Diversity in phosphorus mobilisation and uptake in ectomycorrhizal fungi

Introduction Phosphorus (P) is often the first or second element limiting aboveground net primary productivity of forests. Besides low available inorganic orthophosphate (Pi) concentrations, soil may contain high total P contents, as insoluble mineral P or as organic P. Most plants form mycorrhizal associations that improve their P nutrition. Three main hypotheses have been proposed to explain this positive effect through an increase of (1) P mobilisation from mineral P, (2) P mobilisation from organic P and (3) soil exploration and P uptake. However, the positive effect of mycorrhizal symbiosis may be variable with the fungal species forming the association. This could be due to the different abilities of mycorrhizal fungi to mobilise P and/or to take up Pi from the soil. Objectives The aim of this review was to examine our current knowledge about the capacity of ectomycorrhizal fungi to release organic compounds as low-molecular-weight organic anions and phosphatases thought to have a role for mineral and organic P mobilisation, respectively. The diversity of Pi transporters among mycorrhizal species is also examined. Results The main conclusion is that the study of the functional diversity of ectomycorrhizal fungi in situ is still a challenging question and could be addressed by combining different tools now available to make large-scale studies.