“Beefed Up” Plant-Dwelling Bacteria Boost Phytoremediation

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.

The root of a poplar tree, (magnified 1000X) colonized by endophytic bacteria containing a green fluorescent protein to mark their presence. Equipped with metabolic pathways that enable them to degrade environmental pollutants, these microbes can increase the plants’ effectiveness in phytoremediation.

They then selected the endophytic bacteria 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.

Already, the researchers have identified 150 bacterial species that live as endophytes in poplar, and they are starting experiments to see which species will be most amenable to gene transfer. Then, they will select bacteria able to grow in the contaminated areas that need to be cleaned up, and mix those bacteria with the endophytes to encourage gene sharing. New strains that result will then be inoculated into poplar and tested for their ability to clean up pollutants in field studies.

More Information