Plant growth promotion by Pseudomonas fluorescens : mechanisms, genes and regulation

<em>Pseudomonas fluorescens </em>is a Gram-negative rod shaped bacterium that has a versatile metabolism and is widely spread in soil and water. <em>P. fluorescens</em> strain SBW25 (<em>Pf.</em>SBW25) is a well-known model strain to study bacterial evolution, plant colonization and biocontrol of plant diseases. It produces the biosurfactant viscosin, a lipopeptide that plays a key role in motility, biofilm formation and activity against zoospores of <em>Phytophthora infestans </em>and other oomycete pathogens. In addition to viscosin, <em>Pf.</em>SBW25 produces other metabolites with activity against plant pathogens. The production of these yet unknown metabolites appeared to be regulated by the GacS/GacA two-component regulatory system (the Gac-system). The second <em>P. fluorescens </em>strain SS101 (<em>Pf.</em>SS101) studied in this thesis is known for its plant growth-promoting activities but the underlying mechanisms and genes are largely unknown. Therefore, in this study, we aimed to identify novel metabolites and biosynthetic genes in <em>Pf.</em>SBW25 and <em>Pf.</em>SS101, and to investigate their role in plant growth promotion and biocontrol. To this end, a multidisciplinary approach involving bioinformatic analysis of the genome sequences of strains <em>Pf.</em>SBW25 and <em>Pf.</em>SS101, microarray-based expression profiling, screening of genomic libraries, bioactivity assays, mass spectrometric image analysis (MALDI-IMS) and GC/MSMS analysis was adopted. In conclusion, we showed that the GacS/GacA two-component system as a global regulator of the expression of genes play important roles in antagonism of <em>Pseudomonas fluorescens</em> toward plant pathogenic microbes as well as in plant growth promotion and ISR. Growth promotion by <em>P. fluorescens</em> is associated with alterations in auxin biosynthesis and transport, steroid biosynthesis, carbohydrate metabolism and sulfur assimilation. Moreover, advanced chemical profiling allowed us to compare the metabolite profiles of free-living <em>P. fluorescens </em>and <em>P. fluorescens </em>living in association with plant roots. A better understanding of yet unknown mechanisms exploited by the various <em>Pseudomonas fluorescens </em>strains will lead to new opportunities for the discovery and application of natural bioactive compounds for both industrial and agricultural purposes.