Can Plants with Microbial Enzymes Help Clean up Excess Chemicals?
Microbes and plants have both been studied as means to absorb and clean up specific chemical contaminants in a given ecosystem, a process known as bioremediation. A new Applied and Environmental Microbiology study uses a transgenic plant expressing a bacterial gene to improve the plant bioremediation activity for excess herbicide. This may be one step toward using plant-microbe combinatorial methods to clean other chemical spills.
The research, conducted by first author Xin Yan and senior scientist Jiandong Jiang, started by cloning the Sphingobium N-demethylase pdmAB with a chloroplast transit peptide-coding region into the model plant Arabidopsis thaliana. Chloroplasts are the action site of phenylurea herbicides (PHs), where the PH is converted into an intermediate form that inhibits the chloroplast photosynthesis system and thus inhibits plant growth.
Conceptual framework and experimental design of the transgenic plant-microbe combined remediation systems including (a) definition of 5 different treatments used, (b) testing of soil IPU levels under 5 different treatment, and (c) analysis of the bacterial community structure after each treatment. Source.
Sphingobium is a soil bacterium that can degrade a variety of aromatic compounds, including PHs, and the PdmAB gene alters the active form of the IPU and allows the plants to continuing growing in its presence. Much of the modified compounds are stored in the plant, which can grow as they accumulate, but some is secreted back into the soil. Researchers found that A. thaliana could remove PH and IPU from contaminated soils, and that the addition of Sphingobium to the soil enabled even further breakdown of the modified IPU compound. This combinatorial strategy was more successful than microbial or transgenic plant treatment alone (see schematic, right, for experimental set-up).
Why don’t the researchers simply mix the Sphyingobium or other microbes directly into the soil? While this works in some systems, it can be hard to maintain an introduced microbial population in a nonnative rhizosphere, while different plants can be engineered based on those that grow best in a given ecosystem. The deep root systems of plants also facilitate extensive reach of degradative enzymes, sometimes more than a given microbe’s niche would allow.
The authors suggest a mix of both the transgenic plants and degradative bacteria will be the most efficient way to completely remove pollutants. While the experiments were conducted with PHs, the research lays a foundation for removal of other complex organic compounds, such as those found in other herbicides or pesticides, that can be broken down by microbial enzymes. Future research may help transgenic plants to clean up and increase the safety of contaminated soils.