Tuesday, November 21, 2006

Edible Cotton

Researchers have used a new gene-silencing technique to engineer edible cottonseeds.

By Katherine Bourzac

To make cottonseed edible, researchers reduced its levels of toxic gossypol (black spots in the top picture) using a new gene-silencing technique. Credit: Keerti Rathore, Texas A&M University / PNAS

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."