Mechanistic Role of Plant Root Exudates in the Phytoremediation of Persistent Organic PollutantsEPA Grant Number: R829405
Title: Mechanistic Role of Plant Root Exudates in the Phytoremediation of Persistent Organic Pollutants
Investigators: White, Jason C. , Gage, Daniel J. , Gent, Martin P.N. , Mattina, MaryJane Incorvia , Smets, Barth F.
Institution: Connecticut Agricultural Experiment Station , University of Connecticut
EPA Project Officer: Lasat, Mitch
Project Period: November 1, 2001 through October 31, 2004 (Extended to December 14, 2005)
Project Amount: $401,241
RFA: Phytoremediation (2001) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
This proposal is designed to investigate the role of plant root exudates in the phytoremediation of persistent organic pollutants in soil. Preliminary data have shown that two weathered organic pollutants (p,p'-DDE, chlordane) are readily translocated from soil to the tissues of certain plants. These findings contradict a significant body of scientific evidence indicating time-dependent reductions in contaminant availability in soil (i.e., sequestration). We propose a novel mechanism of phytoremediation whereby plant root exudates increase the bioavailability of weathered contaminants. The following hypotheses will be tested:
(1) The root exudates of certain plant species facilitate the mobility and subsequent availability of weathered organic pollutants.
(2) Contaminant solubilization by exudates occurs by direct or indirect mechanisms. In direct enhancement, the exudate molecules directly induce contaminant release from the soil. Possible mechanisms here include the formation of exudate/contaminant complexes or the partial solubilization/reformation of soil structure organic fractions through chelation of polyvalent metals (iron and aluminum). A second hypothesis considers indirect enhancement, where root exudates stimulate a microbial community that promotes contaminant availability to the plant.
For objective 1, an initial hydroponic experiment will compare the transport of aqueous persistent organic pollutants across the root plasma membranes of good and poor uptakers. Plant root exudates will be isolated and characterized from plants shown to have differential pollutant uptake/remediation abilities in the field. A series of abiotic assays comparing contaminant release in the presence and absence of these exudates will test the central hypothesis. Biosensors will be constructed for in situ visual detection of the spatial and temporal release of critical exudate constituents from roots. For the direct enhancement hypothesis of objective 2, batch-style chelation, solubility enhancement, and desorption assays will be employed. Also, plants will be grown in laboratory-contaminated yet non-chelatable solid media (C 18/sand). For the indirect enhancement hypothesis of objective 2, the microbial communities of plants with differential uptake/remediation potentials will be characterized by 16s rRNA-targeted T-RFLP profiling, and critical microbial exudates will be investigated for their role in promoting pollutant availability.
The first benefit of this work is that it addresses a group of pollutants that are difficult to remediate by conventional means but are of concern due to their potential for toxicity and bioaccumulation. We have data concerning the availability persistent organic pollutants to plants that cannot be reconciled by the traditional mechanisms of phytoremediation. We have proposed a novel mechanism focusing on the role plant root exudates that is of scientific and practical interest. By precisely defining this mechanism, we can address a much broader range of experimental systems to maximize the elucidated processes. That includes not only novel plant-contaminant combinations but also artificially designed systems including the isolation and introduction of critical exudates from "hyperaccumulators," as well as large scale industrial synthesis and application of the most effective exudate materials. Lastly, one could envisage recombinant DNA technology to construct plants or rhizosphere microorganisms that hyper-produce the desired exudate constituents. The second major benefit of this work relates to the issue of sequestration and bioavailability of organic contaminants in regards to establishing risk-based regulatory guidelines for remediation. Current thinking is that bioavailability, toxicity, and the risk of organic contaminants in soil declines with aging time. Consequently, the availability of highly weathered (several decades) pesticide residues like chlordane and p,p'-DDE should be minimal but we have shown that not to be true for certain plants. These findings underscore the fact that although extensive, the compounds and assays used to demonstrate sequestration are far from comprehensive. Our preliminary data suggest that extreme caution should be taken in determining exposure and risk based on any premise other than total contaminant concentration.