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

The plant-arbuscular mycorrhizal (AM) fungi-hyphosphere bacteria forms a cross-kingdom holobiont driven by top-down carbon flow and bottom-up phosphorus (P) fluxes. Hyphosphere keystone bacteria, such as Massilia, can compensate for the limited capacity of AM fungi to mobilize organic phosphorus (Po), 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 fungi pairings (medicago, maize, and sorghum) combined with either single Massilia inoculation or a defined synthetic hyphosphere bacterial community (SynCom). Across all combinations, Massilia significantly enhanced shoot biomass, plant P content, phosphatase activity, and Po mineralization. Interestingly, its effects were amplified by SynCom co-inoculation in maize and sorghum, while in 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 Po mineralization in medicago through phosphatase production, while relying on the growth-stimulation of beneficial bacteria to mediate similar effects in maize and sorghum.