective 2). Finally, the amazing explosion of genetic engineering in plant technology leads us to consider the potential of genetically modified plants for the improvement of their intrinsic degradation capacities. Even though this approach has been pioneered by several research groups, species successfully transformed to date are limited to laboratory model plants and have not led to field applications. Using recent insight about plant factors limiting the transformation process, we will clone useful catabolic enzymes inside plant species that are promising for phytoremediation applications (objective 3). In short, the field of phytoremediation needs to be placed on a more scientific foundation by using fundamental biochemistry and molecular biology.

The Future
The Keck Foundation grant will propel this research to the next level. University of Iowa investigators will characterize the activity of enzymes, identify and purify them. They will analyze metabolites to determine metabolic pathways that occur in whole (intact) plants and tissue cultures, including spherical nodules, a new bioassay technique developed for phytoremediation applications during the past year. The proposal seeks discovery of the range of enzymatic transformations that are possible in the plant world. Basic enzymology of cultivated plants and their catabolic capacities are needed. We must learn which plants (and their enzyme systems) can be utilized to degrade which classes of chemicals. PhytoTechnologies require a whole new area of discovery and application that recognizes the power of plants to treat wastes through nature’s cycles.

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