Biocontrol agents Pseudomonas brassicacearum DF41 & Pseudomonas chlororaphis PA23: Investigation of fungal suppression and defense against Caenorhabditis elegans

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dc.contributor.supervisor de Kievit, Teresa (Microbiology) en_US
dc.contributor.author Nandi, Munmun
dc.date.accessioned 2016-09-12T15:47:09Z
dc.date.available 2016-09-12T15:47:09Z
dc.date.issued 2015-04-22 en_US
dc.identifier.citation Nandi, M., Selin, C., Brassinga, A.K.C., Belmonte, M.F., Fernando, W.G.D., Loewen, P.C. and de Kievit, T.R. 2015. Pyrrolnitrin and hydrogen cyanide production by Pseudomonas chlororaphis strain PA23 exhibits nematicidal and repellent activity against Caenorhabditis elegans. PloSOne/doi:10.1371/Journal.poine.0123184. en_US
dc.identifier.uri http://hdl.handle.net/1993/31674
dc.description.abstract The success of biocontrol bacteria is often restrained due to their low persistence in the rhizosphere and fluctuations in expression of antagonistic compounds. In the first part of this thesis the ability of the biocontrol agents (BCAs) Pseudomonas brassicacearum DF41 and Pseudomonas chlororaphis PA23 to resist grazing by the bacterivorous nematode Caenorhabditis elegans was investigated. We found that both BCAs are capable of killing the nematodes through exposure to toxic metabolites. We discovered that in addition to HCN, pyrrolnitrin (PRN) is a potent nematicide produced by PA23. Unique for a pseudomonad, DF41 was also found to kill the nematodes by forming biofilms on the nematode anterior, causing starvation. Biofilm formation was dependent upon the Gac two-component system and NaCl concentration of the media. Co-culturing these BCAs in the presence of nematodes increased expression of a number of genes associated with biocontrol. We observed elevated exoproduct formation, consistent with our gene expression analysis. The nematicidal activity exhibited by DF41 and PA23 towards C. elegans bodes well for their persistence in the environment. In the second part of this thesis the role of hydrogen cyanide (HCN) and the anaerobic regulator ANR in PA23 biocontrol was explored. An hcn mutant was created and in vitro antifungal (AF) assays revealed the involvement of HCN in Sclerotinia sclerotiorum suppression. Addition of glycine promoted both AF activity and HCN production. In addition, HCN was found to be positively regulated by quorum sensing (QS). Besides a phz box, an anr box was identified in the hcnA promoter, suggesting a role for ANR in regulating hcnA. An anr mutant was generated and phenotypic characterization revealed that ANR is a key regulator governing PA23 secondary metabolite production. Through gene expression analysis, ANR was shown to positively regulate phzI/phzR and PhzR negatively regulate anr. Furthermore, expressing anr in trans partially complemented the QS-deficient phenotype with respect to several biocontrol genes and exoproducts. Overall, the global regulator ANR is vital for PA23-mediated biocontrol and a significant overlap exists between the QS and ANR regulons. en_US
dc.publisher PLoS One en_US
dc.subject Bacterivorous en_US
dc.subject Biocontrol en_US
dc.subject Biofilm en_US
dc.subject Gene expression en_US
dc.subject Metabolites en_US
dc.subject Pseudomonas en_US
dc.subject Quorum sensing en_US
dc.title Biocontrol agents Pseudomonas brassicacearum DF41 & Pseudomonas chlororaphis PA23: Investigation of fungal suppression and defense against Caenorhabditis elegans en_US
dc.degree.discipline Microbiology en_US
dc.contributor.examiningcommittee Internal examiners: Loewen, Peter (Microbiology) Brassinga, Karen (Microbiology) Belmonte, Mark (Biological Sciences) External examiner: Keel, Christoph (Fundamental Microbiology, University of Lausanne) en_US
dc.degree.level Doctor of Philosophy (Ph.D.) en_US
dc.description.note October 2016 en_US

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