Lay summary
We have previously identified syringolin A, an unusual peptide derivative that is synthesized by a mixed non-ribosomal peptide synthetase/polyketide synthetase and that is secreted by the phytopathogenic bacterium Pseudomonas syringae pv. syringae. Spray application of syringolin A has the remarkable property to trigger hypersensitive cell death at infection sites of powdery mildew-infected wheat and Arabidopsis plants. Changes in global gene activity were monitored in wheat and Arabidopsis after spraying of uninfected and powdery mildew-infected plants with syringolin A or a control solution. The results were to a large degree congruent in both species and allowed to formulate a hypothesis about the cellular target and the mode of action of syringolin A that will be tested in this project.Syringolin A is a tripeptide derivative that is synthesized by a mixed non-ribosomal peptide synthetase/polyketide synthetase that is encoded by a gene cluster we previously cloned. The architecture of this gene cluster allowed us to derive a unique model explaining the biosynthesis of syringolin A. Structurally, syringolin is similar in its tripeptide part to the glidobactins, which are acylated tripeptide derivatives. Glidobactins are reported to have antifungal and antitumor activity. The biosynthesis model of the peptide part of syringolin A nicely can accommodate also the tripeptide part of glidobactins. Searching the nearly 600 available eubacterial genome sequences for genes with an architecture similar to the one of the syringolin synthetase genes resulted in the identification of three taxa that are hypothesized to encode synthetases of twelve-membered ring-containing tripeptides: Burkholderia pseudomallei, a dangerous human pathogen and causal agent of melioidosis, Photorhabdus luminescens, an insect pathogen, and Burkholderia mallei, an animal pathogen and causal agent of glanders.A PCR fragment was successfully amplified from genomic DNA of DSM 7029, a bacterium of uncertain origin reported in the literature to synthesize glidobactins. We have cloned that genes encoding the glidobactin synthetase from a bacterium producing the compound. We will also conduct experiments to elucidate the mode of action of glidobatin.The experiments proposed in this project will identify the target on syringolin A and thus identify a new way of how a plant pathogen may manipulate the physiology of the host. Syringolin A belongs to the unique class of compounds containing a twelve-membered ring structure which have an intriguing distribution in nature as far as it is currently known. Elucidation of the biology and mode of action of this class not only increases knowledge about how a plant pathogen interacts with plants, but may also have implications for other pathogens such as Burkholderia pseudomallei, the causing agent of melioidosis.