Apoplast manipulation by Pseudomonas syringae

Plants recognize bacterial pathogens through conserved fragments of flagellin (flg22) that bind the cell surface receptor FLS2 in most flowering plants. We previously discovered that a plant-secreted beta-galactosidase (BGAL1) promotes flagellin detection by removing the terminal glycan from glycosylated flagellin polymers, presumably promoting flagellin monomerisation. Here, I will discuss the identity of a BGAL1 inhibitor secreted by the bacterial model pathogen Pseudomonas syringae pv. tomato DC3000 (PtoDC3000). BGAL1 inhibitor 'glycosyrin' is a novel iminosugar produced by a biosynthesis cluster in PtoDC3000 that is regulated by virulence regulator hrpL and occurs in various P. syringae pathovars isolated from diverse plants. We resolved the glycosyrin biosynthesis, regulation and structure and discovered that it not only suppresses flg22 recognition but also dramatically alters the glycobiology of the host plant. Finally, I will give a brief overview of the research pipeline we are using to discover more extracellular suppression mechanisms in the apoplast of P. syringae-infected N. benthamiana plants involving chemical proteomics combined with large scale in silico protein interaction screens using AlphaFold Multimer.