Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate
Symbiosis is a relationship between two organisms: it can be mutualistic (both .. known as rhizobia during the initiation of nodules on the root of legumes. Rhizobia sp. bacteria can be found in the root nodules of legumes. These are swellings (clusters of cells) that can be found along the roots. The Rhizobia carry . In a symbiotic relationship with the soil bacteria known as 'rhizobia', legumes form nodules on their roots (or stems, see figure below) to 'fix' nitrogen into a form .
In return for the fixed nitrogen that they provide, the rhizobia are provided shelter inside of the plant's nodules and some of the carbon substrates and micronutrients that they need to generate energy and key metabolites for the cellular processes that sustain life Sprent, Nodulation and nitrogen fixation by rhizobia is not exclusive to legumes; rhizobia form root nodules on Parasponis Miq.
The picture on the right shows "stem" nodules on Sesbania rostrata - stem nodules are produced from lateral or adventitious roots and are typically found in those few water-tolerant legume groups Neptunia, Sesbania that prefer wet or water-logged soils Goormachtig et al.
Plants, bacteria, animals, and manmade and natural phenomena all play a role in the nitrogen cycle. The fixation of nitrogen, in which the gaseous form dinitrogen, N2 is converted into forms usable by living organisms, occurs as a consequence of atmospheric processes such as lightning, but most fixation is carried out by free-living and symbiotic bacteria. Plants and bacteria participate in symbiosis such as the one between legumes and rhizobia or contribute through decomposition and other soil reactions.
The plants then use the fixed nitrogen to produce vital cellular products such as proteins. Many species of bacteria adapt to saline conditions by the intracellular accumulation of low-molecular-weight organic solutes called osmolytes The accumulation of osmolytes is thought to counteract the dehydration effect of low water activity in the medium but not to interfere with macromolecular structure or function Rhizobia utilize this mechanism of osmotic adaptation 4243, An osmolyte, N-acetylglutaminyl-glutamine amide, accumulates in cells of R.
The disaccharide trehalose plays a role in osmoregulation when rhizobia are growing under salt or osmotic stress 96 Trehalose accumulates to higher levels in cells of R. Fast-growing peanut rhizobia accumulate trehalose in the presence of many carbon sources mannitol, sucrose, or lactosebut the slow growers accumulate trehalose only when cultured with mannitol as the carbon source. In a medium supplemented with mM NaCl, the content of trehalose increased intracellularly throughout the logarithmic and stationary phases of growth of peanut rhizobia The disaccharides sucrose and ectoine were used as osmoprotectants for Sinorhizobium meliloti However, these compounds, unlike other bacterial osmoprotectants, do not accumulate as cytosolic osmolytes in salt-stressed S.
One salt or osmotic stress response already identified in rhizobia is the intracellular accumulation of glycine betaine, The concentration of glycine betaine increases more in the salt-tolerant strains of R. The addition of sodium salts to bacteroids of Medicago sativa nodules increased the uptake activity of the exogenously added glycine betaine These osmoprotective substances may play a significant role in the maintenance of nitrogenase activity in bacteroids under salt stress.
When externally provided, glycine betaine and choline enhance the growth of Rhizobium tropici, S.
However, the main physiological role of glycine betaine in the family Rhizobiaceae seems to be as an energy source, while its contribution to osmoprotection is restricted to certain strains.
Another osmoprotectant, ectoine, was as effective as glycine betaine in improving the growth of R. Ectoine does not accumulate intracellularly and therefore would not repress the synthesis of endogenous compatible solutes such as glutamate and trehalose; it may play a key role in triggering the synthesis of endogenous osmolytes Therefore, at least two distinct classes of osmoprotectants exist: The content of polyamines, e.
This polyamine may function to maintain the intracellular pH and repair the ionic imbalance caused by osmotic stress. Osmotic stress shock results in the formation of specific proteins in bacteria. Botsford 42 reported that the production of 41 proteins was increased at least fold in salt-stressed cells of Escherichia coli.
The formation of osmotic shock proteins was only recently found in cells of rhizobia. These organic osmolytes amino acids and the inorganic minerals cations may play a role in osmoregulation for this Rhizobium strain. The rhizobial cells responded to high-salt stress by changing their morphology: The cell ultrastructure was severely affected, the cell envelope was distorted, and the homogeneous cytoplasm was disrupted.
