RHIZOBIUM: CHARACTERISTICS, MORPHOLOGY, HABITAT AND BENEFITS - BIOLOGY - 2024

Rhizobium is a genus of bacteria that have the ability to fix nitrogen from the atmosphere. In general, bacteria with the ability to fix nitrogen are known as rhizobia. These relationships between plants and microorganisms have been studied extensively.

These prokaryotes live in symbiotic relationships with different plants: legumes, such as beans, alfalfa, lentils, soybeans, among others.

Source: By Stdout, via Wikimedia Commons

They are specifically associated with its roots and provide the plant with the nitrogen they need. The plant, for its part, offers the bacteria a place of refuge. This close symbiotic relationship causes the secretion of a molecule called leghemoglobin. This symbiosis produces a significant proportion of N ₂ in the biosphere.

In this relationship, the bacterium causes the formation of nodules in the roots, which are differentiated by the so-called "bacteroides".

Most of the studies that have been carried out in this bacterial genus have only taken into account its symbiotic state and its relationship with the plant. For this reason, there is very little information related to the individual lifestyle of the bacterium and its function as a component of the soil microbiome.

characteristics

Bacteria of the genus Rhizobium are known primarily for their ability to fix nitrogen and establish symbiotic relationships with plants. In fact, it is considered one of the most dramatic relationships that exist in nature.

They are heterotrophic, which indicates that they must obtain their energy source from organic matter. Rhizobium grows normally under aerobic conditions and nodules form at a temperature of 25-30 ° C and an optimum pH of 6 or 7.

However, the nitrogen fixation process requires low concentrations of oxygen to protect nitrogenase (the enzyme that catalyzes the process).

To deal with the high amounts of oxygen, there is a protein similar to hemoglobin that is responsible for sequestering the oxygen that could intervene in the process.

The symbiotic relationships that these prokaryotes establish with legumes have a high ecological and economic impact, which is why there is extensive literature on this very specific relationship.

The infection process is not simple, it involves a series of steps where the bacteria and the plant mutually influence cell division activities, gene expression, metabolic functions and morphogenesis.

Infection process

These bacteria are excellent biological models for understanding the interactions that occur between microorganisms and plants.

Rhizobia are found in the soil, where they colonize the roots and enter the plant. Generally, colonization begins in the root hairs, although infection is also possible through small lysions in the epidermis.

When the bacterium manages to penetrate the interior of the plant, it usually stays for a time in the intracellular spaces of the plant. As the nodules develop, the rhizobia enter the cytoplasm of these structures.

Development and type of nodules

The development of the nodules involves a series of synchronous events in both organisms. Nodules are classified as determinate and indeterminate.

The former originate from cell divisions in the inner cortex and have a persistent apical meristem. They are characterized by having a cylindrical shape and two differentiated areas.

On the other hand, the determined nodules result from cell divisions in the middle or outer portion of the root cortex. In these cases there is no persistent meristem and its shape is more spherical. The mature nodule can develop by cell growth.

Bacteroide formation

In the nodule, differentiation into bacteroides occurs: the N _2- fixing form. Bacteroides, along with plant membranes, form the symbiosome.

In these microbe - plant complexes, the plant is responsible for providing carbon and energy, while the bacteria produce ammonia.

Compared with free-living bacteria, the bacterium undergoes a series of changes in its transcriptome, in its entire cellular structure and in metabolic activities. All these changes take place to adapt to an intracellular environment, where their only objective is nitrogen fixation.

The plant can take this nitrogen compound secreted by the bacteria and use it for the synthesis of essential molecules, such as amino acids.

Most Rhizobium species are quite selective in the number of hosts they can infect. Some species have only one host. In contrast, a small number of bacteria are characterized by being promiscuous and having a wide spectrum of potential hosts.

Attraction between rhizobia and roots

The attraction between bacteria and the roots of legumes is mediated by chemical agents, exuded by the roots. When the bacteria and the root are close, a series of events occur at the molecular level.

