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Soil microorganisms exuding organic acids have the potential to solubilize inorganic phosphorus (P), which could improve the P availability to plants growing on calcareous soil. The gluconic acid exuding bacteria Pseudomonas protegens CHA0 can solubilize P under glucose rich in vitro conditions, but evidence on the effectiveness in soil is lacking. This discrepancy in P solubilization between in vitro and in vivo is common for many P solubilizing bacteria. Possible causes for this discrepancy are rarely explored in soil using mechanism oriented approaches. Proposed reasons for limitation of bacterial P solubilization in soil are low persistence of the inoculant or low glucose availability in the plant rhizosphere. To test these two hypotheses we investigated the solubilization of 33P labeled synthetic hydroxyapatite (Ca33P) by the gluconic acid producing P. protegens CHA0 wild type and the mutant strain CHA1198 lacking the capacity to produce this acid, in a plant growth experiment with wheat (Triticum aestivum) and an incubation experiment. Neither in the plant growth- nor in the incubation experiment solubilization of Ca33P by strain CHA0 was detected, in spite the inoculated strain persisted in the rhizoplane of wheat and in the sterilized soil amended with glucose. No detected P solubilization in the sterilized inoculated soil suggests that glucose availability was the main limiting factor. The comparison of the results obtained from the two bacterial inoculants suggested that overall microbial activity, i.e., via protonation due to respiration, increased inorganic P mobilization. P solubilizing bacteria should be evaluated using a tracer and an appropriate bacterial control in order to reveal the mechanisms involved in increased plant available P in soil inoculated with P solubilizing bacteria.

Diabrotica virgifera virgifera LeConte, the western corn rootworm (WCR), is the most destructive pest of maize in North America, and has recently spread across central Europe. Its subterranean larval stages are hard to reach with pesticides and it has evolved resistance to conventional management practices. The application of beneficial soil organisms is being considered as a sustainable and environmental friendly alternative. In a previous study, the combined application in wheat fields of arbuscular mycorrhizal fungi, entomopathogenic Pseudomonas bacteria, and entomopathogenic nematodes was found to promote growth and protection against a natural pest infestation, without negative cross effects. Because of the insect-killing capacity of the bacteria and nematodes, we hypothesized that the application of these organisms would have similar or even greater beneficial effects in WCR-infested maize fields. During three consecutive years (2015–2017), we conducted trials in Missouri (USA) in which we applied the three organisms, alone or in combinations, in plots that were artificially infested with WCR and in non-infested control plots. For two of the three trials, we found that in plots treated with entomopathogenic nematodes and/or entomopathogenic Pseudomonas bacteria, roots were less damaged than the roots of plants in control plots. During one year, WCR survival was significantly lower in plots treated with Pseudomonas than in control plots, and the surviving larvae that were recovered from these plots were lighter. The bacterial and nematodes treatments also enhanced yield, assessed as total grain weight, in one of the trials. The effects of the treatments varied considerable among the three years, but they were always positive for the plants.

Pseudomonas fluorescens CHA0 and the related strain Pf-5 are well-characterized representatives of rhizosphere bacteria that have the capacity to protect crop plants from fungal root diseases, mainly by releasing a variety of exoproducts that are toxic to plant pathogenic fungi. Here, we report that the two plant-beneficial pseudomonads also exhibit potent insecticidal activity. Anti-insect activity is linked to a novel genomic locus encoding a large protein toxin termed Fit (for P. fluorescensinsecticidal toxin) that is related to the insect toxin Mcf (Makes caterpillars floppy) of the entomopathogen Photorhabdus luminescens, a mutualist of insect-invading nematodes. When injected into the haemocoel, even low doses of P. fluorescens CHA0 or Pf-5 killed larvae of the tobacco hornworm Manduca sexta and the greater wax moth Galleria mellonella. In contrast, mutants of CHA0 or Pf-5 with deletions in the Fit toxin gene were significantly less virulent to the larvae. When expressed from an inducible promoter in a non-toxic Escherichia coli host, the Fit toxin gene was sufficient to render the bacterium toxic to both insect hosts. Our findings establish the Fit gene products of P. fluorescens CHA0 and Pf-5 as potent insect toxins that define previously unappreciated anti-insect properties of these plant-colonizing bacteria.

Soil-borne diseases such as damping-off caused by Pythium sp. are responsible for high yield losses in organic vegetable production and are difficult to control. Compost amendments have been shown to improve in many cases survival and growth of plants in soils infested with soil-borne diseases. Yet, not all composts are disease suppressive and suppressiveness of composts can be pathogen-specific. Cress (Lepidium sativum L.) was grown in a standard peat substrate amended either with coco fiber (conducive control) or with composts differing in their suppressiveness towards Pythium ultimum. Bacteria were isolated from the rhizoplane and the most abundant species identified by MALDI-TOF MS. The bacterial composition in the rhizoplane of plants grown in non-suppressive substrates and in the suppressive compost was essentially different. Aeromonas media was the main species isolated from the rhizoplane of plants grown in a suppressive compost, whereas Enterobacter cloacae was the dominating species in the less suppressive compost and in the conducive control. A. media was then added to all substrates to evaluate its role in disease suppression. Addition of A. media improved suppressiveness against P. ultimum in all substrates. The effect was most pronounced in the substrate amended with the compost with low suppressiveness, resulting in levels of suppressiveness comparable to the highly suppressive compost. We conclude that presence of A. media in composts can contribute to disease suppression and might provide a useful marker for qualitative analysis of composts.

