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

Abstract

‘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, lead 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 of rice. In 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. That characteristic could be corelated with the secretory capacity of several enzymes, and be an important selectively acquired characteristic during the breeding in long history.