Azotobacter: A Potential Biofertilizer and Bioinoculants for Sustainable Agriculture

Abstract

Plant growth-promoting rhizobacteria (PGPR) are best known bacterial species among all other microorganisms that have more influence on physiological and structural properties of soil. PGPR helps to replace chemical fertilizer for the sustainable agriculture production by fixing the atmospheric nitrogen and producing growth-promoting substances. Among PGPR group, Azotobacter are ubiquitous, aerobic, free-living, and N2-fixing bacteria commonly living in rhizosphere soil. Being the major group of soilborne bacteria, Azotobacter plays different beneficial roles by producing different types of vitamins, amino acids, plant growth hormones, antifungal substances, hydrogen cyanide, and siderophores. The growth-promoting substances such as indole acetic acid, gibberellic acid, arginine, etc., produced by species of Azotobacter have direct influence on shoot length, root length, and seed germination of several agricultural crops (soil rhizosphere). Some of the species of Azotobacter, viz., A. vinelandii, A. chroococcum, A. salinestris, A. tropicalis, and A. nigricans, are able to produce antimicrobial compounds which inhibit the growth of plant pathogens, viz., Fusarium, Aspergillus, Alternaria, Curvularia, and Rhizoctonia species, which can cause major plant diseases and economic losses. Azotobacter species are efficient in fixation of highest amount of nitrogen (29.21 mu g NmL(-1) day(-1)), production of indole acetic acid (24.50 mu gmL(-1)) and gibberellic acid (15.2 mu g 25 mL(-1)), and phosphate-solubilizing activity (13.4 mm). Species of Pseudomonas, Bacillus, and Azotobacter can grow and survive at extreme environmental conditions, viz., higher salt concentration, high pH environments, and even at higher temperature. Azotobacter is found tolerant to a higher NaCl concentration (6-8%), to maximum temperature (45 degrees C), and also to varied pH ranges (8-9). A. salinestris (GVT-1) culture filtrate has increased the paddy seed vigor index or growth and seed germination rate. Azotobacter species have maintained maximum levels of viable population at different temperatures in different formulations. Azotobacter species can grow and survive for periods in talc-and lignite-based formulations. In view of these properties, Azotobacter isolates can be used for sustainable agriculture as biofertilizer and bioinoculants.