Figure 2.

A likely pathway for Nod factor signaling in legumes based on the cloning of genes from various legume nodulation-defective mutants. Host-specific Nod factors produced by rhizobia are believed to interact with the LysM domains of Nod factor receptors such as LjNFR1 and LjNFR5. The involvement of the LysM domain-proteins in M. truncatula and P. sativum is not known and hence indicated by a question mark. Upon potentially being phosphorylated, one or both of the receptors is predicted to interact with a Nodule Receptor Kinase (MsNORK/MtDMI2/LjSYMRK/PsSYM19), triggering changes in membrane depolarization and ion flux through a potential cation channel and MtDMI1, which enables calcium spiking and oscillations. CASTOR and POLLUX, twin proteins in L. japonicus that are MtDMI ortho-logs, are localized to chloroplast membranes. Increased calcium levels from intracellular stores activate MtDMI3/PsSYM19, proteins highly similar to calcium/calmodulin protein kinase. The lack of an NLS, but experimental localization to the nucleus, suggests a possible transfer of DMI3 from the cytosol to the nucleus. This protein then triggers gene expression through MtNSP1 and MtNPS2 and LjNIN/PsSYM35, and other transcriptional regulators, which affect downstream gene expression.