Screening of free living rhizobacteria associated with wheat rhizosphere for plant growth promoting traits

The use of plant growth promoting rhizobacteria (PGPR) for the benefits of agriculture is gaining worldwide importance and acceptance and appears to be the trend for the future. PGPR are bio-resources which may be viewed as a novel and potential tool for providing substantial benefits to the agriculture. Plant growth promoting rhizobacteria (PGPR) are known to influence plant growth by various direct or indirect mechanisms. In search of efficient PGPR strains with multiple activities, a total of 58 isolates belonging to Pseudomonas, Azotobacter and Bacillus were isolated from wheat rhizospheric soils collected from various districts of Uttar Pradesh. These rhizospheric isolates were biochemically characterized and screened for their plant growth promoting traits like production of indole acetic acid (IAA), ammonia production, siderophore production, phosphate solubilization, salt tolerance and antibiotic sensitivity test activity. The isolates of Pseudomonas (86.36%), and Azotobacter (76.13%) produced IAA, whereas only 38.09% of Bacillus isolates were able to produce IAA. Ammonia production was most common trait of Pseudomonas (90.89%), and Azotobacter (66.43) and Bacillus (76.19%). Phosphorus solubilization was detected in the isolates of Azotobacter (66.23%), Pseudomonas (45.35%), and Bacillus (23.80%). Siderophore production was exhibited by 9.61 to 20.17% of isolates. On the basis of multiple plant growth promoting activities eighteen isolates (nine Azotobacter, six Pseudomonas and three Bacillus) were evaluated for quantitative IAA production, antibacterial and salt tolerance. All the Azotobacter isolates were shown to produce higher range (95.60 to 175.20 µg/ml) of IAA, while Pseudomonas produced (44.40 to 95.60 µg/ml) IAA. The isolate Bc2 also showed potential of producing high amount of IAA. The isolate Azt5, Azt9, Ps2, Bc2 and Bc3 were found resistant even at 20 µg/ml concentration of tetracycline in the medium. Salt tolerance even at 7% NaCl concentration was observed in Azt5, Bc1 and Bc3 isolates. This study has pointed out that few isolates could exhibit PGP traits, which may promote plant growth directly and indirectly.   Key words: Plant growth promoting rhizobacteria (PGPR), wheat, indole acetic acid, ammonia, siderophore, P solubilization, salt tolerance, antibacterial activity.