There is a clear need to better understand the legume-rhizobium symbiosis both in the wild and in agriculture. Finally, it is important to note that the legume-rhizobium symbiosis is not unique. Other plants such as alders can also form nitrogen-fixing symbioses with bacteria; moreover, Sturz, et al. Cleveland, C. Townsend, D. Schimel, et al. Global patterns of terrestrial biological nitrogen N2 fixation in natural ecosystems.
Global Biogeochemical Cycles — This expansive meta-analysis of biological nitrogen fixation discusses the global importance and biome specificity of natural sources of nitrogen fixation. Denison, R. Legume sanctions and the evolution of symbiotic cooperation by rhizobia. American Naturalist — DOI: Denison offered the first general description of host sanctions in this system and provides key information about the symbiosis in the language of evolutionary biologists and ecologists.
Lifestyle alternatives for rhizobia: Mutualism, parasitism, and forgoing symbiosis. Unlike previous work on this topic, this paper explores the different outcomes of symbiosis from the point of view of the bacterium and highlights the diverse ecological contexts that challenge rhizobial bacteria. Ferguson, B.
Indrasumunar, S. Hayashi, et al. Molecular analysis of legume nodule development and autoregulation. To address this hypothesis, they are using a tagged mutant library prepared from TONO and screening for mutants that can form effective nodules on apn1 plants. Genome alignments and expression analyses of the various M. References Hakoyama, T. Nature, , — This explains why the protein-rich legumes lack behind in yield potential compared to cereals.
Production of proteins require 2. The seed biomass production efficiency of legumes is shown to be lower 0. Thus, not only the BNF requires energy, but also it acts as a limiting factor to yields when conditions are uncongenial. Nitrogenase, a major enzyme involved in the nitrogen fixation has 2 components: 1 dinitrogenase reductase, the iron protein and 2 dinitrogenase metal cofactor. The iron protein provides the electrons with a high reducing power to dinitrogenase which in turn reduces N2 to NH3.
Depending on the availability of metal cofactor, three types of N fixing systems have been identified 1 Mo-nitrogenase 2 V-nitrogenase and 3 Fe-nitrogenase. This coexistence enables the induction of nodules depending on plant species.
Although BNF is an energy expensive process, it is the only process through which the atmospheric N is converted to plant usable organic N making the greatest quantitative impact on N cycle. However, various environmental factors limit nitrogen fixation, such as soil moisture deficiency, osmotic stress, extremes of temperature, soil salinity, soil acidity, alkalinity, nutrient deficiency, overdoses of fertilizers and pesticides; since all these soil and environmental factors affect the survival and infectivity rate of rhizobia—an important driver for BNF Zahran Recent research is focused to identify rhizobial strains with resistance to these environmental stresses and explore their potentiality under field conditions.
Details of such rhizobia have been discussed in later parts of this review. Nitrification is an important process in nitrogen cycle in which ammonia is converted to nitrite and nitrate by nitrifying bacteria such as Nitrosomonas and Nitrobacter. Therefore, a reduced rate or inhibition of nitrification provides enough time to plant for assimilation of fixed N. Plants also produce secondary metabolites such as phenolic acids and flavonoids for inhibiting nitrification.
The natural ability of plants to suppress nitrification is not currently recognized or utilized in agricultural production Subbarao et al. However, they have no effects on other soil microbial community. For example, it had been demonstrated that nitrification inhibitor produced by B. This work also demonstrated that, this inhibitory effect vary with the soil type. Nitrification and denitrification remain to be the only known biological processes that generate nitrous oxide N2O , a powerful greenhouse gas contribute to global warming.
Biological nitrification inhibition is seen as a major mitigation process towards global warming besides improving N recovery and N use efficiency of agricultural systems Subbarao et al.
