For centuries, Japanese staples such as sake, miso, and soy sauce have relied on the remarkable abilities of a fungus called koji. In this interview, Norio Takeshita, a microbiologist at the University of Tsukuba, discusses his recent eLife study of Aspergillus oryzae, known as the ‘koji fungus’. He asks how its unusual cell biology could shape its productivity, and reflects on how humans have harnessed fungi long before modern microbiology tools.
How did this project begin?
Norio Takeshita
I have studied filamentous fungi for many years. These organisms grow as hyphae – long, thread-like structures whose cells tend to contain multiple nuclei. In 2016, I moved to the University of Tsukuba in Japan and began research on the species Aspergillus oryzae (A. oryzae), known as the koji fungus. It is an industrially important organism thanks to its ability to produce large quantities of enzymes that break down starch and proteins, useful in fermentation.
Most of the fungi I had worked on previously contained between roughly 10 and 20 nuclei, so I was surprised to see that in a single cell of A. oryzae, more than 200 were densely packed into one cell. This led me to investigate a potential relationship between the number of nuclei in cells, their size, and their enzyme productivity.
What did your study uncover about A. oryzae?
We used a variety of techniques to explore how cell growth, nuclear number, and enzyme production are related. Over time, the hyphae produced by A. oryzae become thicker, and both cell volume and nuclear number increase approximately tenfold. This suggests that, as in other organisms, these two quantities are tightly regulated. Moreover, as the nuclear number increased, so did enzyme activity, likely due to a higher capacity for transcription and translation.
We also identified several genetic and nutrient-sensing factors that appear to be involved in regulating the growth of A. oryzae. While its size and nuclear number are unique, this trend was not limited to A. oryzae, having observed similar patterns in other fungi that are used in industry, including several other species of Aspergillus. This tells us that both the thousand-year history of A. oryzae domestication and strain improvement in industry have converged to improve enzyme production.
Can you tell us more about the significance of Aspergillus fungi in both science and Japanese culture?
I had previously focused on Aspergillus nidulans, which is a foundational model in fungal research. In recent years, other species in the Aspergillus genus have become popular for industrial and clinical research, such as A. niger, used for citric acid production, A. fumigatus, which can cause disease in humans with weak immune systems, and A. flavus, which causes crops to rot.
In Japan, A. oryzae is widely known among the public as the “koji fungus (koji-kin)”. Long before modern microbiology tools, brewers domesticated and bred this organism for its exceptional ability to break down starches and proteins. It has a centuries-long role in traditional Japanese food production, used to ferment products like sake, miso, and soy sauce. In recognition of its importance, the Brewing Society of Japan designated the koji fungus as Japan’s national fungus in 2006. It even appears in manga and anime!
Why do you think your question has been largely unexplored in the past?
The ability of the koji fungus to produce enzymes in large volumes has been studied mainly from a genetic angle, and its cell biology is well understood. The thickening of hyphae in A. oryzae has long been common knowledge, but to recognise that this thickening is unusual, you have to compare A. oryzae with other species. Fortunately, having studied A. nidulans for many years, I realised that the unusually large number of nuclei in A. oryzae might reflect an important biological feature that had not been systematically explored.
Has this project given you broader insights into our relationships with fungi like koji?
Through this work, I have gained a renewed sense of awe for our predecessors who, without modern microbiological knowledge or technology, succeeded in harnessing the remarkable properties of the koji fungus. Our analyses suggest that the traits we observed are not caused by a single mutation but result from multiple changes accumulated over centuries of domestication and selective breeding. Similar trends are also seen in modern industrial strains of other fungi, like Trichoderma and Penicillium, selected for enzyme production. Knowing this gives me a deep respect for the people who developed strains that we continue to study and use today.
Interview by Daisy Veysey.
About Norio
Norio Takeshita is an Associate Professor in the Microbiology Research Center for Sustainability (MiCS) at the University of Tsukuba in Japan.
