Phosphate utilization by the fungal root endophyte Piriformospora indica

Background and aim: The root endophytic fungus Piriformospora indica increases plant resistance and tolerance to stress and promotes plant growth, but its ability to support plant nutrition is still controversially discussed. Irrespective of a potential nutrient transport towards the plant, the fungus might release P from sources unavailable for plant usage by transformation to available forms. Methods: To test this hypothesis, sterile solid and liquid in vitro cultures of P. indica supplied with different organic and inorganic P sources were established. Cultures were investigated for growth, solubilised P, enzyme activities, RNA accumulation of the four genes encoding phosphate transporters and the two genes for acid phosphatases and phytases respectively found in P. indica genome, and for pH values in the media. Results: P. indica growth was higher in the presence of inorganic P than in organic P sources. Significant amounts of P were solubilised by P. indica from Ca₃(PO₄)₂ and rock phosphate. However, no relevant intra- or extracellular enzymatic activity was detected despite RNA accumulation of related genes. In general, the genes were all repressed by higher amounts of inorganic P and were expressed the most when the fungus received phytate. We observed a decrease in medium pH in the presence of P. indica irrespective of the P source. Conclusions: P. indica is able to solubilise phosphate from inorganic, but not from organic P sources. This P solubilisation is not due to enzymatic activities but rather to the lowering of the medium pH.