Rhizobium and the Papilionaceae - a mutualistic association
In a mutualistic association both organisms benefitMutualism can also be called symbiosis
Rhizobium is a nitrogen-fixing bacterium i.e. it can use atmospheric nitrogen as a reagent in the synthesis of amino acids. One of the midpoints of this reaction is the production of ammonia (NH3).
The fixation of nitrogen by Rhizobium requires an anaerobic environment because the enzyme that catalyses the reaction, nitrogenase, is extremely sensitive to oxygen.
A pigment called leghaemoglobin is produced by the plant and binds oxygen. In the diagram that follows two of the root nodules have been crushed and the pink colour provided by the leghaemoglobin can be seen.
Image showing the pink colour of leghaemoglobin taken from http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm
Rhizobium lives in nodules (swellings) in the roots of members of the Papilionaceae (the legumes) such as clover, peas and beans. These plants are so named because their flowers are butterfly-shaped.
Image from http://database.sunsite.ualberta.ca/pls/biod1/ditrl.display.detail?p_mno=135
Plants require a source of ammonia. For the Papilionaceae the presence of Rhizobium provides a ready supply of ammonia, giving them a competitive advantage in nitrogen-poor soil.
Note the ammonia is converted to ammonium (NH4+) before being absorbed by the plant
The plant provides the bacteria with a supply of carbohydrates
Interestingly Rhizobium can also exist free-living in aerobic conditions in soil. Such bacteria do not fix nitrogen, fixation is a process which requires the anaerobic conditions found within the root nodule.
The bacteria are attracted to the roots of the plant by release of hormones by the plant.
Once the bacteria penetrate the plant they stimulate cell growth resulting in the formation of the nodule.
The bacteria change shape and become known as bacteroids.