Lipochitooligosaccharide recognition: an ancient story

Chitin is the second most abundant polysaccharide in nature, found in crustacean shells, insect exoskeletons and fungal cell walls. The action of chitin and chitin derivatives on plants has become a very interesting story of late. Chitin is a 1-4-linked polymer of N-acetyl-d-glucosamine (GlcNAc). In this unmodified form, chitooligosaccharides (degree of polymerization (dp)=6-8)) are strong inducers of plant innate immunity. By contrast, when these chitooligosaccharides are acylated (so-called lipochitooligosaccharides, LCOs) and further modified, they can act as Nod factors, the key signaling molecules that play an important role in the initiation of the legume-rhizobium symbiosis. In a similar form, these molecules can also act as Myc factors, the key signaling molecules involved in the arbuscular mycorrhizal (AM) symbiosis. It has been proposed that Nod factor perception might have evolved from the more ancient AM symbiosis. Increasing evidence now suggests that LCO perception might have evolved from plant innate immunity signaling. In this review, we will discuss the evolutionary origin of symbiotic LCO recognition.