Root nodules occur on the roots of plants that associate with symbiotic bacteria.

Under nitrogen limiting conditions plants from the pea family Fabaceae form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia.

Two main types of nodule have been described.

cross section though a soybean (Glycine max.Essex) root nodule
The bacteria, Bradyrhyzobium japonicum, infects the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteriods within their symbiosomes. In his image, you can also see endoplasmic reticulum, dictysome and cell wall<

Temperate legumes like Pisum, Medicago, Trifolium, and Vicia develop a cylindrical shaped nodule that is called indeterminate because maintains an active apical meristem that produces new cells for growth over the life of the nodule.

Tropical (sub) legumes from the genera Glycine, Phaseolus, Lotus, and Vigna form determinate nodules that lose meristematic activity shortly after initiation, growth is due to cell expansion and mature nodules are spherical in shape.

Legumes release compounds called flavonoids from their roots, which trigger the production of nod factors by the bacteria. When the nod factor is sensed by the root a number of biochemical and morphological changes happen and cell division is triggered in the root to create the nodule. The bacteria enter the developing nodule through a structure called an infection thread, then they are then surrounded by a plant derived membrane and differentiate into bacteroids that fix nitrogen.

Root nodules that occur on non-legume genera like Parasponia in association with Rhizobium bacteria and those that arise from symbiotic interactions with Actinobacteria Frankia in some plant genera such as Alnus vary significantly from those formed in the legume-rhizobia symbiosis. In these symbioses the bacteria are never released from the infection thread.