Location: Horticultural Crops ResearchTitle: Pseudomonas protegens Pf-5 causes discoloration and pitting of mushroom caps due to the production of antifungal metabolites Author
|Davis Ii, Edward|
|Zabriskie, T. Mark|
Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2014
Publication Date: 7/1/2014
Citation: Henkels, M.D., Kidarsa, T.A., Shaffer, B.T., Goebel, N.C., Burlinson, P., Mavrodi, D.V., Bentley, M.A., Rangel, L.I., Davis Ii, E.W., Thomashow, L.S., Zabriskie, T., Preston, G.M., Loper, J.E. 2014. Pseudomonas protegens Pf-5 causes discoloration and pitting of mushroom caps due to the production of antifungal metabolites. Molecular Plant-Microbe Interactions. 27(7):733-746. Interpretive Summary: Spoilage of mushrooms can result from infection by certain bacteria in the Pseudomonas fluorescens group, which cause significant economic losses to the mushroom industry and postharvest losses to consumers. The most well known of these mushroom pathogens is Pseudomonas tolaasii, which causes brown, pitted lesions (called brown blotch) on mushroom caps by producing a lipopeptide called tolaasin. Here, we evaluated a set of well-characterized strains in the P. fluorescens group for the capacity to cause discoloration of mushroom caps. We discovered that one strain, Pseudomonas protegens Pf-5, causes brown blotch symptoms on mushroom caps. We generated mutants in each of the known antifungal compounds produced by Pf-5 and tested the mutants for the capacity to cause brown blotch symptoms on mushroom. We also evaluated the toxicity of purified metabolites on mushroom caps. Results showed that two compounds (2,4-diacetylphloroglucinol and pyoluteorin) caused brown blotch symptoms and were produced by Pf-5 on mushroom caps. In contrast, the lipopeptide orfamide A, which is also produced by Pf-5, did not cause brown blotch symptoms. This study highlights a new mechanism for mushroom toxicity in Pseudomonas spp. that is dependent on the production of antifungal metabolites other than lipopeptides.
Technical Abstract: Bacteria in the diverse P. fluorescens group include mushroom pathogens, such as Pseudomonas tolaasii, and rhizosphere inhabitants known for their antifungal metabolite production and biological control of plant disease, such as Pseudomonas protegens Pf-5. Here, we report that strain Pf-5 causes brown blotch symptoms on basidiocarps of Agaricus bisporus (i.e., mushroom caps). Strain Pf-5 produces six known antifungal metabolites under the control of the GacS/GacA signal transduction system. A gacA mutant produces none of these metabolites and did not cause brown blotch symptoms. By testing a series of mutants, each having a deletion in biosynthetic genes for one or more of the six antifungal metabolites, we found that mutants deficient in the biosynthesis of 2,4-diacetylphloroglucinol and pyoluteorin, but not the lipopeptide orfamide A, caused less severe brown blotch symptoms than the wildtype. Application of purified pyoluteorin and 2,4-diacetylphloroglucinol to mushroom caps mimicked the symptoms caused by Pf-5. Both compounds were isolated from mushroom caps inoculated with Pf-5, providing direct evidence for their in situ production by the bacterium. Although brown blotch has been associated with lipopeptides such as tolaasin, this study demonstrates that certain Pseudomonas sp. cause brown blotch symptoms through a lipopeptide-independent mechanism involving the production of antifungal metabolites.