Auxofuran, a Novel Metabolite That Stimulates the Growth of Fly Agaric, Is Produced by the Mycorrhiza Helper Bacterium Streptomyces Strain AcH 505

Abstract

The mycorrhiza helper bacterium Streptomyces strain AcH 505 improves mycelial growth of ectomycorrhizal fungi and formation of ectomycorrhizas between Amanita muscaria and spruce but suppresses the growth of plant-pathogenic fungi, suggesting that it produces both fungal growth-stimulating and -suppressing compounds. The dominant fungal-growth-promoting substance produced by strain AcH 505, auxofuran, was isolated, and its effect on the levels of gene expression of A. muscaria was investigated. Auxofuran and its synthetic analogue 7-dehydroxy-auxofuran were most effective at a concentration of 15 M, and application of these compounds led to increased lipid metabolism-related gene expression. Cocultivation of strain AcH 505 and A. muscaria stimulated auxofuran production by the streptomycete. The antifungal substances produced by strain AcH 505 were identified as the antibiotics WS-5995 B and C. WS-5995 B completely blocked mycelial growth at a concentration of 60 M and caused a cell stress-related gene expression response in A. muscaria. Characterization of these compounds provides the foundation for molecular analysis of the fungus-bacterium interaction in the ectomycorrhizal symbiosis between fly agaric and spruce.Actinomycetes are known for their capacity to control plant diseases. A number of investigations have reported antagonism of root-pathogenic fungi by soil actinomycetes (6, 40) and that especially streptomycetes are a rich source of antifungal compounds (20).Ectomycorrhizas (ECM) are widespread symbiotic organs that are formed during interactions between most tree species or perennials and soil fungi (44). The establishment of the symbiosis is affected by other soil microbes, such as bacteria and other fungi (18,29). Understanding the interactions between the soil-inhabiting microorganisms in the mycorrhizosphere is very important, as they are key players in nutrient cycling in forest soils (29).Under natural conditions, the plant partner is able to select for bacterial strains that are beneficial for the ECM symbiosis and for plant growth (16). Specifically, the growth of ectomycorrhizal fungi and mycorrhiza formation are promoted by some of the mycorrhizosphere bacteria (mycorrhiza helper bacteria [MHB] [18]), which belong to the genera Bacillus, Burkholderia, Pseudomonas, Rhodococcus, and Streptomyces (15,19,30,37,42).When 12 actinomycete isolates were tested to determine their effects on mycelial growth of ectomycorrhizal fungi (39), the bacterial isolates inhibited, promoted, or had no significant effect on hyphal extension in dual cultures. Thus, there is an opportunity to select for actinomycetes that stimulate certain symbiotic fungi and plant growth but are antagonistic to pathogenic organisms. To do this, we isolated a collection of actinomycetes from the hyphosphere of a spruce (Picea abies) stand and examined their effects on symbiotic and plant-pathogenic fungi (30,31). One member of this collection, streptomycete strain AcH 505, exhibited the desired qualities, as it significantly pro...

Bacterial-Fungal Interactions: Hyphens between Agricultural, Clinical, Environmental, and Food Microbiologists

“…AcH505 was shown to have a pronounced effect on the organization of the cytoskeleton of the ectomycorrhizal fungus Amanita muscaria (351). The same strain also produces auxofuran, a metabolite that contributes to its growth-promoting effect, the synthesis of which is promoted in bacterial-fungal cocultures and at acidic pH values that typify the growth conditions of ectomycorrhizal fungi (323).…”

“…strain AcH505 also leads to the suppression of bacterial antibiotic production. In this case, the fungus represses the biosynthesis of the antibiotics WS-5995 B and WS-5995 C by organic acid production (323). Signaling-based interactions.…”

Mycorrhizal responses to biochar in soil – concepts and mechanisms

“…flavonoids (AMF) and furans (ECM), that facilitate the growth of fungal hyphae and the subsequent colonization of plant roots by ECM (Founoune et al 2002;Duponnois and Plenchette 2003;Aspray et al 2006;Riedlinger et al 2006) and AM fungi (Duponnois and Plenchette 2003;Hildebrandt et al 2002Hildebrandt et al , 2006. Hildebrandt et al (2002Hildebrandt et al ( , 2006 have demonstrated that certain compounds (including raffinose and other unidentified metabolites) produced by strains of Paenibacillus can directly enhance the growth of AMF extraradical mycelium.…”

Fogacin, a Novel Cyclic Octaketide Produced by Streptomyces Strain Tü 6319

“…Two further metabolites did not correspond to any of the 834 reference compounds stored in our database. One of them was identified as anhydroSEK4b (3) [5], the second was named fogacin (1).…”