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Title: An investigation into the fungal suppresive activity of pseudonomas fluorescens SBW25
Author: Bailey, Katy Elizabeth
ISNI:       0000 0001 3436 3561
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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Pseudomonas fluorescens SBW25, isolated from the p.hytosphere of sugar beet, is a plant growth promoting rhizobacterium (PGPR). As a strong rhizosphere colonizer exhibiting broad spectrum activity against phytopathogenic fungi including the commercially important Pylhium ullimum Trow, SBW25 is considered to have potential for use as a biocontrol agent. However, although widely studied its mechanism of fungal suppression remains poorly understood. The research presented in this thesis focused on the elucidation of mechanisms contributing to the antifungal phenotype of SBW25, with the aim of providing scope for future optimization of biocontrol strategies with this bacterium. This was approached through the construction and screening of an SBW25 fosmid library, and the assessment of. compounds produced by SBW25 for antifungal activity. From in vitro agar plate assays, it was observed that SBW25 produced volatile compounds capable of suppressing the phytopathogens Aspergillus niger and Fusarium graminearu11l. Screening of over 70 ecologically and genetically diverse pseudomonads revealed this trait to be common to the majority of strains, irrespective of isolation environment. Application of solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS) identified methanethiol (MTL), dimethyl disulfide (DMDS) and l-undecene to be the dominant . volatile compounds produced by SBW25 and a selection of pseudomonad isolates. Exposure of fungal cultures to these compounds revealed DMDS to exert a fungistatic effect, while the development and application of a quantitative SPME method demonstrated a significant correlation to exist between DMDS production and suppression. Through bioinformatic analysis, a homologue of the methionine y-lyase gene of P. pUlida, the product of which catalyzes the conversion of L-methionine to the PMOS precursor, MTL, was identified in the SBW25 genome. Construction of a methionine y-lyase gene knockout in SBW25 resulted in a complete loss of DMDS production and an accompanying reduction in fungal suppression, therefore confirming the involvement of DMOS in the suppressive phenotype. From reviewing the literature - this'appears to be the first instance of a volatile organic sulfur compound produced by a - pseudomonad being implicated in a biocontrol phenotype. Further chemical analysis suggested ammonia to also play a role in the antifungal activity of this and other pseudomonads assayed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available