Fungi-Streptomyces co-culture metabolites for potential biocontrol against plant pathogenic Fusarium oxysporum

Fungal diseases of plants cause major losses in agriculture, creating a need for safer and more sustainable alternatives to replace chemical fungicides. Fusarium oxysporum is a difficult fungal pathogen to control because it infects many crop plants and can survive in soil for long periods. Microbial secondary metabolites are one possible source of antifungal compounds, and microbial interactions may enhance biosynthesis and production of these yet unknown compounds with biological activities. This study examined whether co-cultures of wood-decaying fungi and bacteria of Streptomyces could produce secondary metabolites with antifungal activity against F. oxysporum. The brown rot fungus Fomitopsis pinicola and the white rot fungus fom radiata were cultivated as single-species cultures and as co-cultures with two representatives of the genus Streptomyces (S. alboniger and Streptomyces sp. S3) on three agar media. Metabolites were extracted, and the extracts were separated and analyzed by applying ultra-performance liquid-chromatography combined with photodiode-array and mass-spectrometry detection (UPLC-PDA-MS). Bioactivities of the metabolite extract fractions were assessed by using iron reduction and antioxidant activity tests, and agar diffusion assay against F. oxysporum. Co-cultivation changed the extractable metabolite profiles, although the effect was not the same in all microbial combinations. The profiles varied according to the fungal and bacterial partners, culture medium and extraction method. Antifungal activity against F. oxysporum was detected in a few extracts, while Fomitopsis pinicola-containing extracts showed the highest iron reducing capacity and antioxidant activity. However, these redox-related activities were not in statistical correlation with the inhibition pattern against F. oxysporum, indicating that antioxidant and antifungal activities were probably linked to different metabolite extract components. The seven previously characterized natural microbial products identified by UPLC-PDA-MS analysis helped explain some organism-related and medium-related patterns in the chemical profiles. However, none of these compounds clearly accounted for the observed antifungal activity. These findings show that combinatory cultures of wood-decaying fungi and bacteria of Streptomyces can produce condition-dependent metabolite mixtures with weak but measurable antifungal activity against F. oxysporum. Weak activity is explained by low abundance of the bioactive compounds. The most promising co-culture metabolite extracts need confirmation in further experiments, followed by compound identification and quantitatively more precise antifungal assays.