Fig. 1.

Proposed model for rhizobia-legume symbiotic signaling pathways. Nod factor from rhizobia stimulates its receptor complex (NFR), MtLYK3/NFP or LjNFR1/5. Signal cascades from NFR are transduced via as-yet-unidentified pathways to induce nuclear calcium spiking and activate DMI3 (CCaMK). Activated DMI3 directly interacts with and activates nodule-related transcription factors, including NSP1/2, via phosphorylation. These transcription factors enhance the expression of Nod factor-responsive genes by directly binding to the NF-box (AATTT). It is not clear how DMI3 regulates nodule-related NIN and ERN, but their up-regulation by Nod factor signaling is essential for nodulation. EFD, an AP2/ERF family member, inhibits nodulation, probably by disrupting cytokinin signaling. The dashed arrows indicate unknown biochemical links.