Serratia plymuthica HK9-3 enhances tomato resistance against Phytophthora capsici by modulating antioxidant defense systems and rhizosphere micro-ecological condition.

Tomatoes are frequently challenged by various pathogens, among which Phytophthora capsici (P. capsici) is a destructive soil-borne pathogen that seriously threatens the safe production of tomatoes. Plant growth-promoting rhizobacteria (PGPR) positively induced plant resistance against multiple pathogens. However, little is known about the role and regulatory mechanism of PGPR in tomato resistance to P. capsici. Here, we identified a new strain Serratia plymuthica (S. plymuthica), HK9-3, which has a significant antibacterial effect on P. capsici infection. Meanwhile, stable colonization in roots by HK9-3, even under P. capsici infection, improved tomato growth parameters, root system architecture, photosynthetic capacity, and boosted biomass. Importantly, HK9-3 colonization significantly alleviated the damage caused by P. capsici infection through enhancing ROS scavenger ability and inducing antioxidant defense system and pathogenesis-related (PR) proteins in leaves, as evidenced by elevating the activities of peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), and chitinase, β-1,3-glucanase, and increasing the transcripts of POD, SOD, CAT, APX1, PAL1, PAL2, PAL5, PPO2, CHI17 and β-1,3-glucanase genes. Notably, HK9-3 colonization not only effectively improved soil microecology and soil fertility, but also significantly enhanced fruit yield by 44.6% and improved quality. Our study presents HK9-3 as a promising and effective solution for controlling P. capsici infection in tomato cultivation while simultaneously promoting plant growth and increasing yield, which may have implications for P. capsici control in vegetable production.