nodulation

Nodulation is the formation of specialized plant structures called nodules on the roots of certain plants, most notably legumes. These nodules host nitrogen-fixing bacteria, enabling a symbiotic relationship in which atmospheric nitrogen is converted to ammonia that the plant can use for growth and metabolism.

Nodulation occurs in legume species such as peas, beans, clovers, and soybeans, and in some non-legumes that partner with actinobacteria of the genus Frankia. Nodules can be categorized as determinate or indeterminate, which describes differences in their development and lifespan: determinate nodules are typically spherical and have a limited growth period, while indeterminate nodules maintain a persistent meristem and elongated structure.

The process begins when plant roots exude flavonoids that induce nod genes in compatible rhizobia. In response, rhizobia produce Nod factors that trigger root hair curling and the formation of an infection thread, through which the bacteria enter the root and migrate toward the cortex. Cortical cells divide to form a nodule primordium, and nodule organogenesis proceeds under the influence of plant hormones such as cytokinins and auxins, coordinating tissue development.

Within mature nodules, rhizobia differentiate into bacteroids that express the nitrogenase enzyme and fix atmospheric nitrogen, reducing N2 to ammonia. Leghemoglobin, a bright red protein in nodules, binds oxygen to maintain a low-oxygen environment necessary for nitrogenase while allowing respiration.

Nodulation is regulated by plant genetics and is influenced by soil nitrogen availability, pH, moisture, and temperature. It is exploited in agriculture through inoculants to improve natural nitrogen supply, reducing the need for synthetic fertilizers in leguminous crops.