Antagonistic Mechanisms of Serratia plymuthica MM Against Phytophthora capsici and Its Growth-Promoting Traits

Phytophthora blight, caused by <i>Phytophthora capsici</i>, an oomycete pathogen belonging to the phylum Oomycota, is a major soil-borne disease that limits the cultivation of pepper (<i>Capsicum annuum</i>). In this study, the bacterium <i>Serratia plymuthica</i> MM was evaluated for both its antagonistic ability and plant growth-promoting (PGP) potential. The sterile fermentation filtrate of <i>S. plymuthica</i> MM exhibited strong antifungal activity in vitro, inhibiting the mycelial growth of <i>P. capsici</i> by up to 88.32%. In pot experiments, <i>Serratia plymuthica</i> MM significantly reduced both disease incidence and disease severity of Phytophthora blight in pepper plants, achieving control efficacies of 88.33% (preventive application) and 55.56% (therapeutic application). Microscopic observations revealed severe hyphal abnormalities, including distortion, shrinkage, collapse, and fragmentation. Furthermore, propidium iodide (PI) and DAPI double staining provided cellular-level evidence of antifungal activity, demonstrating concentration-dependent disruption of membrane integrity and nuclear organization in <i>P. capsici</i> hyphae, which was supported by pronounced increases in ion leakage from pathogen cells. Further, <i>S. plymuthica</i> MM exhibited PGP traits, including nitrogen fixation, phosphate solubilization, siderophore production, and indole-3-acetic acid (IAA) synthesis. Pot experiments using the pepper cultivar 'Longjiao' (<i>Capsicum annuum</i> L. cv. Longjiao) confirmed significant growth promotion and enhanced activities of key defense-related enzymes (POD, PPO, PAL, and CAT). Stable colonization of pepper roots was verified by green fluorescent protein (GFP) labeling, demonstrating the strain's persistence in the rhizosphere. Collectively, these results highlight the dual role of <i>S. plymuthica</i> MM in suppressing <i>P. capsici</i> and promoting pepper growth, supporting its potential as an eco-friendly biocontrol agent for sustainable pepper production.