<b>The Slt2-Swi6 signaling pathway plays a pivotal role in regulating cell wall biosynthesis, secondary metabolite production, and the pathogenicity of </b><b>pear fungal</b><b> </b><b><i>A. alternata</i></b>

The cell wall integrity (CWI) MAPK signal pathway is crucial for the assemblyof fungal cell walls and their associated virulence, as it activates the expression of the downstream transcription factors Mbp1, Swi6 and RlmA. However, the transcription factors directly regulated by this pathway in Alternaria alternata have yet to be identified. In this study, we delineated the functional roles of two transcription factors, AaSwi6 and AaMbp1, through targeted gene deletion. Disruption of either gene resulted in significant defects in mycelial expansion, biomass accumulation, conidiation, spore germination, and appressorium formation. Additionally, the mutant strains demonstrated decreased production levels of multiple secondary metabolites, including melanin and mycotoxins, such as alternariol (AOH), alternariol monomethyl ether (AME)and tenuazonic acid (TeA). These defects were correlated with diminished virulence on tomato and pear fruit. Compositional analysis of the cell wall composition indicated that the deletion of AaSwi6 resulted in decreased levels of chitin, glucan, and mannan, whereas the deletion of AaMbp1 led to reduced levels of chitin and mannan. Yeast two-hybrid experiments further demonstrated that the MAPK kinase AaSlt2 physically interacted with downstreampartners AaSwi6 and AaRlmA, and that AaSwi6 also interacted with AaMbp1 and AaRlmA. In conclusion, our study establishes the AaSlt2-Swi6/RlmA pathway as critical for cell wall synthesis, advances our understanding of A. alternata pathogenic mechanisms, and suggests potential strategiesfor controlling postharvest diseases.