Some soil microorganisms are capable of forging associations with plant roots in the form of symbioses. Certain of these relationships play a highly important ecological and agronomic role. Arbuscular mycorrhizal symbiosis (which links a plant to a fungus) thus gives plants a mechanism for improving their supply of water and mineral nutrition. This association has been in existence for 400 million years and appears to have accompanied plants in their colonization of the terrestrial environment. At present it involves about 80% of plant species. In a more recent era, about 60 million years B. P., the symbiosis which became established between soil bacteria, Rhizobium species, and leguminous plants doted them with the ability, unique among mass-produced crop plants, to capture nutrient nitrogen from the air. Rhizobium forms specialized organs, nodules, on the plant roots. These are capable of transforming atmospheric nitrogen into ammonium that can be directly assimilated by the plant. In return, the plant supplies the microorganisms with nutrients in the form of complex carbohydrates.
Scientists have for many years been seeking to unravel the genetic mechanisms that govern such mutually beneficial relationships, on the one hand between plants and bacteria, on the other between plants and fungi. Investigations by a French team in 2000 had shown that some genetic signalling mechanisms operating in the symbiosis between leguminous plants and Rhizobium type bacteria and such plants and mycorrhizal fungi involved a common genetic element named SymRK. This type of gene was already known to operate in the recognition of Nod factors, signalling substances emitted by the Rhizobium type bacteria which are essential for root nodule formation.
The actinorhizal plants make up another category of plants which have acquired the ability to live symbiotically with a nitrogen fixing bacterium, in this case Frankia. These pioneer plant species, whose host-symbiont mechanis
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Institut de Recherche Pour le Dveloppement