Actinobacteria Warfare Against the Plant Pathogen Sclerotinia sclerotiorum: 2,4,6-Trimethylpyridine Identified as a Bacterial Derived Volatile With Antifungal Activity.

Bacteria and fungi produce a wide range of specialised metabolites, including volatile organic compounds (VOCs) that can act as signals or act directly to inhibit niche-competing microbes. Despite their ecological importance, most VOCs involved as signalling compounds remain uncharacterised. We have previously screened a collection of Actinobacteria strains sourced from Western Australia for their ability in vitro to suppress the growth of plant fungal pathogens. Here we explored the potential of four of the most active strains to produce antifungal metabolites by growing the strains on a range of nutrient-containing media. A casein-based (CYPS) culture medium was found to induce the production of antifungal compounds with high activity against Sclerotinia sclerotiorum, a major necrotrophic fungal pathogen of crops such as canola. We further observed that VOCs were produced that influenced pH and affected the bacterium-fungus interaction. The presence of Sclerotinia induced further VOC production in the Actinobacteria. Solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (GC-MS) analysis identified 2,4,6-trimethylpyridine, a compound not identified previously from Actinobacteria, which showed antifungal activity against different isolates of S. sclerotiorum and increased the pH of the medium. Overall, this study showed that Actinobacteria or their volatile products have the potential to be used in the protection of crops against S. sclerotiorum.