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

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 Lotus japonicus. 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. Editor's summary: Vascular plants regulate the flow of water and nutrients through their roots by creating a barrier in their extracellular space called the Casparian strip. Shen et al. identified genes involved in Casparian strip formation in Lotus japonicus, a legume that forms symbiotic relationships with nitrogen-fixing bacteria. Furthermore, the authors established that mutants lacking a Casparian strip were less able to form nitrogen-fixing root nodules, implying a connection between these processes. The presence of the Casparian strip also affected expression of the root-shoot signaling peptides that regulate nodulation. This work expands the role of the Casparian strip and links it to the spatial regulation of nodulation in plant roots. —Madeleine Seale [ABSTRACT FROM AUTHOR]