Root nodule

A specialized swelling on a plant root that houses nitrogen-fixing bacteria. In legumes, its plant cells provide bacteria with a protected environment for converting atmospheric nitrogen into usable compounds.

Rhizobium: A group of soil bacteria that form nitrogen-fixing symbioses with legumes. These bacteria inhabit many legume nodules and reduce atmospheric nitrogen there.
Legume: A plant in the family Fabaceae, many members of which form nitrogen-fixing root nodules. Most familiar root nodules develop on legumes.
Nodulation: The formation of a nitrogen-fixing association between plants and microorganisms, often involving specialized organs. Root nodules are a prominent outcome of nodulation.
Biological nitrogen fixation: The production of biologically usable nitrogen compounds from atmospheric nitrogen by living organisms. Root nodules are major sites of biological nitrogen fixation in agriculture.
Arbuscular mycorrhiza: A symbiosis in which soil fungi form branched structures inside plant root cells and aid nutrient uptake. It is another root symbiosis, but it chiefly exchanges fungal nutrients for plant carbon.
Nitrogen fixation: The conversion of atmospheric nitrogen gas into chemically usable nitrogen compounds. Nodule bacteria perform this conversion, supplying nitrogen compounds to the host plant.
Root hair: A tubular extension of a root epidermal cell that increases the surface area for absorption. In many legumes, infection begins when bacteria enter a curled root hair.
Symbiosis signaling pathway: A conserved plant signaling network that coordinates cellular responses to beneficial microbial partners. It links perception of bacterial signals to the plant changes needed for nodulation.
Crop rotation: The practice of growing different crops in sequence on the same land. Rotating legumes can increase soil nitrogen available to subsequent crops.
Nodule-like structures: Root swellings that resemble symbiotic nodules but do not house nitrogen-fixing partners. Their appearance alone does not establish that a swelling fixes nitrogen.

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