Toxic seed becomes hope for the hungry
Scientists reengineer cottonseed. Now, they aim to turn more poisonous plants into human food.
Where most people might look at a white-capped cotton plant and see the makings of next year's T-shirts, Keerti Rathore sees food for a hungry world.
Dr. Rathore and his colleagues have figured out how to make poisonous cottonseeds fit for human consumption. The new, nontoxic seeds could give 500 million people an additional source of high-quality protein, the team estimates, if the genetically engineered plant is approved for cultivation.
In principle, this approach could expand the array of plants or plant parts humans could eat without heavy processing or precooking preparation. Rathore's team virtually shuts off the gene responsible for the toxin. The researchers don't replace the gene or add one to get the desired trait. Thus, some researchers suggest, the technique might be more politically palatable to people who oppose genetically modified crops.
"This is a nice piece of work," says Steve Scofield, a molecular biologist at Purdue University in West Lafayette, Ind. Still, he cautions, "the biggest issue they've got is that this will be viewed as a [genetically modified] plant. So there may be a public-acceptance issue."
Crop scientists have been exploring the approach, known as RNA interference, to reduce or eliminate compounds they have tied to allergic reactions to foods.
Coming up with a toxin-free cottonseed "is not that straightforward," says Rathore, who heads the Laboratory for Crop Transformation at Texas A&M University. His team's results are reported in the current issue of the Proceedings of the National Academy of Sciences.
Scientists have pursued the goal for 50 years. Cotton is grown worldwide, and for every two pounds of fiber, a cotton field also yields slightly more than three and a half pounds of seed – totaling some 44 million tons a year.
Cottonseed is the third-largest oil-seed crop in the world, according to the US Department of Agriculture. But its oil requires heavy processing for use in cooking. And the raw seeds are fit to feed only livestock such as cows.
In the 1950s scientists stumbled onto a variety of cotton that didn't have the toxin and bred it with commercial varieties.
"The seeds were good enough to eat," Rathore says. But without the toxin, known as gossypol, the plant cannot defend itself against pests and pathogens.
Thus, for Rathore and colleagues at the USDA's Southern Plains Agricultural Research Center, gossypol became a prime target for RNA silencing – a technique for switching genes off that earned two US scientists a Nobel Prize this year. Genes are long strands of DNA – a twisted, ladder-shaped molecule that carries the blueprint for organic life.
RNA, a close relative of DNA, communicates a gene's information to a cell's proteinmaking apparatus. Shaped like a ladder split down the middle, this messenger RNA is specific to a gene. By joining a strand of messenger RNA with its mirror opposite, scientists learned that the new, combined RNA molecule starts to silence the messenger RNA.
Rathore and his team used this biological "smart bomb" to target the gene linked to gossypol. In addition, the team was able to tailor its approach so that the rest of the plant could produce enough gossypol to meet its defensive needs. But gossypol production in the seeds was suppressed to nontoxic levels.
The team is looking at detoxifying traditional food plants, such as cassava and fava beans, as well as "grass peas" and so called "famine crops," which become a food of last resort for areas hit by drought.
Despite the approach's potential, it's not clear if it will quiet critics of GM crops, notes Douglas Gurian-Sherman, a senior scientist at the Union of Concerned Scientists in Washington.
The team used genetic material foreign to the plant to silence the gene – although eventually the plant's own genetic material could be used for both strands of the RNA off-switch, other scientists say.
Still, "in biology, there is no free lunch," Dr. Gurnian-Sherman says. In this case, he continues, the approach is new and the full range of effects aren't well known.
Some studies have suggested that RNA interference may not be as gene-specific as researchers may think. And lower gossypol levels could still render the seeds more vulnerable to pests or disease.
Instead, the approach may be most acceptable – at least for now – in helping breeders develop more-nutritious crops through traditional crossbreeding.
For instance, scientists at the University of California at Davis, the USDA, and the University of Haifa in Israel have developed a new strain of more-nutritious wheat by crossbreeding it with an ancient type of wheat known to have higher protein, iron, and zinc content. The researchers used RNA interference to ensure that the gene of interest was responsible for the enhancement. Their results appear in the current issue of the journal Science.