legume-rhizobium symbiotic relationship

INTRODUCTION
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 where the bacteria establish 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)
IMPORTANCE OF LEGUMES
Hunger, poverty and disease are major issues in third world regions. Land management, lack of education and nutrient poor soil (especially soil lacking in nitrogen), are precursors to the low food supply in these poverty stricken areas.. Introducing leguminous plants and trees would nourish the soil without the need for expensive fertilizer (Sanchez et al 2007).Wealthier countries, such as Europe, Asia and North America have an abundance/overabundance of soil nitrogen because they can afford the use of synthetic fertilizers (Howarthjavascript:; et al 2005).
Legume plants are versatile groups which are able to withstand stressful environments enabling them to be a very important food source, especially in under privileged countries. By introducing and manipulating these agro-ecological zones with a higher volume of leguminous plants/trees, the African Millennium Project have attempted to eliminate hunger, boost economic situations, generate cash surpluses, enable school food programs, help deter erosion and reduce the frequency of malaria in the African villages of Kenya, Ethiopia and Malawi, (Sanchez et al 2007) Legumes are a main staple for humans as well as cattle for they are high in nutrient value (www.ildis.org).
The Agricultural and Industrial eras are the main culprits for the instability of nitrogen in areas such as Asia, Europe and North America (Howarth et al 2005).. This is due to synthetic fertilizers and the burning of fossil fuels causing an increase of nitrogen within watersheds and the environment. An overabundance of nitrogen causes a variety of problems including the destruction of aquatic and terrestrial habitats because of acid rain (van Kessel & Hartely, 1999).
Increasing the natural use of nitrogen-fixing legumes includes forage cropping, crop rotation and other organic material. (this sentence structure needs work) These are a few answers to the unsettling dependence on synthetic fertilizers. Forage cropping is the reintroduction of prairie grasses in order to reestablish the natural balance of the soils; this reintroduction also encourages grazing by ruminates and other livestock (Forage Cropping 2004). Different types of forage legumes include crimson clover, red clover, crownvetch, and birdsfoot trefoil. Like all leguminous plants, these forages have higher protein content due to its legume-rhizobium relationship. Reintroducing prairie plants also helps the land by controlling the erosion of the soil because of the plants’ deep tap roots, which extend 20-30 feet underground (Forage Cropping 2004). Because of these deep roots which create air pore spaces, the ground acts like a sponge for the excess nutrient filled water. In addition to preventing erosion, the retention of the water is also good for crops and improves the soil structure. (Sierra club presentation).
Green manuring, a type of forage cropping, is another way of improving the soil. By growing crops explicitly for the organic matter help maintain a fertile soil by boosting the nitrogen levels. Green manuring also keep nutrients from leaching beyond the reach of crops providing an excellent habitat and food for microbial soil. (Howarth et al 2005).
Crop rotation is the use of leguminous plants every other growing season in order to introduce nitrogen to the soil keeping a natural balance of nutrients. Alternating crops have several benefits. Growing the same types of crops every year in the same portion of land, attracts pests and diseases specific to those crops. For example, planting onions after a crop of potatoes is beneficial because the potatoes tend to smother weeds. Planting Brassicas crops, such as rapeseed, following legumes benefit the brassicas because of the excess nitrogen in the soil due to the legumes (Conservative Crop Rotation 2000). **(This sentence structure needs work)
Introducing nitrogen in a natural way reduces the dependence and saturation of synthetic fertilizers. An overabundance of fertilizer has no effect on crops causes leaching of the soil (Howarth et al 2005). Problem in Gulf (Sierra Club Presentation)
Besides erosion control and soil nutrient replenishment legume trees also have many other purposes. Leguminous trees have been used medicinally, tannins, for timber and act as nurse plants. Grar, a species of Acacia tree from Ethiopia, is used in a cure for rabies. The Acacia tree is also used in several perfumes because of its strong odor as well making tannins while the wood of Acacia species are also high prized for making furniture because it takes a high polish (http://www.flowers-org.com/acacia.html) Many leguminous trees are also nurse plants. Nurse plants provide sustenance and habitats for animals, shade and a protection from frost at night to the other plants. The absence of leguminous trees would mean eradication of many native plants and animals
IMPORTANCE OF NITROGEN
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. Rhizobiums **(is it Rhizobia or 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).
