title: Pseudomonas syringae virulence factor syringolin A counteracts stomatal immunity by proteasome inhibition.
abstract: The peptide derivative syringolin A, a product of a mixed nonribosomal peptide and polyketide synthetase, is secreted by certain strains of the phytopathogenic bacterium Pseudomonas syringae pv. syringae. Syringolin A was shown to be a virulence factor for P. syringae pv. syringae B728a because disease symptoms on its host Phaseolus vulgaris (bean) were greatly reduced upon inoculation with syringolin A-negative mutants. Syringolin A's mode of action was recently shown to be irreversible proteasome inhibition. Here, we report that syringolin A-producing bacteria are able to open stomata and, thus, counteract stomatal innate immunity in bean and Arabidopsis. Syringolin A-negative mutants, which induce stomatal closure, can be complemented by exogenous addition of not only syringolin A but also MG132, a well-characterized and structurally unrelated proteasome inhibitor. This demonstrates that proteasome activity is crucial for guard cell function. In Arabidopsis, stomatal immunity was salicylic acid (SA)-dependent and required NPR1, a key regulator of the SA-dependent defense pathway whose proteasome-dependent turnover has been reported to be essential for its function. Thus, elimination of NPR1 turnover through proteasome inhibition by syringolin A is an attractive hypothesis to explain the observed inhibition of stomatal immunity by syringolin A.
authors.affiliation: Institute of Plant Biology, University of Zurich, Zurich, Switzerland.
authors.forename: Barbara
authors.initials: B
authors.lastname: Schellenberg
authors.forename: Christina
authors.initials: C
authors.lastname: Ramel
authors.forename: Robert
authors.initials: R
authors.lastname: Dudler
mesh_terms: D017360:Arabidopsis / Q000382:microbiology; D054884:Host-Pathogen Interactions; D010456:Peptides, Cyclic / Q000235:genetics / Q000378:metabolism*; D027805:Phaseolus / Q000382:microbiology*; D010935:Plant Diseases / Q000276:immunology / Q000382:microbiology*; D054046:Plant Stomata; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D044224:Pseudomonas syringae / Q000378:metabolism*
chemical_list: D010456:Peptides, Cyclic; C407267:syringolin A; D046988:Proteasome Endopeptidase Complex