Phosphate solubilizers After nitrogen, phosphorus P is the most limiting nutrient for plant growth. It exists in both inorganic bound, fixed, or labile and organic bound forms and the concentration depends on the parental material. Although the parent material has a strong control over the soil P status of terrestrial ecosystems Buol and Eswaran , the availability of P to plants is influenced by pH, compaction, aeration, moisture, temperature, texture and organic matter of soils, crop residues, extent of plant root systems and root exudate secretions and available soil microbes.
Although the P fertilizer provides the plants with available form of P, excessive application of them is not only expensive, but also damaging to environment. Phosphorus accounts for about 0. The soil solution remains to be the main source of P supply to plants. The P content of agricultural soil solutions are typically in the range of 0. The rest must be obtained from the solid phase through intervention of biotic and abiotic processes where the phosphate solubilizing activity of the microbes has a role to play Sharma et al.
Rhizobia, including R. These bacteria synthesize low molecular organic acids which acts on inorganic phosphorous. For instance, 2-ketogluconic acid with a phosphate-solubilizing ability has been identified in R. Some bacterial strains are found to possess both solubilization and mineralization capacity Tao et al. Importance of this P solubilizing capacity in enhancing plant growth by M.
Siderophore formation Iron, a typical essential plant micronutrient, is present in soils ranging from 0. Under aerobic environments, iron exists as insoluble hydroxides and oxyhydroxides, which are not accessible to both plants and microbes Rajkumar et al. The siderophores are water soluble and are of two types viz. Siderophores can also form stable complex with heavy metals such as Al, Cd, Cu etc.
Thus, the siderophore producing bacteria can relieve plants from heavy metal stress and assist in iron uptake. Rhizobial species, such as R. These include indoleacetic IAA acid auxin , cytokinins, gibberellins and abscisic acid. IAA is involved in cell division, differentiation and vascular bundle formation and an essential hormone for nodule formation. The salient ones are A. Afzal and Bano ; Antoun et al. IAA production in rhizobium takes place via indolepyruvic acid and indoleacetic aldehyde pathway.
On inoculation of R.This coexistence enables the induction of nodules depending on plant species. Although some biologists hold great hopes for the use of rhizobia to enhance legume production, this promise still remains critical thinking ibguides unrealized. The movement of abscisic acid in plants does not exhibit polarity like auxins Walton and Li It had also been shown that the rhizobia producing ACC deaminase are also efficient nitrogen fixers. Here, we begin by inferring the evolutionary history of legume and rhizobial lineages to better understand the diversity of both partners. These include indoleacetic IAA acid auxin , cytokinins, gibberellins and abscisic acid.
Biocontrol abilities of rhizobia Biocontrol is a process through which a living organism limits the growth or propagation of undesired organisms or pathogens. A, Transverse section of a fava bean nodule showing GFP-labeled Klebsiella bacteria within nodule cells. However, N fixation efficiency of legumes varies, and depends on the host genotype, rhizobial efficiency, soil conditions, and climatic factors. DOI: Strains, such as R.
An integrative ecological and evolutionary perspective is useful in the study of the legume-rhizobium symbiosis.
On inoculation of R.
These mechanisms help rhizobia to execute their beneficial PGP traits under stress conditions.
Xanthomonas maltophilia in combination with Mesorhizobium had been shown to enhance plant growth and productivity in chickpea. These mechanisms help rhizobia to execute their beneficial PGP traits under stress conditions. These bacteria live in the soil, and when a legume grows nearby a molecular communication ensues that enables the legume roots to become infected.
These mechanisms help rhizobia to execute their beneficial PGP traits under stress conditions. We review the selective forces that maintain cooperation between symbionts and hosts and explore the utility of evolutionary theory for optimizing agricultural productivity.
Schimel, et al. Recent advancements in the taxonomic research with the aid of specific molecular tools are another reason. This broad view might ultimately enable us to better manipulate the interaction to optimize agricultural productivity. Also, rhizobia could represent a massive boon to agriculture, since they might allow us to avoid the costly process of industrial nitrogen fixation.