Molecular basis of the establishment and functioning of a N2-fixing root nodule.

Compatible interactions between rhizobia and their leguminous host plant(s) culminate in the formation of a new plant organ, the root nodule. Within this structure, the bacteria reduce N2 to NH3 which is then assimilated by the plant. The formation of a N2-fixing nodule requires a continuous process of two-way signalling and cellular recognition between the prokaryote and the plant. Such a process involves the sequential activation and/or repression of host plant- and bacteria-encoded genes. Finally, functioning of a legume-nodule necessitates not only the adaptation of plant and bacterial carbon, nitrogen and oxygen metabolism to an environment allowing N2-fixation to occur, but also requires a tight co-ordination and integration of these plant and bacterial metabolic processes.