Harnessing Epigenetic Modifiers Reveals MAPK-Mediated Regulation Mechanisms in Hadal Fungi of <i>Alternaria alternata</i> Under High Hydrostatic Pressure

High hydrostatic pressure (HHP) significantly modulates microbial metabolism, while chemical epigenetic modifiers are known to reactivate silent biosynthetic gene clusters and induce novel natural products. However, the mechanisms by which these epigenetic modifiers regulate fungal responses under differential pressure conditions, and how such regulation affects natural product biosynthesis, remain completely unexplored. Here, we investigated the hadal fungus <i>Alternaria alternata</i> CIEL23 isolated from 7332 m sediments in the Mariana Trench under epigenetic modifier treatment with contrasting pressures (0.1 MPa vs. 40 MPa). Our results revealed that epigenetic perturbations and high pressure significantly altered fungal phenotypes, gene expression, and secondary metabolite composition. Transcriptome-level analysis of epigenetic regulatory mechanisms under epigenetic modifiers in both pressure conditions (0.1 MPa and 40 MPa) demonstrated that the addition of epigenetic modifiers regulated MAPK pathway-related gene expression in response to the environment stimuli. Under dual stress conditions, the IG, CWI, and HOG branches of the MAPK pathway showed significantly altered activity patterns. These changes were associated with differential the regulation of genes related to hyphal growth, cell wall remodeling, cell cycle progression, and osmolyte synthesis, suggesting the coordinated modulation of multiple cellular processes. These findings provide the mechanistic link between epigenetic modification induced HHP-response changes and regulation in hadal fungi. Our study not only advances understanding of hadal fungal response to dual stressors but also unlocks new possibilities for harnessing their stress-driven metabolic versatility for biotechnological applications.