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Leghemoglobin is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin. Leghemoglobin has close chemical and structural similarities to hemoglobin, and, like hemoglobin, is red in colour. It was originally thought that the heme prosthetic group for plant leghemoglobin was provided by the bacterial symbiont within symbiotic root nodules. However, subsequent work shows that the plant host strongly expresses heme biosynthesis genes within nodules, and that activation of those genes correlates with leghemoglobin gene expression in developing nodules.
Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are gram negative, motile, non-sporulating rods.
Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia. This process has evolved multiple times within the legumes, as well as in other species found within the Rosid clade. Legume crops include beans, peas, and soybeans.
Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen (N2); they must use nitrogen compounds such as nitrates.
Rhizobacteria are root-associated bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). They are an important group of microorganisms used in biofertilizer. Biofertilization accounts for about 65% of the nitrogen supply to crops worldwide. Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature. PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and endophytic. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming root nodules. The dominant species found in the rhizosphere is a microbe from the genus Azospirillum. Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.
A biofertilizer is a substance which contains living micro-organisms which, when applied to seeds, plant surfaces, or soil, colonize the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. Biofertilizers add nutrients through the natural processes of nitrogen fixation, solubilizing phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. The microorganisms in biofertilizers restore the soil's natural nutrient cycle and build soil organic matter. Through the use of biofertilizers, healthy plants can be grown, while enhancing the sustainability and the health of the soil. Biofertilizers can be expected to reduce the use of synthetic fertilizers and pesticides, but they are not yet able to replace their use. Since they play several roles, a preferred scientific term for such beneficial bacteria is "plant-growth promoting rhizobacteria" (PGPR).
Bradyrhizobium japonicum is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacteria. The species is one of many Gram-negative, rod-shaped bacteria commonly referred to as rhizobia. Within that broad classification, which has three groups, taxonomy studies using DNA sequencing indicate that B. japonicum belongs within homology group II.
Sinorhizobium fredii is a nitrogen fixing bacteria of the genus Sinorhizobium. It is a fast-growing root nodule bacteria. Sinorhizobium fredii exhibit a broad host-range and are able to nodulate both determinant hosts such as soy as well as indeterminate hosts including the pigeon pea. Because of their ease of host infection there is interest in their genetics and the symbiotic role in host infection and nodule formation.
Bradyrhizobium arachidis is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacterium. It was first isolated from Arachis hypogaea root nodules in China. Its type strain is CCBAU 051107T.
Bradyrhizobium canariense is a species of legume-root nodulating, endosymbiont nitrogen-fixing bacterium. It is acid-tolerant and nodulates endemic genistoid legumes from the Canary Islands. The type strain is BTA-1T.
Bradyrhizobium yuanmingense is a species of legume-root nodulating, endosymbiont nitrogen-fixing bacterium, associated with Lespedeza and Vigna species. Its type species is CCBAU 10071(T).
Bradyrhizobium iriomotense is a species of legume-root nodulating, endosymbiont nitrogen-fixing bacterium, first isolated from Entada koshunensis. The type strain is EK05T.
Bradyrhizobium betae is a species of legume-root nodulating, microsymbiotic nitrogen-fixing bacterium first isolated from the roots of Beta vulgaris, hence its name. It is slow-growing an endophytic. The type strain is PL7HG1T.
Bradyrhizobium huanghuaihaiense is a bacterium from the genus of Bradyrhizobium.
Sinorhizobium medicae is a species of gram-negative, nitrogen-fixing, rod-shaped bacteria. They can be free-living or symbionts of leguminous plants in root nodules. S.medicae was first isolated from root nodules on plants in the genus Medicago. Some strains of S.medicae, like WSM419, are aerobic. They are chemoorganotrophic mesophiles that prefer temperatures around 28 °C. In addition to their primary genome, these organisms also have three known plasmids, sized 1,570,951 bp, 1,245,408 bp and 219,313 bp.
Streptomyces harbinensis is a bacterium species from the genus of Streptomyces which has been isolated from roots of the soybean Glycine max in Harbin in the Heilongjiang province in China. Streptomyces harbinensis produces ikarugamycin
Streptomyces heilongjiangensis is a bacterium species from the genus of Streptomyces which has been isolated from the root surface of the soybean Glycine max in Hulin in the Heilongjiang province in China. Streptomyces heilongjiangensis produces borrelidin.
Bradyrhizobium ottawaense is a nitrogen fixing bacterium from the genus of Bradyrhizobium which has been isolated from the nodules of soybeans in Ottawa in Canada.
Sphingobacterium yanglingense is a Gram-negative and non-motile bacterium from the genus of Sphingobacterium which has been isolated from the root nodule surface of a soybean in Yangling in China.
Microbacterium shaanxiense is a Gram-positive and non-motile bacterium from the genus of Microbacterium which has been isolated from the surface of a root nodule from a soybean Glycine max.
References
- ↑ Xu, L. M.; Ge, C.; Cui, Z.; Li, J.; Fan, H. (1995). "Bradyrhizobium liaoningense sp. nov., Isolated from the Root Nodules of Soybeans". International Journal of Systematic Bacteriology. 45 (4): 706–711. doi: 10.1099/00207713-45-4-706 . ISSN 0020-7713. PMID 7547289.
