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Ustilaginoidea virens causes rice false smut, one of the most devastating rice diseases. The pathogen produces various types of mycotoxins, such as ustilaginoidins and ustiloxins, which are harmful to both human and animal health. Histone deacetylases in fungi play an important role in regulating chromatin structure and gene expression. However, there is limited knowledge about how histone deacetylases control pathogenicity and mycotoxin biosynthesis in U. virens. Here, we characterize a putative class I histone deacetylase UvHOS2 in U. virens. The UvHos2-deletion mutants exhibit retarded vegetative growth, reduced conidial production and germination, and attenuated virulence. UvHOS2 positively regulates tolerance to various environmental stresses, including cell wall, cell membrane integrity, osmotic and oxidative stresses. UvHOS2 reduces the acetylation levels of histones at multiple Lys sites, including H3K9, H3K14, H3K27, and H3K56. ChIP-PCR assays revealed that UvHOS2-mediated H3K9 deacetylation regulates the expression of ustilaginoidin biosynthesis genes. Consistently, transcriptome analysis indicates that UvHOS2 regulates the expression of the genes involved in secondary metabolism, mycelial growth, conidiogenesis, and pathogenicity, thereby controlling U. virens virulence and mycotoxin (ustilaginoidins and sorbicillinoids) biosynthesis. This study provides a theoretical reference for revealing the epigenetic regulation of pathogenicity and mycotoxin biosynthesis in plant pathogenic fungi. [ABSTRACT FROM AUTHOR]