SYMBIOTIC HOST-SPECIFICITY BETWEEN LEGUMINOUS PLANTS AND RHIZOBIA IS DETERMINED BY SUBSTITUTED AND ACYLATED GLUCOSAMINE OLIGOSACCHARIDE SIGNALS

Rhizobia are nitrogen-fixing bacteria which invade root hairs of legum inous plants and induce, in a specific manner, the formation of root n odules in which they fix nitrogen. The early steps of the symbiosis ca n be considered as a reciprocal molecular communication between the tw o partners. Initially, the plant excretes a gene inducer which stimula tes the expression of bacterial nodulation genes. These nodulation gen es are responsible for the synthesis of extracellular host-specific si gnals, called nodulation factors. The bacterial nodulation factors wer e isolated and structurally identified as substituted and N-acylated c hitin oligosaccharides. These prokaryotic lipo-oligosaccharide signals play a key role in the symbiosis by controlling the host specificity of the bacteria. They constitute a new class of signalling molecules a ble to elicit nodule organogenesis in leguminous plants in the absence of bacteria.