Impact of salt stress on melanin deposition and transcriptomic changes in <i>Alternaria alternata</i>

Salt stress represents a significant environmental challenge that profoundly affects fungal physiology, including growth, development, and pathogenicity. Alternaria alternata, a ubiquitous fungal pathogen and one of the causal agents of Rose black spot disease, demonstrates remarkable adaptability to various environmental stresses. This study employed an integrated approach combining phenotypic characterization with RNA-Seq transcriptomics to investigate the impact of salt stress on A. alternata, with particular focus on melanin biosynthesis and global gene expression patterns. While the radial growth of A. alternata remained unaffected by salt stress, quantitative colorimetric analysis revealed a dose-dependent reduction in melanin deposition (up to 35% decrease, p &lt; 0.01), suggesting specific disruption of the melanin biosynthesis pathway. Comparative transcriptome analysis identified 274 differentially expressed genes, with significant enrichment in ion transport in upregulated genes and detoxification in downregulated genes. Notably, we observed marked upregulation of P-type ENA ATPase genes, known to mediate sodium ion extrusion in yeast. Structural homology analysis revealed that these ATPases share conserved protein domains with their yeast counterparts, further supporting their evolutionarily conserved role in sodium homeostasis across species. These findings not only elucidate the molecular basis of A. alternata’s phenotypic plasticity under salt stress but also provide potential targets for developing novel strategies to manage fungal diseases in saline-affected agricultural systems. The differential regulation of virulence-related pathways suggests that soil salinity may significantly alter fungal pathogenicity, with important implications for crop protection in the context of increasing global soil salinization.