Wheat Root-Exuded Specialized Metabolites (BXs) Drive Rhizosphere Microbial Interactions in Legume Intercropping, Enhancing the Field Growth Performance of Both Crops

Unique rhizosphere metabolites, such as benzoxazinoids (BXs), are secreted by cereal crops, such as wheat, which influence the rhizosphere microbiota and affect the growth of offspring crops. However, the feedback effect of this microbiota interaction on the rhizosphere environment of neighboring intercropped plants and their growth performance remains unclear. This study combined pot and field experiments to explore the intercropping system between wheat (IW) and fava bean (IF). Compared with monoculture wheat (MW), IF significantly increased BXs release from wheat roots and enhanced BXs synthesis in both wheat leaves and roots. BXs not only drove changes in the diversity and richness of the rhizosphere microbiota in wheat but also altered the microbiota composition in the IF rhizosphere through horizontal transfer. Actinomycota, the dominant bacterial phylum, was strongly influenced by BXs in the rhizosphere of IF, showing a positive correlation with Pseudomonadota. In terms of fungi, BXs promoted the enrichment of Penicillium in IW and IF while inhibiting the growth of Fusarium and Gibberella. The BXs-driven rhizosphere effect enhanced enzyme activities, including CAT, urease, sucrase, and neutral phosphatase, in IW and IF rhizosphere soil. These changes improved both aboveground (plant height, leaf length, and fresh weight) and belowground (root length and root weight) growth as well as crop grain yield. In conclusion, this study demonstrates for the first time that BXs secreted by wheat roots promote positive feedback interactions in the fava bean rhizosphere, improving soil enzyme activities and overall plant performance in a wheat–fava bean intercropping system.