| Summary: | <p>Abstract</p> <p>Background</p> <p>Studies on mycorrhiza associated bacteria suggest that bacterial-fungal interactions play important roles during mycorrhiza formation and affect plant health. We surveyed <it>Streptomyces</it> Actinobacteria, known as antibiotic producers and antagonists of fungi, from Norway spruce mycorrhizas with predominantly <it>Piloderma</it> species as the fungal partner.</p> <p>Results</p> <p>Fifteen <it>Streptomyces</it> isolates exhibited substantial variation in inhibition of tested mycorrhizal and plant pathogenic fungi (<it>Amanita muscaria, Fusarium oxysporum</it>, <it>Hebeloma cylindrosporum</it>, <it>Heterobasidion abietinum, Heterobasidion annosum</it>, <it>Laccaria bicolor, Piloderma croceum</it>). The growth of the mycorrhiza-forming fungus <it>Laccaria bicolor</it> was stimulated by some of the streptomycetes, and <it>Piloderma croceum</it> was only moderately affected. Bacteria responded to the streptomycetes differently than the fungi. For instance the strain <it>Streptomyces</it> sp. AcM11, which inhibited most tested fungi, was less inhibitory to bacteria than other tested streptomycetes. The determined patterns of <it>Streptomyces</it>-microbe interactions were associated with distinct patterns of secondary metabolite production. Notably, potentially novel metabolites were produced by strains that were less antagonistic to fungi. Most of the identified metabolites were antibiotics (e.g. cycloheximide, actiphenol) and siderophores (e.g. ferulic acid, desferroxiamines). Plant disease resistance was activated by a single streptomycete strain only.</p> <p>Conclusions</p> <p>Mycorrhiza associated streptomycetes appear to have an important role in inhibiting the growth of fungi and bacteria. Additionally, our study indicates that the <it>Streptomyces</it> strains, which are not general antagonists of fungi, may produce still un-described metabolites.</p>
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