Rhizosphere-associated Pseudomonas Research Articles

Antioxidative Response of Duckweed (Lemna minor L.) to Rhizosphere-Associated Pseudomonas Strains and Exogenous Indole-3-Acetic Acid

Antioxidative Response of Duckweed (Lemna minor L.) to Rhizosphere-Associated Pseudomonas Strains and Exogenous Indole-3-Acetic Acid

Rhizosphere-Associated Pseudomonas Suppress Local Root Immune Responses by Gluconic Acid-Mediated Lowering of Environmental pH

The root microbiome consists of commensal, pathogenic, and plant-beneficial microbes [1]. Most members of the root microbiome possess microbe-associated molecular patterns (MAMPs) similar to those of plant pathogens [2]. Their recognition can lead to the activation of host immunity and suppression of plant growth due to growth-defensetradeoffs [3, 4]. We found that 42% of the tested root microbiota, including the plant growth-promoting rhizobacteria Pseudomonas capeferrum WCS358 [5, 6] and Pseudomonas simiae WCS417 [6, 7], are able to quench local Arabidopsis thaliana root immune responses that aretriggered by flg22 [8], an immunogenic epitope of the MAMP flagellin [9], suggesting that this is animportant function of the root microbiome. Ina screen for WCS358 mutants that lost their capacity to suppress flg22-induced CYP71A12pro:GUS MAMP-reporter gene expression, we identified the bacterial genes pqqF and cyoB in WCS358, which are required for the production of gluconic acid and its derivative 2-keto gluconic acid. Both WCS358 mutants are impaired in the production of these organic acids and consequently lowered their extracellular pH to a lesser extent thanwild-type WCS358. Acidification of the plant growth medium similarly suppressed flg22-induced CYP71A12pro:GUS and MYB51pro:GUS expression, and the flg22-mediated oxidative burst, suggesting a role for rhizobacterial gluconic acid-mediated modulation of the extracellular pH in the suppression of root immunity. Rhizosphere population densities of the mutants were significantly reduced compared to wild-type. Collectively, these findings show thatsuppression of immune responses is an important function of the root microbiome, as it facilitates colonization by beneficial root microbiota.

Detection of 4-formylaminooxyvinylglycine in culture filtrates of Pseudomonas fluorescens WH6 and Pantoea ananatis BRT175 by laser ablation electrospray ionization-mass spectrometry.

The oxyvinylglycine 4-formylaminooxyvinylglycine (FVG) arrests the germination of weedy grasses and inhibits the growth of the bacterial plant pathogen Erwinia amylovora. Both biological and analytical methods have previously been used to detect the presence of FVG in crude and extracted culture filtrates of several Pseudomonas fluorescens strains. Although a combination of these techniques is adequate to detect FVG, none is amenable to high-throughput analysis. Likewise, filtrates often contain complex metabolite mixtures that prevent the detection of FVG using established chromatographic techniques. Here, we report the development of a new method that directly detects FVG in crude filtrates using laser ablation electrospray ionization-mass spectrometry (LAESI-MS). This approach overcomes limitations with our existing methodology and allows for the rapid analysis of complex crude culture filtrates. To validate the utility of the LAESI-MS method, we examined crude filtrates from Pantoea ananatis BRT175 and found that this strain also produces FVG. These findings are consistent with the antimicrobial activity of P. ananatis BRT175 and indicate that the spectrum of bacteria that produce FVG stretches beyond rhizosphere-associated Pseudomonas fluorescens.

Dynamics of Aspen Roots Colonization by Pseudomonads Reveals Strain-Specific and Mycorrhizal-Specific Patterns of Biofilm Formation.

