Spermidine synthase is essential for the trophic growth, stress response, and pathogenicity of Verticillium dahliae

Spermidine synthase (SPDS) is a key enzyme in polyamine biosynthesis, a metabolic pathway that plays a critical regulatory role in pathogen–host interactions. In this study, we characterized spermidine synthase in Verticillium dahliae (VdSPDS) by generating gene knockout and complementation mutants to elucidate its role in stress responses and pathogenicity. The spermidine synthase deletion mutant (ΔVdSPDS) completely lost the ability to undergo vegetative growth and conidiation. These defects were restored by exogenous spermidine (Spd) supplementation. Deletion of VdSPDS also resulted in sparse mycelial growth and increased sensitivity to osmotic, membrane, cell wall, oxidative, and nitrosative stresses. Moreover, the ΔVdSPDS mutant exhibited reduced cellophane penetration and impaired production of cell wall–degrading enzymes. Loss of VdSPDS significantly decreased pathogenicity toward cotton. Collectively, these results demonstrate that VdSPDS is essential for fungal growth, development, stress tolerance, and virulence in Verticillium dahliae.