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Plant Growth–Promoting Rhizobacteria (PGPR) and Bioremediation of Industrial Waste
Published in Ram Chandra, R.C. Sobti, Microbes for Sustainable Development and Bioremediation, 2019
Sangeeta Yadav, Kshitij Singh, Ram Chandra
Azotobacteraceae is the most representative of bacterial genera able to perform free nitrogen fixation. The effect of Azotobacter and Azospirillum is attributed not only to the amounts of fixed nitrogen but also to the production of plant growth regulators [indole acetic acid, gibberellic acid (GA), cytokinins, and vitamins], which result in additional positive effects to the plant. Application of inoculants in agriculture has resulted in notable increases in crop yields, especially in cereals, where Azotobacter chroococcum and Azospirillum brasilense have been very important. These two species include strains capable of releasing substances such as vitamins and plant growth regulators, which have a direct influence on plant growth. The amount of nitrogen from free fixation available to the plant is low because it is used efficiently by the bacteria. Three strategies have been proposed to address this low-yield problem: (i) glutamine synthase bacterial mutants, (ii) formation of paranodules, and (iii) facilitating the penetration of plant tissues by nitrogen-fixing bacterial endophytes that enhance colonization in a low competition niche. Nitrogenase, a major enzyme involved in the nitrogen fixation, has two components: (i) dinitrogenase reductase, the iron protein, and (ii) dinitrogenase (metal cofactor). The iron protein provides the electrons with a high reducing power to dinitrogenase, which in turn reduces N2 to NH3. Depending on the availability of metal cofactor, three types of nitrogen-fixing systems have been identified (i) Mo-nitrogenase, (ii) V-nitrogenase, and (iii) Fe-nitrogenase. Legume-rhizobia symbiosis is a cheaper source of N and an effective agronomic practice ensuring adequate supply of N than the application of fertilizer-N. Furthermore, ammonia is converted to nitrite and nitrate by nitrifying bacteria such as nitrosomonas and nitrobacter during nitrification, which is an important process in nitrogen cycle. Therefore, a reduced rate or inhibition of nitrification provides enough time to plant for assimilation of fixed N. Plants also produce secondary metabolites such as phenolic acids and flavonoids for inhibiting nitrification. The natural ability of plants to suppress nitrification is not currently recognized or utilized in agricultural production. However, they have no effects on other soil microbial community.
Soil Zn Fertilization and Inoculation with Arbuscular Mycorrhizal Fungus and Azotobacter Chroococcum Bacteria Affect the Cd Concentration and Zn Bioavailability in Bread Wheat Grown in a Cd-spiked Soil
Published in Soil and Sediment Contamination: An International Journal, 2021
Amir Hossein Baghaie, Forough Aghili
Azotobacter chroococcum, a free-living bacterium, introduced to enhance the N uptake by the plant and also increase the plant growth by different mechanisms, such as enhancing the N-fixation, phosphate solubilization, and phytohormone production (Behl et al. 2012). It has been reported that dual inoculants of A. chroococcum and AMF considerably improved the biomass production of different crop plants as compared with plants with single inoculation, i.e. A. chroococcum or AMF due to the mutualistic interaction between these two microorganisms (Jahandideh Mahjen Abadi et al. 2016).