Final answer:
Root nodules in members of the pea family form due to bacteria in the genus Rhizobium, specifically through a symbiotic interaction with Bradyrhizobium japonicum, which fix nitrogen in exchange for food and shelter.
Step-by-step explanation:
Root nodules form in members of the pea family in response to bacteria in the genus Rhizobium.
These nodules are novel structures on the roots of certain plants, specifically legumes, which include peas, soybeans, clover, alfalfa, and beans.
The nodules form as a result of a symbiotic interaction between the plant and soil bacteria, where nitrogen fixation occurs.
Nitrogen-fixing bacteria, such as Bradyrhizobium japonicum, live in these root nodules and carry out the chemical work of reducing nitrate to ammonia, providing the host plants with essential nutrients for growth.
In exchange, these bacteria receive food and a protected habitat within the nodules.