By Mark Finlayson, Checkbiotech

Making use of nature’s own arsenal against hazardous compounds is a promising strategy in remediating polluted areas. A Chinese research group has been looking into how plants can contribute towards removing toxins from contaminated soils.

Many hazardous compounds can enter the soil environment as a result of pesticide application, foliar washoff, waste burning, or even accidents at chemical production sites. Some of them are not only hazardous to human health or the environment, but are also very persistent and tend to bind strongly to soil particles, making removal very difficult.

Plenty of research has been done with detoxifying microbial communities in order to find environmentally friendly ways of ridding the ground of unwanted substances. An alternative to bacteria are plants. The biological remediation of environmental problems using plants is called phytoremediation.

During the course of evolution, plants have come up with a wide variety of biochemical responses that enable them to adapt to the changing environment, or to protect themselves from chemical attacks from neighboring plants. The enzymes plants have developed as a defense mechanism often catalyze the transformation of toxic compounds into non-toxic or less toxic compounds, which can also be used to degrade harmful pollutants that humans often create.

Plants can either take up the pollutants and accumulate them before degrading them, or they can simply release the detoxifying enzyme into the soil, thus creating a nontoxic microenvironment. One main advantage of so-called ex-planta detoxification is that the plant cells are not actually directly exposed to the toxin.

In a resent Nature publication, Xiao-Ya Chen’s laboratory at the National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, put this strategy to use, and have conducted trials using one such secretory enzyme, laccase, which occurs naturally in cotton. In the experiment, the gene coding for laccase, LAC1, was transferred to Arabidopsis thaliana plants along with a Cauliflower Mosaic Virus promoter to ensure the plant would produce large amounts.

In the presence of phenolic compounds such as syringic or vanillic acid, the growth of LAC1 seedlings was then compared to that of control plants. Since LAC1 can oxidize, and thus, nutralize these toxic compounds, the seedlings of the transgenic lines grew better and showed more leaf growth and root development. In another experiment, the plants were exposed to highly toxic chlorophenols such as TCP. Once again, the transgenic lines coped better and exhibited a more extensive root system than the control seedlings.

One set-back is that nontoxic phenolic compounds within the plant itself are also degraded. Fortunately, this doesn’t seem to have any affect on the plant’s growth as the compounds in question are dispensable for development.

The experiments showed that the laccase gene can be successfully transferred to other plants that lack its cleansing ability. For larger scale phytoremediation, larger plants can be used, or plants with higher levels of secreted laccase activity can be selected. Also, the treatment could be extended to other soil pollutants, if suitable enzymes are availible.

All in all, engineering plant secretory enzymes provides a novel strategy not only for producing more competitive crops, but also for removing toxins from soil.

To contact Dr. Xiao-Ya Chen, send him an email at xychen@sibs.ac.cn

Mark Finlayson is a Biology student at the University of Basel and a Science Writer for Checkbiotech. Contact him at m.finlayson@stud.unibas.ch about this article.

Link: Ex planta phytoremediation of trichlorophenol and phenolic allelochemicals via an engineered secretory laccase