Lipopeptides are essential for Pseudomonas sp. DF41 biocontrol of Sclerotinia sclerotiorum

Abstract

Pseudomonas sp. DF41 is a biocontrol agent capable of suppressing Sclerotinia sclerotiorum-mediated stem rot of canola. Using transposon mutagenesis, we identified two mutants with greatly reduced antifungal (AF) activity. The first mutant had an insertion in gacS, forming part of the GacS/GacA regulatory system. The second mutation was in a gene involved in lipopeptide (LP) synthesis suggesting LPs are essential for biocontrol. Strain DF41 and the gacS (DF41-469) and lp (DF469-1278) mutants were characterized with respect to their extracellular metabolite production, antifungal (AF) properties and ability to form biofilms. We discovered that DF41 produces a number of compounds that may contribute to biocontrol including hydrogen cyanide (HCN), protease, alginate, and LP molecules. All of these products were found to be under Gac control. In addition, DF41 produces autoinducers, suggesting this bacterium employs quorum sensing as part of its lifestyle. In the greenhouse, the gacS and lp mutants were unable to protect canola from disease incited by S. sclerotiorum. Both mutants were able to sustain themselves in the canola phyllosphere; therefore, the loss of biocontrol activity can be attributed to reduced production of AF compounds and not a declining population size. We conclude that suppression of Sclerotinia stem rot of canola by Pseudomonas strain DF41 depends upon LP production and a functional Gac system.