Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate
This relationship leads to the establishment of specialized structures called nodules. An important feature of rhizobium–legume symbioses is their specificity: each . Contribution of Microbes to the Health of Humans, Animals, and Plants. Symbiosis is a relationship between two organisms: it can be mutualistic (both Most of the microorganisms studied in medical microbiology are parasitic and feed .. Legumes have a symbiotic relationship with bacteria called rhizobia, which. Keywords: nodulation, legume, rhizobia, soil bacteria . diverse communities of microorganisms that affect plant growth and health through Functional studies conducted in L. japonicus in association with its cognate partner.
The plant supplies the rhizobia with energy in the form of amino acids and the rhizobia fix nitrogen from the atmosphere for plant uptake.
The reduction of atmospheric dinitrogen into ammonia is the second most important biological process on earth after photosynthesis Sylvia, The actual process of dinitrogen fixation can only be carried out by diazotrophs that contain the enzyme dinitrogenase. Nitrogen is the most critical nutrient needed to support plant growth. These include electrical N2 fixation by lightning where oxides of N come to ground with rain, the Haber-Bosch process in industrial fertilizer production, and biological N2 fixation in legumes by bacterial symbionts such as Rhizobium etli.
Biological fixation of nitrogen was the leading form of annual nitrogen input until the last decade of the 20th century Russelle, It is gaining attention once again as sustainability becomes a central focus to feed a world population of over 7 billion people. Generally, legumes gain extra nitrogen for plant growth to offset the loss of photosynthate in this mutualistic association.
The rhizobia invade plant roots and induce a nodule in which the bacteria reduce atmospheric nitrogen to ammonia and supply the plant with nitrogenous compounds Young, The plant gains the ability to grow in nitrogen poor soils, and the bacteria gain a protected niche where they multiply and eventually escape back into the surrounding soil when the nodule senesces Young, Because biological N2 fixation requires such a large amount of energy, it is important to understand the energy transfer in the process.
The stepwise reaction of energy transfer is characterized by the following steps: Each yield requires 2 e- for a total of 6 electrons needed. Electrons come in via Fe protein and are donated by ferredoxin. Nodulation The actual process of nodulation is a very coordinated effort between the legume and the Rhizobium bacteria in the soil.
Infection typically occurs in root hairs of legumes. Many rhizobia and host plants are highly specific and legumes can either attract rhizobia to root hairs directly by excretory compounds or by induction of nod gene activity in the bacteria. Molecular determinants of host specificity during nitrogen-fixing symbiosis. Communication between legume and Rhizobium 1. Flavonoids are released by the host root. The flavonoid is at the highest concentration at the root and interacts with the product of bacterial nodD gene.
The nodD gene produces the protein, nodD, which is the sensor that recognizes chemicals excreted by host plant roots Russelle, Rhizobia colonize the soil in the vicinity of the root hair in response to the flavonoids.Legumes of Thailand : The specificity of tropical Legume-Rhizobia symbioses
This process is autoregulated where favonoids stimulate Nod factor production, which stimulates flavonoid secretion Russelle, Recent metagenomic studies of the soil biome have revealed its complexity, which includes microorganisms that affect plant fitness and growth in a beneficial, harmful, or neutral manner. In this complex scenario, understanding the molecular mechanisms by which legumes recognize and discriminate rhizobia from pathogens, but also between distinct rhizobia species and strains that differ in their symbiotic performance, is a considerable challenge.
In this work, we will review how plants are able to recognize and select symbiotic partners from a vast diversity of surrounding bacteria. We will also analyze recent advances that contribute to understand changes in plant gene expression associated with the outcome of the symbiotic interaction. These aspects of nitrogen-fixing symbiosis should contribute to translate the knowledge generated in basic laboratory research into biotechnological advances to improve the efficiency of the nitrogen-fixing symbiosis in agronomic systems.
Introduction The economic and ecological importance of legumes is evidenced by the high number of species that are cultivated and commercialized, as well as by their ability to obtain nitrogen from a symbiotic interaction with soil bacteria known as rhizobia. This family of flowering plants includes species of agronomic importance such as common bean Phaseolus vulgarisalfalfa Medicago sativasoybean Glycine maxpea Pisum sativumand lentil Lens culinarisetc.
Their unique capacity to establish a nitrogen-fixing symbiosis among crops is crucial to alleviate the usage of synthetic fertilizers in agronomic systems.
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- THE NITROGEN FIXATION PROCESS
Nitrogen fertilization is extremely expensive and generates ecological risks such as water eutrophication and emission of atmospheric greenhouse gases that contribute to global warming. 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.
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Mesquite Rhizobium and R. The modification of rhizobial cells by water stress will eventually lead to a reduction in infection and nodulation of legumes.
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.
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One of these solutes is proline, which accumulates in different legumes, e. In these plants, positive correlations were found between proline accumulation and drought tolerance. Potassium is known to improve the resistance of plants to environmental stress. A recent report indicates that K can apparently alleviate the effects of water shortage on symbiotic N2 fixation of V.
The presence of 0. It was also shown that the symbiotic system in these legumes is less tolerant to limiting K supply than are the plants themselves. Species of legumes vary in the type and quantity of the organic solutes which accumulate intracellularly in leguminous plants under water stress.
This could be a criterion for selecting drought-tolerant legume-Rhizobium symbioses that are able to adapt to arid climates. High Temperature and Heat Stress High soil temperatures in tropical and subtropical areas are a major problem for biological nitrogen fixation of legume crops High root temperatures strongly affect bacterial infection and N2 fixation in several legume species, including soybeanguar 22peanutcowpeaand beans Nodule functioning in common beans Phaseolus spp. Nodulation and symbiotic nitrogen fixation depend on the nodulating strain in addition to the plant cultivar 22 Temperature affects root hair infection, bacteroid differentiation, nodule structure, and the functioning of the legume root nodule High not extreme soil temperatures will delay nodulation or restrict it to the subsurface region Strain adaptation to high temperature has also been reported by Hartel and Alexander and Karanja and Wood They attributed these losses in infectiveness to plasmid curing.
Heat treatment of R.