Production of indole acetic acid by Kocuria rosea VB1 and Arthrobacter luteolus VB2 under the influence of L-tryptophan and maize root exudates

Phytohormones play a very important role in enhancing plant growth by direct or indirect mechanisms involving plant-microbe interactions. Indole acetic acid (IAA) is one of the key phytohormones that directly enhance plant development. Kocuria rosea VB1 (GenBank ID: KY608093.1) and Arthrobacter luteolus VB2 (GenBank ID: KY608094.1) from polluted industrial water samples were characterized through a biochemical assay, 16S rDNA sequencing, and for promoting plant growth abilities. IAA production by VB1 and VB2 was tested in pure culture conditions supplemented with various L-tryptophan (Trp) concentrations (0, 50, 100, 200, and 500 μg @ml). A significant difference in indole production by VB2 and VB1 inoculant at various L-Trp concentrations has been observed. VB1 has been reported to produce increased amounts of indole from 0.33 μg @ml to 18.16 μg @ml, while the increase in indole production was from 0.63 μg @ml to 9.22 μg @ml for VB2 for various L-Trp concentrations. The VB1 strain produced 85.07 ng @ml and 123.7 ng @ml IAA, whereas the VB2 strain produced 70.3 ng @ml and 78.4 ng @ml IAA, respectively at 200 and 500 μg @ml Trp concentrations. The growth pouch experiments with maize root exudates also showed a positive effect for both bacterial inoculants tested on IAA biosynthesis in comparison to non-inoculated seeds. Inoculation of maize seeds with VB2 and VB1 bacteria gave a significantly higher level of IAA production in comparison to non-inoculated seeds. Current study outcomes show the beneficial aspects of plant growth regulators produced by free-living bacteria which could play a significant role in plant growth promotion.