Over half the people on the planet eat rice as a staple food. Growing rice emits methane, a potent greenhouse gas — to the tune of 25 million to 100 million metric tons of methane every year, a notable contribution to human-caused greenhouse gas emissions.
As the world's population grows and needs more food, the problem is likely to get worse, but genetic engineering could help, a new study reports. By transferring a barley gene into a rice plant, scientists have created a new variety of rice that produces less methane while still making highly starchy, productive seeds. The development of the new rice strain is described this week in the journal Nature.
Finding a way to boost rice production while reducing methane emissions has been a goal for many years, said Chuanxin Sun, a plant biologist at the Swedish University of Agricultural Sciences and the lead author of the study. By engineering barley genes into the rice plant, "we demonstrated it's possible to get these two traits with this technology," he said.
When rice paddies are flooded, methane-producing bacteria thrive on the carbohydrates secreted by rice roots in the oxygen-free soils. The rice plant itself acts as a conduit, transmitting methane from the soil into the atmosphere.
Methane traps heat in the atmosphere with devastating efficiency: Over 20 years, it is 84 times stronger than carbon dioxide as a greenhouse gas, according to the most recent assessment from the Intergovernmental Panel on Climate Change.
Sun and his team transferred barley genes that cause high-starch production in rice grains and stems. Based on previous research that proposed that rice plants with smaller root systems could produce less methane, the researchers hoped that the genetic transfer would cause their rice plants to allocate less energy toward growing roots and more toward making robust grains.
Then they planted a handful of their genetically engineered rice plants alongside conventionally grown Nipponbare rice in the laboratory and in two field settings in China.
Near the end of the growing season, in summer and fall, the researchers measured how much methane each plant emitted by covering it with a sealed plastic cylinder and using a syringe to extract the accumulated gasses in the trapped air. They also measured how much starch the plants allocated to their seeds, stems and roots, and how many methane-producing bacteria lived around the plant roots.
As they suspected, the genetically engineered rice grew smaller root systems and starchier grains than conventional rice. Far fewer methane-producing bacteria hugged the roots of the new rice. And the new rice variety emitted less than 10% of the methane of conventional rice, they reported.
The genetically modified rice variety provides "a tremendous opportunity for more-sustainable rice cultivation," Paul Bodelier, a microbial ecologist at the Netherlands Institute of Ecology, wrote in an essay that accompanied that research article.
Previous techniques for reducing methane emissions from rice paddies involve alternating flooding with dry periods, said Bruce Linquist, a plant scientist at UC Davis who specializes in rice cultivation and was not involved in the study. Although these techniques can cut methane emissions in half, they also can reduce the plants' productivity. Additionally, in a lot of places where rice is grown, you can't control when rice paddies are wet or dry because irrigation is based on seasonal rains and floods, he said.
The research is too preliminary to make solid conclusions about methane emissions from the genetically engineered rice, Linquist added. More research about how much methane whole rice paddies (and not just individual plants) emit over the entire growing season is necessary, he said. In addition, the new rice plants' smaller root systems could make it difficult for the crop to uptake nutrients.
"It needs to be tested more in the field," he said.
Even if the new rice variety does prove to reduce methane emissions on a larger scale, there are still barriers to it being grown and sold. Genetically engineered rice isn't commercially cultivated anywhere in the world, in part because of ethical and biological concerns about the spread of engineered rice pollen, experts said.
"There's not a market for it," Linquist said.