Apple Valsa canker disease, caused by Valsa mali, is one of the most destructive diseases of apple trees in China and seriously threatens the sustainable development of the apple industry. VmSom1 acts as a core transcription factor in the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) signaling pathway and regulates vegetative growth, development and pathogenicity of this phytopathogen. Transcriptome analysis was performed using the VmSom1 deletion mutant and the wild-type strain sdau11-175, and a significantly differentially expressed transcription factor, VmGAL4, was identified. In this study, the single-gene deletion mutant ΔVmGAL4 and the double-gene deletion mutant ΔVmSom1/VmGAL4 were constructed via homologous recombination, aiming to preliminarily explore the interaction between these two genes. Sequence analysis revealed that the VmGAL4 protein contains a conserved fungal_TF_MHR domain spanning amino acids 164 to 614. Phylogenetic analysis indicated that VmGAL4 shares the closest phylogenetic relationship with homologs from Cytospora schulzeri and Cytospora chrysosperma. Phenotypic assays demonstrated that the VmGAL4 deletion mutant exhibited markedly reduced mycelial growth rate and fewer pycnidia production. Additionally, the mutant displayed enhanced sensitivity to cell wall inhibitors and osmotic stress agents, along with significantly increased capacity to utilize various carbon and nitrogen sources and decreased pathogenicity compared with the wild-type strain. Compared with the single deletion mutant ΔVmSom1, the double mutant ΔVmSom1/VmGAL4 partially rescued the growth defects and also alleviated the reduction in pathogenicity to a certain extent. Nevertheless, conidial production remained severely inhibited in the double mutant. Collectively, VmGAL4 is involved in the regulation of vegetative growth, asexual development, cell wall integrity, osmotic stress response, carbon and nitrogen source utilization, and pathogenicity in V. mali.

Apple valsa canker (AVC), caused by Valsa mali, inflicts substantial annual economic losses in apple production. Transmembrane proteins in plant pathogenic fungi mediate fungus–host interactions and regulate fungal growth and pathogenicity. However, their molecular mechanisms in the V. mali–apple pathosystem remain elusive. In this study, we demonstrated that transmembrane protein VmEM1 exhibits significantly upregulated expression during the early stage of V. mali infection in apple. The virulence of the VmEM1 deletion mutant (ΔVmEM1) was markedly attenuated, indicating that VmEM1 is essential for the full pathogenicity of V. mali. Notably, VmEM1 exerts dual roles: overexpression of VmEM1 attenuates V. mali pathogenicity while activating host immune responses. Purified VmEM1^ECD induced the accumulation of reactive oxygen species (ROS) and callose in Nicotiana benthamiana, enhanced N. benthamiana resistance to pathogens, and triggered cell death in a BAK1-, SOBIR1-, HSP90-, and SGT1-associated manner. Additionally, purified VmEM1^ECD promoted ROS and callose accumulation, as well as the expression of immune-related marker genes in apple tissues, thereby enhancing apple resistance to V. mali. Collectively, our findings reveal that VmEM1 acts as both a key virulence factor and an immune elicitor, providing novel insights into the pathogenic mechanism of V. mali and the molecular basis of the V. mali–apple interaction.