It has been reported 51 that cells of a strain of R. Strains of rhizobia from different species modified their morphology under salt stress, and rhizobia with altered morphology have been isolated from salt-affected soils in Egypt High osmotic stress 0.
The colonies of R. The synthesis pattern in SDS-PAGE of lipopolysaccharides LPS from various species of rhizobia from cultivated legumes and from woody legumes was modified by salt, in the presence of which the length of side chains increased. Changing the surface antigenic polysaccharide and LPS, by salt stress, might impair the Rhizobium-legume interaction.
LPS are very important for the development of root nodules 38 Successful Rhizobium-legume symbioses under salt stress require the selection of salt-tolerant rhizobia from those indigenous to saline soils Rhizobium strains isolated from salt-affected soils in Egypt failed to nodulate their legume host under saline and nonsaline conditions a.
These rhizobia showed alterations in their protein and LPS patterns The genetic structure of these bacteria may also be changed since they showed little DNA-DNA hybridization to reference rhizobia.
The Rhizobium strains that are best able to form effective symbiosis with their host legumes at high salinity levels are not necessarily derived from saline soils Graham reported that salt-tolerant strains of rhizobia represent only a small percentage of all strains isolated and identified; therefore, further research in selecting salt-tolerant and effective strains of rhizobia is strongly recommended.
In fact, and as indicated in recent reports, some strains of salt-tolerant rhizobia are able to establish effective symbiosis, while others formed ineffective symbiosis. Mutant strains of R. These nodules failed to express nitrogenase activity Some strains of Rhizobium tolerated extremely high levels of salt up to 1.
Inoculation of legumes by salt-tolerant strains of R. Salt-tolerant strains isolated from Acacia redolens, growing in saline areas of Australia, produced effective nodules on both A. The growth, nodulation, and N2 fixation N content of Acacia ampliceps, inoculated with salt-tolerant Rhizobium strains in sand culture, were resistant to salt levels up to mM NaCl Under saline conditions, the salt-tolerant strains of Rhizobium sp.
An important result was obtained from the recent work of Lal and Khannawho showed that the rhizobia isolated from Acacia nilotica in different agroclimatic zones, which were tolerant to mM NaCl, formed effective N2-fixing nodules on Acacia trees grown at mM NaCl.
It was concluded from these results that salt-tolerant strains of Rhizobium can nodulate legumes and form effective N2-fixing symbioses in soils with moderate salinity.
Therefore, inoculation of various legumes with salt-tolerant strains of rhizobia will improve N2 fixation in saline environments However, tolerance of the legume host to salt is the most important factor in determining the success of compatible Rhizobium strains to form successful symbiosis under conditions of high soil salinity Evidence presented in the literature suggests a need to select plant genotypes that are tolerant to salt stress and then match them with the salt-tolerant and effective strain of rhizobia 70 In fact, the best results for symbiotic N2 fixation under salt stress are obtained if both symbiotic partners and all the different steps in their interaction nodule formation, activity, etc.
The use of actinorhizal associations to improve N2 fixation in saline environments was also studied but not as extensively as Rhizobium-legume associations.
One of these actinorhizal associations Frankia-Casuarina is known to operate in dry climates and saline lands and was reported to be tolerant to salt up to to mM NaCl 67 Casuarina obesa plants are highly salt tolerantbut growth under saline conditions depends on the effectiveness of symbiotic N2 fixation.
Successful plantings of Casuarina in saline environments require the selection of salt-tolerant Frankia strains to form effective N2-fixing association.
Soil Moisture Deficiency The occurrence of rhizobial populations in desert soils and the effective nodulation of legumes growing therein, emphasize the fact that rhizobia can exist in soils with limiting moisture levels; however, population densities tend to be lowest under the most desiccated conditions and to increase as the moisture stress is relieved It is well known that some free-living rhizobia saprophytic are capable of survival under drought stress or low water potential A strain of Prosopis mesquite rhizobia isolated from the desert soil survived in desert soil for 1 month, whereas a commercial strain was unable to survive under these conditions The survival of a strain of Bradyrhizobium from Cajanus in a sandy loam soil was very poor; this strain did not persist to the next cropping season, when the moisture content was about 2.
