Early-stage reciprocal cooperation between the arbuscular mycorrhizal fungus Rhizophagus irregularis and the phosphate-solubilizing bacterium Rahnella aquatilis is dependent on external phosphorus availability

Plants, arbuscular mycorrhizal (AM) fungi and soil bacteria coexist stably in ecosystems. Resource availability can affect the mutualistic relationship between plants and AM fungi, but it is unclear how it affects the reciprocal cooperation between AM fungi and bacteria. Here, we used an in vitro culture system containing a source of organic phosphorus (P) to explore the early-stage reciprocal cooperation between the AM fungus Rhizophagus irregularis and the phosphate-solubilizing bacterium (PSB) Rahnella aquatilis under two different levels of inorganic P. Under low inorganic P availability, the reciprocal cooperation between the AM fungus and the PSB, materialized by carbon (C)-P exchange, was strengthened. This was evidenced at transcriptional level by the activation of multiple C metabolism and P transport and regulation pathways of both partners. Conversely, under high inorganic P availability, the exchange of P for C with plants was slowed down. This was accompanied by the activation of only P transport and regulation pathways and less C metabolism pathway, weakening the reciprocal cooperation with the PSB. In conclusion, the availability of inorganic P can mediate the reciprocal cooperation between AM fungi and bacteria, which could also extend to host plants since they are inseparable in ecosystems.