More Rice, Less Greenhouse Gas?

Just like CO2, methane is a potent greenhouse gas. The largest single human source of methane is rice cultivation. A new rice variety could change all that, although NIOO-microbiologist Paul Bodelier warns in the current issue of Nature that it's too early to get excited.

Methane has an even higher capacity to absorb the heat from the earth's surface than CO2. But at least there's less of it in the atmosphere. Although global population growth means there's been a rising demand for the world's main staple food: rice.

The dominant form of growing rice worldwide is paddy field farming. The rice plants are flooded, and under these warm and oxygen-less circumstances, microorganisms around the roots of the plants produce large amounts of methane.

Research from 2002 suggested a correlation between the amount of greenhouse gas emitted in this way and the amount of grain carried by the plants. The more carbon is fixed in a plant's grains and stems, the less is available through its roots for soil microbes to turn into methane.

It's taken until this year for researchers to develop an actual new rice variety based on this insight. The researchers produced their new, transgenic rice by adding a barley gene - SUSIBA2 - that increases the production of starch in stems and grains.

Yields are up to 43% higher thanks to the extra starch. It also fixes more carbon, which ultimately means less methane. That's "the cherry on top" writes Paul Bodelier in Nature, commenting on the researchers' letter about their results that is published in the same issue.

At the same time, Bodelier warns that in addition to the obvious ethical concerns raised by the modification, "we do not yet have a clear picture of how it affects rice plants’ survival and general function." According to the NIOO-researcher, long-term measurements will be needed to establish this.

A complicating factor is that the assembly of microbes around the roots of rice plants does more than just produce methane. Some bacteria benefit the plants by decomposing organic material and delivering essential nutrients. Without those nutrients, "larger amounts of nitrogen fertilizer would need to be applied" with deleterious effects for the environment.

Ironically, the modification might also affect aerobic bacteria that consume rather than produce methane. The oxygen these bacteria need flows though the rice plant's stems and roots into the soil by the same route taken by the methane."It is not known how the transport of gases is affected in the transgenic rice."

All the same, comments Bodelier whose own research also focuses on methane and bacteria, it is quite a feat that the researchers have managed to make rice available that is high in starch and low in methane emissions.

"This will spur scientists worldwide to conduct experiments to verify whether this variety will enable more sustainable cultivation of the crop that feeds half the human population."