Invasive growth of Aspergillus oryzae in rice koji and increase of nuclear number

Mizuki Yasui,Shunsuke Masuo,Norio Takeshita,Ken Oda,Takuya Katayama,Jun-ichi Maruyama,Shuji Hosoda,Naoki Takaya

doi:10.1186/s40694-020-00099-9

Abstract

Background‘Rice koji’ is a solid culture of Aspergillus oryzae on steamed rice grains. Multiple parallel fermentation, wherein saccharification of rice by A. oryzae and alcohol fermentation by the budding yeast occur simultaneously, leads to the formation of a variety of ingredients of Japanese sake. In sake brewing, the degree of mycelial invasive growth into the steamed rice, called ‘haze-komi’, highly correlates with the digestibility and quality of rice koji, since the hyphae growing into the rice secrete amylases and digest starch.ResultsIn this study, we investigated mycelial distribution of GFP-tagged A. oryzae in rice koji made with different types of rice, such as sake rice and eating rice, with 50 or 90% polishing rate to remove abundant proteins and lipids near the surface. In addition, we compared transcriptomes of A. oryzae in the different types of rice koji. Finally, we found that A. oryzae increases the nuclear number and hyphal width in the course of 1–3 days cultivation.ConclusionsOur imaging analyses indicate that A. oryzae hyphae grew more deeply into 50% polished rice than 90% polished rice. The increases of nuclear number may be a selectively acquired characteristic for the high secretory capacity during the long history of cultivation of this species.

Highlights

Filamentous fungi secrete a variety of enzymes to degrade extracellular organic compounds, serving as decomposers in nature [1]
To evaluate the degree of invasive growth into the steamed rice, called ‘haze-komi’, we used an A. oryzae strain in which histone H2B is fused with GFP, to make koji
We investigated the nuclear distribution in A. oryzae hyphae grown in detail by using the minimal medium but not the rice koji

Summary

Introduction

Filamentous fungi secrete a variety of enzymes to degrade extracellular organic compounds, serving as decomposers in nature [1]. Modern biotechnologies have utilized filamentous fungi as cell factories for the production of organic acids, drugs including antibiotics and enzymes due to high secretory capacity [2]. The filamentous fungus Aspergillus oryzae has been used in the production of traditional fermented foods such as sake (rice wine), miso (soybean paste) and shoyu (soy sauce) for more than 1000 years in Japan [7]. The safety of A. oryzae is guaranteed by the long history of use in food fermentation industries and molecular genomic and metabolomic analyses [8, 9], which is supported by the World Health Organization [10]. One of the distinctive features in the use of A. oryzae in Japanese traditional fermentation is the use of solid-state cultivation (rice grain, soybean and wheat bran). Since filamentous fungi often secrete more enzymes in solid-state culture than in submerged culture [13,14,15], several commercial enzymes are produced in

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