Nodfactor

Nod factors are signaling molecules produced by nitrogen-fixing rhizobia during the establishment of symbiosis with legume plants. They are lipo-chitooligosaccharides (LCOs) derived from a chitin-like backbone of N-acetylglucosamine units, typically bearing a fatty acyl chain at the non-reducing end and various chemical decorations. When compatible legumes detect these signals, nodulation processes are initiated.

Core structure and biosynthesis: The Nod factor backbone consists of a short chain of N-acetylglucosamine units, commonly four or five in length. The non-reducing end carries a fatty acyl group; additional decorations may include acetyl, sulfate, and sugar substitutions. In rhizobia, nodC constructs the chitin-like backbone, nodB modifies acetylation, and nodA attaches the fatty acyl chain. nodD transcriptional activators respond to plant flavonoids, coordinating expression of the nodulation genes.

Plant perception and response: Plants perceive Nod factors through LysM receptor kinases such as NFR1 and NFR5 in Lotus japonicus or NFP and LYK3 in Medicago truncatula. Perception triggers a signaling cascade that includes calcium spiking, root hair curling, infection thread formation, and cortical cell division, ultimately leading to nodule organogenesis and the development of nitrogen-fixing nodules.

Specificity and significance: Nod factor structure and substitutions influence host range and compatibility, helping determine which rhizobial strains nodulate which legumes. The Nod factor signaling axis is central to biological nitrogen fixation and is a focus of research aimed at improving legume crops or exploring engineering of symbiotic capabilities in non-legumes.

Historical context: The concept of Nod factors emerged from studies of rhizobial-legume symbiosis in the late 20th century, with ongoing work detailing receptor identification, signal transduction, and structural variation.