By Annette Ryser, Checkbiotech

Plants are a very potent tool used to remove toxic chemicals from contaminated areas. In this process, called phytoremediation, transgenic plants play an important role.

Many industrial, municipal and military areas have become contaminated with hazardous pollutants. This constitutes a long lasting potential danger, because emission is possible through air, ground, water or food chain. Therefore, the grounds should be remediated, that is, cleansed from the toxic substances. There are several methods to do this, and they are used differently depending on the pollutant that needs to be removed.

One method is the microbiological remediation system (MRS), where microorganisms are in charge of breaking down the pollutants. This approach has proven to be quite successful. It can also be used in-situ, which means, the contaminated ground can be cleansed without being removed. That is an important factor in cases of broad contamination, because removing the contaminated soil would be too expensive. The MRS can only remove harmful substances that are organic, but it removes them very effectively. This is in contrast to most of the physical and chemical remediation methods, which usually leave by-products.

Besides the MRS method, another option is to use plants. There are, for example, many plants that are known to naturally remove heavy metals. This process is called phytoremediation, and it promises to be an inexpensive and resourceful method. It is also very suitable for continuous remediation and large areas, since plants grow over a long time without needing much more than water.

Many reports for phytoremediation use transgenic plants. By genetically enhancing plants, the cleansing power of fungi, bacteria or in some cases plants can be given to plants that, in the end, more efficiently remove the toxic substance. Since many microorganisms are potent bioremediators, fungal and bacterial enzymes from microorganisms are often used to enhance the cleansing power of plants. An example for this are plants that reduce organic mercury with a bacterial mercury reductase, and are therefore more resistant to toxic levels of mercury (1).

Dr. Y. Iimura’s laboratory, at the National Institute of Advanced Industrial Science and technology (AIST) in Tsukuba Ibaraki, Japan, recently found that genetically engineered tobacco (Nicotiana tabacum) could degrade organic pollutants in the ground such as bisphenol A (BPA), pentachlorophenol (PCP) and other chlorophenols (2). What Dr. Iimura’s laboratory did was, they took a gene from a known, detoxifying, fungal enzyme (Coriolus versicolor) called laccase III, and inserted it into tobacco plants.

During their studies, Dr. Iimura’s laboratory learned that the transgenic tobacco not only produced the new enzyme, but also released it into the surrounding soil – a process that is called exudation. When their modified tobacco plants were put to the test, Dr. Iimura’s laboratory found that the plants were able to purify the soil of BPA and PCP after one week. Moreover, they showed in a different experiment that transgenic plants with the ability to remove BPA effectively also removed PCP from aqueous solution.

In-situ remediation of grounds through plants will hopefully soon become an easy and very useful tool. Dr. Y. Iimura hopes that the results from his laboratory will translate into plant-based technologies that will allow for quicker and more efficient removal of pollutants from contaminated soil.

References:

• Rugh, CL, et al. (1996): Mercuric ion reduction and resistance in transgenic Arabidopsis thaliana plants expressing a modified bacterial merA gene. PNAS 93: 3182-3187.
   
• Sonoki, T., et al. (2004): Transgenic tobacco expressing fungal laccase promotes the detoxification of environmental pollutants. Applied Microbiology and Biotechnology, Nov 12 (Epub ahead of print).

Annette Ryser studied biology at the University of Basel, Switzerland. She now works as a Science Journalist for Checkbiotech.