Soil organic phosphorus is mainly hydrolyzed via phosphatases from ectomycorrhiza-associated bacteria rather than ectomycorrhizal fungi

Mycorrhizal fungi-released phosphatases have long been claimed pivotal to mobilize soil organic phosphorus (P). We hypothesized that ectomycorrhizal (ECM) fungi, compared with their saprotrophic ancestors, evolved adaptive strategies to enhance the release of phosphatases to meet the P demand of ECM fungi and their host plants.

We analyzed genes potentially encoding secreted phosphatases in 103 fungal species, ECM fungi hyphae associated- and ectomycorrhizosphere phosphatase enzyme activities, and gene expression of Lactarius spp. and Laccaria bicolor fungi-secreted phosphatases during ECM formation. We also determined ectomycorrhizosphere abundance of bacterial phosphatase genes, and analyzed Pinus yunnanensis–Lactarius deliciosus ectomycorrhiza-associated P-mobilizing bacteria.

We found that during transition from saprotrophy to ECM symbiosis, genes encoding fungal-secreted phosphatases did not manifest adaptive or convergent evolution. Among 10 genes potentially encoding secreted fungal phosphatases in Lactarius spp. and Laccaria bicolor, only two were up-regulated during ECM formation. Furthermore, unlike saprotrophic fungi, pure-cultured ECM fungi hyphae released very few or no phosphatases to the surroundings, while ECMs generally increased the phosphatase activities in the ectomycorrhizosphere under glasshouse conditions. Additionally, ECM-associated bacteria exhibited an increased abundance of P-cycling genes in the ectomycorrhizosphere under both glasshouse and field conditions. A substantial part of culturable bacteria from ECM tips hydrolyzed organic P and promoted fungal P acquisition.

We found no evidence for adaptive evolution of secreted phosphatases in ECM fungi; some ECM fungi may not even release phosphatases, and ECM-associated bacteria likely play a pivotal role in ECM-promoted organic P hydrolysis and plant P acquisition.