Docking of Chitin Oligomers and Nod Factors on Lectin Domains of the LysM-RLK Receptors in the Medicago-Rhizobium Symbiosis

Legume plants establish a symbiotic association with N2-fixing soil bacteria (rhizobia) to benefit from a nitrogen source independent of soil nitrates for their protein syntheses, growth, and development. The infection and subsequent development of functional rhizobia-containing nodules require a first step by the host plant’s roots in which they specifically recognize the signal molecules produced by the rhizobia, known as Nodulation (Nod) factors [1]. Nod factors consist of amphipathic lipo-chitooligosaccharides built up from an oligochitin backbone of 4-5 β1,4-linked GlcNAc residues carrying a fatty acid chain on the nonreducing end of the GlcNAc residue (Fig. 27.1). Diverse substitutions, e.g. acetyl groups, sulfate groups, or O-linked sugars, on the GlcNAc residues of both the reducing and nonreducing ends of the oligochitin backbone define the different host specificities that are necessary for correct rhizobia–legume recognition.