Chitin deacetylases AaCDAs enable <i>Alternaria alternata</i> to evade host immune responses via cell wall modification

Chitin deacetylases (CDAs) are essential for phytopathogenic fungi to evade plant immune defenses by deacetylating chitin oligomers into chitosan. However, the function of CDAs in Alternaria alternata, the causal agent of pear black spot, remains unclear. In this study, two CDA genes, AaCDA1 and AaCDA2, were cloned and identified. Gene knockout and complementation analyses revealed that ΔAaCDA1 and ΔAaCDA2 mutants showed no significant changes in colony morphology, biomass, or conidiation. However, the ΔAaCDA1 mutant exhibited increased extracellular melanin, linked to the upregulation of the melanin biosynthesis transcription factor AacrmA. The mutants exhibit higher chitin and mannan contents and decreased glucan levels, with corresponding changes in gene expression. Furthermore, they showed reduced appressorium formation and penetration ability, leading to decreased pathogenicity on both pear and tomato fruits. Infected mutants triggered greater reactive oxygen species (ROS) and callose responses, with significant upregulation of host defense genes. Overall, the findings indicate that AaCDA1 and AaCDA2 are crucial for infection structure differentiation and A. alternata pathogenicity by modulating cell wall components and evading host immune responses.