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Nodulation genes in the Rhizobium--plant signal exchange.

The process of the host-plant recognition by rhizobia is complex and multi-step. The interaction between legumes and microorganisms results in the induction of the root nodule. This symbiotic interaction is highly host-specific. Bacteria within nodules fix atmospheric nitrogen. This process is of immense ecological and economic significance. The subject of this presentation is the molecular mechanism by which the bacterium determines its host-specific characteristics. First flavonoids secreted by the plant roots induce the transcription of bacterial genes involved in nodulation, the so-called nod genes. This leads to the next step of the signalling system, i.e. the production and secretion of lipo-oligosaccharide molecules by rhizobia. These signal molecules have various discernible effects on the roots of the host leguminous plants. The bacterial nodulation factors were isolated and structurally identified as substituted and N-acylated chitin oligosaccharides. These prokaryotic signals play a key role in the symbiosis by controlling the host specificity of the bacteria. They constitute a new class of signalling molecules able to elicit nodule organogenesis in leguminous plants in the absence of bacteria. More recent studies implicate involvement of root cell membrane depolarization and ion selective channels in the communication processes that initiate nodule formation.