Undermining the cry for help: the phytopathogenic fungus Verticillium dahliae secretes an antimicrobial effector protein to undermine host recruitment of antagonistic Pseudomonas bacteria.

During pathogen attack, plants recruit beneficial microbes in a 'cry for help' to mitigate disease development. Simultaneously, pathogens secrete effectors to promote host colonisation through various mechanisms, including targeted host microbiota manipulation. Inspired by in silico antimicrobial activity prediction, we investigated the antimicrobial activity of Av2, an effector of Verticillium dahliae, in vitro. Furthermore, its role in V. dahliae virulence was assessed through microbiota sequencing of inoculated plants, microbial co-cultivation assays, and inoculations in a gnotobiotic plant cultivation system. Av2 appears structurally unique and lacks domains that hint towards its function. We show that Av2 inhibits bacterial growth and acts as a virulence factor during host colonisation. Microbiota sequencing revealed involvement of Av2 in suppression of Pseudomonas spp. recruitment upon plant inoculation with V. dahliae, indicating that Av2 suppresses the cry for help. We show that several Pseudomonas spp. are antagonistic to V. dahliae and sensitive to Av2 treatment. We conclude that V. dahliae secretes Av2 to suppress the plant's cry for help by inhibiting the recruitment of antagonistic Pseudomonas spp. to pave the way for successful plant invasion.