ScienceNow: Taking the Toxin out of Cotton-RNAi for Agriculture

By Erik Stokstad
Science NOW Daily News
20 November 2006

King cotton is famous for its puffy-white bolls, but the plant actually contains much more seed than fiber, by weight. Rich in oil and protein, some of that seed is fed directly to livestock and poultry, but oil from the seeds must be purified before people can eat it. That's because the seeds are laced with a natural toxin that can damage the heart and other internal organs, as well as lower fertility. Now a team of researchers has engineered a cotton plant with toxin-free seeds. If the variety is approved by regulators, it could lower the cost of cottonseed oil and provide a plentiful source of cheap protein for subsistence farmers.

Plant breeders have been trying for decades to eliminate the toxin, called gossypol, which cotton plants use to ward off insects. In the 1950s, plant breeders discovered a mutant that completely lacked the compound, but it was vulnerable to pests and a commercial failure. To target seeds while keeping defenses intact in the leaves, molecular biologists have begun to tinker with the metabolic pathways seeds use to make gossypol. Yet so far, they have had minimal success at cutting seed gossypol levels. A team led by molecular biologist Keerti Rathore of Texas A&M University, College Station, decided to try RNA interference (RNAi) technology, which silences specific genes.

Rathore's group targeted the δ-cadinene synthase gene, one of the first steps in the creation of gossypol. They constructed a sequence of interference RNA that would block the gene from making protein and added a genetic element to ensure that this only happened in the seeds. The strategy worked: Transgenic plants had 99% less gossypol in their seeds but regular levels elsewhere, the team reports online this week in Proceedings of the National Academy of Sciences. "The plants--as far as we can tell--are pretty normal," Rathore says. As for the seeds, he says they taste like chickpeas.

Danny Llewellyn, a cotton breeder with CSIRO Plant Industry in Canberra City, Australia, says that the new cotton variety must still pass muster in field trials, but he's excited about the potential of RNAi. "It will make cottonseed a more widely used animal feed and open up uses of cotton for human food production, particularly in developing countries," he notes. However, he cautions that gossypol-free seeds might be more vulnerable to mice because of their reduced toxicity.

Commercial varieties could also lower the cost of cottonseed oil by eliminating the need to extract the gossypol. Rathore is looking for an industry partner to help shoulder the cost of regulatory review. But don't look for cotton seeds on the menu anytime soon: Rathore says it could be 20 years before gossypol-free cotton reaches .

Safe.
Engineered cotton seeds contain extremely little of the toxin gossypol.Credit: G. Sunilkumar; (inset) PNAS