Root flavonoids induce nod genes in bacteria. This leads to the production of oligosaccharides known as LCO or nod factors. LCOs bind to receptors, formed by lysine motifs, in root hairs, thus initiating signaling events.

There are other genes - besides nod - involved in the symbiosis process, such as exo, nif, and fix.

Leghemoglobin

Leghemoglobin is a protein molecule, typical of the symbiotic relationship between rhizobia and legumes. As the name implies, it is quite similar to a more well-known protein: hemoglobin.

Like its blood analog, leghemoglobin has the distinction of having a high affinity for oxygen. As the binding process that occurs in the nodules is adversely affected by high concentrations of oxygen, the protein is responsible for retaining it to keep the system working properly.

Taxonomy

Approximately 30 species of Rhizobium are known, the best known being Rhizobium cellulosilyticum and Rhizobium leguminosarum. These belong to the Rhizobiaceae family, which is also home to other genera: Agrobacterium, Allorhizobium, Pararhizobium, Neorhizobium, Shinella, and Sinorhizobium.

The order is Rhizobiales, the class is Alphaproteobacteria, the Phylum Proteobacteria and the kingdom Bacteria.

Morphology

Rhizobia are bacteria that selectively infect the roots of legumes. They are characterized by being gram negative, have the ability to move and their shape reminds of a cane. Its dimensions are between 0.5 to 0.9 micrometers in width and 1.2 and 3.0 micrometers in length.

It differs from the rest of the bacteria that inhabit the soil by presenting two forms: the free morphology found in soils and the symbiotic form within its plant host.

Beyond the colony morphology and gram staining, there are other methods by which bacteria of the genus Rhizobium can be identified, these include nutrient utilization tests, such as the catalase, oxidase, and uses of carbon and nitrogen.

Similarly, molecular tests have been used for identification, such as the application of molecular markers.

Habitat

In general, rhizobia belonging to the Rhizobiaceae family exhibit the peculiarity of being associated mainly with plants of the Fabaceae family.

The Fabaceae family comprises legumes - grains, lentils, alfalfa, just to mention a few species known for their gastronomic value. The family belongs to the Angiosperms, being the third most numerous family. They are widely distributed in the world, ranging from the tropics to the arctic areas.

Only one species of non-legume plant is known to establish symbiotic relationships with Rhizobium: Parasponea, a genus of plants in the Cannabaceae family.

Furthermore, the number of associations that can be established between the microorganism and the plant depends on many factors. Sometimes the association is restricted by the nature and species of the bacteria, while in other cases it depends on the plant.

On the other hand, in their free form, bacteria are part of the natural flora of the soil - until the nodulation process occurs. Note that although legumes and rhizobia exist in the soil, the formation of nodules is not ensured, since the strains and species of the members of the symbiosis must be compatible.

Benefits and applications

Nitrogen fixation is a crucial biological process. It involves the uptake of nitrogen in the atmosphere, in the form of N ₂ and it is reduced to NH ₄ ⁺. Thus, nitrogen can enter and be used in the ecosystem. The process is of great importance in different types of environments, be it terrestrial, freshwater, marine or Arctic.

Nitrogen seems to be an element that limits, in most cases, the growth of crops and acts as a limiting component.

From a commercial point of view, rhizobia can be used as enhancers in agriculture thanks to their ability to fix nitrogen. For this reason, there is a trade related to the inoculation process of these bacteria.

The inoculation of the rhizobium has very positive effects on the growth of the plant, the weight and the number of seeds it produces. These benefits have been experimentally proven by dozens of studies with legumes.

References

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3. Jordan, DC (1962). The bacteroids of the genus Rhizobium. Bacteriological reviews, 26 (2 Pt 1-2), 119.
4. Leung, K., Wanjage, FN, & Bottomley, PJ (1994). Symbiotic characteristics of Rhizobium leguminosarum bv. trifolii isolates which represent major and minor nodule-occupying chromosomal types of field-grown subclover (Trifolium subterraneum L.). Applied and environmental microbiology, 60 (2), 427-433.
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