BioEd Online

Plant geneticists have induced plants to form 'fertilizer factories' without the aid of bacteria that are normally crucial to the process. If the technology can be transferred to plants such as wheat or rice, industrial fertilization of these crops could be reduced or even abolished.

When bacteria known as rhizobia enter the roots of a leguminous plant, such as a pea or bean, the plant develops lumps, or nodules, on its roots to house the microbes. The bacteria take nitrogen from the air and turn it into ammonia that feeds the plant.

Two research groups have now made legumes that produce nodules in the absence of rhizobia, potentially paving the way for crops that would not need to be treated with nitrogen fertilizer, but instead would rely on nitrogen-processing bacteria that are omnipresent in the soil to colonize their nodules.

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Fertilizing crops is inefficient and environmentally damaging, says Giles Oldroyd of the John Innes Centre in Norwich, UK, who led one of the research teams. Besides polluting waterways, chemical fertilizer production accounts for an estimated half of the fossil fuels burnt by agriculture.

Nitrogen-fixing bacteria are a better way for plants to gain their nitrogen, Oldroyd adds. Many farmers and gardeners alternate legumes such as broad beans or clover with their other crops. Traditional Latin American farmers plant beans alongside their maize.

"Cereals have a huge nitrogen demand. But legumes not only provide nitrogen for themselves, but also for other plants," says Oldroyd. He and his colleagues are attempting to genetically engineer related plants such as tobacco and tomato to produce root nodules.

Self help

Nodule production is normally initiated when nitrogen-processing bacteria enter a plant's root cells. The plant senses the bacteria and its root cells grow to form a nodule.

But the two research groups, Oldroyd's team and a group led by Jens Stougaard of the University of Aarhus, Denmark, found that by mutating a gene that produces a cellular messenger called CCaMK, root cells can be converted into nodule-forming cells, even without the bacteria. They report their discovery in this week's Nature¹,².

The idea that a self-fertilizing facility could be set up in other crop plants has not yet been tested, although Oldroyd says that his work on tobacco and tomato should reveal whether this occurs.

There is no theoretical reason why it shouldn't work, says Oldroyd. "We can make empty nodules, but the plant has to allow the bacteria to invade," he says. "But if legumes can do it, others should be able to as well."

The technology is in its infancy: "A number of key steps need to be achieved," Stougaard warns. But he hopes that it could be used to make self-fertilizing versions of the world's main food staples: maize, wheat, barley and rice.

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