Suppression of Plant Defence Response by a Mycorrhiza Helper Bacterium

• The aim of the present study was to determine whether the mycorrhiza helper bacterium Streptomyces sp. AcH 505 could serve as a biocontrol agent against Heterobasidion root and butt rot. • Bacterial influence on mycelial growth of Heterobasidion sp. isolates, on the colonization of wood discs and Norway spruce (Picea abies) roots was determined. The effect of AcH 505 on plant photosynthesis, peroxidase activity and gene expression, and needle infections were investigated. • AcH 505 was antagonistic to 11 of 12 tested fungal Heterobasidion isolates. The antagonism resulted in a suppression of fungal colonization of Norway spruce roots and wood discs. Mycelial growth rate of the 12th strain, Heterobasidion abietinum 331 was not affected by AcH 505, and colonization of roots by this fungal strain was promoted by AcH 505. Bacterial inoculation led to decreased peroxidase activities and gene expression levels in roots. • AcH 505 promotes plant root colonization by Heterobasidion strains that are tolerant to antifungal metabolites produced by the bacterium. This may result from unknown bacterial factors that suppress the plant defence response.