How is Rhizobium a symbiotic bacteria ?
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Rhizobia are soil bacteria that fix nitrogen(diazotrophs) after becoming established inside root nodules of legumes (Fabaceae). In order to express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are Gram-negative, motile, non-sporulating rods.
Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a symbiotic relationship with legumes. Common crop and forage legumes are peas, beans, clover, and soy.
Infection and signal exchange
The symbiotic relationship implies a signal exchange between both partners that leads to mutual recognition and development of symbiotic structures. Rhizobia live in the soil where they are able to sense flavonoids secreted by the roots of their host legume plant. Flavonoids trigger the secretion of nod factors, which in turn are recognized by the host plant and can lead to root hair deformation and several cellular responses, such as ion fluxes. The best-known infection mechanism is called intracellular infection, in this case the rhizobia enter through a deformed root hair in a similar way to endocytosis, forming an intracellular tube called the infection thread. A second mechanism is called "crack entry"; in this case, no root hair deformation is observed and the bacteria penetrate between cells, through cracks produced by lateral root emergence. Later on, the bacteria become intracellular and an infection thread is formed like in intracellular infections.
The infection triggers cell division in thecortex of the root where a new organ, the nodule, appears as a result of successive processes.
Infection threads grow to the nodule, infect its central tissue and release the rhizobia in these cells, where they differentiate morphologically into bacteroids and fix nitrogen from the atmospheric, elemental N2into a plant-usable form, ammonium (NH3 + H+ → NH4+), using the enzyme nitrogenase. The reaction for all nitrogen-fixing bacteria is:[15]
N2 + 8 H+ + 8 e− → 2 NH3 + H2
In return, the plant supplies the bacteria withcarbohydrates, proteins, and sufficient oxygenso as not to interfere with the fixation process. Leghaemoglobins, plant proteins similar to human hemoglobins, help to provide oxygen for respiration while keeping the free oxygen concentration low enough so as not to inhibit nitrogenase activity. Recently, aBradyrhizobium strain was discovered to form nodules in Aeschynomene without producing nod factors, suggesting the existence of alternative communication signals other than nod factors.[16]
Nature of the MutualismEdit
The legume–rhizobium symbiosis is a classic example of mutualism—rhizobia supply ammonia or amino acids to the plant and in return receive organic acids (principally as thedicarboxylic acids malate and succinate) as a carbon and energy source. However, because several unrelated strains infect each individual plant, a classic tragedy of the commons scenario presents itself. Cheater strains may hoard plant resources such aspolyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen.[17] Given the costs involved in nodulation and the opportunity for rhizobia to cheat, it may be surprising that this symbiosis should exist at all.
The coevolution of cooperation and choice poses a veritable quandary: choice acts to reduce variation, and therefore removes the incentive for its own maintenance. If this is true, choice should be evolutionarily unstable. If rhizobia were perfect cooperators, choosy hosts would suffer adverse fitness consequences if, as with legumes, choice carries energy costs. The continued existence of legume-rhizobia symbiosis has significant parallels to the lek paradox, wherein female selection of showy mates is maintained among birds. Ironically, cheating itself may be a stabilizing force of cooperation. As with birds, variation is introduced in each generation of rhizobia through immigration, mutation, and gene transfer. When the amount of variation in a population is sufficiently high, mutualism is maintained even when choice is costly. In a roundabout way, cooperation owes its very existence to the recurring consequences of consistent parasitism.[18]
Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a symbiotic relationship with legumes. Common crop and forage legumes are peas, beans, clover, and soy.
Infection and signal exchange
The symbiotic relationship implies a signal exchange between both partners that leads to mutual recognition and development of symbiotic structures. Rhizobia live in the soil where they are able to sense flavonoids secreted by the roots of their host legume plant. Flavonoids trigger the secretion of nod factors, which in turn are recognized by the host plant and can lead to root hair deformation and several cellular responses, such as ion fluxes. The best-known infection mechanism is called intracellular infection, in this case the rhizobia enter through a deformed root hair in a similar way to endocytosis, forming an intracellular tube called the infection thread. A second mechanism is called "crack entry"; in this case, no root hair deformation is observed and the bacteria penetrate between cells, through cracks produced by lateral root emergence. Later on, the bacteria become intracellular and an infection thread is formed like in intracellular infections.
The infection triggers cell division in thecortex of the root where a new organ, the nodule, appears as a result of successive processes.
Infection threads grow to the nodule, infect its central tissue and release the rhizobia in these cells, where they differentiate morphologically into bacteroids and fix nitrogen from the atmospheric, elemental N2into a plant-usable form, ammonium (NH3 + H+ → NH4+), using the enzyme nitrogenase. The reaction for all nitrogen-fixing bacteria is:[15]
N2 + 8 H+ + 8 e− → 2 NH3 + H2
In return, the plant supplies the bacteria withcarbohydrates, proteins, and sufficient oxygenso as not to interfere with the fixation process. Leghaemoglobins, plant proteins similar to human hemoglobins, help to provide oxygen for respiration while keeping the free oxygen concentration low enough so as not to inhibit nitrogenase activity. Recently, aBradyrhizobium strain was discovered to form nodules in Aeschynomene without producing nod factors, suggesting the existence of alternative communication signals other than nod factors.[16]
Nature of the MutualismEdit
The legume–rhizobium symbiosis is a classic example of mutualism—rhizobia supply ammonia or amino acids to the plant and in return receive organic acids (principally as thedicarboxylic acids malate and succinate) as a carbon and energy source. However, because several unrelated strains infect each individual plant, a classic tragedy of the commons scenario presents itself. Cheater strains may hoard plant resources such aspolyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen.[17] Given the costs involved in nodulation and the opportunity for rhizobia to cheat, it may be surprising that this symbiosis should exist at all.
The coevolution of cooperation and choice poses a veritable quandary: choice acts to reduce variation, and therefore removes the incentive for its own maintenance. If this is true, choice should be evolutionarily unstable. If rhizobia were perfect cooperators, choosy hosts would suffer adverse fitness consequences if, as with legumes, choice carries energy costs. The continued existence of legume-rhizobia symbiosis has significant parallels to the lek paradox, wherein female selection of showy mates is maintained among birds. Ironically, cheating itself may be a stabilizing force of cooperation. As with birds, variation is introduced in each generation of rhizobia through immigration, mutation, and gene transfer. When the amount of variation in a population is sufficiently high, mutualism is maintained even when choice is costly. In a roundabout way, cooperation owes its very existence to the recurring consequences of consistent parasitism.[18]
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Information about symbiotic bacteria:-
They establish a symbiotic relationship with other organisms. These bacteria converts atmospheric nitrogen into nitrogenous compounds for the plants.
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