the mutualistic relationship between leguminous plants and nitrogen fixing infection of legume roots by nitrogen fixing bacteria leads to the formation of root . Rhizobium is a mutualistic nitrogen fixing bacteria forming a relationship with ( d) Suggest how the relationship between Rhizobium and a legume species is. In microbiology, there are many examples of mutualistic bacteria in the gut that . A hydrothermal vent is a fissure in the earth's surface from which geothermally heated water issues. .. Evaluate legume and nitrogen-fixing bacteria symbiosis .
In turn, they supply their host plants with nitrogen resources produced by nitrogen gas fixation. This mutual nutrient exchange should promote the fitness of both organisms and thereby strengthen the symbiotic relationship.
On the other hand, naturally occurring rhizobium strains vary in their nitrogen fixation activity, and ineffective rhizobia that colonize their host plants without undertaking nitrogen fixation in their root nodules are ubiquitous  — . Because the nitrogen fixation reaction consumes much energy or costssuch parasitic cheaters could use surplus energy for their own growth or for synthesizing storage substances.
Consequently, they are likely to proliferate more efficiently than nitrogen-fixing cooperators, posing a risk to the symbiotic interaction. Rhizobia are therefore exposed to two opposite effects that simultaneously promote by providing benefit and destabilize by incurring cost the mutualistic relationship.
Despite the widespread presence of ineffective rhizobia, the legume—rhizobia symbiosis is evolutionarily stable.
Potential mechanisms that stabilize the symbiotic interaction include partner fidelity feedback  — partner choice  — host sanction  — spatial structure and kin selection or inclusive fitness .
Partner choice and host sanction exert similar effects, in that host plants discriminate beneficial symbionts. However, partner choice is often regarded as a selection process based on recognition signals, while in host sanction, plants punish more parasitic cheaters by reducing nutrient supply based on their symbiotic performance . Thus, in this paper, we consider partner choice and host sanction as pre-infection and post-infection processes, respectively, according to the nitrogen fixation activity of symbionts.
The evolution of cooperation has been theoretically modeled by game theory, in particular, the prisoner's dilemma game and the snowdrift game also known as the hawk-dove game .
If the game involves two players, mutual cheating is an evolutionarily stable strategy ESS in the prisoner's dilemma, while the snowdrift game has the Nash equilibrium of mixed-strategy, in which cooperator and cheater stably coexist.
These games have been extensively studied under various conditions. However, although the simplified framework of binary strategy choice full cooperation or full defection captures the essential nature of cooperation, evolutionary dynamics involving quantitative traits such as nitrogen fixation activity in rhizobia require more complex frameworks. Continuous strategies, such as the continuous versions of snowdrift and prisoner's dilemma, more accurately reflect real-world phenomena  — .
Interestingly, the continuous snowdrift game permits an evolutionary process in which completely non-productive cheaters coexist with cooperators making maximum investment . This evidence provides insight into the evolutionary origin of ineffective symbionts in the legume—rhizobia mutualism. Various mathematical models have been proposed for the evolution of the legume—rhizobia symbiosis. Other models describe the population dynamics of rhizobia strains with fixed nitrogen fixation activity, such as complete cooperators or total cheaters  — .
These models can explain the coexistence of cooperators and cheaters; however, as described above, discrete strategies oversimplify the evolutionary dynamics of quantitative phenotypes. On the other hand, West et al.
Evolutionary Dynamics of Nitrogen Fixation in the Legume–Rhizobia Symbiosis
While finding the optimal strategy is a powerful approach for analyzing evolutionary systems, it cannot sufficiently explain relatively complicated situations, such as frequency-dependent interactions between different rhizobia strains. In fact, the stable co-existence of cooperators and cheaters is difficult to explain using these approaches. In contrast, the legume—rhizobia mutualism is more directly reflected by evolutionary dynamics in which rhizobia proliferate according to the quantitative trait of nitrogen fixation with a frequency-dependent selection.
However, this situation has yet to be fully investigated, although such a model may explain the effect of mixed colonization . The aim of this paper is not to examine the nature or evolution of mechanisms that prevent rhizobia from cheating, but rather to investigate the conditions under which mutualistic and cheating symbionts can coexist.
Evolutionary Dynamics of Nitrogen Fixation in the Legume–Rhizobia Symbiosis
We also aim to understand how and why symbiosis establishes and stably persists despite the ubiquitous distribution of cheating rhizobia. This fact suggests that host plants cannot develop a cheating strategy, whereas ineffective rhizobia are ubiquitously distributed in nature.
