Investigation of the mycelial morphology of Monascus and the expression of pigment biosynthetic genes in high-salt-stress fermentation

Extreme environments, for example high-salt-stress condition, that can induce secondary metabolite biosynthesis in fungi are a promising and effective strategy for producing natural Monascus pigments used as food colourants and nutraceutical supplements. In this study, the relationship between the mycelial morphology and expression of pigment biosynthetic genes in high-salt-stress fermentation (HSF) with Monascus ruber CGMCC 10910 was investigated. The Monascus fungus grew well under HSF conditions with 35 g/l NaCl, and the intracellular yellow pigment yield in HSF was 40% higher than that in conventional batch fermentation (CBF). Moreover, the mycelial morphology was maintained in a better state, with a hyphal diameter of 5-6 mu m in HSF, indicating good biocatalytic activity for pigment synthesis. The rate of the relative content of intracellular orange pigments to yellow pigments (O/Y) significantly (p < 0.05) changed, and the extracellular yellow pigments were transformed into each other, indicating that the pigment biosynthesis pathway was changed to promote yellow pigment accumulation in HSF. The pigment biosynthesis genes MpPKS5, MpFasB2, mppE, mppD and mppB were significantly (p < 0.05) up-regulated by approximately 58.4-106.1%, whereas the regulatory genes mppR1 and mppR2 were significantly (p < 0.05) down-regulated by approximately 23.2% and 59.0% in HSF. Notably, the mppE gene was highly correlated with (r > 0.95, p < 0.05) hyphal diameter. These findings indicated that the cultivation of the Monascus fungus under high-salt-stress conditions was beneficial for pigment biosynthesis by controlling the mycelial morphology to regulate gene expression. This study first described the relationship between the mycelial morphology and expression of pigment biosynthetic genes in Monascus during fermentation.