Rhizobia elicit on their specific leguminous hosts the formation of new organs, called nodules, in which they fix nitrogen. The rhizobial nodulation genes specify the synthesis of lipo-chitooligosaccharide signals, the Nod factors (NFs). Each rhizobial species has a characteristic set of nodulation genes that specifies the length of the chitooligosaccharide backbone and the type of substitutions at both ends of the molecule, thus making the NFs specific for a given plant host. At extremely low concentrations, purified NFs are capable of eliciting on homologous legume hosts many of the plant developmental responses characteristic of the bacteria themselves, including cell divisions, and the triggering of a plant organogenic program. This review summarizes our current knowledge on the biosynthesis, structure, and function of this new class of signaling molecules. Finally we discuss the possibility that these signals could be part of a new family of plant lipo-chitooligosaccharide growth regulators.

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Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factors are O-acetylated and N-acylated by specific Cl6-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhlbited a significantly reduced capacity to elicit infection thread (IT) formation ln alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodFlnodL mutants were unable to penetrate into legume hosts and to form ITs. Nevertheless, these mutants induced widespread cell wall tip growth in trichoblasts and other epidermal cells and were also able to elicit cortical cell activation at a distance. NodRm factor structural requirements are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses.