Malita M. M. Nørgaard

Legume symbiosis with nitrogen-fixing bacteria is controlled by a cascade of signaling events leading to root nodule development. While plant cell-surface receptors initiate this process, the link between receptors and cytoplasmic signaling components remains unclear. Here we identify Early Phosphorylated Protein 1 (EPP1) as a central mediator of this pathway. EPP1 is recruited to the activated SYMRK receptor, where it is phosphorylated on a key serine residue, an event essential and sufficient to propagate symbiotic signaling. We provide structural and functional validation of the SYMRK-EPP1 signaling complex and demonstrate that EPP1 is required for root nodule formation. Synthetic engineering of the SYMRK-EPP1 interaction bypasses the need for symbiotic bacteria to initiate the pathway and triggers nodule organogenesis. These findings establish EPP1 as a crucial cytoplasmic component between receptor activation and intracellular signaling, advancing our understanding of nitrogen-fixing symbiosis.

Symbiosis receptor-like kinase SYMRK is required for root nodule symbiosis between legume plants and nitrogen-fixing bacteria. To understand symbiotic signaling from SYMRK, we determined the crystal structure to 1.95 Å and mapped the phosphorylation sites onto the intracellular domain. We identified four serine residues in a conserved “alpha-I” motif, located on the border between the kinase core domain and the flexible C-terminal tail, that, when phosphorylated, drives organogenesis. Substituting the four serines with alanines abolished symbiotic signaling, while substituting them with phosphorylation-mimicking aspartates induced the formation of spontaneous nodules in the absence of bacteria. These findings show that the signaling pathway controlling root nodule organogenesis is mediated by SYMRK phosphorylation, which may help when engineering this trait into non-legume plants.