Stress Responses of Koji Mold Cells with Highly Polarized and Multicellular Morphology

Abstract

The koji mold Aspergillus oryzae has long been used in traditional Japanese fermentation for the manufacture of sake, soy sauce, and miso, and it is also utilized for the industrial production of enzymes and recombinant proteins. A. oryzae grows by extending and compartmentalizing hyphae into individual cells, a morphological feature that increases the possibility that hyphae differentially encounter environmental and physical stresses. Stress responses of A. oryzae are conventionally analyzed using whole mycelial samples; however, recent studies conducted at the cellular level have provided new insights into the spatial specificity of stress responses in individual hyphae. In contrast to unicellular yeast, A. oryzae has complex morphological features that are characterized by multicellularity and polarized filamentous growth. The intercellular connectivity of hyphae via the septal pore is strictly regulated by the Woronin body, which is a Pezizomycotina-specific organelle that plugs the septal pore upon the physical wounding of cells. AoSO protein, which is required for hyphal fusion, also accumulates at the septal pore in response to stress. Stress granules, which sequester nontranslating mRNAs, form cytoplasmic foci that include a novel component AoSO protein at the hyphal tip, which is the main site of protein secretion. In this review, we present the most recent cell biology-based evidence for the stress response mechanisms in A. oryzae and discuss how this knowledge could contribute to the industrial application of this important fungus.