Societal Impact Statement. Cereal‐legume intercropping is a promising strategy for sustainable agroecosystems, leveraging the biological complementarities between plant species to reduce the need for inputs while enhancing field biodiversity. Here, we focused on maize‐bean intercropping, which is experiencing a revival in conventional agricultural settings. In such settings, intercropping was characterized more by competition than synergy, primarily affecting yield and gene expression in beans, which, despite a reduced yield, produced larger seeds. Our findings highlight the importance of selecting suitable varieties and adjusting agricultural practices to take full advantage of the intercrop. <br /><br />Summary.<br />- Cereal‐legume intercropping is a promising strategy for sustainable agroecosystems. The traditional intercropping of maize and bean is experiencing a revival in some modern agricultural settings, such as in southwestern France, where maize hybrids are intercropped with the commercialized Tarbais bean. We conducted on‐farm surveys and a field assay to address the following questions: How does the cropping system impact yield, nutrient uptake, and rhizosphere bacterial assemblages? Do positive or negative interactions between maize and beans dominate in intercropping? What is the effect of intercropping on plant transcriptomics? <br />- We recorded farming practices, conducted yield and nutrient measurements, and characterized soil bacterial assemblages to compare sole‐cropped maize and beans with intercropped plants. A controlled field assay was also established to extend this comparison to plant gene expression differences.<br />- Intercropping was associated with a trend toward increased bacterial diversity. The cropping system significantly influenced agronomic traits, with frequent farm‐by‐cropping system interactions underscoring the critical role of farming practices. Competition dominated maize‐bean intercropping, with 34 negative correlations among the 47 significant ones between maize and bean traits. This competition affected yield and nutrition, but primarily impacted beans, which produced fewer but bigger/heavier seeds. Transcriptomic results concurred with these findings, revealing no differentially expressed genes in maize but 5,070 in beans under competition.<br />- Overall, our findings suggest that beneficial interactions between the two crops are hindered under current field conditions, underscoring the importance of carefully considering partner varieties and farming practices to revive traditional agricultural systems.

L'association céréale‐légumineuse constitue une stratégie prometteuse pour des agroécosystèmes durables, qui repose sur des complémentarités biologiques entre espèces afin de réduire le besoin en intrants tout en renforçant la biodiversité des parcelles. Nous nous sommes concentrés sur l'association maïs‐haricot, qui connaît un regain d'intérêt dans les systèmes agricoles conventionnels. Dans ces systèmes, cette association se caractérisait davantage par une compétition que par une synergie, affectant principalement le rendement et l'expression génique chez le haricot, lequel, malgré une baisse de rendement, produisait des graines plus grosses. Notre étude souligne l'importance de sélectionner des variétés adaptées et d'ajuster les pratiques agricoles afin de tirer pleinement parti de l'association.