Reducing methane emissions by developing lowfumarate high-ethanol eco-friendly rice

Methane in rice paddies is mainly produced by methanogenic communities feeding on carbon from root exudates and debris. However, the dominant root secretion governing methane emissions is not yet identified after decades of studies, even though secreted carbohydrates and organic acids have been shown to contribute to methane emissions. In this study, we discovered that fumarate and ethanol are two major rice-orchestrated secretions and play a key role in regulating methane emissions. Fumarate released in the rhizosphere is metabolized by microorganisms, supporting the growth of methanogenic archaea that produce methane as an end carbon product, while ethanol mitigates methane emissions through inhibition of methanogenic activity and growth as well as reducing fumarate synthesis in the rice root. Furthermore, we elucidated the route of fumarate metabolism in the anoxic rhizospheric zone. We found that fumarate in the rice root is produced from acetate via propionate and succinate, and when released into soil directly is oxidized to propionate before conversion via acetate into methane as the end product. The knowledge on fumarate and ethanol metabolism in rice was then used for hybrid breeding of new rice varieties with the property of low methane emission. Cultivation of these novel rice lines or employing our findings for rice cultivation managements showed up to 70% reductions in methane production from seven paddy field sites during 3 years of cultivation trials. Taken together, these findings offer great possibilities for effective mitigation of the global climatic impact of rice cultivation.