Fungal-associated molecules induce key genes involved in the biosynthesis of the antifungal secondary metabolites nunamycin and nunapeptin in the biocontrol strain Pseudomonas fluorescens In5

Pseudomonas fluorescens In5 synthesizes the antifungal cyclic lipopeptides (CLPs) nunamycin and nunapeptin, which are similar in structure and genetic organization to the pseudomonas-derived phytotoxins syringomycin and syringopeptin. Regulation of syringomycin and syringopeptin is dependent on the two-component global regulatory system GacS/GacA, and the SalA, SyrF, and SyrG transcription factors, which activate syringomycin synthesis in response to plant signalling molecules.

Previously, we demonstrated that a specific transcription factor, NunF, positively regulates the synthesis of nunamycin and nunapeptin in P. fluorescens In5 and that the nunF gene is upregulated by fungal-associated molecules. This study focusses on further unravelling the complex regulation governing CLP synthesis in P. fluorescens In5.

Promoter fusions were used to show that the specific activator NunF is dependent on the global regulator of secondary metabolism GacA and is regulated by fungal-associated molecules and low temperatures. In contrast, GacA is stimulated by plant signalling molecules leading to the hypothesis that P. fluorescens is a hyphosphere-associated bacterium encoding transcription factor genes that respond to signals indicating the presence of fungi and oomycetes.

Based on these findings, we present a model for how synthesis of nunamycin and nunapeptin is regulated by fungal- and oomycete-associated molecules. Importance: Cyclic lipopeptide (CLP) synthesis gene clusters in pseudomonads display a high degree of synteny and the structure of the peptides synthesized is very similar.

Accordingly, the genomic island encoding the synthesis of syringomycin and syringopeptin in P. syringae pv. syringae closely resembles that of P. fluorescens In5, which code for synthesis of the antifungal and anti-oomycete peptides nunamycin and nunapeptin, respectively. However, the regulation of syringomycin and syringopeptin synthesis is different from that of nunamycin and nunapeptin synthesis.

While CLP synthesis in the plant pathogenic P. syringae pv. syringae is induced by plant signalling molecules, such compounds do not significantly influence synthesis of nunamycin and nunapeptin in P. fluorescens In5. Instead, fungal-associated molecules positively regulate anti-fungal peptide synthesis in P. fluorescens In5 while the synthesis of the global regulator GacA in P. fluorescens In5 is positively regulated by plant signal molecules but not fungal-associated molecules.