Yamagata University finds the secret behind the deliciousness of "Dadachamame": Found in a gene that promotes flowering and pod maturation

A research group including Yamagata University has discovered one of the key factors behind the exceptional deliciousness of "Dadachamame," a famous local specialty from Tsuruoka City, Yamagata Prefecture. By analyzing the plant's DNA, the team found that the gene "tof11" (small tof11), which accelerates flowering and pod maturation, also works to increase the levels of free amino acids, which is the source of edamame's savory flavor. On the other hand, another gene that acts to reduce free amino acids was found located very close to tof11. Researchers suggest that removing this gene could potentially lead to the creation of an even more delicious Dadachamame.

Edamame are harvested immature soybeans. "Dadachamame" got its name long ago, when a local lord was so deeply impressed by its delicious flavor that he asked, "Which dadacha (a Shonai regional dialect term meaning 'father') grew this edamame?"

While edamame is a vegetable unique to Japan, it has recently gained popularity overseas alongside the global boom in Japanese cuisine. Although domestic and international studies have previously compared the nutritional components of regular soybeans and edamame and established that edamame's deliciousness is determined by its sucrose and amino acid content, it remained unclear which specific genes were responsible for these components.

A research group including Professor Tomoki Hoshino, who focusses on Crop Breeding at the Department of Food, Life and Environmental Sciences, Faculty of Agriculture, Yamagata University, has been studying the components that make local agricultural products taste so good. Deliciousness is defined by a wide variety of metrics, including flavor, appearance, aroma, and texture. In this study, the group focused their investigation on the free amino acids that enhance the savoriness (umami) and sweetness of Dadachamame.

At the university's experimental field, the researchers crossed "Shirayama," one of the traditional strains of Dadachamame, with "Enrei," a standard soybean variety. Over a span of 13 years and across 8 generations, they performed "self-pollination cultivation" (fertilization using the plant's own pollen) to produce 344 hybrid lines in which the DNA of Shirayama and Enrei were mixed in various patterns. The team then measured the free amino acid content and extracted DNA from these lines.

The analysis of Dadachamame's free amino acids and DNA revealed that the "tof11" gene, located on the 11th of the plant's 20 chromosomes, was closely linked to the heavy accumulation of free amino acids. The "tof11" gene is a mutated version of "TOF11" (large TOF11) gene found in standard soybeans like Enrei, where a single base deletion caused the gene to lose its original function.

According to previous studies, the TOF11 gene is known to control the timing of flowering and podding in soybeans. It was also known that when TOF11 mutates into tof11, the timing of flowering and pod formation shifts forward.

Compared to regular soybeans, which bear fruit in early autumn, Dadachamame is harvested earlier and shipped during the summer. Hoshino explained, "Summer is peak season for edamame. To achieve that signature summer deliciousness, the early flowering of Dadachamame is precisely controlled by tof11."

This study successfully demonstrated that this tof11 gene increases free amino acid content. Generally, overly intense sunlight suppresses plant photosynthesis, prompting the plant to adjust its osmotic pressure to retain water and protect itself from drought. The tof11 gene is believed to function in protecting the plant from solar stress. "It likely plays a role in locking moisture inside the cells by actively boosting the metabolism of free amino acids for moisture regulation," noted Hoshino.

On the other hand, the researchers also discovered a gene that attempts to reduce free amino acids, located immediately adjacent to the tof11 gene, which increases the amount of free amino acids, in Dadachamame (Shirayama strain). Hoshino explained: "If the gene that increases free amino acids adds a value of plus 10 in Dadachamame, the gene trying to reduce them acts as a minus 3. The net result is a plus 7, which is the "level of deliciousness we traditionally recognize in Dadachamame." If we can develop selective breeding methods that eliminate this minus 3 factor, it opens up the possibility of producing Dadachamame with even more intense umami flavor."

Moving forward, Hoshino intends to investigate whether the same gene operates in other Dadachamame varieties, such as "Kanro," an early-maturing variety, and "Oura," a late-summer variety, while also exploring other elements tied to the beans' flavor profile. Shirayama, Kanro, and Oura are known to each have a distinct taste profile when compared side-by-side.

This research was conducted using seeds provided by the Tsuruoka Agricultural Cooperative (JA Tsuruoka) under the "Tonosama no Dadacha" brand. The study received financial support from the Grants-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science, the Yamagata University Advanced Agri-Food System Research Center, the Tsuruoka Gastronomic Innovation Project, the Takahashi Industrial and Economic Research Foundation, the Yanmar Environmental Sustainability Support Association, the Maeta Technology Research Fund, the Key Coffee Shibata Yutaka Memorial Foundation, the Yamaguchi Educational and Scholarship Foundation, the Tojuro Iijima Foundation for Food Science and Technology, and the Fuji Foundation for Protein Research. The findings were published on March 24 in the online edition of the German scientific journal Theoretical and Applied Genetics and announced jointly by Yamagata University and other partners on April 1.