NITROGENASE
Molybdenum nitrogenase is the enzyme within the rhizobium bacteria which break down the nitrogen. (Patriarca et al 2002) This process takes the following form:
N2 + 8H +8e + 16Mg-ATP 2NH3 +H2 +16Mg-ADP +16 Pi (Patriarca et al 2002)
Only a few prokaryotic organisms have the ability to catalyze nitrogen to ammonia. This is an oxygen-sensitive process. (Energy from Biological Processes, 1980*) The molybdenum nitrogenase enzyme conducts the nitrogen reaction within the inner surface of the rhizobium cell membrane. Simultaneously, the host plant provides a small amount of oxygen to the bacteria. The process of nitrogen-fixation is anaerobic therefore; too much oxygen would denature the bacteria (Patriarca et al 2002).
RHIZOBIUM
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 rhizobia retreat back into the soil, they no longer fix nitrogen. (Sprent & James, 2007)
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). The rhizobium expends its resources in order to fix nitrogen for the host plant but when growth of the plant is limited, the bacteria must replenish its own resources. The rhizobium does this by extracting excess carbon from its environment and storing it a reservoir called Poly-β-Hydroxybutyrate (PHB) and other polyhydroxyalkonoates (PHA) (Cevallos, 1996).
RHIZOBIUM PHB
PHB and PHA are both carbon and reductive-power storage compounds for the rhizobium (Cevallos et al, 1996). Accumulating an excess of these compounds allow the rhizobium survive starvation from the plant (West et al 2001), its symbiosis with the plant during limited growth spurts and the bacteria free-living state . However, **; however, ** when there is no exogenous carbon available to the bacteria, it will use it’s intracellular PHB (Charles et al 1997). Normally, the rhizobium is able to synthesize PHB (Charles et al, 1997) by an intricate process as shown in Figure 1.
Figure 1. PHB synthesis/degradation. (Charles et al, 1997)
Many strains of rhizobium accumulate an abundance of PHB for their use in symbiosis and in free-life. The bacteria are able to obtain excess carbon when living conditions are less than favorable (Cevallos et al, 1996) I KNOW I NEED TO DO MORE EXPLAINING HERE.

LEGUME-RHIZOBIUM RELATIONSHIP
The Legume-rhizobium symbiotic relationship is ecologically important to sustain life. Nitrogen-fixing leguminous plants play an important role in many aspects. Many legumes grow in stressful environments in which the soils have low amounts of nutrients. By introducing legumes, soils are replenished because they add an equal balance to the ecosystem. The legume-rhizobium symbiotic relationship also fixes atmospheric nitrogen to stressful environmental soils. These soils are enriched when the rhizobium form nodules on the roots of legume hosts.
Talk about aquatic legumes – the newly found one too in frequently flooded areas.
NODULE DEVELOPMENT
Nodule development is an intricate process on the roots of legume plants. Once nodules are formed, atmospheric nitrogen is broken down to ammonia by the rhizobium molybdenum nitrogenase enzyme as described above (Patriarca et al 2002). Rhizobium are either fast or slow growers depending on their types of nodules. Fast-growers usually have cylindrical, non-determinant nodules. Examples of these are alfalfa, clover, peas. The exceptions are beans which are still fast growers but are determinant nodules. Slow growing rhizobium have round, determinant nodules. These include lupins, soybeans and cowpeas. An exception to this rule would be sweet clover and alfalfa which are cylindrical and non-determinant nodules. (Brewin, 2004)
Legume plants form a symbiotic relationship with the Rhizobium bacteria in nitrogen poor soil. The process of root nodulation has three stages: preinfection, nodule initiation and differentiation (Dazzo et al 1982). add more to this otherwise add it to another sentence. This isn't a paragraph
Preinfection Stage
Before nodulation, the rhizobium accumulates on the surface of the host plants’ root forming a biofilm, then communicate throught quorum sensing by exchanging homoserine lactone signaling molecules (Wisniewski-Dy and Downine, 2002, Brewin, 2004). This is known as the rhizosphere. Once the bacteria are on the roots they must them initiate their symbiotic nodules.
