RSD-mediated suppression of NIN and NLP2 transcription is crucial for symbiotic nitrogen fixation

Indeterminate nodules establish a developmental gradient along their longitudinal axis, separating cell differentiation from symbiotic nitrogen fixation (SNF). The apex contains differentiating cells, while the differentiated leghemoglobin-rich cells of the nitrogen-fixation zone shield the rhizobial nitrogenase complex from oxygen to facilitate SNF. By combining biochemical, genetic, and genomic approaches, we demonstrate the pivotal role of the Regulator of Symbiosome Differentiation (RSD), a transcriptional repressor, in the transition from symbiosome development to SNF. Interacting Protein of DMI3 (IPD3) activates RSD expression in the invasion zone (ZII) and interzone (IZ). RSD interacts with Nodule Inception (NIN), and NIN-like protein 2 (NLP2) through a novel protein-protein interaction domain. RSD determines cell fate in ZII and the IZ by suppressing several targets of NIN and NLP2, including Leghemoglobins, Nodule-specific Cysteine-Rich genes, and Symbiotic Cysteine-rich Receptor-like Kinase. Our findings underscore the critical role of RSD-mediated suppression of transcription in facilitating the transition from bacteroid differentiation to SNF.