It takes two: A Widespread Temperate Bacteriophage Contributes to Regulation of the Type III Secretion System in Pseudomonas syringae

O_LIThe 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. C_LIO_LIThe 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. C_LIO_LIDeletion 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. C_LIO_LIThese prophage-driven transcriptional changes likely reshape Pams interaction with plant immunity, highlighting how bacteriophages rewire bacterial transcriptomes and contribute to the evolution and emergence of plant diseases. C_LI