Rhizosphere-associated Pseudomonas fluorescens are known plant growth promoting (PGP) and mycorrhizal helper bacteria (MHB) of many plants and ectomycorrhizal fungi. We investigated the spatial and temporal dynamics of colonization of mycorrhizal and non-mycorrhizal Aspen seedlings roots by the P. fluorescens strains SBW25, WH6, Pf0-1, and the P. protegens strain Pf-5. Seedlings were grown in laboratory vertical plates systems, inoculated with a fluorescently labeled Pseudomonas strain, and root colonization was monitored over a period of 5 weeks. We observed unexpected diversity of bacterial assemblies on seedling roots that changed over time and were strongly affected by root mycorrhization. P. fluorescens SBW25 and WH6 stains developed highly structured biofilms with internal void spaces forming channels. On mycorrhizal roots bacteria appeared encased in a mucilaginous substance in which they aligned side by side in parallel arrangements. The different phenotypic classes of bacterial assemblies observed for the four Pseudomonas strains were summarized in a single model describing transitions between phenotypic classes. Our findings also reveal that bacterial assembly phenotypes are driven by interactions with mucilaginous materials present at roots.

Rhizosphere-associated Pseudomonas induce systemic resistance to herbivores at the cost of susceptibility to bacterial pathogens.

Plant-associated soil microbes are important mediators of plant defence responses to diverse above-ground pathogen and insect challengers. For example, closely related strains of beneficial rhizosphere Pseudomonas spp. can induce systemic resistance (ISR), systemic susceptibility (ISS) or neither against the bacterial foliar pathogen Pseudomonas syringae pv. tomato DC3000 (Pto DC3000). Using a model system composed of root-associated Pseudomonas spp. strains, the foliar pathogen Pto DC3000 and the herbivore Trichoplusia ni (cabbage looper), we found that rhizosphere-associated Pseudomonas spp. that induce either ISS and ISR against Pto DC3000 all increased resistance to herbivory by T.ni. We found that resistance to T.ni and resistance to Pto DC3000 are quantitative metrics of the jasmonic acid (JA)/salicylic acid (SA) trade-off and distinct strains of rhizosphere-associated Pseudomonas spp. have distinct effects on the JA/SA trade-off. Using genetic analysis and transcriptional profiling, we provide evidence that treatment of Arabidopsis with Pseudomonas sp. CH267, which induces ISS against bacterial pathogens, tips the JA/SA trade-off towards JA-dependent defences against herbivores at the cost of a subset of SA-mediated defences against bacterial pathogens. In contrast, treatment of Arabidopsis with the ISR strain Pseudomonas sp. WCS417 disrupts JA/SA antagonism and simultaneously primes plants for both JA- and SA-mediated defences. Our findings show that ISS against the bacterial foliar pathogens triggered by Pseudomonas sp. CH267, which is a seemingly deleterious phenotype, may in fact be an adaptive consequence of increased resistance to herbivory. Our work shows that pleiotropic effects of microbiome modulation of plant defences are important to consider when using microbes to modify plant traits in agriculture.

Functional analysis of a biosynthetic cluster essential for production of 4-formylaminooxyvinylglycine, a germination-arrest factor from Pseudomonas fluorescens WH6.

Rhizosphere-associated Pseudomonas fluorescens WH6 produces the germination-arrest factor 4-formylaminooxyvinylglycine (FVG). FVG has previously been shown to both arrest the germination of weedy grasses and inhibit the growth of the bacterial plant pathogen Erwinia amylovora. Very little is known about the mechanism by which FVG is produced. Although a previous study identified a region of the genome that may be involved in FVG biosynthesis, it has not yet been determined which genes within that region are sufficient and necessary for FVG production. In the current study, we explored the role of each of the putative genes encoded in that region by constructing deletion mutations. Mutant strains were assayed for their ability to produce FVG with a combination of biological assays and TLC analyses. This work defined the core FVG biosynthetic gene cluster and revealed several interesting characteristics of FVG production. We determined that FVG biosynthesis requires two small ORFs of less than 150 nucleotides and that multiple transporters have overlapping but distinct functionality. In addition, two genes in the centre of the biosynthetic gene cluster are not required for FVG production, suggesting that additional products may be produced from the cluster. Transcriptional analysis indicated that at least three active promoters play a role in the expression of genes within this cluster. The results of this study enrich our knowledge regarding the diversity of mechanisms by which bacteria produce non-proteinogenic amino acids like vinylglycines.