AZOSPIRILLUM: CHARACTERISTICS, HABITAT, METABOLISM - BIOLOGY - 2024

Azospirillum is a genus of free-living gram-negative bacteria capable of fixing nitrogen. It has been known for many years as a plant growth promoter, as it is a beneficial organism for crops.

Therefore, they belong to the group of plant growth-promoting rhizobacteria and have been isolated from the rhizosphere of grasses and cereals. From the point of view of agriculture, Azospirillum is a genus that has been widely studied for its properties.

By Frank Vincentz, from Wikimedia Commons

This bacterium is capable of using the nutrients excreted by plants and is responsible for fixing atmospheric nitrogen. Thanks to all these favorable characteristics, it is included in the formulation of biofertilizers to be applied in alternative agricultural systems.

Taxonomy

In 1925 the first species of this genus was isolated and it was called Spirillum lipoferum. It was not until 1978 when the genus Azospirillum was postulated.

Twelve species belonging to this bacterial genus are currently recognized: A. lipoferum and A. brasilense, A. amazonense, A. halopraeferens, A. irakense, A. largimobile, A. doebereinerae, A. oryzae, A. melinis, A. canadense, A. zeae and A. rugosum.

These genera belong to the order of Rhodospirillales and to the subclass of alphaproteobacteria. This group is characterized by believing with minute concentrations of nutrients and by establishing symbiotic relationships with plants, plant pathogenic microorganisms and even with human beings.

General characteristics and morphology

The genus is easily identified by its vibroid or thick rod shape, pleomorphism, and spiral mobility. They can be straight or slightly curved, their diameter is approximately 1 um and 2.1 to 3.8 in length. The tips are generally sharp.

Bacteria of the genus Azospirillum show evident motility, presenting a pattern of polar and lateral flagella. The first group of flagella is used primarily for swimming, while the second is related to movement on solid surfaces. Some species only have the polar flagellum.

This motility allows the bacteria to move to areas where the conditions are proper for their growth. In addition, they have chemical attraction towards organic acids, aromatic compounds, sugars and amino acids. They are also capable of moving into regions with optimal oxygen contractions.

When faced with adverse conditions - such as desiccation or a shortage of nutrients - the bacteria can take the forms of cysts and develop an outer covering composed of polysaccharides.

The genomes of these bacteria are large and have multiple replicons, which is evidence of the plasticity of the organism. Finally, they are characterized by the presence of poly-b-hydroxybutyrate grains.

Habitat

Azospirillum is found in the rhizosphere, some strains predominantly inhabit the surface of the roots, although there are some types capable of infecting other areas of the plant.

It has been isolated from different plant species throughout the world, from environments with tropical climates to regions with temperate temperatures.

They have been isolated from cereals such as corn, wheat, rice, sorghum, oats, from grasses such as Cynodon dactylon and Poa pratensis. They have also been reported in the agave and in different cacti.

They are not found homogeneously in the root, certain strains exhibit specific mechanisms to infect and colonize the interior of the root, and others specialize in the colonization of the mucilaginous portion or damaged cells of the root.

Metabolism

Azospirillum exhibits a very diverse and versatile carbon and nitrogen metabolism, which allows this organism to adapt and compete with the other species in the rhizosphere. They can proliferate in anaerobic and aerobic environments.

Bacteria are nitrogen fixers and can use ammonium, nitrites, nitrates, amino acids and molecular nitrogen as a source of this element.

The conversion of atmospheric nitrogen to ammonia is mediated by an enzyme complex composed of the protein dinitrogenase, which contains molybdenum and iron as a cofactor, and another protein portion called dinitrogenase reductase, which transfers electrons from the donor to the protein.

Similarly, the enzymes glutamine synthetase and glutamate synthetase are involved in the assimilation of ammonium.

Interaction with the plant

The association between the bacterium and the plant can occur successfully only if the bacterium is able to survive in the soil and find a significant population of roots.

In the rhizosphere, the decreasing gradient of nutrients from the root to its surroundings is generated by the exudates of the plant.

Due to the chemotaxis and motility mechanisms mentioned above, the bacteria are able to travel to the plant and use the exudates as a carbon source.

The specific mechanisms that the bacteria use to interact with the plant have not yet been fully described. However, certain genes in the bacterium are known to be involved in this process, including pelA, sala, salB, mot 1, 2 and 3, laf 1, etc.

Applications

Plant growth-promoting rhizobacteria, abbreviated PGPR for its acronym in English, comprise a group of bacteria that promote plant growth.

The association of bacteria with plants has been reported to be beneficial for plant growth. This phenomenon occurs thanks to different mechanisms, which produce nitrogen fixation and the production of plant hormones such as auxins, giberillins, cytokinins and absisic acid, which contribute to the development of the plant.

Quantitatively, the most important hormone is auxin - indoleacetic acid (IAA), derived from the amino acid tryptophan - and it is synthesized by at least two metabolic pathways within the bacteria. However, there is no direct evidence of the participation of auxin in increasing plant growth.

The giberillins, in addition to participating in growth, stimulate cell division and germination of the seed.

The characteristics of the plants inoculated by this bacterium include an increase in the length and number of laterally located roots, an increase in the number of root hairs, and an increase in the dry weight of the root. They also increase cellular respiration processes.

References

1. Caballero-Mellado, J. (2002). The genus Azospirillum. Mexico, D F. UNAM.
2. Cecagno, R., Fritsch, TE, & Schrank, IS (2015). The Plant Growth-Promoting Bacteria Azospirillum amazonense: Genomic Versatility and Phytohormone Pathway. BioMed Research International, 2015, 898592.
3. Gómez, MM, Mercado, EC, & Pineda, EG (2015). Azospirillum a rhizobacterium with potential use in agriculture. Biological Journal of DES Agricultural Biological Sciences Universidad Michoacana de San Nicolás de Hidalgo, 16 (1), 11–18.
4. Kannaiyan, S. (Ed.). (2002). Biotechnology of biofertilizers. Alpha Science Int'l Ltd.
5. Steenhoudt, O., & Vanderleyden, J. (2000). Azospirillum, a free-living nitrogen-fixing bacterium closely associated with grasses: genetic, biochemical and ecological aspects. FEMS microbiology reviews, 24 (4), 487–506.
6. Tortora, GJ, Funke, BR, & Case, CL (2007). Introduction to microbiology. Panamerican Medical Ed.