Biotransformation of the Fungal Phytotoxin Fomannoxin by Soil Streptomycetes Nadine Horlacher & Jonny Nachtigall & Dirk Schulz & Roderich D. Süssmuth & Rüdiger Hampp & Hans-Peter Fiedler & Silvia D. Schrey Received: 5 March 2013 / Revised: 12 April 2013 / Accepted: 19 April 2013 / Published online: 15 May 2013 # Springer Science+Business Media New York 2013 Abstract Rhizosphere-associated Streptomyces sp. AcH 505 (AcH 505) promotes infection of Norway spruce (Picea abies) with the pathogenic fungus Heterobasidion abietinum 331, while Streptomyces sp. GB 4–2 (GB 4–2) enhances spruce defense against the fungus. To identify whether these bacteria influence the availability of the fungal phytotoxin fomannoxin and hence spruce infection, we analyzed the fomannoxin yield in H. abietinum 331-AcH 505 dual cul- tures. Further, the fate of fomannoxin was studied by adding the compound to cultures of AcH 505, GB 4–2 and nine other soil streptomycetes. Culture filtrates were extracted with ethyl acetate and analyzed by HPLC. Structures of novel com- pounds were elucidated by HPLC-HR-ESI-Orbitrap-MS and NMR spectroscopy. Phytotoxicity of the compounds was determined by in vivo measurement of maximum photosys- tem II efficiency of Arabidopsis thaliana seedlings. The amount of fomannoxin in H. abietinum 331-AcH 505 dual cultures was reduced compared to axenic fungus cultures and fungus-plant dual cultures. Following addition of fomannoxin to AcH 505 cultures, the compound disappeared and three novel fomannoxin derivatives without phytotoxic activity were detected. Another novel compound, fomannoxin amide, was discovered following fomannoxin addition to GB 4–2 cultures. Nine other streptomycetes converted fomannoxin into fomannoxin acid or fomannoxin amide. Both com- pounds exhibit the same phytotoxicity as fomannoxin. We, thus, conclude that the streptomycete-mediated modulation of spruce infection with H. abietinum 331 does not depend on the availability of fomannoxin. We further add evidence to the observation that the lipophilic side chain of fomannoxin is an important structural element for its phytotoxicity. Keywords Streptomyces . Biotransformation . Fomannoxin . Phytotoxicity . Heterobasidion spp. Introduction Within the rhizosphere, the narrow zone surrounding plant roots, plant root exudates form the basis for a specialized microbe community (Hiltner, 1904). Interactions between bacteria and fungi in this zone are ubiquitous, and can result in modified fungal growth, morphology, and physiology, thus altering the outcome of infection with pathogenic or symbiotic organisms (Whipps, 2001; Duffy et al., 2003; de Boer et al., 2005; Frey-Klett et al., 2011). Heterobasidion annosum sensu lato (Heterobasidion spp.) is a common pathogenic fungus that causes ‘annosum root rot’ in conifers in the Northern hemisphere (Asiegbu et al., 2005), resulting in large annual losses in the forest industry (Woodward et al., 1998). The basidiomycete Heterobasidion spp. forms a species complex in Europe, consisting of three species (Korhonen, 1978), (i) species P (H. annosum sensu stricto), (ii) species S (H. parviporum), and (iii) species F (H. abietinum). These are differentiated based on their main host preference for pine, spruce, and fir, Electronic supplementary material The online version of this article (doi:10.1007/s10886-013-0290-3) contains supplementary material, which is available to authorized users. N. Horlacher : R. Hampp : S. D. Schrey (*) IMIT, Universität Tübingen, Auf der Morgenstelle 1, 72076 Tübingen, Germany e-mail: silvia.schrey@uni-tuebingen.de N. Horlacher : D. Schulz : H.<P. Fiedler IMIT, Universität Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany J. Nachtigall : R. D. Süssmuth Institut für Chemie, TU Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany J Chem Ecol (2013) 39:931–941 DOI 10.1007/s10886-013-0290-3