Processing of NODULE INCEPTION controls the transition to nitrogen fixation in root nodules

Legume nodules create an environment for intracellular bacterial symbionts to fix atmospheric nitrogen. The master regulator NODULE INCEPTION (NIN) controls many aspects of nodule initiation, and we demonstrate that it also regulates the transition to nitrogen fixation via proteolytic processing by a signal peptidase complex. Processing of NIN results in a carboxyl-terminal NIN fragment containing the DNA binding motifs, which activates a suite of genes associated with symbiosome development and nitrogen fixation. Similar NIN processing is observed in Medicago truncatula and Lotus japonicus, implying a conserved mechanism of cell state transition. These findings explain how legume nodules transition to a nitrogen-fixing state and a mechanism by which a single transcription factor can regulate many different developmental processes necessary in the activation and regulation of nitrogen fixation.