Root exudate is one of the ways for plant communication to the neighboring plant and adjoining of microorganisms present in the rhizosphere of the root. The chemicals ingredients of the root exudates are specific to a particular plant species and also depend on the nearby biotic and abiotic environment.

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The rhizosphere is an important region of microbial interactions where exudates released by plant roots are the main source of food for microorganisms, which play a vital role in increasing their population density and activities. An isolate of bacterium Pseudomonas sp., designated as KS51, that expressed plant growth-promoting traits and antagonistic activity was isolated from the rhizospheric soil of Calotropis sp., grown in the northern and central regions of India. The isolate was recognized as Gram negative rod (0.67 9 2.89 lm) shaped bacterium. It grew at optimum temperature ranging from 25 to 30°C, exhibited tolerance to a wide pH range of 6-10 and tolerated salt concentrations up to 5 % (wt/vol). Though, it was sensitive to chloramphenicol, ampicillin, tetracycline, gentamicin, ceftriaxone, and erythromycin, it showed resistance to co-trimoxazole, cefuroxime, ciprofloxacin, penicillin G, augmentin, fusidic acid and vancomycin. The bacterium was biochemically characterized and examined in vitro for their plant growth-promoting traits like production of indole acetic acid (IAA) (8 lg ml -1 day -1 ), hydrogen cyanide (HCN), siderophore and phosphate solubilization (268 lg ml -1 after 144 h). The mean growth rate constant (K) of isolate was found to increase with successive increments in substrate concentration of naphthalene and anthracene (0.5-1.0 mg/50 ml). KS51 was also found to be a good degrader for naphthalene (78.44 %) and anthracene (63.53 %) as determined by HPLC analysis. Based on the 16S rRNA analysis, KS51 showed the maximum similarity with Pseudomonas sp. On the basis of its growth-promoting, biocontrol and bioremediation potential properties, KS51 could be applied in biotechnological applications.

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