Superfund Research Program
Plant Root Exudates Facilitate Phytoremediation
An increasing body of evidence indicates that phytoremediation is an effective in situ tool for the removal or degradation of toxic contaminants in soil, sediment, and water. Phytoremediation can target heavy metals, metalloids, petroleum, hydrocarbons, pesticides, explosives, chlorinated solvents, and industrial by-products. Removal or degradation of these compounds is thought to occur in the rhizosphere - the region of the soil influenced by the presence of plant roots.
The physical and/or chemical mechanisms by which plants impact biodegradative activity have not been identified. Scientists must study not only how the individual rhizosphere components function in isolation, but also how the plants, microorganisms, and soil interact to function together in the environment. As an ecological system, the rhizosphere probably has emergent properties that differ from the sum of its individual parts. Researchers at the University of Cincinnati (UC) are conducting experiments to identify the factors that drive or inhibit the activities of rhizosphere systems.
The UC team is working to understand the role played by plant-released materials (i.e. root exudates) in rhizosphere degradation. Their focus is to determine how root exudates affect microbial communities. In on-going studies, the UC researchers are investigating the effect of soluble root exudates on polycyclic aromatic hydrocarbon (PAH) degradation. The mechanisms by which the exudates impact biodegradation are not clearly understood. Exudates may act as carbon substrates or cofactors that support larger microbial populations in the soil or they may create soil conditions that favor specific degradative microbial communities. It is possible that exudates may contain enzymes that work directly on contaminants in the soil or they might act indirectly by changing the bioavailability of organic contaminants in soil.
To determine if root exudates alone can induce a "rhizosphere effect", the UC researchers compared mineralization of pyrene in root exudate-treated soil columns to mineralization in actual rhizosphere soil. Sterile exudates from corn (Zea mays L.) plants flowed directly onto unplanted soil, eliminating all rhizosphere factors except the chemical influence of the soluble exudates. This study is unique because unlike many studies that evaluate the addition of simple carbon substrates in short pulses, the UC study attempted to expose the soil microbial community to real root exudates in concentrations and time periods that mimic actual conditions. Although the researchers noted that the study likely provided low substrate loadings and may have underestimated the rhizosphere effect, they determined that the addition of root exudates stimulated the mineralization of pyrene in unplanted soil to the same degree as observed in actual rhizosphere soil.
The UC team conducted analyses to evaluate the ability of microbial communities found in soil to use root exudates as sources of carbon. Soil microorganisms were clearly able to utilize root exudates when they were provided as sole carbon sources. In liquid cultures, root exudates supported the growth of soil microorganisms better than glucose. Root exudates are complex mixtures that have been found to contain many nutrients necessary for growth, such as vitamins and co-factors. As a result, exudates have the potential to both enhance the growth of microbial populations and stimulate the cometabolic attack of xenobiotics in the rhizosphere.
In separate studies, analyses were conducted to evaluate the impact of root exudates on the microbial community. Long term application of exudates to bulk soil containing pyrene resulted in an increased mineralization of pyrene, but minimal stimulation of biomass and activity. This indicates that simple increases in microbial numbers or activities were not responsible for the increased mineralization. Multivariate analysis of the data revealed that exudate amendment resulted in functional shifts in microbial communities. This suggests exudate pressures may shift community structure in favor of the populations of bacteria capable of degrading pyrene.
Separating the chemical impact of the root exudates from any root surface phenomena is an important step in isolating the potential of plant root exudates to stimulate degradation. The effective and successful application of phytoremediation depends on our understanding of the roles of plants and microbes, as well as their ecological constraints. While much remains to be learned about the basic mechanisms of phytoremediation in ecological systems, the studies conducted at UC are providing a solid foundation of information. The UC research provides direct evidence that plant-derived chemicals are a significant factor in enhanced rhizosphere degradation. Further studies will focus on the components in root exudates that most increase degradation. Plant species for use in phytoremediation could then be selected on the basis of exudate composition and production. This information would be directly applicable to the management of actual brownfields or Superfund sites and would further the development of phytoremediation as an in situ technology.
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To learn more about this research, please refer to the following sources:
- Yoshitomi KJ, Shann JR. 2001. Evidence of corn (Zea mays L.) root exudate carbon utilization by microorganisms and its impact on 14C-Pyrene mineralization. Soil Biol Biochem 33:1769-1776.
- Boyle JJ, Shann JR. 1998. The influence of planting and soil characteristics on mineralization of 2,4,5-T in rhizosphere soil. J Environ Qual 27:704-709.
- Cunningham SD, Shann JR, Crowley DE, Anderson TA. 1997. Phytoremediation of contaminated water and soil. In: Phytoremediation of Soil and Water Contamination: ACE Symposium Series 664. American Chemical Society,
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