Synergistic Biocontrol of Agrobacterium tumefaciens by Phage PAT1 and Ascaphin-8: Enhanced Antimicrobial Activity and Virulence Attenuation via HupB Loss

<i>Agrobacterium tumefaciens</i> (<i>A. tumefaciens</i>), the causal agent of crown gall disease on several plant species, is responsible for substantial yield losses worldwide. The limitations of conventional pesticides in controlling this disease highlight the need for alternative antibacterial solutions. Phage biocontrol can be an option, effectively managing bacterial plant diseases, by reducing pathogen loads while driving evolutionary trade-offs, often enhancing synergy with other antibacterial strategies. In this study, we aimed to explore and develop a sustainable strategy to control <i>A. tumefaciens</i>, by combining <i>Agrobacterium</i> phage PAT1 with the natural antimicrobial peptide "Ascaphin 8" and leveraging the fitness trade-offs resulting from phage resistance. In vitro and in planta investigations showed that PAT1 in combination with Ascaphin 8 at the sublethal concentration of 3 μM could effectively eradicate <i>A. tumefaciens</i> in YPG broth and reduce tumor formation by 46.33% on tomato plants, unlike their individual applications, indicating that the combination was synergistic against <i>A. tumefaciens</i>. This synergy was attributed to the fitness trade-offs in <i>A. tumefaciens</i> induced by phage resistance, which led to increased sensitivity to antimicrobial peptides, slower growth rate, and an 89.96% attenuation of virulence in the PAT1-resistant mutant (AT-M1). Transmission electron microscopy analyses showed that treatment with 1 µM of Ascaphin 8 induced cytoplasmic condensation in 80% of AT-M1 cells, whereas only 16% of the wild-type CFBP 5770 cells exhibited similar alterations under identical conditions. Furthermore, proteomic analyses performed on AT-M1 and CFBP 5770 revealed that the mutant AT-M1 exhibited a loss of DNA-binding protein HupB and downregulation of SDR family oxidoreductase and superoxide dismutase. These molecular alterations are potentially associated with the reduced virulence and heightened AT-M1 sensitivity. This study investigated the fitness costs associated with phage resistance in <i>A. tumefaciens</i> and laid the first foundation for potential biocontrol of plant bacterial diseases, particularly <i>A. tumefaciens</i> infections, using phage-peptide combination.