Group Leader John Innes Centre Norwich, England, United Kingdom
Body of Abstract: Pseudomonas syringae (Psy) is a widespread bacterial pathogen that causes significant damage to plants worldwide. Though much attention is paid to interactions with angiosperms, recent ecological surveys suggest a more expansive host range that includes non-flowering plants. To better understand the core aspects underpinning broad host pathogenicity, we tested diverse Psy pathovars for their ability to infect three divergent plants: the bryophyte Marchantiapolymorpha, the fern Ceratopteris richardii, and the dicot Nicotiana benthamiana. While most pathovars displayed variable disease potentials, members of the phylogroup 2 (PG2) clade were particularly virulent in all three systems. Since PG2 is well known for its expanded repertoire of phytotoxins and certain type-3 effectors, we assessed the contribution of these factors to broad host infection using established mutants defective in lipopeptide toxin production (syringomycin) or effector translocation. Importantly, both toxins and effectors were essential for virulence in divergent host plants. Here, we discuss comparative RNA-sequencing analyses aimed at understanding how toxins and effectors subvert conserved immune responses in land plants. Moreover, we explore the extent to which syringomycin can expand the host range of toxin-deficient Psy phylogroups. Collectively, our work suggests that toxins are a key virulence determinant that enables the broad host success of PG2 Psy across the plant kingdom.