Maddock, D.; Liberto, S.; Ognian, B.; Sundin, G.; Hulin, M.

The Pseudomonas syringae species complex includes major crop pathogens that use a type III secretion system (T3SS) to inject effectors into plant cells, suppressing immunity and promoting disease. The cherry canker pathogen Pseudomonas amygdali pv. morsprunorum (Pam) carries the effector gene hopAR1 on a prophage, PamPP1, which belongs to a novel Caudoviricetes family widespread across the P. syringae complex and likely acquired before pathovar divergence. Deletion of PamPP1 shows that this prophage enhances Pam virulence independently of hopAR1, instead it alters the T3SS operon expression both in vitro and in planta. These prophage-driven transcriptional changes likely reshape how Pam interacts with plant immunity, highlighting how bacteriophages rewire bacterial transcriptomes and contribute to the evolution and emergence of plant diseases.