The survival and activity of microorganisms may depend on their distribution among microhabitats and changes in soil moisture The distribution of R. Moderate moisture tension slowed the movement of R.
The migration of strains of B. One of the immediate responses of rhizobia to water stress low water potential concerns the morphological changes. Mesquite Rhizobium and R. The modification of rhizobial cells by water stress will eventually lead to a reduction in infection and nodulation of legumes.
Nitrogen Fixation and the Nitrogen Cycle
Low water content in soil was suggested to be involved in the lack of success of soybean inoculation in soils with a high indigenous population of R.
Further, a reduction in the soil moisture from 5. Similarly, water deficit, simulated with polyethylene glycol, significantly reduced infection thread formation and nodulation of Vicia faba plants A favorable rhizosphere environment is vital to legume-Rhizobium interaction; however, the magnitude of the stress effects and the rate of inhibition of the symbiosis usually depend on the phase of growth and development, as well as the severity of the stress.
For example, mild water stress reduces only the number of nodules formed on roots of soybean, while moderate and severe water stress reduces both the number and size of nodules Symbiotic N2 fixation of legumes is also highly sensitive to soil water deficiency. A number of temperate and tropical legumes, e.
Soil moisture deficiency has a pronounced effect on N2 fixation because nodule initiation, growth, and activity are all more sensitive to water stress than are general root and shoot metabolism 14 The response of nodulation and N2 fixation to water stress depends on the growth stage of the plants.
It was found that water stress imposed during vegetative growth was more detrimental to nodulation and nitrogen fixation than that imposed during the reproduction stage There was little chance for recovery from water stress in the reproductive stage. Nodule P concentrations and P use efficiency declined linearly with soil and root water content during the harvest period of soybean-Bradyrhizobium symbiosis More recently, Sellstedt et al.
The wide range of moisture levels characteristic of ecosystems where legumes have been shown to fix nitrogen suggests that rhizobial strains with different sensitivity to soil moisture can be selected. Laboratory studies have shown that sensitivity to moisture stress varies for a variety of rhizobial strains, e. Thus, we can reasonably assume that rhizobial strains can be selected with moisture stress tolerance within the range of their legume host.
Optimization of soil moisture for growth of the host plant, which is generally more sensitive to moisture stress than bacteria, results in maximal development of fixed-nitrogen inputs into the soil system by the Rhizobium-legume symbiosis Drought-tolerant, N2-fixing legumes can be selected, although the majority of legumes are sensitive to drought stress. Moisture stress had little or no effect on N2 fixation by some forage crop legumes, e.
One legume, guar Cyamopsis tetragonolobais drought tolerant and is known to be adapted to the conditions prevailing in arid regions Variability in nitrogen fixation under drought stress was found among genotypes of Vigna radiata and Trifolium repens These results assume a significant role of N2-fixing Rhizobium-legume symbioses in the improvement of soil fertility in arid and semiarid habitats.
Several mechanisms have been suggested to explain the varied physiological responses of several legumes to water stress. The legumes with a high tolerance to water stress usually exhibit osmotic adjustment; this adjustment is partly accounted for by changing cell turgor and by accumulation of some osmotically active solutes The accumulation of specific organic solutes osmotica is a characteristic response of plants subject to prolonged severe water stress.
One of these solutes is proline, which accumulates in different legumes, e. Infection and signal exchange[ edit ] The formation of the symbiotic relationship involves a signal exchange between both partners that leads to mutual recognition and development of symbiotic structures.Nitrogen Nodes in Leguminous Plants - jogglerwiki.info
The most well understood mechanism for the establishment of this symbiosis is through intracellular infection. Rhizobia are free living in the soil until they are able to sense flavonoidsderivatives of 2-phenyl This is followed by continuous cell proliferation resulting in the formation of the root nodule.
In this case, no root hair deformation is observed. Instead the bacteria penetrate between cells, through cracks produced by lateral root emergence. Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant. This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity. Nature of the mutualism[ edit ] 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 the dicarboxylic 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 as polyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen.
The sanctions hypothesis[ edit ] There are two main hypotheses for the mechanism that maintains legume-rhizobium symbiosis though both may occur in nature. The sanctions hypothesis theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia, and must counter the parasitism by post-infection legume sanctions.