Production of indole acetic acid by Rhizobium isolates from Vigna trilobata (L) Verdc.

Abstract

Rhizobium strains were isolated from root nodules of five cultivars of Vigna trilobatanamely: cultivar-1 from Prakasham District, cultivar-2 from East Godavari District, cultivar-3 from Guntur District, cultivar-4 from West Godavari District and cultivar-5 from Krishna District on yeast extract mannitol broth (YEMB) medium with bromothymol blue indicator. The strains were examined for production of acid and indole acetic acid (IAA) by utilizing different carbon sources. After 48 h of incubation, almost the five isolates produced acid ranging between 4.31 to 7.11. Isolate-4 (from CV-4) showed maximum growth on arabinose, with maximum acidic pH of 4.31. Isolate-1 and 3 (from CV-1 and 3) grow better in all carbon sources, whereas, isolate 2 and 5 (from CV-2 and 5) showed maximum growth on glucose. All the five isolates initiated IAA production immediately after inoculation and maximum was produced at 72 h in YEM broth and showed a decline afterwards. Maximum amount of IAA was produced (92.6 μg/ml) in isolate-3 at 72 h of incubation. These isolates were further tested for the production of IAA in a medium with 0.5, 1.0, 1.5, 2.0 and 2.5 mg/ml of L-Tryptophan. All the five isolates produced maximum growth in a medium containing 1 mg/ml conc. of L-Tryptophan. Among the isolates; isolate-3 produced a maximum of 80.96 μg/ml of IAA. The effect of different carbon sources on IAA production was studied by replacing mannitol in YEM broth with equal quantities of different carbon sources. Isolate-2 produced IAA at a maximum of 82.89 μg/ml followed by isolate-3, 5, 1 and 4. Key words: Rhizobium, Vigna trilobata, indole acetic acid, acid production.