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United States Department of Agriculture

Agricultural Research Service

Research Project: Improved Strategies for Management of Soilborne Diseases of Horticultural Crops

Location: Horticultural Crops Research

Title: The Gac Regulon of Pseudomonas fluorescens SBW25

Authors
item Cheng, Xu -
item DE Bruijn, Irene -
item Van Der Voort, Menno -
item Loper, Joyce
item Raaijmakers, Jos -

Submitted to: Environment Microbiology Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 1, 2013
Publication Date: April 30, 2013
Citation: Cheng, X., De Bruijn, I., Van Der Voort, M., Loper, J.E., Raaijmakers, J.M. 2013. The Gac Regulon of Pseudomonas fluorescens SBW25. Environment Microbiology Reports. 5:608-619.

Interpretive Summary: Biological control provides a promising strategy for managing plant diseases, but has not yet been utilized widely in agriculture due, in part, to unexplained variation in its success in managing disease. Our research goals are to identify sources of variation in biological control, and devise ways to make it more reliable. We focus on Pseudomonas fluorescens, which is a species of bacteria that occurs naturally on plant surfaces such as leaves and roots. In this study, we evaluated a central regulator (called GacS) of gene expression in the plant-associated bacterium P. fluorescens SBW25. We show that GacS influenced the transcription of 702 genes that have diverse functions in this bacterium. The transcriptional analysis highlighted a gene cluster that is likely to confer the production of an iron-binding compound whose structure can be predicted from the nucleotide sequence. The genome-wide analysis indicated that a mutation in gacS causes major transcriptional changes in SBW25 and significantly enhances its antimicrobial activities by yet unknown mechanisms. These results add to the information about factors influencing gene expression in plant-associated Pseudomonas spp.

Technical Abstract: Transcriptome analysis of Pseudomonas fluorescens SBW25 showed that 702 genes were differentially regulated (FC>4, P<0.0001) in a gacS::Tn5 mutant, with 300 and 402 genes up- and down-regulated, respectively. Similar to the Gac-regulon of four other Pseudomonas species, genes involved in motility, biofilm formation, siderophore biosynthesis and oxidative stress were differentially regulated in the gacS mutant of SBW25. Our analysis also revealed that transcription of nineteen rhizophere-induced genes and of genes involved in type II secretion, (exo)polysaccharide and pectate lyase biosynthesis, motility and nonribosomal peptide synthesis (NRPS) were significantly affected in the gacS mutant. Furthermore, the gacS mutant of SBW25 inhibited growth of oomycete, fungal and bacterial pathogens significantly more than wild type SBW25. Since RP-HPLC analysis did not reveal any candidate metabolites, the potential role of an NRPS gene cluster was investigated. This NRPS cluster was predicted to encode an ornicorrugatin-like peptide. Site-directed mutagenesis showed that the peptide product of the NRPS was is not involved in the enhanced antimicrobial activity of the gacS mutant but may function as a siderophore. Collectively, this genome-wide analysis indicated that a mutation in the Gac-regulatory system causes major transcriptional changes in SBW25 and significantly enhances its antimicrobial activities by yet unknown mechanisms.

Last Modified: 12/20/2014
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