Host-dependent roles of hyphosphere keystone Massilia in organic phosphorus mineralization and arbuscular mycorrhizal fungal growth

The plant-arbuscular mycorrhizal (AM) fungi-hyphosphere bacteria form a cross-kingdom holobiont driven by top-down carbon flow and bottom-up phosphorus fluxes. Hyphosphere keystone bacteria, such as Massilia, can compensate for the limited capacity of AM fungi to mobilize organic phosphorus, thereby enhancing fungal development and plant performance. However, how Massilia modulates its functional role across plant-fungal combinations remains unclear. To address this, we employed three plant-AM fungus (Rhizophagus irregularis) pairings [medicago (Medicago sativa), maize (Zea mays), and sorghum (Sorghum bicolor)] combined with either single Massilia inoculation or a defined synthetic hyphosphere bacterial community (SynCom). Across all combinations, Massilia significantly enhanced shoot biomass, plant phosphorus content, phosphatase activity, and organic phosphorus mineralization. Interestingly, its effects were amplified by SynCom co-inoculation in maize and sorghum, while in the medicago hyphosphere, Massilia alone was more effective. Community profiling revealed host-specific Massilia-mediated recruitment of bacteria with high phosphatase activity and indole-3-acetic acid production. Our findings demonstrate that, as a hyphosphere keystone taxon, Massilia adopts host-dependent functional strategies-promoting AM fungal growth and organic phosphorus mineralization in medicago through phosphatase production, while relying on the growth stimulation of beneficial bacteria to mediate similar effects in maize and sorghum.