In agricultural ecosystems, pest insects, pathogens, and reduced soil fertility pose major challenges to crop productivity and are responsible for significant yield losses worldwide. Management of belowground pests and diseases remains particularly challenging due to the complex nature of the soil and the limited reach of conventional agrochemicals. Boosting the presence of beneficial rhizosphere organisms is a potentially sustainable alternative and may help to optimize crop health and productivity. Field application of single beneficial soil organisms has shown satisfactory results under optimal conditions. This might be further enhanced by combining multiple beneficial soil organisms, but this remains poorly investigated. Here, we inoculated wheat plots with combinations of three beneficial soil organisms that have different rhizosphere functions and studied their effects on crop performance. Plant beneficial Pseudomonas bacteria, arbuscular mycorrhizal fungi (AMF), and entomopathogenic nematodes (EPN), were inoculated individually or in combinations at seeding, and their effects on plant performance were evaluated throughout the season. We used traditional and molecular identification tools to monitor their persistence over the cropping season in augmented and control treatments, and to estimate the possible displacement of native populations. In three separate trials, beneficial soil organisms were successfully introduced into the native populations and readily survived the field conditions. Various Pseudomonas, mycorrhiza, and nematode treatments improved plant health and productivity, while their combinations provided no significant additive or synergistic benefits compared to when applied alone. EPN application temporarily displaced some of the native EPN, but had no significant long-term effect on the associated food web. The strongest positive effect on wheat survival was observed for Pseudomonas and AMF during a season with heavy natural infestation by th

Pseudomonas fluorescens CHA0 protects various crop plants against root diseases caused by pathogenic fungi. Among a range of exoproducts excreted by strain CHA0, the antifungal compounds 2,4-diacetylphloroglticinol (DAPG) and pyoluteorin (PLT) are particularly relevant to the strain's biocontrol potential. Here, we report on the characterization of MvaT and MvaV as novel regulators of biocontrol activity in strain CHA0. We establish the two proteins as further members of an emerging family of MvaT-like regulators in pseudomonads that are structurally and functionally related to the DNA-binding protein H-NS. In mvaT and mvaV in frame-deletion mutants of strain CHA0, PLT production was enhanced about four-and 1.5-fold, respectively, whereas DAPG production remained at wild-type levels. Remarkably, PLT production was increased tip to 20-fold in an mvaT mvaV double mutant. DAPG biosynthesis was almost completely repressed in this mutant. The effects on antibiotic production could be confirmed by following expression of gfp-based reporter fusions to the corresponding biosynthetic genes. MvaT and MvaV also influenced levels of other exoproducts, motility, and physicochemical cell-surface properties to various extents. Compared with the wild type, mvaT and mvaV mutants had an about 20% reduced capacity (in terms of plant fresh weight) to protect cucumber from a root rot caused by Pythium ultimum. Biocontrol activity was nearly completely abolished in the double mutant Our findings indicate that MvaT and MvaV act together as further global regulatory elements in the complex network controlling expression of biocontrol traits in plant-beneficial pseudomonads.

The photosensitizing perylenequinone toxin elsinochrome A (EA) is produced in culture by the bindweed biocontrol fungus Stagonospora convolvuli LA39 where it apparently plays a pathogenicity related role. We investigated the fate of EA with reference to its stability under different temperature and light conditions. EA remained stable when boiled in water at 100°C for 2 h. Similarly, exposing EA to 3-27°C in the dark for up to 16 weeks did not affect its stability either in dry or in aqueous form. However, results from irradiation experiments indicate that direct photolysis may be a significant degradation pathway for EA in the environment. EA either in dry form or dissolved in water was degraded by different irradiation wavelengths and intensities, with degradation plots fitting a first order rate kinetics. EA degraded faster if exposed in aqueous form, and at higher quantum flux density (μmol s−1 m−2). Sunlight was more effective in degrading EA than artificial white light and ultraviolet radiations (UV-A or UV-B). Exposing EA to natural sunlight, particularly, during the intense sunshine (1,420-1,640 μmol −1 m−2) days of 30 July to 5 August 2004 in Zurich caused the substance to degrade rapidly with half-life under such condition only 14 h. This implies that should EA gets into the environment, particularly on exposed environmental niches, such as on plant surfaces through biocontrol product spray, or released from shed diseased leaves, it may have no chance of accumulating to ‘level of concern'. Furthermore, a toxicity assay using Trichoderma atroviride P1 as biosensor showed that photo-degraded EA was not toxic, indicating that no stable toxic by-products were left