A new type of genetically modified (GM) rice might significantly lessen the impact of agriculture on the climate. The plant, equipped with DNA from barley, emits as little as 1% of the methane—a powerful greenhouse gas—of a conventional variety, while also producing more rice. Experts say the approach has great potential for boosting food sustainability, but requires more research to check whether the new rice performs well in paddies and fields. “They are extraordinary results,” says Timothy Searchinger, who researches climate and agriculture at Princeton University and wasn't involved in the study.
Methane has caused roughly 20% of global warming since the industrial revolution. The major anthropogenic source of methane is agriculture, principally from the guts and manure of livestock and from rice. Why rice? Most of the crop is grown in flooded soil, which lacks oxygen and is an ideal home for methane-producing microbes. Between 80% and 90% of methane emitted from rice fields is produced by microbes living on plant roots; some of the gas dissolves into the water and bubbles up, but most is absorbed along with water by plant roots, travels up to the stems and leaves, and escapes into the atmosphere.
There is already a way to significantly cut methane emissions from rice paddies: Briefly draining the fields adds oxygen to the soil and knocks back the methane-producing microbes. This has other benefits as well: Farmers in China already drain their fields because it can boost yields, and in California and elsewhere, draining helps conserve water. But this type of water management isn’t easy, especially in places where fields drain unevenly or where there is lots of rain. And if done wrong, it can harm yields. “It’s a heck of lot easier to just change a seed,” Searchinger says.
Hence the desire for a new kind of rice. In 2002, scientists noticed that rice plants with more grain also emit less methane. The reason is that carbon locked up as starch in the rice grains (and other tissues, except roots) isn't available to the microbes in the soil. Rice and other plants normally release carbon-rich sugars and other compounds through their roots, contributing to the soil ecosystem. These nourish beneficial microbes, but also those that make methane. In addition, microbes can use the carbon released when roots decompose.
The new rice was created by a group led by Chuanxin Sun, a plant biochemist at Swedish University of Agricultural Sciences, Uppsala. What makes it work is DNA from barley. In 2003, Sun and colleagues had discovered a so-called transcription factor, which turns on genes involved in making starch. They tacked on another stretch of DNA, called a promoter, which would make sure the starch is produced mostly in seeds. Then they inserted this combo into a major type of rice called Japonica. Just as intended, the starch content was higher in the modified rice seeds, making up 86.9% of their dry weight, compared with 76.7% in the conventional variety, the team reports in Nature online today.
As for methane, genetic tests suggested that there were far fewer methane-producing microbes living on the roots of the modified rice than on the conventional rice. Measurements in the greenhouse and in small field plots confirmed that the modified plants released 0.3% to 10% as much methane, depending on the time of the season. Methane reduction was greater in hotter weather, which could make the modified rice an even more valuable tool to mitigate emissions as the planet warms, Sun says. “These reductions are huge,” says Bruce Linquist, an agronomist at the University of California, Davis, but he suspects they won’t be quite as large in practice.
The modified rice could boost food security as well. One measure of yield, the dry weight of the rice grains, shot up from 16 grams per plant to 24 grams in the transgenic variety, a massive increase. “I was impressed,” Sun says, but he and others note that much work needs to be done to see if that holds up in realistic field trials. Linquist says: “I’ve seen a lot of promising things come out of controlled situations that just don’t work in the field.”
Another possible issue is the long-term health of the soil. By hosting fewer methane-producing microbes, the GM rice might alter the soil ecosystem in unknown ways, notes microbial ecologist Paul Bodelier of the Netherlands Institute of Ecology at Wageningen University in a commentary.
Moreover, if the new rice supplies less carbon and other nutrients to the soil, farmers might eventually want to use more nitrogen-based fertilizers, resulting in the release of nitrous oxide, another strong greenhouse gas. Indeed, the root mass of the transgenic rice weighed about 35% less than that of the conventional variety, which means that microbes have less to eat after the plants die. Nevertheless, Bodelier calls the approach “a tremendous opportunity for more sustainable rice cultivation.”
A more immediate challenge to the use of GM plants is consumer opposition. “Right now, Chinese society is very sensitive” to concerns about GM food, Sun says; the country hasn't allowed a single GM rice variety on its fields. One alternate approach, he adds, may be to look for corresponding transcription factors in rice itself, which might be turned on or otherwise tweaked without having to use genetic engineering.
Meanwhile, Sun’s group is also working on transforming another major kind of rice, Indica, which emits more methane. They are also discussing how to conduct larger experiments, on the scale of a hectare, and measure methane emissions more comprehensively.