Phages in Action: Their Role in Bacterial Virulence on Plants

Cherry orchards worldwide frequently suffer from outbreaks of Pseudomonas amygdali. pv morsprunorum (PSM) which causes leaf lesions, gummosis of branches, stem canker, girdling and death of trees, drastically reducing the production of cherries. Michigan’s cherry industry, valued at $36.5 million, significantly contributes to a $108 million sector with U.S agriculture, highlighting a need to understand this devastating pathogen. Previous work has shown that a temperate bacteriophage carrying the type 3 effector (T3E) hopAR1 amongst other beneficial bacterial genes is conserved within the genome of PSM. Via scarless cloning, mutant PSM with the prophages full 50 KB region removed were generated. Detached leaf pathology assays revealed a significant reduction in virulence in the prophage deletion mutants. RNA-seq analysis of the mutant vs wild type in hrp inducing minimal media showed altered transcription of the hrp operon and associated T3Es amongst other genes of interest. A fluorescent reporter strain expressing mStayGold from the hrpA promoter revealed a timing based regulatory difference which was associated with phage encoded transcription factors. This suggests that not only does the PSM associated prophage carry beneficial moron genes for the host, but it actively enhances the hosts pathogenicity via transcriptional regulation. This work demonstrates the importance of mobile genetic elements in the evolution of phytopathogens and begins to elucidate other mechanisms underlying pathogenicity within Pseudomonas species.