Apoplastic barriers are essential for nodule formation and nitrogen fixation in .

Establishment of the apoplastic root barrier known as the Casparian strip occurs early in root development. In legumes, this area overlaps with nitrogen-fixing nodule formation, which raises the possibility that nodulation and barrier formation are connected. Nodules also contain Casparian strips, yet, in this case, their role is unknown. We established mutants with defective barriers in . This revealed that effective apoplastic blockage in the endodermis is important for root-to-shoot signals underlying nodulation. Our findings further revealed that in nodules, the genetic machinery for Casparian strip formation is shared with roots. Apoplastic blockage controls the metabolic source-sink status required for nitrogen fixation. This identifies Casparian strips as a model system to study spatially constrained symbiotic plant-microbe relationships.