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Fluorescent pseudomonads pursue media-dependent strategies to inhibit growth of pathogenic Verticillium fungi

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Abstract

Verticillium species represent economically important phytopathogenic fungi with bacteria as natural rhizosphere antagonists. Growth inhibition patterns of Verticillium in different media were compared to saprophytic Aspergillus strains and were significantly more pronounced in various co-cultivations with different Pseudomonas strains. The Brassica napus rhizosphere bacterium Pseudomonas fluorescens DSM8569 is able to inhibit growth of rapeseed (Verticillium longisporum) or tomato (Verticillium dahliae) pathogens without the potential for phenazine or 2,4-diacetylphloroglucinol (DAPG) mycotoxin biosynthesis. Bacterial inhibition of Verticillium growth remained even after the removal of pseudomonads from co-cultures. Fungal growth response in the presence of the bacterium is independent of the fungal control genes of secondary metabolism LAE1 and CSN5. The phenazine producer P. fluorescens 2-79 (P_phen) inhibits Verticillium growth especially on high glucose solid agar surfaces. Additional phenazine-independent mechanisms in the same strain are able to reduce fungal surface growth in the presence of pectin and amino acids. The DAPG-producing Pseudomonas protegens CHA0 (P_DAPG), which can also produce hydrogen cyanide or pyoluteorin, has an additional inhibitory potential on fungal growth, which is independent of these antifungal compounds, but which requires the bacterial GacA/GacS control system. This translational two-component system is present in many Gram-negative bacteria and coordinates the production of multiple secondary metabolites. Our data suggest that pseudomonads pursue different media-dependent strategies that inhibit fungal growth. Metabolites such as phenazines are able to completely inhibit fungal surface growth in the presence of glucose, whereas GacA/GacS controlled inhibitors provide the same fungal growth effect on pectin/amino acid agar.

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Acknowledgements

The authors thank Linda Thomashow for strain Pf2-79∆phz, Dieter Haas for PfCHA-mutants, and Tuan Tran for plasmid pKO2. We are grateful to Josua Schinke and Bastian Joehnk for proofreading and Sarah Zunken for technical support.

Funding

This study was funded by the Deutsche Forschungsgemeinschaft (DFG) grant BR1502/15–1 and by the Federal Ministry of Education and Research (BMBF) BioFung project.

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Correspondence to Gerhard H. Braus.

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The authors declare that they have no competing interests.

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This article does not contain any studies with human participants or animals performed by any of the authors.

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Nesemann, K., Braus-Stromeyer, S.A., Harting, R. et al. Fluorescent pseudomonads pursue media-dependent strategies to inhibit growth of pathogenic Verticillium fungi. Appl Microbiol Biotechnol 102, 817–831 (2018). https://doi.org/10.1007/s00253-017-8618-5

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  • DOI: https://doi.org/10.1007/s00253-017-8618-5

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