Rice paddies play a dual role, being both a crucial source of methane (CH4) emissions and a protector of global food security. More than half of the world's population relies on rice as a dietary staple. Projections indicate that an additional 180 million tons of rice must be produced each year by 2050 to meet the growing global food demands. At the same time, around 30% of CH4 emissions globally originate from rice paddy fields. Importantly CH4 has a global warming potential 20-25 times higher than that of carbon dioxide (CO2). Methane stands as the second most significant anthropogenic greenhouse gas, following CO2. The concentration in the atmosphere is on the rise (IPCC, 2013).
Flooded rice paddy fields have a significant impact on global anthropogenic CH4 emissions, accounting for 12% of these emissions according to recent data. Collectively, rice fields are indeed significant contributors to greenhouse gas emissions. According to the Food and Agriculture Organization (FAO) in 2023, when convert to CO2, CH4 releases approximately 745 million tons of CO2 annually. To put this into perspective, global CO2 emissions from passenger cars were around 3 billion tons. This means that, on average, 1 hectare of rice paddy (for flooding period, 4 month) emits greenhouse gases at a rate equivalent to that of 2.5 cars per year. This comparison underscores the importance of addressing emissions from rice cultivation to mitigate their impact on climate change. If reduce 30% of methane emission from paddy fields, same with reduction of 40 million cars per year (Fig.1). Therefore, it is imperative to reduce CH4 emissions from paddy fields.
Why does rice paddy emit such tremendous CH4 emission? The simple answer is a group of microorganism called as “methanogens”. In the anaerobic conditions of flooded rice paddies, methanogens thrive around rice roots, producing CH4 by utilizing root exudates as a food source for methanogens, including organic acids and carbon compounds transported through rice stem channels1. About 90% of CH4 is emitted through rice plant not from soil surface. Higher yield cultivar of rice generally secretes higher amount of root exudates resulting higher CH4 emission in the field. It resembles to chase two rabbits that run different direction in the yard.
To overcome such fundamental problem, we focused on the allocation of photosynthesis production from carbon fixation. We hypothesized that a rice trait can allocate the sink and source tissues such as photosynthates into rice grain and secretion of root exudates. We developed ‘Milany360’ by introgressing the loss of function GS3 allele in natural variation using conventional breeding program (Fig. 2). The GS3 gene exerts its influence on organ size through its Organ Size Regulation (OSR) domain, acting as a negative regulator. Through natural genetic variation, a loss of function in the GS3 allele showed a notable 16% reduction in CH4 emissions, coupled with a 6% increase in crop yield. This outcome highlights the potential for a dual benefit, akin to catching "two rabbits at once".
Another challenge in the modern rice cultivation is application of increasing nitrogen fertilizer. Nitrogen is a vital nutrient for ensuring food security, yet it poses a significant threat to the ongoing climate crisis. A reduction of nitrogen fertilization by 50% resulted in a notable 11% decrease in CH4 emissions, albeit with the trade-off of a 15-20% loss in crop yields2.
Remarkably, the gs3 allele demonstrated the potential to achieve a triple benefit, akin to catching "three rabbits at once". When coupled with nitrogen reduction, it showcased an additive effect, resulting in a remarkable 24% reduction in methane CH4 emissions, while the accompanying trade-off in crop yield was limited to just 7% reduction in yield. This was attributed to the fact that environmental factors did not have an impact on grain size and weight. These low-CH4 emitting rice varieties can contribute to both net-zero emissions goals, by reducing greenhouse gas emissions, and food security, by maintaining or increasing rice production without the need for additional labor and resources. This dual benefit highlights the potential of such innovations to address multiple challenges simultaneously.
In conclusion, the introduction of the gs3 allele in rice effectively reduced CH4 emissions and demonstrated an additive effect when combined with reduced nitrogen input. The null gs3 allele increased sink strength by directing more photosynthates to the aboveground parts (grain) rather than the belowground parts (root), which contributed to a decrease in root exudate release. In summary, our findings indicate that Milyang360, carrying the gs3 allele, could serve as a model for managing CH4 emissions through conventional breeding programs (Fig.3). It can reduce CH4 emissions without the need for additional, labor-intensive measures such as water management or the application of chemicals and biochar. In 2020, rice paddies globally emitted 29.8Tg of CH4 according to FAO Tier 1 data. If we incorporate the gs3 allele's effects globally, 120 million ton of CO2 annually (same with reduction of 25.5 million cars). Further, if combine gs3 allele and reduction of nitrogen input, 179 million ton of CO2 (same with reduction of 38 million cars). Therefore, the integration of the gs3 allele into rice germplasm not only aids in reducing greenhouse gas emissions and nitrogen fertilizer use but also addresses the growing global food demand.
Behind the work
In the summer of 2011, we crossed Saeilmi with Shindongjin to create a gs3 allele plant (Milyang360). We conducted two backcrosses in December 2012 and February 2013, all within a greenhouse. Additionally, we diligently performed background selection to identify well-recovered near-isogenic lines. This work proved to be quite challenging as it involved genotyping hundreds of lines with tens of thousands of genetic markers
Collecting CH4 gas was a demanding task. In our case, every Wednesday, we gathered CH4 gas samples at 10 am, taking three replicates at two different locations. On rainy days, we had to conduct two additional collections for calibration purposes.
Starting in 2020, we initiated a regional yield trial across 16 locations in South Korea to assess adaptability, which will continue until 2024. The culmination of this effort will be the introduction of a new cultivar named Milyang360 in 2024, marking a 14-year journey from the initial cross.
To assess the impact of the gs3 allele on the national greenhouse gas emission factor, we've been conducting CH4 emission analysis in three regions from 2023, and this endeavor will persist until 2025. While this work has been challenging, we eagerly anticipate that our results will make a valuable contribution to addressing global warming.