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  • Title: Pseudomonas syringae virulence factor syringolin A counteracts stomatal immunity by proteasome inhibition.
    Author: Schellenberg B, Ramel C, Dudler R.
    Journal: Mol Plant Microbe Interact; 2010 Oct; 23(10):1287-93. PubMed ID: 20831408.
    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.
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