what is rhizobium and there importance?
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Answer:
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of legumes and Parasponia.
The bacteria colonize plant cells within root nodules, where they convert atmospheric nitrogen into ammonia using the enzyme nitrogenase and then provide organic nitrogenous compounds such as glutamine or ureides to the plant. The plant, in turn, provides the bacteria with organic compounds made by photosynthesis. This mutually beneficial relationship is true of all of the rhizobia, of which the genus Rhizobium is a typical example.
Answer:
Signaling and Host Range in Rhizobium–Legume Symbioses
Rhizobium–legume symbioses are of great ecological and agronomic importance, due to their ability to fix large amounts of atmospheric nitrogen. These symbioses result in the formation on legume roots of differentiated organs called nodules, in which the bacteria reduce nitrogen into ammonia used by the host plant. Infection of legumes by rhizobia generally involves the curling of root hairs, formation of infection threads within root hairs and the root cortex, and induction of a meristem in the inner root cortex, giving rise to the nodule. An important feature of rhizobium–legume symbioses is their specificity: each rhizobium has a defined host range varying from a few legume genera to more than a hundred. For example Sinorhizobium meliloti nodulates only Medicago, Melilotus, and Trigonella species, while Rhizobium sp. NGR234 nodulates plants in more than 110 legume genera. Genetic analysis of nodulation in several rhizobium species has identified a number of nodulation (nod) genes which specify host range, infection, and nodule formation. Some of these genes such as nodD and nodABC are present in all rhizobia, while others, called host-specific nod genes, are found in various combinations in the different rhizobium species. The nod genes control an exchange of signals between the rhizobium and its host plant. The regulatory nodD genes, in the presence of flavonoid plant signals, activate the expression of the other (structural) nod genes, which are involved in the synthesis and excretion of extracellular signals, called Nod factors, which are specifically active on host plants.
Potential of Management Practices and Amendments for Preventing Nutrient Deficiencies in Field Crops under Organic Cropping Systems
Sukhdev S. Malhi, ... Kabal S. Gill, in Agricultural Sustainability, 2013
5.3.1 Rhizobium, Azospirillum, Azotobacter, Azolla, and Blue Green Algae for Biological N Fixation
Rhizobium inoculation of legume crops is commonly practiced in most countries, and the importance of using rhizobial inoculants specific to the legume being planted is well established. Rhizobium inoculation has shown beneficial effects on nodulation, grain yield, and protein content of peas (McKenzie et al., 2001; Kutcher et al., 2002). In warm and humid regions, inoculation of Azospirillum, Azotobacter, blue green algae (BGA), and Azolla (a water fern) was found to increase N supply through BNF in rice-based cropping systems (Singh, 1981). Azolla is used as green manure in rice production systems in many Asian countries.
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