Kefir

APPLIED AND ENVIRONMENTAL MICROBIOLOGY, May 2006, p. 3550–3557 0099-2240/06/$08.00 0 doi:10.1128/AEM.72.5.3550–3557.2006 Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Vol. 72, No. 5

Auxofuran, a Novel Metabolite That Stimulates the Growth of Fly Agaric, Is Produced by the Mycorrhiza Helper Bacterium Streptomyces Strain AcH 505†
Julia Riedlinger,1 Silvia D.Schrey,2‡ Mika T. Tarkka,2‡* Rudiger Hampp,2 Manmohan Kapur,3 ¨ and Hans-Peter Fiedler1*
Mikrobiologisches Institut,1 Botanisches Institut,2 and Institut fur Organische Chemie,3 Universitat Tubingen, Tubingen, Germany ¨ ¨ ¨ ¨
Received 12 October 2005/Accepted 24 February 2006

The mycorrhiza helper bacterium Streptomyces strain AcH 505 improves mycelial growth of ectomycorrhizal fungi and formationof 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 itssynthetic 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 Bcompletely 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 ofinvestigations 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, suchas 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 ofectomycorrhizal 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 isolatesinhibited, 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 andplant-pathogenic fungi (30, 31). One member of this collection, streptomycete strain AcH 505, exhibited the desired qualities, as it significantly promoted mycelial growth and the mycorrhization rate of Amanita muscaria (fly agaric) and Suillus bovinus but suppressed mycelial extension of the plant pathogens Armillariella obscura and Heterobasidion annosum (31, 42). A dual culture with strain AcH 505...