From roots to nodules: regulation of organogenesis in nitrogen-fixing symbiosis

Plants in the nitrogen-fixing clade have evolved symbiotic root nodules to overcome nitrogen limitations in the soil. These nodules host nitrogen-fixing bacteria that convert atmospheric nitrogen into ammonia, supplying essential nutrients to the plant. Nodule formation is triggered by plant-bacteria interactions and relies on genetic adaptations, including the recruitment of existing regulatory pathways. The transcription factor NODULE INCEPTION (NIN) is a key regulator required for bacterial infection, nodule initiation, and organ differentiation. Nodule development shares key features with lateral root formation, particularly in organ initiation and early growth stages, as both arise from the same root tissue layers. This overlap raises intriguing questions about how nodules evolved distinct forms and functions. This review highlights recent discoveries in the molecular and cellular mechanisms of nodule development, especially in the Papilionoideae clade. By comparing nodules and lateral roots, we explore the regulatory changes that led to their evolutionary divergence. We highlight emerging tools-single-cell and spatial transcriptomics, and advanced imaging-that are deepening insights into nodulation, alongside phylogenomics revealing its evolutionary history.