Flavonoid‐Mediated Recruitment of <i>Bradyrhizobium</i> Enhances Maize Root Development and Nutrient Acquisition in Maize–Soybean Intercropping Systems

Intercropping between legumes and cereals enhances nutrient acquisition. However, the mechanisms by which legume-associated microbes influence non-legume root development remain unclear. Bradyrhizobium, traditionally recognised as a legume symbiont, may also perform non-symbiotic roles in shaping cereal root architecture and nutrient uptake. We investigated how soybean-maize intercropping recruits Bradyrhizobium through flavonoid exudation and how this bacterium modulates maize flavonoid metabolism, root growth, and nutrient acquisition. Pot intercropping experiments were conducted with maize grown alone or with soybean. Root exudates, rhizosphere microbial communities, and soil nutrient profiles were analysed, and Bradyrhizobium isolates were tested for flavonoid responses and maize inoculation effects. Intercropping markedly enriched Bradyrhizobium in the maize-soybean interaction-zone rhizosphere. Soybean roots released 5-8 times more flavonoids than maize, which recruited Bradyrhizobium and enhanced soil phosphate availability and nutrient-cycling potential. Inoculation with Bradyrhizobium promoted maize root elongation and nutrient uptake. Transcriptomic analyses revealed activation of the phenylpropanoid-flavonoid pathway, repression of flavonol biosynthesis, and induction of auxin-responsive and nutrient transport genes, suggesting that Bradyrhizobium stimulates maize root growth via a flavonoid-auxin regulatory module. Soybean-derived flavonoids recruit Bradyrhizobium to maize rhizospheres, where the bacterium enhances maize root development and nutrient acquisition, uncovering a cross-species microbial mechanism underlying legume-cereal intercropping benefits.