Transcription Factor VmGAL4 Governs Vegetative Growth, Development, and Virulence in Valsa mali

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.