Azotobacter: PGPR activities with special reference to effect of pesticides and biodegradation

Abstract

Among all the microorganisms, plant growth-promoting rhizobacteria (PGPR) have significant influence on soil physiological and structural properties. PGPR help to replace chemical fertilizer for the sustainable agriculture production by fixing the atmospheric nitrogen and producing growth-promoting substances. Among the PGPR group, Azotobacter are ubiquitous, aerobic, free-living, and N 2 -fixing bacteria commonly living in soil, water, and sediments. Being the major group of soilborne bacteria, Azotobacter plays different benefi cial roles and is known to produce varieties of vitamins, amino acids, plant growth hormones, antifungal substances, hydrogen cyanide, and siderophores. The growth-promoting substances such as indoleacetic acid, gibberellic acid, arginine, etc., produced by Azotobacter have direct infl uence on shoot and root length as well as seed germination of several agricultural crops. Azotobacter species are effi cient in fi xation of highest amount of nitrogen (29.21μg NmL-1 day-1), production of indoleacetic acid (24.50μgmL-1) and gibberellic acid (15.2 μg 25 mL -1), and formation of larger phosphate solubilizing zone (13.4 mm). Many species of Pseudomonas, Bacillus, and Azotobacter can grow and survive at extreme environmental conditions, viz., tolerant to higher salt concentration, pH values, and even at dry soils with maximum temperature. Different factors affect Azotobacter population in soil such as pH, phosphorus content, soil aeration and moisture contents, etc. A. chroococcum found tolerant to a maximum NaCl concentration of 6 % with a temperature of 45° C and also up to pH of 8. Azotobacter species such as A. vinelandii, A. chroococcum, A. salinestris, A. tropicalis, and A. nigricans are able to produce antimicrobial compounds which inhibit the growth of common plant pathogens, viz., Fusarium, Aspergillus, Alternaria, Curvularia, and Rhizoctonia species. Pesticides used to control pests, insects, and phytopathogens are known to cause direct effect on soil microbiological aspects, environmental pollution, and health hazards in all living beings of the soil ecosystem. The species of Azotobacter are known to tolerate up to 5 % pesticide concentration and also to degrade heavy metals and pesticides. A. chroococcum and A. vinelandii proved their biodegradation effi ciency of many commonly used pesticides, viz., endosulfan, chlorpyrifos, pendimethalin, phorate, glyphosate, and carbendazim. From these results, it is clear that the Azotobacter strains not only produce plant growth-promoting substances (PGPS) but are also tolerant to abiotic stress under different physiological conditions.