Rhizobium soil bacteria

B. D. H. Jarvis, L. J. H. Ward, and E. A. Slade, Expression by soil bacteria of nodulation genes from Rhizobium leguminosarum biovar trifolii. Appl. Environ. Microbiol. 55 1426-1434 (1989). 

Nitrogen fixation is the conversion of N2 gas into ammonia, a process carried out by some soil bacteria, cyanobacteria and the symbiotic bacteria Rhizobium that invade the root nodules of leguminous plants. This process is carried out by the nitrogenase complex, which consists of a reductase and an iron-molybdenum-containing nitrogenase. At least 16 ATP molecules are hydrolyzed to form two molecules of ammonia. Leghemoglobin is used to protect the nitrogenase in the Rhizobium from inactivation by 02. 

Nitrogen undergoes a complicated series of cyclic pathways in the ecosystem (Fig. 8.2). The atmospheric form of free nitrogen must be fixed —incorporated into chemical compounds (e.g., NH3) which can be utilized by plants. This nitrogen fixation can be accomplished by bacterial action of both free-living soil bacteria such as azotobacter and chlostridium and symbiotic bacteria such as rhizobium. It can also be... 

Asummary of the nitrogen cycle is shown in Fig. 3.17. Nitrogen fixation, which converts N2 into NH4+, is performed by a restricted number of eubacteria and ar-chaebacteria (sometimes in symbiotic associations). Anaerobes such as the photosynthetic bacteria, together with some species of Clostridium and sulphate reducers, can perform the process. However, the most important nitrogen-fixing bacteria are aerobes the cyanobacteria (e.g. Oscillatoria) in aquatic environments and Rhizobium in plant root nodules in soils (free-living soil bacteria such as Azotobacterare... 

In the case of agricultural applications it may be possible to improve on the natural functions of certain bacterial polysaccharides. At Norwich studies have concentrated on the possible biological and ecological roles played by the EPS secreted by the soil bacteria Agrobacterium and Rhizobium. 

Agrobacterium tumefaciens and Rhizobium leguminosarium are cellulose-producing soil bacteria. They bind to plant roots in two steps. In the first step, bacterial proteins bind to plant lectins and acidic surface polymers. Although this is an essential step, the binding is very weak. In the second step, bacterial cellulose fibrils firmly attach the bacterial cells to the plant cell walls. Notably, cellulose-deficient mutants of A. tumefaciens are far less infectious than the wild type. Cellulose production in R. leguminosarium is induced by plant contact, and it has been suggested that this contact induces the bacteria to produce a cellulose... 

Since most soils contain numerous different bacteria, the host plant and rhizobia must have mechanisms that allow homologous rhizobial strains to penetrate and subsequently develop nodules, whereas heterologous strains and other soil bacteria are not allowed entry (Vance, 1983). Processes that regulate this specificity are thought to occur prior to, or upon, initial contact of the legume host root hair and Rhizobium (Bauer, 1981 Downie and Johnston, 1988 Rolfe and Gresshoff, 1988). 

Soil-borne bacteria of the family Rhizobiaceae and leguminous plants form a symbiotic relationship during which a new organ, the root nodule, is developed. Within these root nodules the bacteria fix atmospheric dinitrogen and the product of nitrogen fixation, ammonia, is exported to the plant [69,70]. Root nodules develop from primordia which are established at specific sites in the root cortex shortly after Rhizobium infection. The peptide enod40 is believed to play a critical role in inducing the de-differentiation and the mitotic division of root cortical cells, i.e. the initial steps in nodule development. This however, is not entirely undisputed [3,4,69-72]. 

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