Summary
Legume roots form nodules housing nitrogen-fixing rhizobia after exchanging signalling molecules.
In depth
Roots release flavonoids; rhizobia respond with Nod factors, causing root hair curling and infection thread formation. Nodule cells become bacteroids that fix nitrogen. Plants get ammonia while bacteria get sugars.
The plant limits nodule number by autoregulation. Leghaemoglobin gives nodules a pink colour and buffers oxygen. Nitrogen fixation costs about 16 ATP per N2, a heavy energy investment.
Example: alder trees use actinobacteria (Frankia) for nitrogen fixation.
The partners exchange signals: legumes secrete flavonoids that induce Nod genes in Rhizobium, which produce lipochitooligosaccharide Nod factors that are perceived by receptor kinases (NFR1/NFR5, LysM). A common symbiosis signalling pathway (shared with mycorrhizae, evolved earlier) triggers calcium spiking. Bacteroids in nodule cells, enclosed by a plant membrane, fix nitrogen using plant sugars, and the plant limits oxygen with leghaemoglobin, which gives nodules a pink colour.
Nitrogen fixation in nodules provides about 40 to 100 kg N per hectare per year. Plants regulate nodule number to control costs (autoregulation) and can punish ineffective strains. Non-legumes such as alder (Frankia) and Gunnera (Nostoc) also form symbioses.
Key idea: root nodules show how mutualism depends on signal exchange, with ammonia traded for carbon.
Frequently asked questions
What is Root Nodules and Symbiosis?
Legume roots form nodules housing nitrogen-fixing rhizobia after exchanging signalling molecules.
Which area of biology covers Root Nodules and Symbiosis, and how advanced is it?
Root Nodules and Symbiosis is one of the 56 topics in Plant Biology. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Roots, Algal Blooms and Plant Evolution, Photosynthesis.
Can you give an example of Root Nodules and Symbiosis?
Crop rotation with clover or beans improves soil.
What should I learn before Root Nodules and Symbiosis?
Work through the Beginner topics in Plant Biology first, such as Plant Kingdom, Photosynthesis, Chlorophyll, then return to Root Nodules and Symbiosis. Advanced topics assume the Intermediate vocabulary.
What topics are related to Root Nodules and Symbiosis?
The closest topics to Root Nodules and Symbiosis are Roots, Algal Blooms and Plant Evolution, Photosynthesis, Plant Defence Responses. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Root Nodules and Symbiosis?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Root Nodules and Symbiosis. To practise the whole of Plant Biology, use the Plant Biology quiz.
How is Root Nodules and Symbiosis different from Roots?
Root Nodules and Symbiosis: Legume roots form nodules housing nitrogen-fixing rhizobia after exchanging signalling molecules. Roots: Organs that anchor the plant and absorb water and minerals from the soil. Read both side by side and note what each one explains.
How should I study Root Nodules and Symbiosis step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Roots and explain the difference aloud. Finally, take the Plant Biology quiz and come back to this page for any question you miss.