Nodule Initiation
The initiation stage of root nodulation has four modes of entry. First mode of entry is an intracellular infection as the rhizobium enter through a deformed hair and entrapped by infected threads (ITs) forming sybiosomes (Quadri et al 2008) . This mode of entry is common in clover and bean plants (Brewin, 2004). The second method of root nodulation is when the bacteria enter through undeformed underformed root hair but through “wounds” of the lateral roots. The bacteria become intracellular, form ITs and then nodules; this mode of method is common in peanuts. In the third mode of entry, the rhizobium colonize the surface of the primary root, penetrate through the plants adhesive (mucilage) and primary wall of the plant’s epidermal cells (Brewin, 2004). As the rhizobium penetrate further through the plant’s primary wall layers and intercellular air, the bacteria occasionally penetrate intracellular epidermal and cortical cells. This process always froms a boundary between the two neighboring cells; this mode of entry is common on stems of Sesbania, a family of leguminous trees. In the fourth mode of entry, rhizobium induce root hair deformations (also called Shephard’s Crook, Esseling et al 2003) shown in Figure 3. and penetrate root hair cell wall, where instead of forming infection thread, bacteria colonize the space between the cell wall and plasma membrane. There are no intracellular or transcellular infectrion threads and bacteria are released directly from infection droplets into the cytoplasm of cells in the root cortex, with them become meristematic (Brewin, 2004).
The initiation process of the legume-rhizobium relationship begins when the rhizobium recognizes its plant host via flavanoids, a type of hormone the plant host emits. (Raghavendra et al 1994) These flavonoids are synthesized by the plants in response to two muligene-encoded enzymes: phenylalanine ammonia lyase and chalcone synthase, which interact with the od genes emitted by the rhizobia (Raghavendra et al 1994). Specific species of rhizobium are attracted to specific plant host (Brewin, 2004) although several species of genres will inhabit the same plant host. These genres are categorized by the rates at which they grow. Once the rhizobium recognizes their host, they move towards their host through the soil water by propelling their flagella. When the rhizobium cells reach their destination they form a biofilm on the roots of the host plant. Rhizobium communicates through quorum sensing by exchanging homoserine lactone signaling molecules (Brewin, 2004). The rhizobium also produce lipooligosaccharides (nod factors) which in turn initiate eformation of the root hairs. Also called Shephard's Crook(***), this is the division of cortical cells in the root cells (Brewin, 2004)

The IT is initiated by a protein called recadhesin and polysaccharides from rhizobium and the lectins from plants which adhere the bacterium to the root hair. This in turn causes the curling of root hairs (Shepherd Crook**) and the hydrolysis of the epidermis of the root. The rhizobium move towards the center of the root hair towards the root cortex, in response the plant produces a tube-like structure called an infection thread (IT) While in the cortex, the rhizobium enter an enclosed area with a plant-derived peribacteroid membrane, also called a symbiosome. This membrane protects the rhizobium from plant defense responses.
Differentiation
Following the formation of root nodules, cell division and enlargement of the rhizobium, determinate nodules, round and have no pronounced meristematic region. Non-determinate nodules are elongated with a pronounced meristematic region. (Wojciechowski & Mahn
), As shown in figure 1, nodulation takes many steps:****
Root nodules are classified as indeterminate, which are cylindrical and often times branched, producing amide products and determinate, which are spherical with prominent lenticels, producing lenticels, producing ureide products. Indeterminate nodules are characteristic of peas and alfalfa,plants of temperate regions, and determinate nodules are characteristic of soybeans and similar plants, plants of tropical and sub-tropical regions. (Sprent, 2001; p. 35) Indeterminate nodules are the only type found in Caesalpiniodeae and Minosoidease and the most common type in Papilionoideae (Sprent, 2001; pg. 32)
CONCLUSION
Introducing leguminous plants to third world countries could improve the quality of life, provide nutrient rich soil, prevent erosion and bring a cash crop to the country. In other parts of the country in which the soil have been depleted of nutrients because of excessive amounts of synthetic fertilizers, restoring the soil’s natural balance could also improve the quality of life in wealthier countries.

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