Search

Pseudomonas chlororaphis strain PA23 is a biocontrol agent capable of protecting canola from stem rot disease caused by the fungal pathogen Sclerotinia sclerotiorum. PA23 produces several inhibitory compounds that are under control of a complex regulatory network. Included in this cascade is the PhzRI quorum sensing (QS) system, which plays an essential role in PA23 biocontrol, as well as CsaRI and AurRI, which have not yet been characterized in PA23. The focus of the current study was to employ RNA sequencing to explore the spectrum of PA23 genes under QS control. In this work, we investigated genes under the control of the main QS transcriptional regulator, PhzR, as well as those differentially expressed in an AHL-deficient strain, PA23-6863, which constitutively expresses an AiiA lactonase, rendering the strain QS defective. Transcriptomic profiling revealed 545 differentially expressed genes (365 downregulated; 180 upregulated) in the phzR mutant and 534 genes (382 downregulated; 152 upregulated) in the AHL-deficient PA23-6863. In both strains, decreased expression of phenazine, pyrrolnitrin, and exoprotease biosynthetic genes was observed. We have previously reported that QS activates expression of these genes and their encoded products. In addition, elevated siderophore and decreased chitinase gene expression was observed in the QS-deficient stains, which was confirmed by phenotypic analysis. Inspection of the promoter regions revealed the presence of "phz-box" sequences in only 58 of the 807 differentially expressed genes, suggesting that much of the QS regulon is indirectly regulated. Consistent with this notion, 41 transcriptional regulators displayed altered expression in one or both of the QS-deficient strains. Collectively, our findings indicate that QS governs expression of approximately 13% of the PA23 genome affecting diverse functions ranging from secondary metabolite production to general metabolism., Competing Interests: The authors have declared that no competing interests exist.

In Pseudomonas aeruginosa the LasR–LasI and RhlR–RhlI quorum-sensing (QS) systems control expression of numerous virulence genes in a population density-dependent fashion. In this study, we investigated regulation of the autoinducer synthase gene rhlI, which is responsible for C4-HSL signal production. Primer extension analysis was used to map the rhlI transcriptional start site and an upstream regulatory region was identified. Expression studies revealed that (i) this regulatory region is important for rhlI expression and (ii) although the rhl QS system will induce rhlI, las is the dominant regulator. Furthermore, we found that control of rhlI in Escherichia coli is markedly different than in P. aeruginosa. [Copyright &y& Elsevier]

Pseudomonas brassicacearum DF41 is a biocontrol agent that suppresses disease caused by the fungal pathogen Sclerotinia sclerotiorum. A number of exometabolites are produced by DF41, including the lipopeptide sclerosin, hydrogen cyanide (HCN), and degradative enzymes. The production of these compounds is controlled at both the transcriptional and posttranscriptional levels by quorum sensing (QS) and the Gac two-component regulatory system. In order to be successful, a biocontrol agent must persist in the environment at levels sufficient for pathogen control. Bacterivorous predators, including nematodes, represent a challenge to the establishment of introduced microorganisms. In the current study, DF41 was investigated for its ability to resist predation by Caenorhabditis elegans. We discovered that this bacterium is capable of killing C. elegans through two different mechanisms: the first involves exposure to toxic metabolites, and the second entails biofilm formation on the nematode head blocking the buccal cavity. Biofilm formation on nematodes, which has been reported only for Yersinia spp. and Xenorhabdus nematophila, is dependent upon the Gac system. Biofilms were not observed when bacteria were grown on NaCl-containing medium or on C. elegans biofilm-resistant mutants. Coculturing with nematodes led to the increased expression of the pdfRI-rfiA QS genes and hcnA, which is under QS control. HCN was the most nematicidal of the exometabolites, suggesting that this bacterium can respond to predator cues and upregulate expression of toxins accordingly. In summary, DF41 is able to respond to the presence of C. elegans, and through two distinct mechanisms, it can escape predation. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Pseudomonas sp. DF41 quorum-sensing system consists of PdfR and PdfI. [•] A gene encoding a novel LuxR-type protein, RfiA, is co-transcribed with pdfI. [•] Quorum sensing indirectly controls DF41 antifungal activity through RfiA. [•] pdfI and acyl homoserine lactone production are positively regulated by the Gac–Rsm system. [ABSTRACT FROM AUTHOR]

Abstract: Pseudomonas sp. DF41 is a biocontrol agent capable of suppressing Sclerotinia sclerotiorum-mediated stem rot of canola. Using transposon mutagenesis, we identified two mutants with greatly reduced antifungal (AF) activity. The first mutant had an insertion in gacS, forming part of the GacS/GacA regulatory system. The second mutation was in a gene involved in lipopeptide (LP) synthesis suggesting LPs are essential for biocontrol. Strain DF41 and the gacS (DF41-469) and lp (DF469-1278) mutants were characterized with respect to their extracellular metabolite production, antifungal (AF) properties and ability to form biofilms. We discovered that DF41 produces a number of compounds that may contribute to biocontrol including hydrogen cyanide (HCN), protease, alginate, and LP molecules. All of these products were found to be under Gac control. In addition, DF41 produces autoinducers, suggesting this bacterium employs quorum sensing as part of its lifestyle. In the greenhouse, the gacS and lp mutants were unable to protect canola from disease incited by S. sclerotiorum. Both mutants were able to sustain themselves in the canola phyllosphere; therefore, the loss of biocontrol activity can be attributed to reduced production of AF compounds and not a declining population size. We conclude that suppression of Sclerotinia stem rot of canola by Pseudomonas strain DF41 depends upon LP production and a functional Gac system. [ABSTRACT FROM AUTHOR]