Deciphering small molecules as virulence factors by the bacterial sugarcane pathogen Xanthomonas albilineans

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In order to successfully colonize plants, plant pathogenic bacteria have evolved a variety of virulence factors to subvert host defences or to obtain nutrients. The understanding of these pathogenic processes is of particular importance since bacteria are capable of causing enormous economic losses on crop plants. Xanthomonas albilineans (Xa) is a systemic, xylem-invading pathogen that causes leaf scald, a lethal disease of sugarcane (Saccharum interspecific hybrids). The Xa-sugarcane pathosystem is an original model in plant pathology because: (i) Xa is constrained within the xylem, but disease symptoms result from changes in chlorenchyma cells; (ii) the Xa infection can be latent for many weeks or months before occurrence of acute disease. Sequencing of the genome of Xa revealed the absence of the type III secretion system Hrp and rather suggested that pathogenicity is dependent on the biosynthesis of secondary metabolites. The most prominent representative is albicidin, a specific toxin that causes foliar leaf scald symptoms. Furthermore, genome sequencing results indicate the presence of genes encoding for additional three non-ribosomal peptide synthetase gene clusters (Metacols A, B and C). The production of secondary metabolites implies additional functions as effectors or toxins in pathogenesis of Xa. Hence, the project is directed to the isolation and structure elucidation of albicidin, its derivatives and of secondary metabolites Metacols A-C. The study will further aim at the characterization of the bioactivities of Metacols A-C to better understand how X. albilineans successfully spreads in sugarcane xylem vessels. The work will have an impact on the understanding of the Xa-sugarcane pathosystem and also two other pathosystems involving species of Xanthomonas, which possess a Metacol-B like NRPS gene cluster (the rice pathogen Xanthomonas oryzae and the wheat pathogen Xanthomonas transluscens). Finally, albicidin may server as a lead structure for novel antibiotics.