Symbiosis between bacteria and some plants, leading to nodulous swellings of the roots.
Some plants (in particular some members of the fabaceae) are forming nodule-like root deformations after contact with bacteria from the genus rhizobium. The bacteria are colonizing the nodules and are fixing atmospheric nitrogen there (this way transforming it into a form usable by plants). In exchange, the bacteria obtain carbohydrates from plant photosynthesis.