The world's growing population faces a constant string of tradeoffs. On the one hand, we need more rice to feed ourselves. On the other hand, control of greenhouse gases is a major priority and rice growing generates a lot of methane. It seems like a Catch 22, but a team led by the US Department of Energy's Pacific Northwest National Laboratory (PNNL) has come up with a genetically engineered strain of rice that not only produces almost no methane, but also more grains.

Rice feeds over half the people on Earth and, according to the Food and Agricultural Organization (FAO), is the world's second most valuable and third largest crop. The trouble is that rice paddies with their warm, wet soils, harbor colonies of microbes that feed off the carbon dioxide dissolved in the mud and convert it to methane. Such paddies are one of the main sources of atmospheric methane, accounting for up to 17 percent of global emissions. This is significant because while it isn't the most prevalent greenhouse gas, methane traps 20 times more heat than carbon dioxide.

The trick is to starve the microbes of carbon dioxide by diverting it out of the paddy and into the plants.

Normally, during photosynthesis, carbon dioxide is converted to sugar and starch. Directing more carbon dioxide to the rice grains makes them plumper and more nutritious, and funneling it to the stems and leaves creates more biomass for fuel and livestock feed. The PNNL team reasoned that if they could persuade rice to act more like other cereals, it would not only increase yields, but also help cut down on methane emissions.

SUgar SIgnaling in BArley 2 (SUSIBA2) is a new genetically modified rice strain that's the result of over tens years of work involving scientists from the United States, Sweden, and China, plus three years of Chinese field studies. According to the team, it kills two birds with one stone, so its rice paddies emit almost no methane during growth while producing more starch and biomass that can be used for fuel and feed.

The strain was created by the introduction of a single gene from barley to common rice. This master regulator gene triggers several other genes and causes the rice plant to divert more carbon to its grains, leaves, and stems as happens in barley. This starves the microbes in the paddy soils around the rice plant roots, which normally generate methane while increasing crop yields.

The team plans to continue studying SUSIBA2 and the mechanisms it uses for carbon reallocation as well as learning more about how the rice roots interact with the resident microbes.

"The need to increase starch content and lower methane emissions from rice production is widely recognized, but the ability to do both simultaneously has eluded researchers," says Christer Jansson, director of plant sciences at PNNL. "As the world's population grows, so will rice production. And as the Earth warms, so will rice paddies, resulting in even more methane emissions. It's an issue that must be addressed."

The team's results were published in Nature.

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