Auxins are a class of plant hormones or plant growth substances. Auxins have a cardinal role in the regulation of many growth and developmental processes in the plant. Soil samples were serially diluted and screened for auxin production by Salkowski method. Isolates that have the ability to produce auxin were identified by culture characters, morphology, and bio-chemical profile. From 150 isolates, 04 bacteria were selected (AUX-36, AUX-53, AUX-137, and AUX-142). The bacteria were identified by following the flowcharts described in “Berges Mannual of Determinative Bacteriology”, 9th addition. These isolates were identified as Bacillus megaterium, Escherichia coli, Klebsiella pneumonia and Bacillus marinus respectively. Next, different physical and chemical parameters for growth of bacteria and auxin biosynthesis were optimized. For the optimization of bacterial growth OD values of the culture broth (at wavelength 600nm) was taken by spectrophotometer. To estimate the amount of auxin (ìg/ml) produced in the culture broth; a standard curve (concentration of auxin ìg/ml at x-axis and OD value at y-axis) was prepared by using commercially available auxin.
The optimum conditions for growth and auxin production by AUX-36 was found to be pH 7, 0.98% osmotic pressure at 37 °C after 72 hours of incubation. If the medium is supplemented with 0.1 and 1.0% glucose, sucrose and peptone then it increased the bacterial growth which ultimately increased the auxin concentration in the broth medium. The growth and auxin
Production by AUX-53 was 0.98% NaCl concentration at 37 °C after 72 hours of incubation. The optimum pH was found to be 7 but it showed good growth at acidic as well as alkaline pH. The addition of glucose and sucrose in the growth medium increased the growth as well as auxin production. The optimum conditions for the growth of AUX-137 were as follows: pH=7, 0.98% osmotic pressure, temperature 37 °C. However the isolate had good growth at 28 °C and 2% NaCl concentration as well. The bacterial cell density and auxin increased with incubation time up to 72 hours. The isolate produced highest concentration of auxin under the same conditions. Similarly, the cell density and auxin increased with the increasing concentration of glucose in the growth medium. Sucrose increased the auxin only in the culture filtrate. While the bacteria AUX-142 showed highest growth as well as auxin production at 42 °C after 72 hours of incubation. The optimum pH and osmotic pressure was found to be 7 and 2% respectively. The cell density and concentration of auxin increased with the increasing concentration of peptone in the growth medium. Addition of tryptophan (1-2%) increased the auxin concentration in the culture supernatant of all isolates.
Next, the seed germination test and plant pot experiment were performed of selected isolates to observe the effect of bacterial inoculation on wheat plants. In seed germination test treatment of seeds with AUX-36, AUX-53 and AUX-142 significantly increased the root length and number of root hairs as compared to non-treated seeds. In plant pot experiment comparison of various growth parameters of inoculated plants with non-inoculated plants revealed the improvement in plant growth by bacterial inoculation.