Pollution of air, soil and water with toxic metals such as lead, arsenic, mercury and zinc is a major environmental problem that poses a significant threat to human health. Increasing industrialization, automobile usage, mining and agriculture are some of the sources of toxic metal pollution (1). Mercury for example is a very common contaminant in industrial effluents and is also a component of many pesticides. Mercury leaches from the contaminated sites into the water system and often gets concentrated in the food chain. In the United States alone, the cleanup of sites contaminated with heavy metals is estimated to cost $7.1 billion (1).
While there are several ways of treating organic pollutants through degradation, there are few, if any, safe and inexpensive approaches for reducing the toxic metal pollution in our ecosystem as the metals cannot be degraded chemically. A recent report from the University of Georgia group led by Dr. Richard Meagher offers a new hope that genetically engineered plants can one day be used in our fight against metal ion pollution in the environment (2). The research article describes the efforts of Dr. Meagher's group in successfully developing transgenic Arabidopsis plants that convert toxic ionic mercury to a less-toxic vapor form.
The merA gene employed in this research is of bacterial origin and encodes the enzyme mercuric ion reductase, which catalyzes the reduction of toxic Hg2+ to less toxic, relatively inert, nonionic Hg. This gene in its native form did not function when introduced into plants. Thus, the Georgia group using polymerase chain reaction approach modified the nucleotide sequence of this gene to be more 'plant-like' by changing the codon usage, altering the flanking sequences and decreasing the total G+C content from 65% to 47%. The new merApe9 gene was introduced into Arabidopsis using the standard Agrobacterium mediated gene transformation procedure. When the resulting transgenic plants were grown in a medium laced with toxic levels of HgCl2 (5-20 ppm), they developed normally, producing flowers and seeds. The control untransformed plants, however, either did not germinate or died quickly when exposed to even low levels of mercury. Interestingly, a few transgenic plant lines seemed to prefer mercury because they grew better on media with HgCl2 while performing poorly on media without the toxic metal. The transgenic Arabidopsis plants with the mercury reductase gene were also resistant to gold ion (Au3+), suggesting a broad range of substrates for the mercuric ion reductase.
When tested with a mercury vapor analyzer, transgenic plants grown on HgCl2 medium released volatile mercury in the air, which indicates that these plants were converting the toxic mercury supplied in the nutrient medium to a vapor form. The level of such reduction to the nonionic form of mercury was seven times greater in transgenic plants than in the control plants. Further, the level of nonionic mercury evolution was directly proportional to the steady-state mRNA levels of the merA gene, providing evidence that mercury detoxification observed in transgenic plants was due to the action of the introduced gene.
Dr. Meagher's group proposes that when planted in mercury-contaminated soils, transgenic plants expressing the mercury detoxification gene may hasten the biological transformation of this toxic compound. Especially when planted around the wetlands where the mercury-problem is most acute, transgenic plants would convert the toxic mercury to less-toxic vapor form which is then released into the atmosphere where it would be further diluted through distribution. If successful under further field studies, the 'mercury-eating' plants may thus help reduce the toxic metal pollution of the biosphere. Phytoremediation through biotechnology clearly has a potential to ameliorate the excesses of other technologies.
1. Salt, D. E. et al. 1995. Phytoremediation: A novel strategy for
the removal of toxic metals from the environment using plants.
2. Rugh, C. L. et al. 1996. Mercuric ion reduction and resistance in transgenic Arabidopsis thaliana plants expressing a modified bacterial merA gene. Proc. Natl. Acad. Sci., USA 93: 3182-3187.
C. S. Prakash
Center for Plant Biotechnology Research