By Ben Harder
A human gene that Japanese researchers have inserted into rice enables the plant to break down a portfolio of chemicals now used on farms to kill weeds. The unusual breadth of that herbicide resistance could circumvent a major shortcoming of existing genetically engineered crops and also open new avenues for cleaning up contaminated soils.
Some scientists, however, are concerned that weeds growing with the rice could eventually acquire the human gene and become herbicide-resistant superweeds.
The herbicide resistance of many crops, including much of U.S. soy and cotton, results from genetic elements that scientists have transferred from other species. These engineered plants can tolerate powerful weed-control chemicals. To date, most plants tweaked this way are resistant to only one type of chemical, so farmers must use both the herbicide-resistant crop and the matching herbicide to keep weeds at bay without killing the plants they want to harvest.
However, weeds exposed to the same herbicide, season after season, are more apt to develop chemical resistance than are weeds treated with alternating herbicides, says molecular biologist Sharon L. Doty of the University of Washington in Seattle.
Plants and animals make numerous enzymes, called cytochrome P450s, that break down various harmful chemicals. One such human enzyme, CYP2B6, disables more than a dozen herbicides, pesticides, and industrial chemicals. For some of these compounds, CYP2B6 works better than natural rice enzymes do.
So, plant biologist Sakiko Hirose of the National Institute of Agrobiological Sciences in Tsukuba, Japan, and her colleagues added to rice seeds the human gene that makes CYP2B6.
They found that the engineered and traditional plants grew similarly in the presence of any of 4 herbicides and that the engineered plants were healthier in tests of 13 other herbicides, including one called metolachlor.
In additional experiments, the scientists determined that the transgenic plants disarm metolachlor more rapidly than the other plants do. After 3 days growing in a solution containing metolachlor, transgenic seedlings contained only 0.2 percent of the herbicide initially present, whereas standard rice contained 4 percent.
Moreover, almost none of the original metolachlor remained in the transgenic plant’s growth medium, while one-quarter of the herbicide persisted in media hosting standard plants, the researchers report in an upcoming Journal of Agricultural and Food Chemistry.
The Japanese team’s approach to genetic engineering could produce crops tolerant of a rotating regimen of herbicides, a practice that should delay the emergence of herbicide resistance in agricultural weeds, Doty says.
On the other hand, she adds, “strict measures” must be taken to confine the gene for CYP2B6 to the desired crops. “If the transgene is transferred to weeds, then the weeds will be resistant to a very broad spectrum of herbicides,” Doty notes.
Beyond potentially giving crops an edge over weeds, the new technique could make plants more useful in laundering environmental contaminants, including herbicides used on cropland. Says Richard Meilan of Purdue University in West Lafayette, Ind., “It might allow us to grow more rice efficiently and go light on the environment at the same time.”
