points of simbiotic nitrogen fixation
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Symbiotic nitrogen fixation occurs in plants that harbor nitrogen-fixing bacteria within their tissues. The best-studied example is the association between legumes and bacteria in the genus Rhizobium.
Each of these is able to survive independently (soil nitrates must then be available to the legume), but life together is clearly beneficial to both. Only together can nitrogen fixation take place.
A symbiotic relationship in which both partners benefits is called mutualism.
Link to discussion of the role of nitrogen fixation in the nitrogen cycle.
Rhizobia
Rhizobia are Gram-negative bacilli that live freely in the soil (especially where legumes have been grown). However, they cannot fix atmospheric nitrogen until they have invaded the roots of the appropriate legume.
The Infection Thread
The interaction between a particular strain of rhizobia and the "appropriate" legume is mediated by:
a "Nod factor" secreted by the rhizobia and
transmembrane receptors on the cells of the root hairs of the legume.
Different strains of rhizobia produce different Nod factors, and
different legumes produce receptors of different specificity.
Each of these is able to survive independently (soil nitrates must then be available to the legume), but life together is clearly beneficial to both. Only together can nitrogen fixation take place.
A symbiotic relationship in which both partners benefits is called mutualism.
Link to discussion of the role of nitrogen fixation in the nitrogen cycle.
Rhizobia
Rhizobia are Gram-negative bacilli that live freely in the soil (especially where legumes have been grown). However, they cannot fix atmospheric nitrogen until they have invaded the roots of the appropriate legume.
The Infection Thread
The interaction between a particular strain of rhizobia and the "appropriate" legume is mediated by:
a "Nod factor" secreted by the rhizobia and
transmembrane receptors on the cells of the root hairs of the legume.
Different strains of rhizobia produce different Nod factors, and
different legumes produce receptors of different specificity.
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