Functional dissection of the smi1 regulator delineates its pivotal roles in modulating cellular physiology, Monascus pigments production and citrinin accumulation in Monascus purpureus.

Abstract

β-1,3-glucan, a polysaccharide and a major structural component of the fungal cell wall, is crucial for microbial growth and secondary metabolism, yet its role in Monascus purpureus remains unclear. In this study, we identified the smi1 gene in M. purpureus, which encodes a cell wall assembly regulator. Recombinant strains were then constructed to investigate the role of the Smi1 protein in β-1,3-glucan biosynthesis, cell growth and secondary metabolism. Results showed that the overexpression strain exhibited total Monascus pigments (MPs) production increased by 76.02%, whereas the extracellular MPs enhanced by 71.95% in the knockout strain compared with that produced by the parent strain. Morphological analysis revealed that smi1 overexpression significantly thickened the cell wall and induced surface wrinkling, whereas knockout resulted in thinner walls and smoother surfaces, indicating Smi1's essential role in cell wall integrity and morphological regulation. Furthermore, smi1 overexpression upregulated β-1,3-glucan precursor synthesis genes and increased β-1,3-glucan synthase activity by 37.37% and 30.58% on days 3 and 6, respectively, whereas it decreased significantly in the knockout strain. Comparative transcriptomic analysis revealed that both overexpression and deletion of smi1 induced significant changes involved in β-1,3-glucan synthesis, chitin synthesis, the biosynthesis of MPs and citrinin, as well as fatty acid and amino acid metabolism. These results indicate that smi1 not only participates in cell wall biosynthesis and morphological regulation in M. purpureus, but also potentially regulates the synthesis of MPs and citrinin by affecting central and secondary metabolic networks.