2.2: Root Nodules

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Several different groups of prokaryotes form mutualistic relationships with plant roots. Nitrogen is an essential nutrient for plants, yet it is difficult to obtain in many ecosystems. Though it is abundant in our atmosphere, this form a nitrogen (N₂) is triple-bonded to itself, a bond which most organisms cannot break. However, certain bacteria have an enzyme called nitrogenase that can break the triple bond and convert nitrogen into usable forms for plants, such as ammonia (NH₃). These bacteria can be found free-living in the environment or in mutualistic relationships with certain plants. A common relationship between plants and these nitrogen-fixing bacteria is the formation of root nodules--swellings in the plant roots that connect to the vascular tissue, allowing for the exchange of sugars and nutrients between the two different organisms.

Root Nodules in Legumes

Plants in the bean family (Fabaceae) form mutualistic relationships in the form of root nodules with nitrogen-fixing bacteria in the order Rhizobiales.

A network of roots, pulled from the ground, covered in small tan orbs. — Figure \(\PageIndex{1}\): Nodules formed on the roots of Robinia provide a safe place and plenty of sugars for the nitrogen-fixing *Rhizobium* bacteria living inside. Photo by Thomas Koffel, CC-BY-NC.

A single pinkish nodule, shaped a bit like a bean, removed from the root system — Figure \(\PageIndex{2}\): Root nodules from a leguminous plant. In the first image, several amorphous nodules can be seen emerging from the roots. In the second image, one of these nodules has been removed. The nodules appear pinkish due to the activity of leghemoglobin transporting oxygen away from the nodule, indicating that the enzyme nitrogenase is actively fixing nitrogen inside. Photos by Olive Long, CC-BY-NC.

Root Nodules in Alder

Frankia is a genus of bacteria that grows filamentously, called an actinomycete. Root nodules present on alder and few other groups of woody plants contain Frankia.

Frankia nodules on living alder roots — Figure \(\PageIndex{3}\): The image on the left shows excavated roots from a living alder tree (*Alnus rubra*). There are many yellow-orange nodules in clusters on the roots where *Frankia* is actively fixing nitrogen. In the image on the right, a dead alder tree is washed up on the beach in California. The arrow is indicating clusters of root nodules that were formed when the tree was alive and housed nitrogen-fixing *Frankia*. Photos by Maria Morrow CC BY-NC.