Endofungal bacteria and ectomycorrhizal fungi synergistically promote the absorption of organic phosphorus in Pinus massoniana.

Ectomycorrhizal fungi (ECMFs) that are involved in phosphorus mobilisation and turnover have limited ability to mineralise phytate alone. The endofungal bacteria in the ectomycorrhizal fruiting body may contribute to achieving this ecological function of ECMFs. We investigated the synergistic effect and mechanisms of endofungal bacteria and ECMF Suillus grevillea on phytate mineralisation. The results showed that soluble phosphorus content in the combined system of endofungal bacterium Cedecea lapagei and S. grevillea was 1.8 times higher than the sum of C. lapagei and S. grevillea alone treatment under the phytate mineralisation experiment. The S. grevillea could first chemotactically assist C. lapagei in adhering to the surface of S. grevillea. Then, the mineralisation of phytate was synergistically promoted by increasing the biomass of C. lapagei and the phosphatase and phytase activities of S. grevillea. The expression of genes related to chemotaxis, colonisation, and proliferation of C. lapagei and genes related to phosphatase and phytase activity of S. grevillea was also significantly upregulated. Furthermore, in the pot experiment, we verified that there might exist a ternary symbiotic system in the natural forest in which endofungal bacteria and ECMFs could synergistically promote phytate uptake in the plant Pinus massoniana via the ectomycorrhizal system.

Endofungal Bacterial Microbiota Promotes the Absorption of Chelated Inorganic Phosphorus by Host Pine through the Ectomycorrhizal System.

Ectomycorrhizal fungi play an irreplaceable role in phosphorus cycling. However, ectomycorrhizal fungi have a limited ability to dissolve chelated inorganic phosphorus, which is the main component of soil phosphorus. Endofungal bacteria in ectomycorrhizal fruiting bodies are always closely related to the ecological function of ectomycorrhizal fungi. In this study, we explore endofungal bacteria in the fruiting body of Tylopilus neofelleus and their function during the absorption of chelated inorganic phosphorus by host pine through the ectomycorrhizal system. The results showed that the endofungal bacterial microbiota in the fruiting body of T. neofelleus might be related to the dissolution of chelated inorganic phosphorus in soil. The soluble phosphorus content in the combined system of T. neofelleus and endofungal bacteria Bacillus sp. strain B5 was five times higher than the sum of T. neofelleus-only treatment and Bacillus sp. strain B5-only treatment in the dissolution experiment of chelated inorganic phosphorus. The results showed that T. neofelleus not only promoted the proliferation of Bacillus sp. strain B5 in the combined system but also improved the expression of genes related to organic acid metabolism, as assesed by transcriptomic analysis. Lactic acid content was five times higher in the combined system than the sum of T. neofelleus-only treatment and Bacillus sp. strain B5-only treatment. Two essential genes related to lactate metabolism of Bacillus sp. strain B5, gapA and pckA, were significantly upregulated. Finally, in a pot experiment, we verified that T. neofelleus and Bacillus sp. strain B5 could synergistically promote the absorption of chelated inorganic phosphorus by Pinus sylvestris in a ternary symbiotic system. IMPORTANCE Ectomycorrhizal fungi (ECMF) have a limited ability to dissolve chelated inorganic phosphorus, which is the main component of soil phosphorus. In the natural environment, the extraradical hyphae of ECMF alone may not satisfy the phosphorus demand of the plant ectomycorrhizal system. In this study, our results innovatively show that the ectomycorrhizal system might be a ternary symbiont in which ectomycorrhizal fungi might recruit endofungal bacteria that could synergistically promote the mineralization of chelated inorganic phosphorus, which ultimately promotes plant phosphorus absorption by the ectomycorrhizal system.