Aspergillus species are one of the most important filamentous fungi from the point of view of industry, pathogenesis, and mycotoxin production. Some aspergilla are used to produce traditional Japanese fermented foods such as sake, shoyu (soy sauce), and miso. In the fermentation steps, fungi are cultivated on solid substrates (steamed or baked cereal grains), under which conditions fungi produce and secrete a large amount of hydrolytic enzymes. During this solid-state fermentation, fungi must cope with various abiotic stresses including temperature, pH, osmotic stress, and low oxygen. To understand fungal biology and to make more use of fungi for fermentation and enzyme production, intensive research on stress adaptation mechanisms have been performed in Aspergillus species. This review focuses on the responses to cell wall stress es and osmotic stresses to which aspergilli should adapt during solid-state cultivation. In a cell wall integrity signaling pathway, the MpkA mitogen-activated protein kinase (MAPK) cascade plays a central role in the regulation of α-1,3-glucan synthase genes and consequently affects fungal cell wall composition. An osmotic stress signaling pathway is composed of the combination of the upstream two-component phosphorelay signaling and the downstream HogA/SakA MAPK cascade, and the signaling pathway is responsible for adaptation to environmental osmotic changes. Here, we provide recent findings on the two stress response signaling pathways in aspergilli. The accumulated knowledge will be useful for understanding and improving the fermentation processes of industrial aspergilli in solid-state cultivation.
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