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.

Temperate legumes like Pisum, Medicago, Trifolium, and Vicia develop a cylindrical shaped nodule that is called "indeterminate" because it 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: cell division is triggered in the root to create the nodule, and the root hair growth is redirected to wind around the bacteria multiple times until it fully encapsulates 1 or more bacteria. The bacteria encapsulated divide multiple times, forming a microcolony. From this microcolony, the bacteria enter the developing nodule through a structure called an infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into the root cortex; 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.

Plant physiology | Legumes | Nitrogen metabolism | Soil biology

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