Legumes have been cultivated since ancient times as a source of food (peas, soybeans, alfalfa, etc.). They are also of interest for improving soil fertility, serving as a pioneer species for nitrogen-poor soils. The beneficial role of a family of bacteria, the rhizobia, has only recently been discovered.
Nitrogen is an essential element for plant development. It is very abundant in the atmosphere in the form of N2 (dinitrogen). However, plants are not able to absorb nitrogen in its gaseous form. They can only absorb it in a dissolved form (nitrate).
This is where rhizobium-type bacteria come in. These nitrogen-fixing bacteria are capable of transforming this dinitrogen into NO3- (Nitrate) or NH4+ (Ammonium ion) which can be assimilated by the plant. This transformation is energy consuming because the triple bond N≡N is very stable. In the case of rhizobial symbiosis, the plant is the source that provides the necessary energy in the form of carbohydrates from photosynthesis. It takes no less than 16 ATP to reduce a N≡N molecule
N2 + 8H+ + 8e- + 16 ATP → 2 NH3 + H2 + 16 ADP
It was recently discovered that a real molecular dialogue is taking place between the root and the rhizobium. To stimulate the nodulation process, the plant secretes flavonoids in the soil which are perceived by the rhizobium via a regulatory protein. In response, rhizobium triggers the expression of genes that initiate the nodulation process.
Source: Symbiose rhizobium / légumineuse . un nouveau Sesame (Éric Giraud - Laboratoire des Symbioses Tropicales et Méditerranéennes)