What started in a Petri dish like this one could ultimately provide protein for 500 million people every year (Image: Kathleen Phillips / Texas Agricultural Experiment Station)

Cotton that has been genetically engineered so its seed is no longer toxic could provide protein-rich food for poor countries. The researchers say the technology used could make other toxic plants safe to eat.

Cottonseed contains about 22% protein, and the cotton already produced worldwide has enough protein to meet the requirements of 500 million people. But it also contains the toxin gossypol, making it poisonous to animals, including humans.

In people, gossypol lowers blood potassium to dangerous levels, resulting in fatigue and even paralysis. A surprising side effect is that gossypol is an effective male contraceptive, but research on this aspect was abandoned in the late 1990s. Attempts to eliminate gossypol from cotton plants in the 1960s and 1970s failed: insects that had previously been kept at bay by the toxin happily ate the modified plant.

Keerti Rathore of Texas A & M University in the US has managed to remove gossypol from cotton seed without affecting the toxin load in the rest of the plant, meaning the plant will contain edible seed but not be destroyed by crop pests.

Reversed sequence

Rathore and his colleagues achieved this – after what he says has been a 10-year race with teams in China and Australia – using RNA interference (RNAi) technology.

The team chose to target an enzyme that is critical in the production of gossypol. They constructed a genetic sequence that would only be active in cotton seeds and which contained a portion of the enzyme’s gene, followed by the same sequence in reverse.

They introduced their construct into cottonseed cells. There, it generated a strand of messenger RNA with two halves which stuck to each-other in a hair-pin shape.

In cells, this hairpin RNA gets chopped up into small bits by a “dicer enzyme” and, through a mechanism that is not completely understood, the small bits stick to the original enzyme’s messenger RNA. This tags the RNA, labelling it for destruction by a cellular mechanism known as the RISC complex. The net result is that the production of gossypol in the modified cottonseeds is interrupted.

Roast and grind

Rathore says the kernel of the non-toxic cotton seed can be roasted and salted and has a “nutty flavour”. He says the kernel can also been ground into a flour and combined with wheat and corn flours to enrich them with protein.

Danny Llewellyn, a plant biologist at CSIRO in Australia who has also been trying to remove gossypol from cottonseed, says the research will “allow cottonseed to be used more widely as an animal feed and extend its uses as a substitute for other high-value oils, like canola (rapeseed) oil.”

Already, unmodified cottonseed is used in limited amounts to feed cattle, because bacteria in their rumen detoxify it. But the modified seed could make cotton plants more valuable to farmers because they could sell the seed as feed for pigs and chickens, or even human food, once they have sold the fibres around the seeds.

“It will certainly be a useful addition to cotton farmers in developing countries who grow cotton as a cash crop,” says Llewellyn. China and India are the world’s first and third largest cotton producers, respectively.

However, many developing countries have long resisted the introduction of genetically modified crops and cotton farming has been criticised for the large amounts of irrigation water it requires.

Indian pea

“RNAi technology is probably the most exciting technology of modern times,” says Rathore. “Many scientists are trying to use it to address the problems of cancer, AIDS and heart disease, but the fact is that the same technology can be used to fight food shortages too.”

He says it could be used to make another crop safe to eat, such as Lathyrus sativus, also knows as grass pea, chickling vetch, or Indian pea. This legume is an emergency crop which farmers in Asia and Africa plant in times of drought because it is resists the toughest growing conditions.

L. sativus produces a high-protein seed, but also contains a neurotoxin which paralyses the lower body when eaten in large quantities. “A lot of times when you see people with this sort of paralysis in Asia it’s because they are forced to plant grass pea,” says Rathore. He says RNAi is “perfect” to address the problem.

While Rathore’s team has created gossypol-free cottonseed in the laboratory, the research remains in its early stages. The trait has been shown to be stable in three generations of plants, but the team must demonstrate it is stable in field trials, and there will be a whole series of safety tests. He does not expect his gossypol-free cottonseed to be on sale for at least 10 years.

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Journal reference: Proceedings of the National Academy of Sciences (vol 103, p 18057)