Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate
Legumes form a unique symbiotic relationship with bacteria known as rhizobia, which they allow to infect their roots. This leads to root nodule. The legumes and their association with Rhizobiumspp. in the broad sense have . Root hair curling (which involves entrapment of the bacteria) and infection . be an intimate connection between the rhizobial cell surface and the plant cell wall. .. of his research for treating the consequences of Parkinson's disease, stroke. Deficiency in mineral nitrogen often limits plant growth, and so symbiotic relationships The symbioses between Rhizobium or Bradyrhizobium and legumes are a the world's land surface can be categorized as having potential salinity problems Although the root nodule-colonizing bacteria of the genera Rhizobium and.
Common crop and forage legumes are peas, beans, clover, and soy. 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. 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.
Study examines the effects of exposing legumes to nitrogen fertilizer
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. It is used as a biocontrol treatment against crown gall disease in stone fruits, nut trees and roses. NOGALL is applied as an aqueous suspension to seeds, seedlings and cuttings before planting, and works by protecting wound sites from infection.
For effective action it has to be applied within two hours of damage caused by taking cuttings, repotting, lifting and planting techniques. The NOGALL bacteria belong to a non-disease causing species of the crown gall bacteria group and act as a biological control agent primarily due to the production of a neutral antibiotic called agrocin The key to the development of the strain KI was the removal of a small piece of DNA from the bacterium.
This prevents the transfer of the agrocin 84 genes to other soil bacteria, and hence prevents any immunity build-up against the protective nature of strain KI Crown gall disease causes hundreds of millions of dollars worth of damage to crops worldwide.
With the exceptional protective benefits of NOGALL, horticulturalists around the world can expect to obtain greatly improved crop returns and significant savings for consumers. How is transfer of a resistance plasmid to the pathogenic species of Agrobacterium prevented? Mutants producing higher yields of delta-endotoxins would give a more efficient and economical production of B. This in turn would be an advantage for the user as reduced volumes of pesticide formulation have to be stored and handled for a given acreage.
One of the immediate responses of rhizobia to water stress low water potential concerns the morphological changes. Mesquite Rhizobium and R.
Lecture 9 - microbial-plant relationships
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.
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. Rhizobial survival in soil exposed to high temperature is greater in soil aggregates than in nonaggregated soil and is favored by dry rather than moist conditions Ten inoculant strains of Rhizobium spp. High soil temperature could contribute to the frequency of noninfective isolates in soil; Segovia et al.
Heat shock proteins have been found in Rhizobium 1 but have not been studied in detail The synthesis of heat shock proteins was detected in both heat-tolerant and heat-sensitive bean-nodulating Rhizobium strains at different temperatures.
Heat-tolerant rhizobia are likely to be found in environments affected by temperature stress. Rhizobia isolated from the root nodules of Acacia senegal and Prosopis chilensis, growing in hot, dry regions of Sudan, had high maximum growth temperatures The same authors found that temperature stress consistently promoted the production of a protein with a relative mobility of 65 kDa in four strains of tree legume rhizobia.
The kDa protein that was detected under heat stress was heavily overproduced. This protein was not overproduced during salt or osmotic stresswhich indicates that it is a specific response to heat stress.
Soil Acidity and Alkalinity Soil acidity is a significant problem facing agricultural production in many areas of the world and limits legume productivity 416573 Most leguminous plants require a neutral or slightly acidic soil for growth, especially when they depend on symbiotic N2 fixation 41 It has been recently reportedthat pasture and grain legumes acidify soil to a greater extent and that the legume species differ in their capacity to produce acids.
Legumes and their rhizobia exhibit varied responses to acidity. Some species, like lucerne M. Soil acidity constrains symbiotic N2 fixation in both tropical and temperate soilslimiting Rhizobium survival and persistence in soils and reducing nodulation 47, Rhizobia with a higher tolerance to acidity have been identified These strains usually but not always perform better under acidic soil conditions in the field It has been found that R.
Strains of a given species vary widely in certain cases in their pH tolerance. The fast-growing strains of rhizobia have generally been considered less tolerant to acid pH than have slowly growing strains of Bradyrhizobiumalthough some strains of the fast-growing rhizobia, e. Recent reports, however, support the existence of acid-tolerant fast-growing strains, since both fast- and slow-growing strains of Vigna unguiculata which are tolerant to pH values as low as 4.
The basis for differences in pH tolerance among strains of Rhizobium and Bradyrhizobium is still not clear 73, although several workers have shown that the cytoplasmic pH of acid-tolerant strains is less strongly affected by external acidity 6062, Aarons and Graham 1 reported high cytoplasmic potassium and glutamate levels in acid-stressed cells of R. Differences in LPS composition, proton exclusion and extrusion 6062accumulation of cellular polyaminesand synthesis of acid shock proteins have been associated with the growth of cells at acid pH.
The composition and structure of the outer membrane could also be a factor in pH tolerance Studies on the genetic basis of tolerance to low pH suggest that at least two loci of either megaplasmid or chromosomal location for pH genes are necessary for the growth of rhizobia at low pH 60 — Acid tolerance in R.
The expression of these proteins increased when the cells were grown at pH 4. The same authors 73 suggested that acid tolerance in R.
The failure of legumes to nodulate under acid-soil conditions is common, especially in soils of pH less than 5.
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The inability of some rhizobia to persist under such conditions is one cause of nodulation failure 3055, but poor nodulation can occur even where a viable Rhizobium population can be demonstrated Despite this, elevated inoculation levels have enhanced the nodulation response under acidic conditions in some studies The growth, nodulation, and yield of V. It appears that the pH-sensitive stage in nodulation occurs early in the infection process and that Rhizobium attachment to root hairs is one of the stages affected by acidic conditions in soils 54 They suggested that colonization of soils and soybean roots by B.
The host cultivar-rhizobial strain interaction at acid pH has also been investigated. Vargas and Graham examined the cultivar and pH effects on competition for nodule sites between isolates of Rhizobium in beans P. They found a significant effect of host cultivar, ratio of inoculation, and pH on the percentage of nodule occupancy by each strain. However, it has been suggested that only one of the symbionts needed to be acid tolerant for good nodulation to be achieved at pH 4.
Inoculation of Medicago polymorpha by an acid-tolerant R. The performance of the R. Rhizobia appear to be vary in their symbiotic efficiency under acidic conditions. Van Rossum et al. Acid-tolerant alfalfa-nodulating strains of rhizobia, isolated from acidic soils, were able to grow at pH 5. The results also demonstrate the complexity of the rhizobial populations present in the acidic soils, represented by a major group of nitrogen-fixing rhizobia and a second group of ineffective and less predominant isolates.
The host legume appears to be the limiting factor for establishing Rhizobium-legume symbiosis under acidic conditions. Legume species differ greatly in their response to low pH with regard to growth and nodulation Recently, it has been found that the amount of N2 fixed by forage legumes on low-fertility acidic soil is dependent on legume growth and persistence However, selection of acid-tolerant rhizobia to inoculate legume hosts under acidic conditions will ensure the establishment of the symbiosis and also successful performance 73,