Researchers at Texas A&M University have selectively reduced the levels of a toxic chemical in cotton, making the seeds edible and potentially transforming cotton into an important new food source.
Researchers led by Keerti Rathore, associate professor in the university’s Institute for Plant Genomics and Biotechnology, used a new gene-silencing technique called RNA interference (RNAi) to eliminate the toxin. Biologists can silence a specific plant gene by inserting a gene that codes for double-stranded RNA whose sequence is very similar to that of the gene of interest. The presence of the double-stranded RNA causes cells to destroy any messenger RNA from the targeted gene, effectively silencing it.
In cotton, Rathore constructed an RNA gene that could silence the gene for an enzyme crucial to the biosynthesis of gossypol, a toxic compound found in cotton. Rathore used a promoter specific to seed tissue in cotton plants so that the gene silencer would not be turned on in other cotton tissues. Gossypol is normally produced in almost every tissue of cotton plants and seems to protect them from insects and from fungal and bacterial infections.
“This is an excellent example of the usefulness of RNA-silencing technology to improve crop quality and food healthfulness by removing toxic or unhealthy compounds, which are far more prevalent in food plants than most people realize,” says Richard Jorgensen, professor of plant sciences at the University of Arizona. Jorgensen was the first to observe this kind of gene silencing in petunia plants in 1990.
Animals with multiple stomachs, like cows, can eat cottonseeds, but gossypol is a heart and liver toxin in humans and other animals, including poultry. “If you feed a chicken a cottonseed-only diet, within a week it will die,” says Rathore.
For every pound of fiber produced by cotton farming, 1.6 pounds of seed are produced. Cottonseed is potentially very nutritious: 23 percent of the seed is high-quality protein. But much of what is not used for replanting is thrown away or given to cows. The real value of edible cottonseed, says Rathore, is for the many farmers in poor countries who are growing one or two acres of the plant for its fiber but are not able to use the crop for food.
Cotton plants are not the only candidate for this kind of genetic engineering, says Jodi Scheffler, a research geneticist in the USDA’s Crop Genetics and Production Research Unit. In fact, she says, “this application [of RNAi] may be more important for other crop species. There are many other species with seeds with toxic components.”
Traditional breeding doesn’t work for eliminating toxins like gossypol because completely eliminating them leaves plants vulnerable. In 1954, researchers found a cotton plant that did not make gossypol, and in the early 1960s they crossbred it with the agricultural version of the plant. The seeds of this plant could be roasted and eaten like a nut or crushed and mixed with flour to make high-protein bread. “Farmers realized the plants were being chewed up by insects and didn’t want to grow them,” says Rathore.
The advantage of Rathore’s RNAi approach is its specificity. The seeds are nearly free of toxin, but the rest of the plant has normal levels of gossypol, so it should be hardy. The same holds true for the next two generations of plants grown from the seeds, Rathore says.
Abhaya Dandekar, professor of pomology at the University of California-Davis, who works on transgenic plants, says the gene-silencing technique could be broadly useful. “The only issue is how stable this will be in production,” he says. “Environmental conditions and viruses can interfere with the gene-silencing mechanism in plants.”
Scheffler says that the Texas A&M researchers face an additional challenge in getting the edible cotton plants onto farms. The cotton plants used for research like Rathore’s are not the same ones in agricultural use. Traditional breeding will be needed to transfer the trait to the agricultural cotton species. Researchers will then need to test the resulting plants for the stability of the RNAi.
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