Fig. 1.

Schematic representation of Rj proteins involved in nitrogen-fixing root nodule formation in soybean. (A) The putative Nod factor receptor complex, Rj1/GmNFR1 and Rj5/Rj6/GmNFR5. Binding of Nod factors to extracellular LysM domains of the complex and subsequent transducing the signal through the intracellular kinase of Rj1/GmNFR1 to downstream signalling cascades leads to rhizobial infection and nodule organogenesis. (B) The AON signalling mediated by the putative Rj7/NTS1/GmNARK and CLV2-like protein complex. Recognition of root-derived signal(s) produced in response to NF perception by extracellular LRR domains of the complex in shoots results in production of the AON signal(s) through the intracellular kinase of Rj7/NTS1/GmNARK. The AON signal(s) inhibits formation of new nodules. (C) A TIR-NBS-LRR class of plant resistance (R) protein, Rj2/Rfg1, involved in host-restriction of nodulation with specific rhizobial strains. Determination of symbiotic specificity might be achieved in the manner of ETI responses.