Rhizobium-Legume Relationship
Nitrogen Fixation
Nitrogen comprises nearly 80% of atmospheric gases; however, the triple bond between the nitrogen molecules prevents it from being broke down by the majority of organisms except for a type of bacteria called rhizobium. Rhizobium are specialized bacteria which attach to leguminous plants allowing them to utilize nitrogen. Besides rhizobium, the only other way to break nitrogen’s triple bond is by a process called the Haber-Bosch synthesis. This process uses an iron catalyst, a very high temperature of 400-500◦C, and 100-200 atmospheric pressure in order to convert the nitrogen to ammonia. (Patriarca et al, 2002)
Nitrogen plays a major role in every aspect of life as it is the building blocks in protein, amino and nucleic acids (Patriarca et al, 2002).
Rhizobium/Legume symbiosis
Vascular plants have always formed relationships with microorganisms in order to obtain a wide variety of nutrients such as nitrogen, phosphorus, micronutrients, biological control and phytohormones. The oldest type of relationship among plants is believed to be the mycorrhizal-AMF (Arbusclur Mycorrhizal Fungi). The AMF is a symbiotic relationship which occurs between a fungus and plant roots either intercelluarly or extracellularly. Another symbiotic relationship, believed to have evolved from the AMF, is between legumes and rhizobium bacteria, and this relationship establishes root nodules in or on the plants’ roots. In both relationships, the bacteria (or fungus) receive nutrients from the plant host in exchange for the ability to obtain nutrients from the soil. Both types of relationships formed as a competition for food (Sprent & James, 2007)

Rhizobiums, also called diazotrophs, are free-living gram negative bacteria that can live within the soil for years. When suitable leguminous hosts are available the Rhizobium form nodules onto their roots in order to break down atmospheric nitrogen through a process called nitrogenase. Leguminous plants have a symbiotic relationship with Rhizobium bacteria which enriches the soil by adding essential nutrients, especially nitrogen. Leguminous plants/trees are a very important source of nitrogen because it is the only way nitrogen is introduced into the food chain. As the rhizobium retreat back into the soil, they no longer fix nitrogen. (Sprent & James, 2007)
Leguminous plants are versatile plants which are able to thrive under stressful situations others plants are not; this is partly due to their relationship with rhizobium. By using a number of intricate steps the rhizobium attach to the roots of leguminous plants expending resources to break down the nitrogen for the plant while at the same time utilizing resources provided to it by the plant (Brewin, 2004).
So how do Rhizobium get into plant roots?
NOD proteins important for root nodule development in legume roots
NOD proteins (Nucleotide-binding Oligomerization Doman Proteins) (Opitz et al 2004)
Similarities between NOD proteins
Unanswered questions

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