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

The cell wall integrity (CWI) MAPK signal pathway is crucial for the assembly of 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 impaired hyphal extension, reduced mycelial accumulation, decreased conidiation, altered differentiation of infection structures, and impaired melanin synthesis. Furthermore, when compared to the WT, the levels of alternariol (AOH), alternariol monomethyl ether (AME), and tenuazonic acid (TeA) toxins in the two ΔAaSwi6 and ΔAaMbp1 mutants decreased by 63.98% and 86.32%, 36.71% and 82.85% and 33.28% and 33%, respectively. 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 downstream partners AaSwi6 and AaRlmA, and that AaSwi6 also interacted with AaMbp1 and AaRlmA. In conclusion, our study identifies a physical interaction between AaSlt2 and Swi6/RlmA, suggesting that these components are critical for cell wall synthesis. The finding advances our understanding the pathogenic mechanisms of A. alternata and proposes potential strategies for controlling postharvest diseases.