Phevamine A, a small molecule that suppresses plant immune responses

Bacterial plant pathogens cause significant crop damage worldwide. They invade plant cells by producing a variety of virulence factors, including small-molecule toxins and phytohormone mimics. Virulence of the model pathogen Pseudomonas syringae pv. tomato DC3000 ( Pto ) is regulated in part by the sigma factor HrpL. Our study of the HrpL regulon identified an uncharacterized, three-gene operon in Pto that is controlled by HrpL and related to the Erwinia hrp -associated systemic virulence ( hsv ) operon. Here, we demonstrate that the hsv operon contributes to the virulence of Pto on Arabidopsis thaliana and suppresses bacteria-induced immune responses. We show that the hsv -encoded enzymes in Pto synthesize a small molecule, phevamine A. This molecule consists of l-phenylalanine, l-valine, and a modified spermidine, and is different from known small molecules produced by phytopathogens. We show that phevamine A suppresses a potentiation effect of spermidine and l-arginine on the reactive oxygen species burst generated upon recognition of bacterial flagellin. The hsv operon is found in the genomes of divergent bacterial genera, including ∼37% of P. syringae genomes, suggesting that phevamine A is a widely distributed virulence factor in phytopathogens. Our work identifies a small-molecule virulence factor and reveals a mechanism by which bacterial pathogens overcome plant defense. This work highlights the power of omics approaches in identifying important small molecules in bacteria–host interactions.