Root nodules occur on the roots of plants (primarily Fabaceae

Fabaceae

The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...

) that associate with symbiotic nitrogen-fixing

Nitrogen fixation

Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

 bacteria. Under nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia

Rhizobia

Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...

. This process has evolved multiple times within the Fabaceae, as well as in other species found within the Rosid clade.

Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamin

Vitamin

A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s, flavones, and hormones.