Thus, the evolutionary dynamics of the mutualism are most likely driven by the strategies of the symbionts, while the host plants play a minor role. Accordingly, our model is based on the strategies adopted by the rhizobia. The evolutionary strategy of each rhizobium is its nitrogen fixation activity in a root nodule, which is continuous between 0 no activity or full defection and 1 maximum activity or full cooperation and which is genetically transmitted to its progeny.
Other plants benefit from nitrogen fixing bacteria when the bacteria die and release nitrogen to the environment, or when the bacteria live in close association with the plant.
In legumes and a few other plants, the bacteria live in small growths on the roots called nodules. Within these nodules, nitrogen fixation is done by the bacteria, and the NH3 produced is absorbed by the plant. Nitrogen fixation by legumes is a partnership between a bacterium and a plant. Biological nitrogen fixation can take many forms in nature including bluegreen algae a bacteriumlichens, and free-living soil bacteria.
These types of nitrogen fixation contribute significant quantities of NH3 to natural ecosystems, but not to most cropping systems, with the exception of paddy rice. Their contributions are less than 5 lbs of nitrogen per acre per year. However, nitrogen fixation by legumes can be in the range of pounds of nitrogen per acre per year in a natural ecosystem, and several hundred pounds in a cropping system. A common soil bacterium, Rhizobium, invades the root and multiplies within the cortex cells.
The plant supplies all the necessary nutrients and energy for the bacteria. Within a week after infection, small nodules are visible with the naked eye. In the field, small nodules can be seen weeks after planting, depending on legume species and germination conditions. When nodules are young and not yet fixing nitrogen, they are usually white or grey inside. As nodules grow in size they gradually turn pink or reddish in color, indicating nitrogen fixation has started. The pink or red color is caused by leghemoglobin similar to hemoglobin in blood that controls oxygen flow to the bacteria.
Nodules on many perennial legumes such as alfalfa and clover are finger-like in shape. Nodules on perennials are long-lived and will fix nitrogen through the entire growing season, as long as conditions are favorable. Most of the nodules per large alfalfa plant will be centered around the tap root. Nodules on annual legumes such as beans, peanuts, and soybeans are round and can reach the size of a large pea.
Nodules on annuals are short-lived and will be replaced constantly during the growing season. At the time of pod fill, nodules on annual legumes generally lose their ability to fix nitrogen because the plant feeds the developing seed rather than the nodule. Beans will generally have less than nodules per plant, soybeans will have several hundred per plant, and peanuts may have 1, or more nodules on a well-developed plant.
Legume nodules that are no longer fixing nitrogen usually turn green, and may actually be discarded by the plant. Pink or red nodules should predominate on a legume in the middle of the growing season.
If white, grey, or green nodules predominate, little nitrogen fixation is occurring as a result of an inefficient Rhizobium strain, poor plant nutrition, pod filling, or other plant stress. The nitrogen fixed is not free. The plant must contribute a significant amount of energy in the form of photosynthate photosynthesis derived sugars and other nutritional factors for the bacteria.
Any stress that reduces plant activity will reduce nitrogen fixation. Factors like temperature and water may not be under the control of the farmer. But nutrition stress especially phosphorus, potassium, zinc, iron, molybdenum, and cobalt can be corrected with fertilizers. When a nutritional stress is corrected, the legume responds directly to the nutrient, and indirectly to the increased nitrogen nutrition resulting from enhanced nitrogen fixation.
Poor nitrogen fixation in the field can be easily corrected by inoculation, fertilization, irrigation, or other management practices. Common beans are poor fixers less than 50 lbs per acre and fix less than their nitrogen needs.
Maximum economic yield for beans in New Mexico requires an additional lbs of fertilizer nitrogen per acre.
Nitrogen Fixation by Legumes
However, if beans are not nodulated, yields often remain low, regardless of the amount of nitrogen applied. Nodules apparently help the plant use fertilizer nitrogen efficiently. Other grain legumes such as peanuts, cowpeas, soybeans, and faba beans are good nitrogen fixers, and will fix all of their nitrogen needs other than that absorbed from the soil.
These legumes may fix up to lbs of nitrogen per acre and are not usually fertilized. In fact, they usually don't respond to nitrogen fertilizer as long as they are capable of fixing nitrogen. Nitrogen fertilizer is applied at planting to these legumes when grown on sandy or low organic matter soils to supply nitrogen to the plant before nitrogen fixation starts.
If nitrogen is applied, the rate is low, lbs per acre. When large amounts of nitrogen are applied, the plant literally slows or shuts down the nitrogen fixation process.