Characterizing the CsaI-CsaR quorum sensing system in Pseudomonas chlororaphis strain PA23 and investigating its role in regulating the production of secondary metabolites
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Date
2020-07-24
Authors
Saliga, Grace
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Abstract
Pseudomonas chlororaphis strain PA23 is able to inhibit fungal phytopathogens, primarily through production of excreted secondary metabolites, deeming this organism a promising biocontrol candidate. Antibiotics, including phenazine (PHZ) and pyrrolnitrin (PRN), in addition to degradative enzymes and volatile organic compounds, make up the arsenal that combats pathogenic fungi like Sclerotinia sclerotiorum. Production of these exometabolites is tightly controlled via a hierarchy, which includes the Phz quorum sensing (QS) system. Previous RNA-seq analysis focusing on two QS-deficient strains, namely PA23phzR and PA2--AHL, revealed that 13% of the PA23 genome is QS-regulated. A number of unique differentially expressed genes in the QS-deficient strain compared to the phzR mutant suggested that additional regulators are involved in PA23 QS. A second system, called Csa (cell surface alterations) has been characterized in the closely related P. chlororaphis 30-84. While csaI and csaR are present in the PA23 genome, the role of this system in PA23 biocontrol has not yet been investigated. The purpose of this research was to characterize the PA23 Csa QS system and elucidate its role in regulating secondary metabolite production. Single mutants of the Csa and Phz QS systems, PA23csaR, PA23csaI and PA23phzI, in addition to double mutants, PA23phzRcsaR and PA23phzIcsaI, were generated and subjected to phenotypic analysis. Results indicated that the Csa system negatively regulates PA23 AF activity. PA23csaR and PA23csaI, as well as the phzIcsaI double mutant, all exhibited increased S. sclerotiorum inhibition in plate assays. While phenotypic analysis revealed very minor differences in chitinase, protease, and HCN production as well as biofilm formation in one or both csa mutants, these differences
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alone are insufficient to account for the increased antifungal (AF) activity. Expression of prnA, which encodes the primary antibiotic involved in PA23 biocontrol, was unchanged in the csa mutant background suggesting that PRN is likely not affected. Therefore, the traits responsible for the enhanced AF activity have yet to be identified. Additionally, the relationship between the Csa QS system and other components of the regulatory cascade require further investigation to elucidate to what degree, directly or indirectly, the Csa QS regulates PA23 biocontrol.
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Keywords
Biocontrol, Pseudomonas, PA23, Sclerotinia, Canola
Citation
APA