Secondary metabolites of Pseudomonas putida and Pseudomonas fluorescens involved in antagonistic interactions with phylogenetically related plant-associated bacteria
Secundaire metabolieten van Pseudomonas putida en Pseudomonas fluorescens betrokken bij antagonistische interacties met fylogenetisch verwante plant-geassocieerde bacteriën
Rokni Zadeh, Hassan; S0171835
Secondary metabolites of Pseudomonas putida and Pseudomonas fluorescens involved in antagonistic interactions with phylogenetically related plant-associated bacteria Several species of the genus Pseudomonas produce compounds with antimicrobial activity, targeting different microorganisms. Bacteriocins are used to kill related bacteria while antibiotics mediate inhibition of bacterial or fungal growth. In this work the objective was to identify novel antibacterial activities elaborated by non-pathogenic plant-associated fluorescent Pseudomonads against related phytopathogenic bacteria, focusing on a member of the Pseudomonas syringae group (Pseudomonas savastanoi, causative agent of olive knot disease) and pathogenic Xanthomonas species. Screening of a collection of Pseudomonas rhizosphere isolates for constitutive and inducible anti-P. savastanoi activity retained two strains for further characterization. Pseudomonas fluorescens SWRI196, a wheat rhizosphere isolate from Iran, exhibits an antagonistic activity that is not restricted to Pseudomonas species but also affects other g-proteobacteria as well as alfa- and ß-proteobacteria. Mutational analysis revealed a putative type-II polyketide system, with no known equivalent in other microorganisms, to be required for this antagonism. The corresponding biosynthetic gene cluster encodes components of a divergent polyketide synthase (PKS) linked to several putative tailoring genes and is associated with an acyl-homoserine lactone-responsive quorum sensing (QS) system. In addition, the Gac/Rsm signaling cascade is required as an additional regulatory system.Pseudomonas putida RW10S2, a rice rhizosphere isolate from Sri Lanka, displays both anti-Pseudomonas and -Xanthomonas activity which was shown to be achieved through different mechanisms. Antagonism against Pseudomonas is observed only when both producer and indicator are cocultured and also requires a functional Gac/Rsm system. This target-induced killing is specifically activated by QS via the RW10S2 Pmr system, producing and responding to 3-OH-C12-homoserine lactone. The pmr genes are part of an operon with seven orfs. Both this gene cluster and the P. fluorescens SWRI196 PKS-based gene cluster, contain genes with low homology to non-Pseudomonas genes, indicative of acquisition by horizontal gene transfer from other bacteria. In both cases, this precludes a reliable prediction of the chemical nature of the compounds synthesized, and structure elucidation will be imperative to resolve the biosynthetic functions of the respective genes. Using a different strategy, P. putida RW10S2 kills some Xanthomonas species by constitutive production of the cyclic lipopeptide WLIP (white line-inducing principle). Here also, global regulation involves the GacS/Rsm system and, in addition, a cognate non-QS type of LuxR family activator controls production. WLIP biosynthesis is performed by three non-ribosomal peptide synthetases (NRPSs), encoded by the wlp genes located in two separate genomic regions that equally carry genes for an export system and the dedicated LuxR-type regulator. One of these NRPS genes (wlpC) is also involved in mounting the inducible anti-P. savastanoi activity of P. putida RW10S2 but the mechanism behind this remains to be elucidated. In addition to its function as an antibacterial compound, WLIP contributes to proper biofilm formation and swarming, suggestive of a role in competitive solid surface colonization in structured environments. By characterization of the WLIP biosynthetic genes of P. fluorescens LMG 5329, a second WLIP-producing system (Wip) was identified, being clearly distinct from that of P. putida RW10S2 (Wlp). The Wlp components are more related to those of the putisolvin and entolysin systems, whereas a strong similarity exists between the Wip and viscosin systems.