A new plant-bacterial symbiotic mechanism...( sured in leguminous plants which have n...)

The researchers sequenced (3) the genes of two bacterial strains of Bradyrhizobium, ORS278 and BTAi1, in order to find out their genetic make-up and identify the genes involved in this rather special form of symbiosis. These bacteria were found to have no nod genes, usually essential for nodulation. Bradyrhizobium consequently appeared to use mechanisms that involved other genes. This surprising result calls into question the universally recognized model of molecular communication that initiates the rhizobia-legume symbiosis. This common model requires the presence of several nod genes which allow synthesis of the Nod factor, a compound elaborated by the bacterium which enables the plant to recognize it, by molecular recognition, thereby allowing the microorganism to penetrate inside the plant by the root hairs. The finding raises the question as to what signalling pathway Bradyrhizodium might use to gain entry to the plant and set off nodulation.

The first observation was that the bacteria did not penetrate the roots of its host-plant by the hairs. It took advantage of crack zones comparable with wound areas. The set of results obtained from subsequent work, seeking to identify the genes involved in producing the unknown signal molecule that plays the role of Nod factor, prompted the teams hypothesis that a molecule similar to a plant hormone (4), cytokinin, could act in the mechanisms by triggering nodulation. The discovery of the nature of the signal molecule itself, which remains to be fully determined, brings a glimpse of future agricultural applications.

Many plants live in symbiosis with bacteria, but the mechanisms are known for only a small number of these interactions. The demonstration of alternative pathways capable of triggering the nodulation signal in certain rhizobia is promising for future techniques for bringing these bacteria into association with different legum
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Contact: Marie Guillaume-Signoret
guillaum@paris.ird.fr
33-148-037-607
Institut de Recherche Pour le Dveloppement
15-Jul-2007