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Rhizobia are one of the groups of microorganisms living in soil. They are single celled bacteria, approximately one thousandth of a millimetre in length. Rhizobia belong to the family of bacteria called Rhizobiaceae. There are a number of groups (genera and species) of bacteria in this family.
Rhizobia belong to a specific group of bacteria that form a mutually beneficial association, or symbiosis, with legume plants. These bacteria take nitrogen from the air (which plants cannot use) and convert it into a form of nitrogen called ammonium (NH4+), which plants can use. The bacterial enzyme nitrogenase controls the process, called nitrogen fixation, and these bacteria are often called "nitrogen fixers".
Rhizobia are found in soils of many natural ecosystems. They may also be present in agricultural areas where they are associated with both crop legumes (like soybean) and pasture legumes (like clover). Usually, the rhizobia in agricultural areas have been introduced at sowing by applying an inoculum to the exterior of the seeds as liquid formations or pellets.
The nodulation process is a series of events in which rhizobia interact with the roots of legume plants to form a specialised structure called a root nodule. These are visible, spherical structures on the surface of roots that are formed by the plant in response to the presence of the bacteria.
The process involves complicated signals between the bacteria and the host roots. In the first stages, the bacteria multiply near the root and then adhere to it. The small hairs on the root's surface curl around the bacteria and they enter the root. Alternatively, the bacteria may enter directly through points on the root surface. The method of entry of the bacteria into the root depends on the type of plant. Once inside the root, the bacteria multiply within thin threads. Signals stimulate cell multiplication of both the plant's cells and the bacteria and this repeated division results in a mass of root cells containing many bacterial cells. Some of these bacteria then change into a form that is able to convert gaseous nitrogen into ammonium nitrogen (that is, they can "fix" nitrogen). These bacteria are then called bacteroids.
The shape of the nodules is controlled by the plant and nodules can vary considerably - both in size and shape.
Most plants need specific kinds of rhizobia to form nodules. The rhizobia that form nodules on peas, for example, cannot form nodules on clover.
Nodulation can be impeded by low pH, aluminium toxicity, nutrient deficiencies, salinity, waterlogging, and the presence of root parasites such as nematodes or genetic incompatibility with the plant host.
The rhizobia produce an enzyme called nitrogenase. Nitrogenase is extremely sensitive to oxygen and is only active at low oxygen levels, or anaerobic conditions. The physical structure of the nodule acts as a barrier to oxygen and another enzyme called leghaemaglobin binds oxygen and transports it away from nitrogenase to respiratory sites. Leghaemaglobin gives the inside of nodules their reddish-pink colour.
Associations between bacteria and plants that fix atmospheric nitrogen include an association between species of Frankia bacteria and several tree species such as those of the genera Casuarina and Allocasuarina. Another example is between that of Azospirillum and grasses.
One example of a group of bacteria that can fix atmospheric nitrogen without being in a symbiosis with a legume is Azotobacter. This genus of bacteria are able to convert atmospheric nitrogen into ammonium that plants can take up. However, this process can only take place if the following conditions are met:
Azobacter is inhibited by a large range of toxic mineral and organic compounds, but may tolerate relatively high salinity and their activities are enhanced in the presence of clays.