Figure 3.

The interplay between nodulation and defense signaling pathways. Plants perceive bacterial molecules (pathogen-associated molecular patterns, PAMPs) using pattern recognition receptors (PPRs) that activate mitogen activated kinase (MAPK) cascades that trigger host defense responses. Adapted pathogens use the type III secretion system (T3SS) to deliver effector proteins into the cytosol of host cells. Bacterial effectors can inhibit the MAPK cascade, leading to suppression of host defenses. In some plants varieties, these effectors are recognized by nucleotide binding site- leucine rich repeat domains (NBS-LRR) receptors, which trigger a second tier of host defense responses. Recognition of Nod factors produced by compatible rhizobia by specific receptors (NFR) triggers a signaling cascade leading to nodulation (NF-Pathway). Rhizobial effectors can also promote nodulation by directly activating the NF-Pathway. The symbiosis receptor-like kinase (SYMRK) is also necessary for nodule formation, but the nature of its putative ligand is unknown. In a second stage of rhizobia recognition, exopolysaccharides (EPS) produced by rhizobia are perceived by exopolysaccharide protein receptor 3 (EPR3), inactivating the defense signaling pathway through unknown mechanisms.