The genus Azospirillum encompasses microorganisms that live together in association capable of externally and internally colonizing plant organs, including roots, part area and internal structures. They mainly influence the production of compounds of hormonal metabolism and biological nitrogen fixation.
Azospirillum is widespread in grasses, especially corn, wheat and sugarcane, but it has shown promising results when associated with Bradyrhizobium in the cultivation of legumes. These bacteria are found in diverse environments, even in extreme conditions. In general, they are organisms that develop under conditions of very low oxygen concentrations and are capable of biological nitrogen fixation.
They are bacteria capable of fixing atmospheric nitrogen, making it available to plants in the form of ammonium. The mineralization performed by these bacteria contributes with additional nitrogen inputs to the plants, partially supplying the amount necessary for the development of non-legume plants.
These bacteria promote growth by producing and releasing amino acids and polyamines that favor the root system and intensify the absorption of water and nutrients from plants, consequently improving parameters such as the measurement of stomata opening in leaves, water potential, water content in the apoplast, elasticity of cell wall and water stress tolerance.
In addition, the use of inoculation with Azospirillum has resulted in higher levels of photosynthetic pigments, essential for photosynthesis to occur. The different methods of growth promotion mediated by Azospirillum brasilense happen concomitantly in the plant and have reflected in an increase in the accumulation of biomass and productivity.
Pseudomonas fluorescens
The bacterium Pseudomonas fluorescens is commonly found in soil, water and/or plant surfaces and are organisms that survive on dead matter. It is a bacterium that needs oxygen, but some strains can utilize nitrate and perform anaerobic respiration (in the absence of oxygen).
They colonize the rhizosphere efficiently, promoting development and occupying sites of infection (competing with other microorganisms and occupying entry ports that would be used by pathogens), helping in biological control. They are producers of phytohormones, such as auxin and cytokinin, and mobilize iron in the soil. They are efficient phosphorus solubilizers, as they have the ability to produce organic acids and phosphatases.
The biosynthesis of auxin, cytokinin and gibberellin (phytohormones) mediated by Pseudomonas fluorescens promotes plant growth under favorable conditions or not for plant development, influencing the absorption of water and nutrients. Studies have indicated an increase in growth promotion responses in inoculated plants subjected to stress situations, such as salinity, positively influencing seed germination, seedling growth and establishment, root development and productivity.
Among these phytohormones, auxin affects all stages of plant growth and development, especially the root system, stimulating the formation of lateral and adventitious roots. The increase in auxin production in plants inoculated with P. fluorescens has reflected in the increase in soil exploitation by secondary roots, favoring access to water and nutrients, especially phosphorus.
Most of the phosphorus added as mineral fertilizer is lost naturally in the soil, limiting the efficiency of fertilization, making it necessary to increase the dose, impacting economically and environmentally. The bacterial species Pseudomonas fluorescens is one of the bacterial groups that promote plant growth present in the soil, highly effective in the solubilization of insoluble phosphates (present in the soil, but not available to plants), combining the production of organic acids (which reduce the pH of the soil). ) with the chelation of cations (which compete with phosphate for adsorption sites in the soil).
Studies have revealed that inoculation with Pseudomonas fluorescens acts on plant metabolism, positively affecting plant development in unfavorable conditions for growth. Under these conditions, plants are able to synthesize signaling molecules that damage cell membranes and reduce carbon assimilation rates (photosynthesis), reducing the vegetative cycle, affecting source-sink relationships, altering the composition of the compounds produced, among other damages. When under abiotic stress, these microorganisms synthesize exopolysaccharides and the enzyme ACC deaminase, which are key mechanisms of growth promotion.
Exopolysaccharides are polymers that accumulate water with reduced evapotranspiration loss, helping to maintain cell water when there is stress due to drought or salinity. In addition, they participate in the formation of biofilms, which is another strategy to assist in bacterial establishment in plants. When there is low water availability, there is an increase in the water potential around the roots, increasing the absorption of water and nutrients, reflecting in the dry matter. When there is salinity, exopolysaccharides can reduce the concentration of salts available to plants and make it difficult for them to be absorbed by the roots. The main types of exopolysaccharides produced by Pseudomonas are levan, marginalan, cellulose and alginate, and their proportion varies as a function of stress.
Ethylene synthesis resulting from stress in the plant organism has aminocyclopropane carboxylic acid (ACC) as its immediate precursor. Growth-promoting bacteria such as Pseudomonas produce the enzyme ACC deaminase, which sequesters the ACC molecule produced by the plant and produces ketobutyrate and ammonium, used in bacterial growth, decreasing the concentration of ethylene in plant tissues. This activity reflects positively on growth, biomass accumulation and plant protection, mitigating the inhibitory effects of ethylene.
The low natural availability of available iron in the soil combined with competition with microorganisms led to the adaptation of these organisms for the production of siderophores (organic compound that captures iron for microorganisms). These bacteria produce siderophores of high and low affinity, leading to the loss of elements from ferric ions, helping to establish the bacteria in the rhizosphere. It is a key mechanism in the efficient establishment of Pseudomonas in plant roots.
The Pseudomonas plant association increases iron absorption in poor soils, stimulates development and is advantageous for strains introduced into the rhizosphere that need to establish and defend their site of infection (rhizosphere competence).
Biotic and abiotic factors such as cell density, carbon and amino acid content, regulate the production of various metabolites by Pseudomonas, directly or indirectly, linked to the promotion of plant growth such as peptides, organic acids, antibiotics, siderophores and lytic enzymes.
