Genetic Tweaks Reduce Pollution from Growing Rice

Rice is one of the world’s most widely consumed staple foods, but its prevalence comes with an environmental price tag. Rice paddies account for between 7 and 17 percent of the methane in the atmosphere, making them the world’s largest source of man-made methane emissions. Though the gas represents a much smaller percentage of overall greenhouse gases than carbon dioxide, it is about 20 times more effective at trapping infrared radiation from the sun.

For the past several years, scientists have been looking for the most effective way to reduce the amount of the greenhouse gas that rice cultivation produces. Most current approaches had focused on making changes in water management, tillage and fertilizer as a means to reduce methane. The downside is these management changes require additional labor and may not work in all environments.

Now for the first time, a team of researchers based in China, Sweden and the United States has found a way to change the plant’s biology so that rice paddies release less of the greenhouse gas. Its findings were published this week in the journal .

The researchers were able to accomplish this feat by changing how rice plants store energy through the addition of a transcription factor gene found in barley, called SUSIBA2.

Normally, when rice performs photosynthesis, the plant’s stem and leaves will take in carbon dioxide and the plant converts it into starch, which is stored in the roots, stem and grains. In the researchers’ genetically modified rice, the plant produces even more starch, storing it mostly in the stems and grains, rather than distributing it proportionally throughout the plant, including in the roots, said Chuanxin Sun, the lead author of the study and a plant biologist at the Swedish University of Agricultural Sciences in Uppsala.

“This type of rice may be particularly useful in a predicted climate with higher temperatures, which will accelerate methane emissions from paddies, as methane emissions are temperature dependent. The rice may counteract the acceleration,” Sun said.

The addition of the gene not only altered how the plant stored starch but also changed the amount of carbon released from the plant’s roots in the form of sugars, amino acids and organic acids. This carbon feeds a host of soil microbes that surround the plant’s root system in the area known as the rhizosphere. Among those microbes are some that are responsible for producing the methane associated with rice cultivation.

When the researchers measured the microbial population on and around the transgenic plants’ roots, they found that there were fewer methane-producing microbes than with the unmodified rice they tested as a control. The shift in carbon eventually led to the die-off of the methane-producing bacteria. This had a significant impact on methane production over the course of the study, which included three consecutive seasons of field trials in China.

In one of the two genetically modified rice varieties the researchers tested, SUSIBA2-77, emissions fell to just 10 percent compared with the control before flowering, and methane declined to almost zero about a month after flowering. This indicates that the introduced barley gene had a greater impact on methane as the rice plant matured and began to make seeds and the plant was more metabolically active, according to Christer Jansson, director of plant sciences at the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory and a co-author on the study.

Until this study, researchers hadn’t come up with low-methane rice because no one had been able to find a genetic control for a low-methane trait. Many high-yielding rice varieties are available, but their methane emissions were usually high, Sun said.

Inserting a ‘master regulator’

While this study represents a step forward in reducing methane emissions, the researchers consider the work a proof of concept, rather than a chef d’oeuvre.

“It’s too short to show that this is a lasting effect,” said Paul Bodelier, a senior scientist at the Netherlands Institute of Ecology in Amsterdam, who wrote the commentary accompanying the study. “I think the findings are very cool, maybe preliminary, but very cool.”

Going forward, the researchers plan to study how the transgenic plants affect other microbial life in the rhizosphere, not just the methanogenic varieties, Jansson said.

Bodelier also questioned whether by reducing the biomass of the roots, the rice plants wouldn’t be able to take up enough nutrients, potentially requiring the introduction of nitrogen fertilizer—another source of greenhouse gas emissions. So far, the researchers haven’t found this to be a problem, according to Sun. He suggested another alternative.

“Soil carbon can be recycled by returning straw if and when needed, instead of increasing nitrogen-based fertilizer,” he said. “In addition, rice varieties with much less requirement for nitrogen-based fertilizers are already available. The trait can be further introduced into low-methane rice.”

The researchers would need to see the results replicated in a much larger study that covers hectares of land, and measures both yield and methane emissions, Sun said.

Jansson said he expected that the methane reductions in an expanded study would likely be the same as those recorded in the field trials.

The researchers’ initial success is somewhat tempered by the fact that their genetically modified organism will likely meet sharp public opposition both in Europe and in Asia. “As GMO [rice] has no commercial markets at the moment, we have to breed an acceptable non-GMO variety according to our findings,” Sun said. “It could take five to 10 years to sell society-acceptable low-methane rice seeds.”

To do this, the researchers will use the transgenic plant as a starting point, and then continually cross it with nonmodified rice plants until the barley gene becomes increasingly diluted, Jansson said.

Both Jansson and Sun expressed cautious optimism that attitudes about GMOs may become more open over time, particularly as concerns about food security and environmental improvement become more serious in the future. “The more transgenic crops that are obviously good for consumers, and in this case the environment, the more accepted they will be,” Jansson said.

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