Rhizobium
Rhizobia (genus Rhizobium) are aerobic soil bacteria belonging to the Rhizobiaceae family. These bacteria have the ability to enter into symbiosis with plants from the Fabaceae by forming nodules, found specifically in leguminous plants such as : peas, beans, soya, peanuts, clover, alfalfa, etc. They form part of the rhizosphere.
This symbiosis gives Fabaceae a unique ability among field crops to fixnitrogen from the air and feed on it. Only one non-legume has been identified as capable of associating with rhizobia to formnitrogen-fixing nodules : Parasponia.
Under combined nitrogen-limiting conditions, rhizobia will induce the formation of nodules in the roots or stems of legumes. These nodules act as metabolic exchange organs between the bacteria and the plants.
This mutually beneficial symbiosis will enable the bacteria to benefit from an exceptionally favourable micro-habitat , with the Fabaceae providing them with carbonaceous substrates produced by photosynthesis. In exchange, the bacteria will fix and reduce atmospheric nitrogen to ammonium, which can be directly assimilated by the host plants.
There are distinct strains of Rhizobium for each type of legume. When these strains are lacking in their natural state or are not very effective, it is possible to artificially introduce more effective inoculums, but these sometimes have difficulty overcoming the strains already in place.
Certain varieties of Rhizobium can create nodules while fixing mediocre or even zero quantities of nitrogen, while others can fix up to 600 kg of nitrogen/hectare.
The piliferous cells of the roots of Fabaceae (Fabaceae) emit recognition chemicals (flavonoids/isoflavonoids). These exudates attract the bacteria, which in turn synthesise and secrete nodulation factors (nod factors). These " nod factors " differ according to the rhizobial species and have a specific antigenic structure recognised by the plant.
The bacterium is specifically recognised by the angiosperm, which forms an infectious cord by invagination of the plasma membrane. This is always first formed at the apex of an absorbent hair and then expands to reach the cells of the root cortex. When the bacteria reach this area, they infect the cells. Their intracellular presence induces the expression of nodulin genes, proteins that trigger the dedifferentiation of cortical cells. The cortical cells multiply and form an outgrowth called a nodule, which is connected to the plant's conductive vessels that supply the system with energy.
The bacteria remain isolated from the cytoplasm by the host's plasma membrane. They are gradually grouped together in vesicles called symbiosomes. At the same time, under the action of a nodulin, the nodule cells synthesise leghemoglobin, which depletes the nodule of oxygen by trapping it. Oxygen denatures the enzyme (nitrogenase), so the environment must be anaerobic. Finally, the plant secretes peptides which enter the bacterial cell and block its division at the end of mitosis, so that the rhizobium continues to grow progressively.()
