807-P2 - Pseudomonas syringae produces an inhibitor of a plant defence-related Beta-galactosidase

Abstract

During infection, plants and bacteria participate in a dynamic interaction in the apoplast. Secreted hydrolases are among the prominent players in plant defence and are therefore targets of virulence factors. Using Nicotiana benthamiana and Pseudomonas syringae pv. tomato (Pst) DC3000 as a model system, we recently discovered a plant β-galactosidase (BGAL1) that functions in immunity against bacteria by facilitating the hydrolytic release of elicitor peptides from glycosylated flagellin, which activates plant defences. Interestingly, PstDC3000 produce an inhibitor of BGAL1. Here, we identify the biosynthetic gene cluster that is responsible for BGAL1 inhibitor production, which is under the control of type III secretion system regulators hrpR/S/L. Mutant bacteria lacking this gene cluster show reduced virulence in N. benthamiana. Comparative genomics suggests acquisition of this gene cluster via horizontal gene transfer and indicates a correlation between inhibitor production and the identity of flagellin glycans. Partial purification and characterization of the inhibitor has revealed that it is a hydrophilic, heat stable and basic small molecule, similar to a sugar derivative. Our work has uncovered a novel small molecule virulence factor that is secreted into the apoplast to inhibit a plant defence-related enzyme and highlights the role of glycans and apoplastic enzymes in plant-pathogen interactions.