Phosphate-solubilizing bacteria isolated from ectomycorrhizal mycelium of Picea glauca are highly efficient at fluorapatite weathering

Fluorapatite-solubilizing bacteria were isolated from the hyphosphere of the ectomycorrhizal (ECM) fungus Wilcoxina sp., a dominant species in the Picea glauca (Moench) Voss rhizosphere. Hundreds of strains isolated from the ascomycete Wilcoxina sp. could dissolve tricalcium phosphate, while only 27 of them could produce clarification halos on fluorapatite-amended solid medium. Most of the fluorapatite-solubilizing strains belonged to the Burkholderia genus. Scanning electron microscopy observations have shown that these efficient phosphate-solubilizing bacteria (PSB) were able to completely solubilize fluorapatite crystals within 22 h. The efficient PSB Burkholderia sp. strain 205 and Curtobacterium sp. strain 168 were tested for their ability to associate with a genetically distant fungal host while fulfilling their phosphate-solubilizing function. Burkholderia sp. strain 205 successfully associated with the basidiomycete Laccaria bicolor (Maire) P.D. Orton when hydroxyapatite was the only phosphorus source available to the fungus, while there was no bacterial development when Laccaria bicolor could access soluble phosphorus as well. Optical microscopic observation of Laccaria bicolor associated with Burkholderia sp. revealed extensive colonization of fungal hyphae by the bacterium. These results suggest an important role of bacteria - ECM fungi associations in white spruce phosphate nutrition.