Rhizobia

Rhizobia are bacteria that fix nitrogen in soil that aid in the growth and development of plants. Rhizobia can only fix nitrogen when associated with a plant that provides it with carbohydrates and are only associated with legumes, but not all legumes associate with rhizobia.

Taxonomy

Scientific Classification

Domain:	Bacteria
Phylum:	Pseudomonadota
Class:	Alphaproteobacteria
Order:	Hyphomicrobiales
Family:	Rhizobiaceae
Genus:	Rhizobium

Symbiotic Relationship with Legumes

The symbiosis between rhizobia and legumes begins with a signal exchange between the host and plant and its microsymbiont, where the host induces cell divisions to form root nodule primordia, and simultaneously initiates an infection process to deliver the bacteria into the nodule cells [4]. Root hairs then curl around the bacteria, forming infection threads that guide rhizobia into the root, where they are enclosed in membrane-bound structures called symbiosomes, where the bacteria transforms into nitrogen-fixing bacteria [4]. This symbiotic relationship is highly specific because no single rhizobial strain can form symbiosis with all legumes, and compatibility varies at species and genotypic levels [4].

Through their symbiotic relationship with legumes, they enhance the availability of nutrients by breaking down organic matter and releasing the nutrients into the soil [2]. Rhizobia and legumes work together to promote the growth of numerous plant species, thus strengthening the ecosystem [2].

Soil Benefits

Nitrogen is an important nutrient for plant growth and development, but plants do not have the ability to use the nitrogen in the atmosphere directly [2]. Rhizobium forms symbiotic relationships with legume plants by developing root nodules, where it converts the nitrogen in the atmosphere into a form that plants can absorb [2]. This process improves soil health by increasing nitrogen availability, contribute to increased soil microbial diversity, and improving soil structure.