Symbiotic effectiveness of indigenous arctic rhizobia on a temperate forage legume: Sainfoin (Onobrychis viciifolia)
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Leguminous plants possess the almost unique ability to enter symbiosis with soil-resident, nitrogen fixing bacteria called rhizobia. During this symbiosis, the bacteria physically colonize specialized organs on the roots of the host plant called nodules, where they reduce atmospheric nitrogen into forms that can be assimilated by the host plant and receive photosynthates in return. In order for nodule development to occur, there is extensive chemical cross-talk between both parties during the formative stages of the symbiosis. The vast majority of the legume family are capable of forming root nodules and typically rhizobia are only able to fix nitrogen within the context of this symbiotic association. However, many legume species only enter productive symbiosis with a few, or even single rhizobial species or strains, and vice-versa. Permitting symbiosis with only rhizobial strains that will be able to fix nitrogen with high efficiency is a crucial strategy for the host plant to prevent cheating by rhizobia. This selectivity is enforced at all stages of the symbiosis, with partner choice beginning during the initial communication between the plant and rhizobia. However, it can also be influenced even once nitrogen-fixing nodules have developed on the root. This review sets out current knowledge about the molecular mechanisms employed by both parties to influence host range during legume-rhizobia symbiosis.
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Many factors that contribute to alfalfa's excellence, but recognition must be given to the microsymbiont, the nodule bacteria that fix free nitrogen from the atmosphere. Early studies of the rhizobia:legume association revealed that there were many kinds of nodule bacteria and that various leguminous plants had their preference. Biochemical studies on N fixation in legume root nodules have been made with the soybean because it is an easy plant to culture, shows a marked response to effective rhizobia, and yields abundant nodule tissue. The principles elucidated are applicable to alfalfa and other nodulating leguminous plants. The number of rhizobia required for effective nodulation under any particular set of conditions depends greatly upon the type of inoculum employed. Inoculation is not considered necessary on the high-Ca soils of the West but is strongly recommended on the more acid soils of the Northwest USA.
Nodule (geology)
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Frankia
Actinorhizal plant
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Legume plants are able to establish root nodule symbioses with nitrogen-fixing bacteria, called rhizobia. Recent studies revealed that the root nodule symbiosis has co-opted the signaling pathway that mediates the ancestral mycorrhizal symbiosis that occurs in most land plants. Despite being unable to induce nodulation, rhizobia have been shown to be able to infect and colonize the roots of non-legumes such as rice. One fascinating question is whether establishment of such associations requires the common symbiosis (Sym) genes that are essential for infection of plant cells by mycorrhizal fungi and rhizobia in legumes. Here, we demonstrated that the common Sym genes are not required for endophytic colonization of rice roots by nitrogen-fixing rhizobia.
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Bradyrhizobium japonicum
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When nitrogen is limited, many legume plants form beneficial associations with nitrogen-fixing rhizobia. Efficient reduction of atmospheric nitrogen by rhizobial nitrogenase is restricted to plant nodules, inside which rhizobia establish persistent colonies of intracellular bacteroids. Of all known nodulating rhizobia, Sinorhizobium fredii strain NGR234 fixes nitrogen with >130 legumes. Genetic basis for such symbiotic promiscuity was linked to a 536 kb plasmid called pNGR234a. In this study, we aimed at downsizing pNGR234a to a core of essential loci sufficient to sustain proficient interactions with legumes. Hence, synthetic plasmids carrying loci needed for the flavonoid-dependent synthesis of basic nodulation factor structures (pMiniSym2) and functions required for nitrogen fixation (pMiniSym4) were constructed. When mobilized into a derivative of NGR234 cured of pNGR234a or into non-symbiotic soil bacteria, pMiniSym2 conferred nodulation and infection abilities to recipient strains. However, pMiniSym4 failed to confer a robust SNF, suggesting additional work is needed to secure SNF.
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This chapter contains sections titled: Introduction to Rhizobia and Symbioses Invasion of Plant Cells by Rhizobia and Arbuscular Mycorrhizae Plant Genes Involved in Symbiosis Initiation Symbiotic Signals Produced by Rhizobia Summary and Outlook References
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