Scientists 'Beef Up' Plant-dwelling Bacteria To Boost Phytoremediation

"By introducing genes for the appropriate degradation pathways into natural plant-dwelling bacteria, known as endophytes, we should be able to tailor-make plants capable of cleaning up a variety of organic pollutants," said Brookhaven biologist Daniel (Niels) van der Lelie, one of the lead authors on the paper. He also envisions introducing pollutant-degrading pathways into bacteria that live in crop plants to reduce the residues of pesticides and herbicides that make their way into our food.

Van der Lelie maintains that the technique should win widespread acceptance because it uses only naturally occurring bacteria and natural gene-transfer methods.

The scientists started with a type of bacteria that naturally colonizes the roots and stems of their test plant, yellow lupine. They mixed these bacteria with a related soil-dwelling strain known to degrade toluene. This allowed the strains to share genetic material through a natural process known as bacterial conjugation.

They then selected for the endophytic bacterium that had acquired the capability to grow on toluene, and used this strain to inoculate yellow lupine plants. After allowing the inoculated plants to grow for 21 days, the scientists analyzed the bacterial content of their roots and shoots using selective growth media containing toluene to confirm plant colonization by the so called "ENDEGRADER" bacteria.

The scientists then compared the ability of these plants to grow in an environment containing toluene (both hydroponically and in non-sterile soil in greenhouse studies) with that of non-inoculated plants and plants inoculated with the soil bacteria. They also measured the amount of toluene released from the plants' leaves via evapotranspiration.

Plants inoculated with endophytic bacteria that had acquired the toluene-degradation pathway were able to grow in the toluene-contaminated environment under both hydroponic and greenhouse conditions, even when the levels of toluene present killed the other test plants. Furthermore, plants inoculated with the toluene-degrading endophytic bacteria released three to four times less toluene into the atmosphere.

"These results confirm our hypothesis that endophytic bacteria, when equipped with the appropriate degradation pathway, can help plants survive under conditions with elevated levels of pollutants, and improve the performance of plants used to remove these contaminants from the environment," van der Lelie said.

The next step will be to test the technique in poplar and willow trees, deep-rooting species already used in phytoremediation. "In trees, the time between the uptake of the pollutant by the roots and its arrival in the leaves can take several hours to days, allowing sufficient time for efficient degradation by endophytic bacteria in the plant tissue," van der Lelie said.

The scientists have already isolated 150 bacterial species that live as endophytes in poplar and are beginning experiments to see which will be most amenable to gene transfer. The bacteria containing the degradation pathways will be isolated from contaminated environments.

The collaborators on this research, which is part of the European Research Project ENDEGRADE, are from the University of Limburg (LUC) in Diepenbeek, Belgium, and the Flemish Institute for Technological Research (VITO), where Daniel van der Lelie and his wife Safiyh Taghavi hold joint appointments and where they initiated this research. The work was supported by the European Commission under the Fifth Framework Program, Quality of Life; the Ford Motor Company; and Brookhaven Laboratory discretionary funding.

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