Auxin-producing Sphingobium promotes maize growth heterosis under phosphorus deficiency

Phosphorus (P) deficiency is a major constraint on crop productivity, yet the microbial functions contributing to heterosis (hybrid vigor, the superior F1 performance versus inbred parents) under P limitation remain largely unknown. Here, we investigated rhizosphere microbiome assembly across 93 maize hybrid-inbred triplets grown under contrasting P conditions. Integrating plant performance, phosphorus accumulation, and the microbial abundance identified Sphingobium as a bacterial taxon consistently enriched in hybrid rhizospheres under P deficiency. A hybrid-enriched Sphingobium isolate (W6) preferentially promoted hybrid growth under low-P conditions, enhancing lateral root development, phosphorus acquisition, and biomass heterosis. Inhibition of auxin transport by N-1-naphthylphthalamic acid (NPA) abolished the W6-mediated growth promotion, whereas DR5::GUS assays and transcriptomic analyses revealed enhanced auxin responses following W6 inoculation. Genome analysis further identified the indole-3-pyruvate pathway as the predominant route for auxin biosynthesis in W6. Together, our findings demonstrate that a hybrid-enriched rhizobacterial function promoting auxin-dependent root architectural plasticity, rather than phosphorus mobilization alone, contributes to maize heterosis under phosphorus deficiency, providing mechanistic insight into how host–microbiome interactions improve crop adaptation to nutrient-limited environments.