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Figure 2 Regulation of legume nodule development. Legume roots exude flavonoid molecules, which attracts compatible rhizobia and triggers them to produce a Nod factor signal. Stress and Nitrogen locally inhibit nodule development. Nitrogen triggers the production of a CLE peptide, called GmNIC1 in soybean, that acts through the GmNARK receptor to suppress nodulation. The Autoregulation Of Nodulation (AON) acts systemically through the shoot. CLE peptides, called GmRIC1 and GmRIC2 in soybean, are produced in the root in response to the first nodulation events. These signals are transported to the shoot where they are perceived by GmNARK, which triggers the production of a shoot-derived inhibitor (SDI) signal that travels to the root to prevent further nodulation.