Transgenic Plants Sop Up Soil Pollutants

Researchers have inserted four bacterial genes into rice and a flowering plant to turn the organisms into pollutant-cleaning crops (Environ. Sci. Technol. 2014, DOI: 10.1021/es5015357). The plants, which break down an organic pollutant, could someday play a role in phytoremediation, a process that uses vegetation to clean up contaminated ground for agricultural or other human uses.

Plant species used for phytoremediation mostly absorb heavy metals and organic contaminants from soil. Unfortunately, sopping up pollutants can sicken the plants, making them yellow or stunting their growth. Bacteria, which can sequester heavy metals and metabolize complex organic pollutants into less toxic by-products, could be applied directly to soil to absorb contaminants. However, environmental scientists would still have the difficult task of removing the microbes from the soil. With plants, the final cleanup is relatively easy: Yank out the crops.

Transferring the bacterial genes responsible for pollutant metabolism into plants could solve the problem of plants feeling the toxic effects. But researchers thought it would be difficult to get the bacterial proteins to express and then assemble into the necessary large enzyme complexes.

Quan-Hong Yao of Shanghai Academy of Agricultural Sciences and his colleagues set their sights on engineering plants that could metabolize the soil pollutant phenanthrene, a polycyclic aromatic hydrocarbon (PAH) that is released when coal, oil, or garbage are incompletely burned. For the bacterial genes, they turned to the soil microbe Pseudomonas putida, which uses four genes to form a naphthalene dioxygenase enzyme complex to break down phenanthrene to less harmful metabolites. The researchers inserted all four of these bacterial genes for the first time in the model plant species Arabidopsis and in rice plants (Oryza sativa).

Rice and Arabidopsis plants engineered with the bacterial genes absorbed about three to six times more phenanthrene from soil and water than plants without the bacterial transplant. But even though they drew more of the pollutants out of the soil, the transgenic plants didn’t suffer toxic side effects as wild-type plants did.

The transgenic plants are “a step in the right direction,” says plant scientist Om Parkash of the University of Massachusetts, Amherst. However, if the plants are to be used in the field, the researchers will need to make them more efficient at absorbing and metabolizing the pollutants, Parkash says. In the paper, Yao and his coauthors suggest that transferring these genes into perennial grasses or other robust plants may lead to increased efficiencies.

One other hurdle the team must deal with is the low water solubility of phenanthrene and other PAHs, which means the compounds aren’t easily available for absorption by plants. Yao and his colleagues helped improve absorption in their experiments by adding organic solvents to create a slushy soil. But for real field conditions, they will need another method—perhaps supplementing the soil with specific microbes, Parkash says.