The symbiosis between rhizobia and plants is a typical mutualistic model, with its core functions and derivative values as follows:

Core Function: Biological Nitrogen Fixation to Supply Nitrogen Nutrition    Approximately 78% of the atmosphere consists of nitrogen gas (N₂), which plants cannot directly utilize due to its inert nature. After invading plant roots, rhizobia differentiate into bacteroids within root nodules and synthesize nitrogenase. Nitrogenase can reduce atmospheric N₂ to ammonia (NH₃) in an oxygen-free environment; ammonia is further converted into ammonium salts or amino acids, directly providing nitrogen required for plant growth.

For leguminous plants, nitrogen fixed by rhizobia can meet 30%–80% of their growth demand, significantly reducing reliance on chemical nitrogen fertilizers.

Improving Soil Fertility and Promoting Ecological Cycles

After harvesting leguminous plants, residual roots and root nodules decompose in soil, releasing fixed nitrogen to increase soil nitrogen content. This provides a nitrogen source for subsequent crops (e.g., corn, wheat), achieving the effect of soil improvement. This nitrogen fixation also constitutes a key link in the nitrogen cycle of agricultural ecosystems.

Enhancing Plant Stress Resistance

During symbiosis with plants, rhizobia secrete various bioactive substances (e.g., auxins, cytokinins), which promote plant root development and enhance plant resistance to drought, salinity, diseases, and pests. For example, a well-developed root system enables plants to absorb soil water and minerals more efficiently.

Reducing Agricultural Production Costs and Environmental Risks

The production and application of chemical nitrogen fertilizers not only consume large amounts of energy but also may cause environmental issues such as soil compaction and water eutrophication. Utilizing the nitrogen fixation function of rhizobia can reduce fertilizer dosage, lower agricultural production costs, and mitigate ecological environmental pollution.

Specificity of Symbiotic Relationship

The symbiosis between rhizobia and plants is highly specific. For instance, Rhizobium japonicum can only form root nodules with a limited number of leguminous plants such as soybeans and wild soybeans, while Rhizobium leguminosarum is compatible with crops like peas and broad beans. This specificity is jointly regulated by flavonoids secreted by plant roots and nodulation factors on the surface of rhizobia.